Start X11 and web frontends for xC

For this, we needed a wire protocol.  After surveying available options,
it was decided to implement an XDR-like protocol code generator
in portable AWK.  It now has two backends, per each of:

 - xF, the X11 frontend, is in C, and is meant to be the primary
   user interface in the future.

 - xP, the web frontend, relies on a protocol proxy written in Go,
   and is meant for use on-the-go (no pun intended).

They are very much work-in-progress proofs of concept right now,
and the relay protocol is certain to change.

20 files changed, 2798 insertions, 91 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4de1ad2..f678592 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,10 +1,12 @@
 # Ubuntu 18.04 LTS and OpenBSD 6.4
 cmake_minimum_required (VERSION 3.10)
-project (xK VERSION 1.5.0 DESCRIPTION "IRC client, daemon and bot" LANGUAGES C)
+project (xK VERSION 1.5.0
+	DESCRIPTION "IRC daemon, bot, TUI client and X11/web frontends" LANGUAGES C)
 
 # Options
 option (WANT_READLINE "Use GNU Readline for the UI (better)" ON)
 option (WANT_LIBEDIT "Use BSD libedit for the UI" OFF)
+option (WANT_XF "Build xF" OFF)
 
 # Moar warnings
 set (CMAKE_C_STANDARD 99)
@@ -143,7 +145,8 @@ set (HAVE_EDITLINE "${WANT_LIBEDIT}")
 set (HAVE_LUA      "${WITH_LUA}")
 
 include (GNUInstallDirs)
-configure_file (${PROJECT_SOURCE_DIR}/config.h.in ${PROJECT_BINARY_DIR}/config.h)
+set (project_config ${PROJECT_BINARY_DIR}/config.h)
+configure_file (${PROJECT_SOURCE_DIR}/config.h.in ${project_config})
 include_directories (${PROJECT_SOURCE_DIR} ${PROJECT_BINARY_DIR})
 
 # Generate IRC replies--we need a custom target because of the multiple outputs
@@ -157,17 +160,40 @@ add_custom_command (OUTPUT xD-replies.c xD.msg
 	COMMENT "Generating files from the list of server numerics")
 add_custom_target (replies DEPENDS ${PROJECT_BINARY_DIR}/xD-replies.c)
 
+add_custom_command (OUTPUT xC-proto.c
+	COMMAND env LC_ALL=C awk
+		-f ${PROJECT_SOURCE_DIR}/xC-gen-proto.awk
+		-f ${PROJECT_SOURCE_DIR}/xC-gen-proto-c.awk
+		${PROJECT_SOURCE_DIR}/xC-proto > xC-proto.c
+	DEPENDS
+		${PROJECT_SOURCE_DIR}/xC-gen-proto.awk
+		${PROJECT_SOURCE_DIR}/xC-gen-proto-c.awk
+		${PROJECT_SOURCE_DIR}/xC-proto
+	COMMENT "Generating xC relay protocol code")
+add_custom_target (xC-proto DEPENDS ${PROJECT_BINARY_DIR}/xC-proto.c)
+
 # Build
 foreach (name xB xC xD)
-	add_executable (${name} ${name}.c ${PROJECT_BINARY_DIR}/config.h)
+	add_executable (${name} ${name}.c ${project_config})
 	target_link_libraries (${name} ${project_libraries})
 	add_threads (${name})
 endforeach ()
 
 add_dependencies (xD replies)
-add_dependencies (xC replies)
+add_dependencies (xC replies xC-proto)
 target_link_libraries (xC ${xC_libraries})
 
+if (WANT_XF)
+	pkg_check_modules (x11 REQUIRED x11 xrender xft fontconfig)
+	include_directories (${x11_INCLUDE_DIRS})
+	link_directories (${x11_LIBRARY_DIRS})
+
+	add_executable (xF xF.c ${project_config})
+	add_dependencies (xF xC-proto)
+	target_link_libraries (xF ${x11_LIBRARIES} ${project_libraries})
+	add_threads (xF)
+endif ()
+
 # Tests
 include (CTest)
 if (BUILD_TESTING)
diff --git a/NEWS b/NEWS
index e521b33..5b187c9 100644
--- a/NEWS
+++ b/NEWS
@@ -1,5 +1,9 @@
 Unreleased
 
+ * xD: implemented WALLOPS, choosing to make it target even non-operators
+
+ * xC: made it show WALLOPS messages, as PRIVMSG for the server buffer
+
  * xC: all behaviour.* configuration options have been renamed to general.*,
    with the exception of editor_command/editor, backlog_helper/pager,
    and backlog_helper_strip_formatting/pager_strip_formatting
@@ -10,11 +14,13 @@ Unreleased
 
  * xC: normalized editline's history behaviour, making it a viable frontend
 
- * xC: made it show WALLOPS messages, as PRIVMSG for the server buffer
-
  * xC: various bugfixes
 
- * xD: implemented WALLOPS, choosing to make it target even non-operators
+ * xC: added a relay interface, enabled through the general.relay_bind option
+
+ * Added an experimental X11 frontend for xC called xF
+
+ * Added an experimental web frontend for xC called xP
 
 
 1.5.0 (2021-12-21) "The Show Must Go On"
diff --git a/README.adoc b/README.adoc
index 11eb609..559c69d 100644
--- a/README.adoc
+++ b/README.adoc
@@ -1,9 +1,9 @@
 xK
 ==
 
-'xK' (chat kit) is an IRC software suite consisting of a terminal client,
-daemon, and bot.  It's all you're ever going to need for chatting,
-so long as you can make do with slightly minimalist software.
+'xK' (chat kit) is an IRC software suite consisting of a daemon, bot, terminal
+client, and X11/web frontends for the client.  It's all you're ever going to
+need for chatting, so long as you can make do with slightly minimalist software.
 
 They're all lean on dependencies, and offer a maximally permissive licence.
 
@@ -20,8 +20,18 @@ a powerful configuration system, integrated help, text formatting, automatic
 message splitting, multiline editing, bracketed paste support, word wrapping
 that doesn't break links, autocomplete, logging, CTCP queries, auto-away,
 command aliases, SOCKS proxying, SASL EXTERNAL authentication using TLS client
-certificates, or basic support for Lua scripting.  As a unique bonus, you can
-launch a full text editor from within.
+certificates, a remote relay interface, or basic support for Lua scripting.
+As a unique bonus, you can launch a full text editor from within.
+
+xF
+--
+The X11 frontend for 'xC', making use of its networked relay interface.
+It's currently in development, and is hidden behind a CMake option.
+
+xP
+--
+The web frontend for 'xC', making use of its networked relay interface.
+It's currently rather elementary, and can be built from within its directory.
 
 xD
 --
@@ -38,9 +48,8 @@ What it notably doesn't support is online changes to configuration, any limits
 besides the total number of connections and mode `+l`, or server linking
 (which also means no services).
 
-This program has been
-https://git.janouch.name/p/haven/src/branch/master/hid[ported to Go],
-and development continues over there.
+This program has been https://git.janouch.name/p/haven/src/branch/master/hid[
+ported to Go] in a different project, and development continues over there.
 
 xB
 --
@@ -60,11 +69,12 @@ a package with the latest development version from Archlinux's AUR.
 
 Building
 --------
-Build dependencies: CMake, pkg-config, asciidoctor or asciidoc, awk,
-                    liberty (included) +
-Runtime dependencies: openssl +
-Additionally for 'xC': curses, libffi, lua >= 5.3 (optional),
-                       readline >= 6.0 or libedit >= 2013-07-12
+Build-only dependencies:
+ CMake, pkg-config, asciidoctor or asciidoc, awk, liberty (included) +
+Common runtime dependencies: openssl +
+Additionally for 'xC': curses, libffi, +
+ readline >= 6.0 or libedit >= 2013-07-12, lua >= 5.3 (optional) +
+Additionally for 'xF': x11, xft
 
  $ git clone --recursive https://git.janouch.name/p/xK.git
  $ mkdir xK/build
diff --git a/common.c b/common.c
index d9d2a61..6f99509 100644
--- a/common.c
+++ b/common.c
@@ -1,7 +1,7 @@
 /*
  * common.c: common functionality
  *
- * Copyright (c) 2014 - 2020, Přemysl Eric Janouch <p@janouch.name>
+ * Copyright (c) 2014 - 2022, Přemysl Eric Janouch <p@janouch.name>
  *
  * Permission to use, copy, modify, and/or distribute this software for any
  * purpose with or without fee is hereby granted.
@@ -48,6 +48,66 @@ init_openssl (void)
 #endif
 }
 
+static char *
+gai_reconstruct_address (struct addrinfo *ai)
+{
+	char host[NI_MAXHOST] = {}, port[NI_MAXSERV] = {};
+	int err = getnameinfo (ai->ai_addr, ai->ai_addrlen,
+		host, sizeof host, port, sizeof port,
+		NI_NUMERICHOST | NI_NUMERICSERV);
+	if (err)
+	{
+		print_debug ("%s: %s", "getnameinfo", gai_strerror (err));
+		return xstrdup ("?");
+	}
+	return format_host_port_pair (host, port);
+}
+
+static bool
+accept_error_is_transient (int err)
+{
+	// OS kernels may return a wide range of unforeseeable errors.
+	// Assuming that they're either transient or caused by
+	// a connection that we've just extracted from the queue.
+	switch (err)
+	{
+	case EBADF:
+	case EINVAL:
+	case ENOTSOCK:
+	case EOPNOTSUPP:
+		return false;
+	default:
+		return true;
+	}
+}
+
+/// Destructively tokenize an address into a host part, and a port part.
+/// The port is only overwritten if that part is found, allowing for defaults.
+static const char *
+tokenize_host_port (char *address, const char **port)
+{
+	// Unwrap IPv6 addresses in format_host_port_pair() format.
+	char *rbracket = strchr (address, ']');
+	if (*address == '[' && rbracket)
+	{
+		if (rbracket[1] == ':')
+		{
+			*port = rbracket + 2;
+			return *rbracket = 0, address + 1;
+		}
+		if (!rbracket[1])
+			return *rbracket = 0, address + 1;
+	}
+
+	char *colon = strchr (address, ':');
+	if (colon)
+	{
+		*port = colon + 1;
+		return *colon = 0, address;
+	}
+	return address;
+}
+
 // --- To be moved to liberty --------------------------------------------------
 
 // FIXME: in xssl_get_error() we rely on error reasons never being NULL (i.e.,
@@ -74,6 +134,15 @@ xerr_describe_error (void)
 	return reason;
 }
 
+static struct str
+str_from_cstr (const char *cstr)
+{
+	struct str self;
+	self.alloc = (self.len = strlen (cstr)) + 1;
+	self.str = memcpy (xmalloc (self.alloc), cstr, self.alloc);
+	return self;
+}
+
 static ssize_t
 strv_find (const struct strv *v, const char *s)
 {
diff --git a/xC-gen-proto-c.awk b/xC-gen-proto-c.awk
new file mode 100644
index 0000000..e7faef0
--- /dev/null
+++ b/xC-gen-proto-c.awk
@@ -0,0 +1,325 @@
+# xC-gen-proto-c.awk: C backend for xC-gen-proto.awk.
+#
+# Copyright (c) 2022, Přemysl Eric Janouch <p@janouch.name>
+# SPDX-License-Identifier: 0BSD
+#
+# Neither *_new() nor *_destroy() functions are provided, because they'd only
+# be useful for top-levels, and are merely extra malloc()/free() calls.
+# Users are expected to reuse buffers.
+#
+# Similarly, no constructors are produced--those are easy to write manually.
+#
+# All arrays are deserialized zero-terminated, so u8<> and i8<> can be directly
+# used as C strings.
+#
+# All types must be able to dispose partially zero values going from the back,
+# i.e., in the reverse order of deserialization.
+
+function define_internal(name, ctype) {
+	Types[name] = "internal"
+	CodegenCType[name] = ctype
+}
+
+function define_int(shortname, ctype) {
+	define_internal(shortname, ctype)
+	CodegenSerialize[shortname] = \
+		"\tstr_pack_" shortname "(w, %s);\n"
+	CodegenDeserialize[shortname] = \
+		"\tif (!msg_unpacker_" shortname "(r, &%s))\n" \
+		"\t\treturn false;\n"
+}
+
+function define_sint(size) { define_int("i" size, "int" size "_t") }
+function define_uint(size) { define_int("u" size, "uint" size "_t") }
+
+function codegen_begin() {
+	define_sint("8")
+	define_sint("16")
+	define_sint("32")
+	define_sint("64")
+	define_uint("8")
+	define_uint("16")
+	define_uint("32")
+	define_uint("64")
+
+	define_internal("string", "struct str")
+	CodegenDispose["string"] = "\tstr_free(&%s);\n"
+	CodegenSerialize["string"] = \
+		"\tif (!proto_string_serialize(&%s, w))\n" \
+		"\t\treturn false;\n"
+	CodegenDeserialize["string"] = \
+		"\tif (!proto_string_deserialize(&%s, r))\n" \
+		"\t\treturn false;\n"
+
+	define_internal("bool", "bool")
+	CodegenSerialize["bool"] = \
+		"\tstr_pack_u8(w, !!%s);\n"
+	CodegenDeserialize["bool"] = \
+		"\t{\n" \
+		"\t\tuint8_t v = 0;\n" \
+		"\t\tif (!msg_unpacker_u8(r, &v))\n" \
+		"\t\t\treturn false;\n" \
+		"\t\t%s = !!v;\n" \
+		"\t}\n"
+
+	print "// This file directly depends on liberty.c, but doesn't include it."
+
+	print ""
+	print "static bool"
+	print "proto_string_serialize(const struct str *s, struct str *w) {"
+	print "\tif (s->len > UINT32_MAX)"
+	print "\t\treturn false;"
+	print "\tstr_pack_u32(w, s->len);"
+	print "\tstr_append_str(w, s);"
+	print "\treturn true;"
+	print "}"
+
+	print ""
+	print "static bool"
+	print "proto_string_deserialize(struct str *s, struct msg_unpacker *r) {"
+	print "\tuint32_t len = 0;"
+	print "\tif (!msg_unpacker_u32(r, &len))"
+	print "\t\treturn false;"
+	print "\tif (msg_unpacker_get_available(r) < len)"
+	print "\t\treturn false;"
+	print "\t*s = str_make();"
+	print "\tstr_append_data(s, r->data + r->offset, len);"
+	print "\tr->offset += len;"
+	print "\tif (!utf8_validate (s->str, s->len))"
+	print "\t\treturn false;"
+	print "\treturn true;"
+	print "}"
+}
+
+function codegen_constant(name, value) {
+	print ""
+	print "enum { " PrefixUpper name " = " value " };"
+}
+
+function codegen_enum_value(name, subname, value, cg) {
+	append(cg, "fields",
+		"\t" PrefixUpper toupper(cameltosnake(name)) "_" subname \
+		" = " value ",\n")
+}
+
+function codegen_enum(name, cg,    ctype) {
+	ctype = "enum " PrefixLower cameltosnake(name)
+	print ""
+	print ctype " {"
+	print cg["fields"] "};"
+
+	# XXX: This should also check if it isn't out-of-range for any reason,
+	# but our usage of sprintf() stands in the way a bit.
+	CodegenSerialize[name] = "\tstr_pack_i32(w, %s);\n"
+	CodegenDeserialize[name] = \
+		"\t{\n" \
+		"\t\tint32_t v = 0;\n" \
+		"\t\tif (!msg_unpacker_i32(r, &v) || !v)\n" \
+		"\t\t\treturn false;\n" \
+		"\t\t%s = v;\n" \
+		"\t}\n"
+
+	CodegenCType[name] = ctype
+	for (i in cg)
+		delete cg[i]
+}
+
+function codegen_struct_tag(d, cg,    f) {
+	f = "self->" d["name"]
+	append(cg, "fields", "\t" CodegenCType[d["type"]] " " d["name"] ";\n")
+	append(cg, "dispose", sprintf(CodegenDispose[d["type"]], f))
+	append(cg, "serialize", sprintf(CodegenSerialize[d["type"]], f))
+	# Do not deserialize here, that would be out of order.
+}
+
+function codegen_struct_field(d, cg,    f, dispose, serialize, deserialize) {
+	f = "self->" d["name"]
+	dispose = CodegenDispose[d["type"]]
+	serialize = CodegenSerialize[d["type"]]
+	deserialize = CodegenDeserialize[d["type"]]
+	if (!d["isarray"]) {
+		append(cg, "fields", "\t" CodegenCType[d["type"]] " " d["name"] ";\n")
+		append(cg, "dispose", sprintf(dispose, f))
+		append(cg, "serialize", sprintf(serialize, f))
+		append(cg, "deserialize", sprintf(deserialize, f))
+		return
+	}
+
+	append(cg, "fields",
+		"\t" CodegenCType["u32"] " " d["name"] "_len;\n" \
+		"\t" CodegenCType[d["type"]] " *" d["name"] ";\n")
+
+	if (dispose)
+		append(cg, "dispose", "\tif (" f ")\n" \
+			"\t\tfor (size_t i = 0; i < " f "_len; i++)\n" \
+			indent(indent(sprintf(dispose, f "[i]"))))
+	append(cg, "dispose", "\tfree(" f ");\n")
+
+	append(cg, "serialize", sprintf(CodegenSerialize["u32"], f "_len"))
+	if (d["type"] == "u8" || d["type"] == "i8") {
+		append(cg, "serialize",
+			"\tstr_append_data(w, " f ", " f "_len);\n")
+	} else if (serialize) {
+		append(cg, "serialize",
+			"\tfor (size_t i = 0; i < " f "_len; i++)\n" \
+			indent(sprintf(serialize, f "[i]")))
+	}
+
+	append(cg, "deserialize", sprintf(CodegenDeserialize["u32"], f "_len") \
+		"\tif (!(" f " = calloc(" f "_len + 1, sizeof *" f ")))\n" \
+		"\t\treturn false;\n")
+	if (d["type"] == "u8" || d["type"] == "i8") {
+		append(cg, "deserialize",
+			"\tif (msg_unpacker_get_available(r) < " f "_len)\n" \
+			"\t\treturn false;\n" \
+			"\tmemcpy(" f ", r->data + r->offset, " f "_len);\n" \
+			"\tr->offset += " f "_len;\n")
+	} else if (deserialize) {
+		append(cg, "deserialize",
+			"\tfor (size_t i = 0; i < " f "_len; i++)\n" \
+			indent(sprintf(deserialize, f "[i]")))
+	}
+}
+
+function codegen_struct(name, cg,    ctype, funcname) {
+	ctype = "struct " PrefixLower cameltosnake(name)
+	print ""
+	print ctype " {"
+	print cg["fields"] "};"
+
+	if (cg["dispose"]) {
+		funcname = PrefixLower cameltosnake(name) "_free"
+		print ""
+		print "static void\n" funcname "(" ctype " *self) {"
+		print cg["dispose"] "}"
+
+		CodegenDispose[name] = "\t" funcname "(&%s);\n"
+	}
+	if (cg["serialize"]) {
+		funcname = PrefixLower cameltosnake(name) "_serialize"
+		print ""
+		print "static bool\n" \
+			  funcname "(\n\t\t" ctype " *self, struct str *w) {"
+		print cg["serialize"] "\treturn true;"
+		print "}"
+
+		CodegenSerialize[name] = "\tif (!" funcname "(&%s, w))\n" \
+			"\t\treturn false;\n"
+	}
+	if (cg["deserialize"]) {
+		funcname = PrefixLower cameltosnake(name) "_deserialize"
+		print ""
+		print "static bool\n" \
+			  funcname "(\n\t\t" ctype " *self, struct msg_unpacker *r) {"
+		print cg["deserialize"] "\treturn true;"
+		print "}"
+
+		CodegenDeserialize[name] = "\tif (!" funcname "(&%s, r))\n" \
+			"\t\treturn false;\n"
+	}
+
+	CodegenCType[name] = ctype
+	for (i in cg)
+		delete cg[i]
+}
+
+function codegen_union_tag(d, cg) {
+	cg["tagtype"] = d["type"]
+	cg["tagname"] = d["name"]
+	append(cg, "fields", "\t" CodegenCType[d["type"]] " " d["name"] ";\n")
+}
+
+function codegen_union_struct( \
+		name, casename, cg, scg,     structname, fieldname, fullcasename) {
+	# Don't generate obviously useless structs.
+	fullcasename = toupper(cameltosnake(cg["tagtype"])) "_" casename
+	if (!scg["dispose"] && !scg["deserialize"]) {
+		append(cg, "structless", "\tcase " PrefixUpper fullcasename ":\n")
+		for (i in scg)
+			delete scg[i]
+		return
+	}
+
+	# And thus not all generated structs are present in Types.
+	structname = name "_" casename
+	fieldname = tolower(casename)
+	codegen_struct(structname, scg)
+
+	append(cg, "fields", "\t" CodegenCType[structname] " " fieldname ";\n")
+	if (CodegenDispose[structname])
+		append(cg, "dispose", "\tcase " PrefixUpper fullcasename ":\n" \
+			indent(sprintf(CodegenDispose[structname], "self->" fieldname)) \
+			"\t\tbreak;\n")
+
+	# With no de/serialization code, this will simply recognize the tag.
+	append(cg, "serialize", "\tcase " PrefixUpper fullcasename ":\n" \
+		indent(sprintf(CodegenSerialize[structname], "self->" fieldname)) \
+		"\t\tbreak;\n")
+	append(cg, "deserialize", "\tcase " PrefixUpper fullcasename ":\n" \
+		indent(sprintf(CodegenDeserialize[structname], "self->" fieldname)) \
+		"\t\tbreak;\n")
+}
+
+function codegen_union(name, cg,    f, ctype, funcname) {
+	ctype = "union " PrefixLower cameltosnake(name)
+	print ""
+	print ctype " {"
+	print cg["fields"] "};"
+
+	f = "self->" cg["tagname"]
+	if (cg["dispose"]) {
+		funcname = PrefixLower cameltosnake(name) "_free"
+		print ""
+		print "static void\n" funcname "(" ctype " *self) {"
+		print "\tswitch (" f ") {"
+		if (cg["structless"])
+			print cg["structless"] \
+				indent(sprintf(CodegenDispose[cg["tagtype"]], f)) "\t\tbreak;"
+		print cg["dispose"] "\tdefault:"
+		print "\t\tbreak;"
+		print "\t}"
+		print "}"
+
+		CodegenDispose[name] = "\t" funcname "(&%s);\n"
+	}
+	if (cg["serialize"]) {
+		funcname = PrefixLower cameltosnake(name) "_serialize"
+		print ""
+		print "static bool\n" \
+			  funcname "(\n\t\t" ctype " *self, struct str *w) {"
+		print "\tswitch (" f ") {"
+		if (cg["structless"])
+			print cg["structless"] \
+				indent(sprintf(CodegenSerialize[cg["tagtype"]], f)) "\t\tbreak;"
+		print cg["serialize"] "\tdefault:"
+		print "\t\treturn false;"
+		print "\t}"
+		print "\treturn true;"
+		print "}"
+
+		CodegenSerialize[name] = "\tif (!" funcname "(&%s, w))\n" \
+			"\t\treturn false;\n"
+	}
+	if (cg["deserialize"]) {
+		funcname = PrefixLower cameltosnake(name) "_deserialize"
+		print ""
+		print "static bool\n" \
+			  funcname "(\n\t\t" ctype " *self, struct msg_unpacker *r) {"
+		print sprintf(CodegenDeserialize[cg["tagtype"]], f)
+		print "\tswitch (" f ") {"
+		if (cg["structless"])
+			print cg["structless"] "\t\tbreak;"
+		print cg["deserialize"] "\tdefault:"
+		print "\t\treturn false;"
+		print "\t}"
+		print "\treturn true;"
+		print "}"
+
+		CodegenDeserialize[name] = "\tif (!" funcname "(&%s, r))\n" \
+			"\t\treturn false;\n"
+	}
+
+	CodegenCType[name] = ctype
+	for (i in cg)
+		delete cg[i]
+}
diff --git a/xC-gen-proto-go.awk b/xC-gen-proto-go.awk
new file mode 100644
index 0000000..1880de7
--- /dev/null
+++ b/xC-gen-proto-go.awk
@@ -0,0 +1,447 @@
+# xC-gen-proto-go.awk: Go backend for xC-gen-proto.awk.
+#
+# Copyright (c) 2022, Přemysl Eric Janouch <p@janouch.name>
+# SPDX-License-Identifier: 0BSD
+#
+# This backend also enables proxying to other endpoints using JSON.
+
+function define_internal(name, gotype) {
+	Types[name] = "internal"
+	CodegenGoType[name] = gotype
+}
+
+function define_sint(size,    shortname, gotype) {
+	shortname = "i" size
+	gotype = "int" size
+	define_internal(shortname, gotype)
+
+	if (size == 8) {
+		CodegenSerialize[shortname] = "\tdata = append(data, uint8(%s))\n"
+		CodegenDeserialize[shortname] = \
+			"\tif len(data) >= 1 {\n" \
+			"\t\t%s, data = int8(data[0]), data[1:]\n" \
+			"\t} else {\n" \
+			"\t\treturn nil, false\n" \
+			"\t}\n"
+		return
+	}
+
+	CodegenSerialize[shortname] = \
+		"\tdata = binary.BigEndian.AppendUint" size "(data, uint" size "(%s))\n"
+	CodegenDeserialize[shortname] = \
+		"\tif len(data) >= " (size / 8) " {\n" \
+		"\t\t%s = " gotype "(binary.BigEndian.Uint" size "(data))\n" \
+		"\t\tdata = data[" (size / 8) ":]\n" \
+		"\t} else {\n" \
+		"\t\treturn nil, false\n" \
+		"\t}\n"
+}
+
+function define_uint(size,    shortname, gotype) {
+	shortname = "u" size
+	gotype = "uint" size
+	define_internal(shortname, gotype)
+
+	# Both byte and uint8 luckily marshal as base64-encoded JSON strings.
+	if (size == 8) {
+		CodegenSerialize[shortname] = "\tdata = append(data, %s)\n"
+		CodegenDeserialize[shortname] = \
+			"\tif len(data) >= 1 {\n" \
+			"\t\t%s, data = data[0], data[1:]\n" \
+			"\t} else {\n" \
+			"\t\treturn nil, false\n" \
+			"\t}\n"
+		return
+	}
+
+	CodegenSerialize[shortname] = \
+		"\tdata = binary.BigEndian.AppendUint" size "(data, %s)\n"
+	CodegenDeserialize[shortname] = \
+		"\tif len(data) >= " (size / 8) " {\n" \
+		"\t\t%s = binary.BigEndian.Uint" size "(data)\n" \
+		"\t\tdata = data[" (size / 8) ":]\n" \
+		"\t} else {\n" \
+		"\t\treturn nil, false\n" \
+		"\t}\n"
+}
+
+function codegen_begin() {
+	define_sint("8")
+	define_sint("16")
+	define_sint("32")
+	define_sint("64")
+	define_uint("8")
+	define_uint("16")
+	define_uint("32")
+	define_uint("64")
+
+	define_internal("bool", "bool")
+	CodegenSerialize["bool"] = \
+		"\tif %s {\n" \
+		"\t\tdata = append(data, 1)\n" \
+		"\t} else {\n" \
+		"\t\tdata = append(data, 0)\n" \
+		"\t}\n"
+	CodegenDeserialize["bool"] = \
+		"\tif data, ok = protoConsumeBoolFrom(data, &%s); !ok {\n" \
+		"\t\treturn nil, ok\n" \
+		"\t}\n"
+
+	define_internal("string", "string")
+	CodegenSerialize["string"] = \
+		"\tif data, ok = protoAppendStringTo(data, %s); !ok {\n" \
+		"\t\treturn nil, ok\n" \
+		"\t}\n"
+	CodegenDeserialize["string"] = \
+		"\tif data, ok = protoConsumeStringFrom(data, &%s); !ok {\n" \
+		"\t\treturn nil, ok\n" \
+		"\t}\n"
+
+	print "package main"
+	print ""
+	print "import ("
+	print "\t`encoding/binary`"
+	print "\t`encoding/json`"
+	print "\t`errors`"
+	print "\t`math`"
+	print "\t`strconv`"
+	print "\t`unicode/utf8`"
+	print ")"
+	print ""
+
+	print "// protoConsumeBoolFrom tries to deserialize a boolean value"
+	print "// from the beginning of a byte stream. When successful,"
+	print "// it returns a subslice with any data that might follow."
+	print "func protoConsumeBoolFrom(data []byte, b *bool) ([]byte, bool) {"
+	print "\tif len(data) < 1 {"
+	print "\t\treturn nil, false"
+	print "\t}"
+	print "\tif data[0] != 0 {"
+	print "\t\t*b = true"
+	print "\t} else {"
+	print "\t\t*b = false"
+	print "\t}"
+	print "\treturn data[1:], true"
+	print "}"
+	print ""
+
+	print "// protoAppendStringTo tries to serialize a string value,"
+	print "// appending it to the end of a byte stream."
+	print "func protoAppendStringTo(data []byte, s string) ([]byte, bool) {"
+	print "\tif len(s) > math.MaxUint32 {"
+	print "\t\treturn nil, false"
+	print "\t}"
+	print "\tdata = binary.BigEndian.AppendUint32(data, uint32(len(s)))"
+	print "\treturn append(data, s...), true"
+	print "}"
+	print ""
+
+	print "// protoConsumeStringFrom tries to deserialize a string value"
+	print "// from the beginning of a byte stream. When successful,"
+	print "// it returns a subslice with any data that might follow."
+	print "func protoConsumeStringFrom(data []byte, s *string) ([]byte, bool) {"
+	print "\tif len(data) < 4 {"
+	print "\t\treturn nil, false"
+	print "\t}"
+	print "\tlength := binary.BigEndian.Uint32(data)"
+	print "\tif data = data[4:]; uint64(len(data)) < uint64(length) {"
+	print "\t\treturn nil, false"
+	print "\t}"
+	print "\t*s = string(data[:length])"
+	print "\tif !utf8.ValidString(*s) {"
+	print "\t\treturn nil, false"
+	print "\t}"
+	print "\treturn data[length:], true"
+	print "}"
+	print ""
+
+	print "// protoUnmarshalEnumJSON converts a JSON fragment to an integer,"
+	print "// ensuring that it's within the expected range of enum values."
+	print "func protoUnmarshalEnumJSON(data []byte) (int64, error) {"
+	print "\tvar n int64"
+	print "\tif err := json.Unmarshal(data, &n); err != nil {"
+	print "\t\treturn 0, err"
+	print "\t} else if n > math.MaxInt32 || n < math.MinInt32 {"
+	print "\t\treturn 0, errors.New(`integer out of range`)"
+	print "\t} else {"
+	print "\t\treturn n, nil"
+	print "\t}"
+	print "}"
+	print ""
+}
+
+function codegen_constant(name, value) {
+	print "const " PrefixCamel snaketocamel(name) " = " value
+	print ""
+}
+
+function codegen_enum_value(name, subname, value, cg,    goname) {
+	goname = PrefixCamel name snaketocamel(subname)
+	append(cg, "fields",
+		"\t" goname " = " value "\n")
+	append(cg, "stringer",
+		"\tcase " goname ":\n" \
+		"\t\treturn `" snaketocamel(subname) "`\n")
+	append(cg, "marshal",
+		goname ",\n")
+	append(cg, "unmarshal",
+		"\tcase `" snaketocamel(subname) "`:\n" \
+		"\t\t*v = " goname "\n")
+}
+
+function codegen_enum(name, cg,    gotype, fields) {
+	gotype = PrefixCamel name
+	print "type " gotype " int"
+	print ""
+
+	print "const ("
+	print cg["fields"] ")"
+	print ""
+
+	print "func (v " gotype ") String() string {"
+	print "\tswitch v {"
+	print cg["stringer"] "\tdefault:"
+	print "\t\treturn strconv.Itoa(int(v))"
+	print "\t}"
+	print "}"
+	print ""
+
+	fields = cg["marshal"]
+	sub(/,\n$/, ":", fields)
+	gsub(/\n/, "\n\t", fields)
+	print "func (v " gotype ") MarshalJSON() ([]byte, error) {"
+	print "\tswitch v {"
+	print indent("case " fields)
+	print "\t\treturn json.Marshal(v.String())"
+	print "\t}"
+	print "\treturn json.Marshal(int(v))"
+	print "}"
+	print ""
+
+	print "func (v *" gotype ") UnmarshalJSON(data []byte) error {"
+	print "\tvar s string"
+	print "\tif json.Unmarshal(data, &s) == nil {"
+	print "\t\t// Handled below."
+	print "\t} else if n, err := protoUnmarshalEnumJSON(data); err != nil {"
+	print "\t\treturn err"
+	print "\t} else {"
+	print "\t\t*v = " gotype "(n)"
+	print "\t\treturn nil"
+	print "\t}"
+	print ""
+	print "\tswitch s {"
+	print cg["unmarshal"] "\tdefault:"
+	print "\t\treturn errors.New(`unrecognized value: ` + s)"
+	print "\t}"
+	print "\treturn nil"
+	print "}"
+	print ""
+
+	# XXX: This should also check if it isn't out-of-range for any reason,
+	# but our usage of sprintf() stands in the way a bit.
+	CodegenSerialize[name] = \
+		"\tdata = binary.BigEndian.AppendUint32(data, uint32(%s))\n"
+	CodegenDeserialize[name] = \
+		"\tif len(data) >= 4 {\n" \
+		"\t\t%s = " gotype "(int32(binary.BigEndian.Uint32(data)))\n" \
+		"\t\tdata = data[4:]\n" \
+		"\t} else {\n" \
+		"\t\treturn nil, false\n" \
+		"\t}\n"
+
+	CodegenGoType[name] = gotype
+	for (i in cg)
+		delete cg[i]
+}
+
+function codegen_struct_field(d, cg,    camel, f, serialize, deserialize) {
+	camel = snaketocamel(d["name"])
+	f = "s." camel
+	serialize = CodegenSerialize[d["type"]]
+	deserialize = CodegenDeserialize[d["type"]]
+	if (!d["isarray"]) {
+		append(cg, "fields", "\t" camel " " CodegenGoType[d["type"]] \
+			" `json:\"" decapitalize(camel) "\"`\n")
+		append(cg, "serialize", sprintf(serialize, f))
+		append(cg, "deserialize", sprintf(deserialize, f))
+		return
+	}
+
+	append(cg, "fields", "\t" camel " []" CodegenGoType[d["type"]] \
+		" `json:\"" decapitalize(camel) "\"`\n")
+
+	# XXX: This should also check if it isn't out-of-range for any reason.
+	append(cg, "serialize",
+		sprintf(CodegenSerialize["u32"], "uint32(len(" f "))"))
+	if (d["type"] == "u8") {
+		append(cg, "serialize",
+			"\tdata = append(data, " f "...)\n")
+	} else {
+		append(cg, "serialize",
+			"\tfor i := 0; i < len(" f "); i++ {\n" \
+			indent(sprintf(serialize, f "[i]")) \
+			"\t}\n")
+	}
+
+	append(cg, "deserialize",
+		"\t{\n" \
+		"\t\tvar length uint32\n" \
+		indent(sprintf(CodegenDeserialize["u32"], "length")))
+	if (d["type"] == "u8") {
+		append(cg, "deserialize",
+			"\t\tif uint64(len(data)) < uint64(length) {\n" \
+			"\t\t\treturn nil, false\n" \
+			"\t\t}\n" \
+			"\t\t" f ", data = data[:length], data[length:]\n" \
+			"\t}\n")
+	} else {
+		append(cg, "deserialize",
+			"\t\t" f " = make([]" CodegenGoType[d["type"]] ", length)\n" \
+			"\t}\n" \
+			"\tfor i := 0; i < len(" f "); i++ {\n" \
+			indent(sprintf(deserialize, f "[i]")) \
+			"\t}\n")
+	}
+}
+
+function codegen_struct_tag(d, cg,    camel, f) {
+	camel = snaketocamel(d["name"])
+	f = "s." camel
+	append(cg, "fields", "\t" camel " " CodegenGoType[d["type"]] \
+		" `json:\"" decapitalize(camel) "\"`\n")
+	append(cg, "serialize", sprintf(CodegenSerialize[d["type"]], f))
+	# Do not deserialize here, that is already done by the containing union.
+}
+
+function codegen_struct(name, cg,    gotype) {
+	gotype = PrefixCamel name
+	print "type " gotype " struct {\n" cg["fields"] "}\n"
+
+	if (cg["serialize"]) {
+		print "func (s *" gotype ") AppendTo(data []byte) ([]byte, bool) {"
+		print "\tok := true"
+		print cg["serialize"] "\treturn data, ok"
+		print "}"
+		print ""
+
+		CodegenSerialize[name] = \
+			"\tif data, ok = %s.AppendTo(data); !ok {\n" \
+			"\t\treturn nil, ok\n" \
+			"\t}\n"
+	}
+	if (cg["deserialize"]) {
+		print "func (s *" gotype ") ConsumeFrom(data []byte) ([]byte, bool) {"
+		print "\tok := true"
+		print cg["deserialize"] "\treturn data, ok"
+		print "}"
+		print ""
+
+		CodegenDeserialize[name] = \
+			"\tif data, ok = %s.ConsumeFrom(data); !ok {\n" \
+			"\t\treturn nil, ok\n" \
+			"\t}\n"
+	}
+
+	CodegenGoType[name] = gotype
+	for (i in cg)
+		delete cg[i]
+}
+
+function codegen_union_tag(d, cg) {
+	cg["tagtype"] = d["type"]
+	cg["tagname"] = d["name"]
+	# The tag is implied from the type of struct stored in the interface.
+}
+
+function codegen_union_struct(name, casename, cg, scg,     structname, init) {
+	# And thus not all generated structs are present in Types.
+	structname = name snaketocamel(casename)
+	codegen_struct(structname, scg)
+
+	init = CodegenGoType[structname] "{" snaketocamel(cg["tagname"]) \
+		": " decapitalize(snaketocamel(cg["tagname"])) "}"
+	append(cg, "unmarshal",
+		"\tcase " CodegenGoType[cg["tagtype"]] snaketocamel(casename) ":\n" \
+		"\t\ts := " init "\n" \
+		"\t\terr = json.Unmarshal(data, &s)\n" \
+		"\t\tu.Interface = s\n")
+	append(cg, "serialize",
+		"\tcase " CodegenGoType[structname] ":\n" \
+		indent(sprintf(CodegenSerialize[structname], "union")))
+	append(cg, "deserialize",
+		"\tcase " CodegenGoType[cg["tagtype"]] snaketocamel(casename) ":\n" \
+		"\t\ts := " init "\n" \
+		indent(sprintf(CodegenDeserialize[structname], "s")) \
+		"\t\tu.Interface = s\n")
+}
+
+function codegen_union(name, cg,    gotype, tagfield, tagvar) {
+	gotype = PrefixCamel name
+	print "type " gotype " struct {"
+	print "\tInterface any"
+	print "}"
+	print ""
+
+	print "func (u *" gotype ") MarshalJSON() ([]byte, error) {"
+	print "\treturn json.Marshal(u.Interface)"
+	print "}"
+	print ""
+
+	tagfield = snaketocamel(cg["tagname"])
+	tagvar = decapitalize(tagfield)
+	print "func (u *" gotype ") UnmarshalJSON(data []byte) (err error) {"
+	print "\tvar t struct {"
+	print "\t\t" tagfield " " CodegenGoType[cg["tagtype"]] \
+		" `json:\"" tagvar "\"`"
+	print "\t}"
+	print "\tif err := json.Unmarshal(data, &t); err != nil {"
+	print "\t\treturn err"
+	print "\t}"
+	print ""
+	print "\tswitch " tagvar " := t." tagfield "; " tagvar " {"
+	print cg["unmarshal"] "\tdefault:"
+	print "\t\terr = errors.New(`unsupported value: ` + " tagvar ".String())"
+	print "\t}"
+	print "\treturn err"
+	print "}"
+	print ""
+
+	# XXX: Consider changing the interface into an AppendTo/ConsumeFrom one,
+	# that would eliminate these type case switches entirely.
+	# On the other hand, it would make it possible to send unsuitable structs.
+	print "func (u *" gotype ") AppendTo(data []byte) ([]byte, bool) {"
+	print "\tok := true"
+	print "\tswitch union := u.Interface.(type) {"
+	print cg["serialize"] "\tdefault:"
+	print "\t\treturn nil, false"
+	print "\t}"
+	print "\treturn data, ok"
+	print "}"
+	print ""
+
+	CodegenSerialize[name] = \
+		"\tif data, ok = %s.AppendTo(data); !ok {\n" \
+		"\t\treturn nil, ok\n" \
+		"\t}\n"
+
+	print "func (u *" gotype ") ConsumeFrom(data []byte) ([]byte, bool) {"
+	print "\tok := true"
+	print "\tvar " tagvar " " CodegenGoType[cg["tagtype"]]
+	print sprintf(CodegenDeserialize[cg["tagtype"]], tagvar)
+	print "\tswitch " tagvar " {"
+	print cg["deserialize"] "\tdefault:"
+	print "\t\treturn nil, false"
+	print "\t}"
+	print "\treturn data, ok"
+	print "}"
+	print ""
+
+	CodegenDeserialize[name] = \
+		"\tif data, ok = %s.ConsumeFrom(data); !ok {\n" \
+		"\t\treturn nil, ok\n" \
+		"\t}\n"
+
+	CodegenGoType[name] = gotype
+	for (i in cg)
+		delete cg[i]
+}
diff --git a/xC-gen-proto.awk b/xC-gen-proto.awk
new file mode 100644
index 0000000..d5f19fb
--- /dev/null
+++ b/xC-gen-proto.awk
@@ -0,0 +1,303 @@
+# xC-gen-proto.awk: an XDR-derived code generator for network protocols.
+#
+# Copyright (c) 2022, Přemysl Eric Janouch <p@janouch.name>
+# SPDX-License-Identifier: 0BSD
+#
+# You may read RFC 4506 for context, however it is only a source of inspiration.
+# Grammar is easy to deduce from the parser.
+#
+# Native types: bool, u{8,16,32,64}, i{8,16,32,64}, string
+#
+# Don't define any new types, unless you hate yourself, then it's okay to do so.
+# Both backends are a pain in the arse, for different reasons.
+#
+# All numbers are encoded in big-endian byte order.
+# Booleans are one byte each.
+# Strings must be valid UTF-8, use u8<> to lift that restriction.
+# String and array lengths are encoded as u32.
+# Enumeration values automatically start at 1, and are encoded as i32.
+# Any struct or union field may be a variable-length array.
+#
+# Message framing is done externally, but also happens to prefix u32 lengths.
+#
+# Usage: env LC_ALL=C awk -v prefix=Relay \
+#  -f xC-gen-proto.awk        < xC-proto \
+#  -f xC-gen-proto-{c,go}.awk > xC-proto.{c,go} | {clang-format,gofmt}
+
+# --- Utilities ----------------------------------------------------------------
+
+function cameltosnake(s) {
+	while (match(s, /[[:lower:]][[:upper:]]/)) {
+		s = substr(s, 1, RSTART) "_" \
+			tolower(substr(s, RSTART + 1, RLENGTH - 1)) \
+			substr(s, RSTART + RLENGTH)
+	}
+	return tolower(s)
+}
+
+function snaketocamel(s) {
+	s = toupper(substr(s, 1, 1)) tolower(substr(s, 2))
+	while (match(s, /_[[:alnum:]]/)) {
+		s = substr(s, 1, RSTART - 1) \
+			toupper(substr(s, RSTART + 1, RLENGTH - 1)) \
+			substr(s, RSTART + RLENGTH)
+	}
+	return s
+}
+
+function decapitalize(s) {
+	if (match(s, /[[:upper:]][[:lower:]]/)) {
+		return tolower(substr(s, 1, 1)) substr(s, 2)
+	}
+	return s
+}
+
+function indent(s) {
+	if (!s)
+		return s
+
+	gsub(/\n/, "\n\t", s)
+	sub(/\t*$/, "", s)
+	return "\t" s
+}
+
+function append(a, key, value) {
+	a[key] = a[key] value
+}
+
+# --- Parsing ------------------------------------------------------------------
+
+function fatal(message) {
+	print "// " FILENAME ":" FNR ": fatal error: " message
+	print FILENAME ":" FNR ": fatal error: " message > "/dev/stderr"
+	exit 1
+}
+
+function skipcomment() {
+	do {
+		if (match($0, /[*][/]/)) {
+			$0 = substr($0, RSTART + RLENGTH)
+			return
+		}
+	} while (getline > 0)
+	fatal("unterminated block comment")
+}
+
+function nexttoken() {
+	do {
+		if (match($0, /^[[:space:]]+/)) {
+			$0 = substr($0, RLENGTH + 1)
+		} else if (match($0, /^[/][/].*/)) {
+			$0 = ""
+		} else if (match($0, /^[/][*]/)) {
+			$0 = substr($0, RLENGTH + 1)
+			skipcomment()
+		} else if (match($0, /^[[:alpha:]][[:alnum:]_]*/)) {
+			Token = substr($0, 1, RLENGTH)
+			$0 = substr($0, RLENGTH + 1)
+			return Token
+		} else if (match($0, /^(0[xX][0-9a-fA-F]+|[1-9][0-9]*)/)) {
+			Token = substr($0, 1, RLENGTH)
+			$0 = substr($0, RLENGTH + 1)
+			return Token
+		} else if (/./) {
+			Token = substr($0, 1, 1)
+			$0 = substr($0, 2)
+			return Token
+		}
+	} while (/./ || getline > 0)
+	Token = ""
+	return Token
+}
+
+function expect(v) {
+	if (!v)
+		fatal("broken expectations at `" Token "' before `" $0 "'")
+	return v
+}
+
+function accept(what) {
+	if (Token != what)
+		return 0
+	nexttoken()
+	return 1
+}
+
+function identifier(    v) {
+	if (Token !~ /^[[:alpha:]]/)
+		return 0
+	v = Token
+	nexttoken()
+	return v
+}
+
+function number(    v) {
+	if (Token !~ /^[0-9]/)
+		return 0
+	v = Token
+	nexttoken()
+	return v
+}
+
+function readnumber(    ident) {
+	ident = identifier()
+	if (!ident)
+		return expect(number())
+	if (!(ident in Consts))
+		fatal("unknown constant: " ident)
+	return Consts[ident]
+}
+
+function defconst(    ident, num) {
+	if (!accept("const"))
+		return 0
+
+	ident = expect(identifier())
+	expect(accept("="))
+	num = readnumber()
+	if (ident in Consts)
+		fatal("constant redefined: " ident)
+
+	Consts[ident] = num
+	codegen_constant(ident, num)
+	return 1
+}
+
+function readtype(    ident) {
+	ident = deftype()
+	if (ident)
+		return ident
+
+	ident = identifier()
+	if (!ident)
+		return 0
+
+	if (!(ident in Types))
+		fatal("unknown type: " ident)
+	return ident
+}
+
+function defenum(    name, ident, value, cg) {
+	delete cg[0]
+
+	name = expect(identifier())
+	expect(accept("{"))
+	while (!accept("}")) {
+		ident = expect(identifier())
+		value = value + 1
+		if (accept("="))
+			value = readnumber()
+		if (!value)
+			fatal("enumeration values cannot be zero")
+		expect(accept(","))
+		append(EnumValues, name, SUBSEP ident)
+		if (EnumValues[name, ident]++)
+			fatal("duplicate enum value: " ident)
+		codegen_enum_value(name, ident, value, cg)
+	}
+
+	Types[name] = "enum"
+	codegen_enum(name, cg)
+	return name
+}
+
+function readfield(out,    nonvoid) {
+	nonvoid = !accept("void")
+	if (nonvoid) {
+		out["type"] = expect(readtype())
+		out["name"] = expect(identifier())
+		# TODO: Consider supporting XDR's VLA length limits here.
+		# TODO: Consider supporting XDR's fixed-length syntax for string limits.
+		out["isarray"] = accept("<") && expect(accept(">"))
+	}
+	expect(accept(";"))
+	return nonvoid
+}
+
+function defstruct(    name, d, cg) {
+	delete d[0]
+	delete cg[0]
+
+	name = expect(identifier())
+	expect(accept("{"))
+	while (!accept("}")) {
+		if (readfield(d))
+			codegen_struct_field(d, cg)
+	}
+
+	Types[name] = "struct"
+	codegen_struct(name, cg)
+	return name
+}
+
+function defunion(    name, tag, tagtype, tagvalue, cg, scg, d, a, i, unseen) {
+	delete cg[0]
+	delete scg[0]
+	delete d[0]
+
+	name = expect(identifier())
+	expect(accept("switch"))
+	expect(accept("("))
+	tag["type"] = tagtype = expect(readtype())
+	tag["name"] = expect(identifier())
+	expect(accept(")"))
+
+	if (Types[tagtype] != "enum")
+		fatal("not an enum type: " tagtype)
+	codegen_union_tag(tag, cg)
+
+	split(EnumValues[tagtype], a, SUBSEP)
+	for (i in a)
+		unseen[a[i]]++
+
+	expect(accept("{"))
+	while (!accept("}")) {
+		if (accept("case")) {
+			if (tagvalue)
+				codegen_union_struct(name, tagvalue, cg, scg)
+
+			tagvalue = expect(identifier())
+			expect(accept(":"))
+			if (!unseen[tagvalue]--)
+				fatal("no such value or duplicate case: " tagtype "." tagvalue)
+			codegen_struct_tag(tag, scg)
+		} else if (tagvalue) {
+			if (readfield(d))
+				codegen_struct_field(d, scg)
+		} else {
+			fatal("union fields must fall under a case")
+		}
+	}
+	if (tagvalue)
+		codegen_union_struct(name, tagvalue, cg, scg)
+
+	# What remains non-zero in unseen[2..] is simply not recognized/allowed.
+	Types[name] = "union"
+	codegen_union(name, cg)
+	return name
+}
+
+function deftype() {
+	if (accept("enum"))
+		return defenum()
+	if (accept("struct"))
+		return defstruct()
+	if (accept("union"))
+		return defunion()
+	return 0
+}
+
+BEGIN {
+	PrefixLower = "relay_"
+	PrefixUpper = "RELAY_"
+	PrefixCamel = "Relay"
+
+	print "// Generated by xC-gen-proto.awk. DO NOT MODIFY."
+	codegen_begin()
+
+	nexttoken()
+	while (Token != "") {
+		expect(defconst() || deftype())
+		expect(accept(";"))
+	}
+}
diff --git a/xC-proto b/xC-proto
new file mode 100644
index 0000000..31534d4
--- /dev/null
+++ b/xC-proto
@@ -0,0 +1,120 @@
+// Backwards-compatible protocol version.
+const VERSION = 1;
+
+// From the frontend to the relay.
+struct CommandMessage {
+	u32 command_seq;
+	union CommandData switch (enum Command {
+		HELLO,
+		PING,
+		ACTIVE,
+		BUFFER_COMPLETE,
+		BUFFER_INPUT,
+		BUFFER_ACTIVATE,
+		BUFFER_LOG,
+	} command) {
+	case HELLO:
+		u32 version;
+	case PING:
+		void;
+	case ACTIVE:
+		void;
+	case BUFFER_COMPLETE:
+		string buffer_name;
+		string text;
+		u32 position;
+	case BUFFER_INPUT:
+		string buffer_name;
+		string text;
+	case BUFFER_ACTIVATE:
+		string buffer_name;
+	case BUFFER_LOG:
+		string buffer_name;
+	} data;
+};
+
+// From the relay to the frontend.
+struct EventMessage {
+	u32 event_seq;
+	union EventData switch (enum Event {
+		PING,
+		BUFFER_UPDATE,
+		BUFFER_RENAME,
+		BUFFER_REMOVE,
+		BUFFER_ACTIVATE,
+		BUFFER_LINE,
+		BUFFER_CLEAR,
+		ERROR,
+		RESPONSE,
+	} event) {
+	case PING:
+		void;
+	case BUFFER_UPDATE:
+		string buffer_name;
+	case BUFFER_RENAME:
+		string buffer_name;
+		string new;
+	case BUFFER_REMOVE:
+		string buffer_name;
+	case BUFFER_ACTIVATE:
+		string buffer_name;
+	case BUFFER_LINE:
+		string buffer_name;
+		bool is_unimportant;
+		bool is_highlight;
+		enum Rendition {
+			BARE,
+			INDENT,
+			STATUS,
+			ERROR,
+			JOIN,
+			PART,
+		} rendition;
+		// Unix timestamp in seconds.
+		u64 when;
+		// Broken-up text, with in-band formatting.
+		union ItemData switch (enum Item {
+			TEXT,
+			RESET,
+			FG_COLOR,
+			BG_COLOR,
+			FLIP_BOLD,
+			FLIP_ITALIC,
+			FLIP_UNDERLINE,
+			FLIP_INVERSE,
+			FLIP_CROSSED_OUT,
+			FLIP_MONOSPACE,
+		} kind) {
+		case TEXT:
+			string text;
+		case RESET:
+			void;
+		case FG_COLOR:
+			i16 color;
+		case BG_COLOR:
+			i16 color;
+		case FLIP_BOLD:
+		case FLIP_ITALIC:
+		case FLIP_UNDERLINE:
+		case FLIP_INVERSE:
+		case FLIP_CROSSED_OUT:
+		case FLIP_MONOSPACE:
+			void;
+		} items<>;
+	case BUFFER_CLEAR:
+		string buffer_name;
+	case ERROR:
+		u32 command_seq;
+		string error;
+	case RESPONSE:
+		u32 command_seq;
+		union ResponseData switch (Command command) {
+		case BUFFER_COMPLETE:
+			u32 start;
+			string completions<>;
+		case BUFFER_LOG:
+			// UTF-8, but not guaranteed.
+			u8 log<>;
+		} data;
+	} data;
+};
diff --git a/xC.c b/xC.c
index c80499c..e0c9e01 100644
--- a/xC.c
+++ b/xC.c
@@ -50,6 +50,7 @@ enum
 
 #include "common.c"
 #include "xD-replies.c"
+#include "xC-proto.c"
 
 #include <math.h>
 #include <langinfo.h>
@@ -1526,6 +1527,7 @@ enum buffer_line_flags
 	BUFFER_LINE_HIGHLIGHT   = 1 << 2,   ///< The user was highlighted by this
 };
 
+// NOTE: This sequence must match up with xC-proto, only one lower.
 enum buffer_line_rendition
 {
 	BUFFER_LINE_BARE,                   ///< Unadorned
@@ -1666,6 +1668,50 @@ buffer_destroy (struct buffer *self)
 REF_COUNTABLE_METHODS (buffer)
 #define buffer_ref do_not_use_dangerous
 
+// ~~~ Relay ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+struct client
+{
+	LIST_HEADER (struct client)
+	struct app_context *ctx;            ///< Application context
+
+	// TODO: Convert this all to TLS, and only TLS, with required client cert.
+	//   That means replacing plumbing functions with the /other/ set from xD.
+
+	int socket_fd;                      ///< The TCP socket
+	struct str read_buffer;             ///< Unprocessed input
+	struct str write_buffer;            ///< Output yet to be sent out
+
+	uint32_t event_seq;                 ///< Outgoing message counter
+	bool initialized;                   ///< Initial sync took place
+
+	struct poller_fd socket_event;      ///< The socket can be read/written to
+};
+
+static struct client *
+client_new (void)
+{
+	struct client *self = xcalloc (1, sizeof *self);
+	self->socket_fd = -1;
+	self->read_buffer = str_make ();
+	self->write_buffer = str_make ();
+	return self;
+}
+
+static void
+client_destroy (struct client *self)
+{
+	if (!soft_assert (self->socket_fd == -1))
+		xclose (self->socket_fd);
+
+	str_free (&self->read_buffer);
+	str_free (&self->write_buffer);
+	free (self);
+}
+
+static void client_kill (struct client *c);
+static bool client_process_buffer (struct client *c);
+
 // ~~~ Server ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 // The only real purpose of this is to abstract away TLS
@@ -2079,10 +2125,19 @@ struct app_context
 
 	struct str_map servers;             ///< Our servers
 
+	// Relay:
+
+	int relay_fd;                       ///< Listening socket FD
+	struct client *clients;             ///< Our relay clients
+
+	/// A single message buffer to prepare all outcoming messages within
+	struct relay_event_message relay_message;
+
 	// Events:
 
 	struct poller_fd tty_event;         ///< Terminal input event
 	struct poller_fd signal_event;      ///< Signal FD event
+	struct poller_fd relay_event;       ///< New relay connection available
 
 	struct poller_timer flush_timer;    ///< Flush all open files (e.g. logs)
 	struct poller_timer date_chg_tmr;   ///< Print a date change
@@ -2129,6 +2184,8 @@ struct app_context
 	char *editor_filename;              ///< The file being edited by user
 	int terminal_suspended;             ///< Terminal suspension level
 
+	// Plugins:
+
 	struct plugin *plugins;             ///< Loaded plugins
 	struct hook *input_hooks;           ///< Input hooks
 	struct hook *irc_hooks;             ///< IRC hooks
@@ -2197,6 +2254,8 @@ app_context_init (struct app_context *self)
 	self->config = config_make ();
 	poller_init (&self->poller);
 
+	self->relay_fd = -1;
+
 	self->servers = str_map_make ((str_map_free_fn) server_unref);
 	self->servers.key_xfrm = tolower_ascii_strxfrm;
 
@@ -2223,6 +2282,17 @@ app_context_init (struct app_context *self)
 }
 
 static void
+app_context_relay_stop (struct app_context *self)
+{
+	if (self->relay_fd != -1)
+	{
+		poller_fd_reset (&self->relay_event);
+		xclose (self->relay_fd);
+		self->relay_fd = -1;
+	}
+}
+
+static void
 app_context_free (struct app_context *self)
 {
 	// Plugins can try to use of the other fields when destroyed
@@ -2247,6 +2317,11 @@ app_context_free (struct app_context *self)
 	}
 	str_map_free (&self->buffers_by_name);
 
+	app_context_relay_stop (self);
+	LIST_FOR_EACH (struct client, c, self->clients)
+		client_kill (c);
+	relay_event_message_free (&self->relay_message);
+
 	str_map_free (&self->servers);
 	poller_free (&self->poller);
 
@@ -2285,6 +2360,7 @@ on_config_show_all_prefixes_change (struct config_item *item)
 	refresh_prompt (ctx);
 }
 
+static void on_config_relay_bind_change (struct config_item *item);
 static void on_config_backlog_limit_change (struct config_item *item);
 static void on_config_attribute_change (struct config_item *item);
 static void on_config_logging_change (struct config_item *item);
@@ -2479,6 +2555,11 @@ static struct config_schema g_config_general[] =
 	  .comment   = "Plugins to automatically load on start",
 	  .type      = CONFIG_ITEM_STRING_ARRAY,
 	  .validate  = config_validate_nonjunk_string },
+	{ .name      = "relay_bind",
+	  .comment   = "Address to bind to for a user interface relay point",
+	  .type      = CONFIG_ITEM_STRING,
+	  .validate  = config_validate_nonjunk_string,
+	  .on_change = on_config_relay_bind_change },
 
 	// Buffer history:
 	{ .name      = "backlog_limit",
@@ -2681,6 +2762,418 @@ serialize_configuration (struct config_item *root, struct str *output)
 	config_item_write (root, true, output);
 }
 
+// --- Relay plumbing ----------------------------------------------------------
+
+static void
+client_kill (struct client *c)
+{
+	struct app_context *ctx = c->ctx;
+	poller_fd_reset (&c->socket_event);
+	xclose (c->socket_fd);
+	c->socket_fd = -1;
+
+	LIST_UNLINK (ctx->clients, c);
+	client_destroy (c);
+}
+
+static bool
+client_try_read (struct client *c)
+{
+	struct str *buf = &c->read_buffer;
+	ssize_t n_read;
+
+	while ((n_read = read (c->socket_fd, buf->str + buf->len,
+		buf->alloc - buf->len - 1 /* null byte */)) > 0)
+	{
+		buf->len += n_read;
+		if (!client_process_buffer (c))
+			break;
+		str_reserve (buf, 512);
+	}
+
+	if (n_read < 0)
+	{
+		if (errno == EAGAIN || errno == EINTR)
+			return true;
+
+		print_debug ("%s: %s: %s", __func__, "read", strerror (errno));
+	}
+
+	client_kill (c);
+	return false;
+}
+
+static bool
+client_try_write (struct client *c)
+{
+	struct str *buf = &c->write_buffer;
+	ssize_t n_written;
+
+	while (buf->len)
+	{
+		n_written = write (c->socket_fd, buf->str, buf->len);
+		if (n_written >= 0)
+		{
+			str_remove_slice (buf, 0, n_written);
+			continue;
+		}
+		if (errno == EAGAIN || errno == EINTR)
+			return true;
+
+		print_debug ("%s: %s: %s", __func__, "write", strerror (errno));
+		client_kill (c);
+		return false;
+	}
+	return true;
+}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+static void
+client_update_poller (struct client *c, const struct pollfd *pfd)
+{
+	int new_events = POLLIN;
+	if (c->write_buffer.len)
+		new_events |= POLLOUT;
+
+	hard_assert (new_events != 0);
+	if (!pfd || pfd->events != new_events)
+		poller_fd_set (&c->socket_event, new_events);
+}
+
+static void
+on_client_ready (const struct pollfd *pfd, void *user_data)
+{
+	struct client *c = user_data;
+	if (client_try_read (c) && client_try_write (c))
+		client_update_poller (c, pfd);
+}
+
+static bool
+relay_try_fetch_client (struct app_context *ctx, int listen_fd)
+{
+	// XXX: `struct sockaddr_storage' is not the most portable thing
+	struct sockaddr_storage peer;
+	socklen_t peer_len = sizeof peer;
+
+	int fd = accept (listen_fd, (struct sockaddr *) &peer, &peer_len);
+	if (fd == -1)
+	{
+		if (errno == EAGAIN || errno == EWOULDBLOCK)
+			return false;
+		if (errno == EINTR)
+			return true;
+
+		if (accept_error_is_transient (errno))
+			print_warning ("%s: %s", "accept", strerror (errno));
+		else
+			print_fatal ("%s: %s", "accept", strerror (errno));
+		return true;
+	}
+
+	hard_assert (peer_len <= sizeof peer);
+	set_blocking (fd, false);
+	set_cloexec (fd);
+
+	// We already buffer our output, so reduce latencies.
+	int yes = 1;
+	soft_assert (setsockopt (fd, IPPROTO_TCP, TCP_NODELAY,
+		&yes, sizeof yes) != -1);
+
+	struct client *c = client_new ();
+	c->ctx = ctx;
+	c->socket_fd = fd;
+	LIST_PREPEND (ctx->clients, c);
+
+	c->socket_event = poller_fd_make (&c->ctx->poller, c->socket_fd);
+	c->socket_event.dispatcher = (poller_fd_fn) on_client_ready;
+	c->socket_event.user_data = c;
+
+	client_update_poller (c, NULL);
+	return true;
+}
+
+static void
+on_relay_client_available (const struct pollfd *pfd, void *user_data)
+{
+	struct app_context *ctx = user_data;
+	while (relay_try_fetch_client (ctx, pfd->fd))
+		;
+}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+static int
+relay_listen (struct addrinfo *ai, struct error **e)
+{
+	int fd = socket (ai->ai_family, ai->ai_socktype, ai->ai_protocol);
+	if (fd == -1)
+	{
+		error_set (e, "socket: %s", strerror (errno));
+		return -1;
+	}
+
+	set_cloexec (fd);
+
+	int yes = 1;
+	soft_assert (setsockopt (fd, SOL_SOCKET, SO_KEEPALIVE,
+		&yes, sizeof yes) != -1);
+	soft_assert (setsockopt (fd, SOL_SOCKET, SO_REUSEADDR,
+		&yes, sizeof yes) != -1);
+
+	if (bind (fd, ai->ai_addr, ai->ai_addrlen))
+		error_set (e, "bind: %s", strerror (errno));
+	else if (listen (fd, 16 /* arbitrary number */))
+		error_set (e, "listen: %s", strerror (errno));
+	else
+		return fd;
+
+	xclose (fd);
+	return -1;
+}
+
+static int
+relay_listen_with_context (struct addrinfo *ai, struct error **e)
+{
+	char *address = gai_reconstruct_address (ai);
+	print_debug ("binding to `%s'", address);
+
+	struct error *error = NULL;
+	int fd = relay_listen (ai, &error);
+	if (fd == -1)
+	{
+		error_set (e, "binding to `%s' failed: %s", address, error->message);
+		error_free (error);
+	}
+	free (address);
+	return fd;
+}
+
+static bool
+relay_start (struct app_context *ctx, char *address, struct error **e)
+{
+	const char *port = NULL, *host = tokenize_host_port (address, &port);
+	if (!port || !*port)
+		return error_set (e, "missing port");
+
+	struct addrinfo hints = {}, *result = NULL;
+	hints.ai_socktype = SOCK_STREAM;
+	hints.ai_flags = AI_PASSIVE;
+
+	int err = getaddrinfo (*host ? host : NULL, port, &hints, &result);
+	if (err)
+	{
+		return error_set (e, "failed to resolve `%s', port `%s': %s: %s",
+			host, port, "getaddrinfo", gai_strerror (err));
+	}
+
+	// Just try the first one, disregarding IPv4/IPv6 ordering.
+	int fd = relay_listen_with_context (result, e);
+	freeaddrinfo (result);
+	if (fd == -1)
+		return false;
+
+	set_blocking (fd, false);
+
+	struct poller_fd *event = &ctx->relay_event;
+	*event = poller_fd_make (&ctx->poller, fd);
+	event->dispatcher = (poller_fd_fn) on_relay_client_available;
+	event->user_data = ctx;
+
+	ctx->relay_fd = fd;
+	poller_fd_set (event, POLLIN);
+	return true;
+}
+
+static void
+on_config_relay_bind_change (struct config_item *item)
+{
+	struct app_context *ctx = item->user_data;
+	char *value = item->value.string.str;
+	app_context_relay_stop (ctx);
+	if (!value)
+		return;
+
+	struct error *e = NULL;
+	char *address = xstrdup (value);
+	if (!relay_start (ctx, address, &e))
+	{
+		// TODO: Try to make sure this finds its way to the global buffer.
+		print_error ("%s: %s", item->schema->name, e->message);
+		error_free (e);
+	}
+	free (address);
+}
+
+// --- Relay output ------------------------------------------------------------
+
+static void
+relay_send (struct client *c)
+{
+	struct relay_event_message *m = &c->ctx->relay_message;
+	m->event_seq = c->event_seq++;
+
+	// TODO: Also don't try sending anything if half-closed.
+	if (!c->initialized || c->socket_fd == -1)
+		return;
+
+	// liberty has msg_{reader,writer} already, but they use 8-byte lengths.
+	size_t frame_len_pos = c->write_buffer.len, frame_len = 0;
+	str_pack_u32 (&c->write_buffer, 0);
+	if (!relay_event_message_serialize (m, &c->write_buffer)
+	 || (frame_len = c->write_buffer.len - frame_len_pos - 4) > UINT32_MAX)
+	{
+		print_error ("serialization failed, killing client");
+		client_kill (c);
+		return;
+	}
+
+	uint32_t len = htonl (frame_len);
+	memcpy (c->write_buffer.str + frame_len_pos, &len, sizeof len);
+	client_update_poller (c, NULL);
+}
+
+static void
+relay_broadcast (struct app_context *ctx)
+{
+	LIST_FOR_EACH (struct client, c, ctx->clients)
+		relay_send (c);
+}
+
+static struct relay_event_message *
+relay_prepare (struct app_context *ctx)
+{
+	struct relay_event_message *m = &ctx->relay_message;
+	relay_event_message_free (m);
+	memset (m, 0, sizeof *m);
+	return m;
+}
+
+static void
+relay_prepare_ping (struct app_context *ctx)
+{
+	relay_prepare (ctx)->data.event = RELAY_EVENT_PING;
+}
+
+static void
+relay_prepare_buffer_update (struct app_context *ctx, struct buffer *buffer)
+{
+	struct relay_event_message *m = relay_prepare (ctx);
+	struct relay_event_data_buffer_update *e = &m->data.buffer_update;
+	e->event = RELAY_EVENT_BUFFER_UPDATE;
+	e->buffer_name = str_from_cstr (buffer->name);
+}
+
+static void
+relay_prepare_buffer_rename (struct app_context *ctx, struct buffer *buffer,
+	const char *new_name)
+{
+	struct relay_event_message *m = relay_prepare (ctx);
+	struct relay_event_data_buffer_rename *e = &m->data.buffer_rename;
+	e->event = RELAY_EVENT_BUFFER_RENAME;
+	e->buffer_name = str_from_cstr (buffer->name);
+	e->new = str_from_cstr (new_name);
+}
+
+static void
+relay_prepare_buffer_remove (struct app_context *ctx, struct buffer *buffer)
+{
+	struct relay_event_message *m = relay_prepare (ctx);
+	struct relay_event_data_buffer_remove *e = &m->data.buffer_remove;
+	e->event = RELAY_EVENT_BUFFER_REMOVE;
+	e->buffer_name = str_from_cstr (buffer->name);
+}
+
+static void
+relay_prepare_buffer_activate (struct app_context *ctx, struct buffer *buffer)
+{
+	struct relay_event_message *m = relay_prepare (ctx);
+	struct relay_event_data_buffer_activate *e = &m->data.buffer_activate;
+	e->event = RELAY_EVENT_BUFFER_ACTIVATE;
+	e->buffer_name = str_from_cstr (buffer->name);
+}
+
+static void
+relay_prepare_buffer_line (struct app_context *ctx, struct buffer *buffer,
+	struct buffer_line *line)
+{
+	struct relay_event_message *m = relay_prepare (ctx);
+	struct relay_event_data_buffer_line *e = &m->data.buffer_line;
+	e->event = RELAY_EVENT_BUFFER_LINE;
+	e->buffer_name = str_from_cstr (buffer->name);
+	e->is_unimportant = !!(line->flags & BUFFER_LINE_UNIMPORTANT);
+	e->is_highlight = !!(line->flags & BUFFER_LINE_HIGHLIGHT);
+	e->rendition = 1 + line->r;
+	e->when = line->when;
+
+	size_t len = 0;
+	for (size_t i = 0; line->items[i].type; i++)
+		len++;
+
+	// XXX: This way helps xP's JSON conversion, but is super annoying for us.
+	union relay_item_data *p = e->items = xcalloc (len * 6, sizeof *e->items);
+	for (struct formatter_item *i = line->items; len--; i++)
+	{
+		switch (i->type)
+		{
+		case FORMATTER_ITEM_TEXT:
+			p->text.text = str_from_cstr (i->text);
+			(p++)->kind = RELAY_ITEM_TEXT;
+			break;
+		case FORMATTER_ITEM_ATTR:
+			// For future consideration.
+			(p++)->kind = RELAY_ITEM_RESET;
+			break;
+		case FORMATTER_ITEM_FG_COLOR:
+			p->fg_color.color = i->color;
+			(p++)->kind = RELAY_ITEM_FG_COLOR;
+			break;
+		case FORMATTER_ITEM_BG_COLOR:
+			p->bg_color.color = i->color;
+			(p++)->kind = RELAY_ITEM_BG_COLOR;
+			break;
+		case FORMATTER_ITEM_SIMPLE:
+			if (i->attribute & TEXT_BOLD)
+				(p++)->kind = RELAY_ITEM_FLIP_BOLD;
+			if (i->attribute & TEXT_ITALIC)
+				(p++)->kind = RELAY_ITEM_FLIP_ITALIC;
+			if (i->attribute & TEXT_UNDERLINE)
+				(p++)->kind = RELAY_ITEM_FLIP_UNDERLINE;
+			if (i->attribute & TEXT_INVERSE)
+				(p++)->kind = RELAY_ITEM_FLIP_INVERSE;
+			if (i->attribute & TEXT_CROSSED_OUT)
+				(p++)->kind = RELAY_ITEM_FLIP_CROSSED_OUT;
+			if (i->attribute & TEXT_MONOSPACE)
+				(p++)->kind = RELAY_ITEM_FLIP_MONOSPACE;
+			break;
+		default:
+			break;
+		}
+	}
+
+	e->items_len = p - e->items;
+}
+
+static void
+relay_prepare_buffer_clear (struct app_context *ctx,
+	struct buffer *buffer)
+{
+	struct relay_event_message *m = relay_prepare (ctx);
+	struct relay_event_data_buffer_clear *e = &m->data.buffer_clear;
+	e->event = RELAY_EVENT_BUFFER_CLEAR;
+	e->buffer_name = str_from_cstr (buffer->name);
+}
+
+static void
+relay_prepare_error (struct app_context *ctx, uint32_t seq, const char *message)
+{
+	struct relay_event_message *m = relay_prepare (ctx);
+	struct relay_event_data_error *e = &m->data.error;
+	e->event = RELAY_EVENT_ERROR;
+	e->command_seq = seq;
+	e->error = str_from_cstr (message);
+}
+
 // --- Terminal output ---------------------------------------------------------
 
 /// Default colour pair
@@ -4089,6 +4582,9 @@ log_formatter (struct app_context *ctx, struct buffer *buffer,
 	if (buffer->log_file)
 		buffer_line_write_to_log (ctx, line, buffer->log_file);
 
+	relay_prepare_buffer_line (ctx, buffer, line);
+	relay_broadcast (ctx);
+
 	bool unseen_pm = buffer->type == BUFFER_PM
 		&& buffer != ctx->current_buffer
 		&& !(flags & BUFFER_LINE_UNIMPORTANT);
@@ -4302,6 +4798,9 @@ buffer_add (struct app_context *ctx, struct buffer *buffer)
 
 	buffer_open_log_file (ctx, buffer);
 
+	relay_prepare_buffer_update (ctx, buffer);
+	relay_broadcast (ctx);
+
 	// Normally this doesn't cause changes in the prompt but a prompt hook
 	// could decide to show some information for all buffers nonetheless
 	refresh_prompt (ctx);
@@ -4328,6 +4827,9 @@ buffer_remove (struct app_context *ctx, struct buffer *buffer)
 	if (buffer->type == BUFFER_SERVER)
 		buffer->server->buffer = NULL;
 
+	relay_prepare_buffer_remove (ctx, buffer);
+	relay_broadcast (ctx);
+
 	str_map_set (&ctx->buffers_by_name, buffer->name, NULL);
 	LIST_UNLINK_WITH_TAIL (ctx->buffers, ctx->buffers_tail, buffer);
 	buffer_unref (buffer);
@@ -4457,6 +4959,9 @@ buffer_activate (struct app_context *ctx, struct buffer *buffer)
 	ctx->last_buffer = ctx->current_buffer;
 	ctx->current_buffer = buffer;
 
+	relay_prepare_buffer_activate (ctx, buffer);
+	relay_broadcast (ctx);
+
 	refresh_prompt (ctx);
 }
 
@@ -4491,12 +4996,19 @@ buffer_merge (struct app_context *ctx,
 	merged->lines_tail = start->prev;
 	merged->lines_count -= n;
 
-	// And append them to current lines in the buffer
+	// Append them to current lines in the buffer
 	buffer->lines_tail->next = start;
 	start->prev = buffer->lines_tail;
 	buffer->lines_tail = tail;
 	buffer->lines_count += n;
 
+	// And since there is no log_*() call, send them to relays manually
+	LIST_FOR_EACH (struct buffer_line, line, start)
+	{
+		relay_prepare_buffer_line (ctx, buffer, line);
+		relay_broadcast (ctx);
+	}
+
 	log_full (ctx, NULL, buffer, BUFFER_LINE_SKIP_FILE, BUFFER_LINE_STATUS,
 		"End of merged content");
 }
@@ -4511,6 +5023,9 @@ buffer_rename (struct app_context *ctx,
 
 	hard_assert (!collision);
 
+	relay_prepare_buffer_rename (ctx, buffer, new_name);
+	relay_broadcast (ctx);
+
 	str_map_set (&ctx->buffers_by_name, buffer->name, NULL);
 	str_map_set (&ctx->buffers_by_name, new_name, buffer);
 
@@ -4524,13 +5039,16 @@ buffer_rename (struct app_context *ctx,
 }
 
 static void
-buffer_clear (struct buffer *buffer)
+buffer_clear (struct app_context *ctx, struct buffer *buffer)
 {
 	LIST_FOR_EACH (struct buffer_line, iter, buffer->lines)
 		buffer_line_destroy (iter);
 
 	buffer->lines = buffer->lines_tail = NULL;
 	buffer->lines_count = 0;
+
+	relay_prepare_buffer_clear (ctx, buffer);
+	relay_broadcast (ctx);
 }
 
 static struct buffer *
@@ -5947,29 +6465,6 @@ irc_finish_connection (struct server *s, int socket, const char *hostname)
 	refresh_prompt (s->ctx);
 }
 
-/// Unwrap IPv6 addresses in format_host_port_pair() format
-static void
-irc_split_host_port (char *s, char **host, char **port)
-{
-	*host = s;
-	*port = "6667";
-
-	char *right_bracket = strchr (s, ']');
-	if (s[0] == '[' && right_bracket)
-	{
-		*right_bracket = '\0';
-		*host = s + 1;
-		s = right_bracket + 1;
-	}
-
-	char *colon = strchr (s, ':');
-	if (colon)
-	{
-		*colon = '\0';
-		*port = colon + 1;
-	}
-}
-
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 
 static void
@@ -6019,8 +6514,8 @@ irc_setup_connector (struct server *s, const struct strv *addresses)
 
 	for (size_t i = 0; i < addresses->len; i++)
 	{
-		char *host, *port;
-		irc_split_host_port (addresses->vector[i], &host, &port);
+		const char *port = "6667",
+			*host = tokenize_host_port (addresses->vector[i], &port);
 		connector_add_target (connector, host, port);
 	}
 }
@@ -6062,9 +6557,8 @@ irc_setup_connector_socks (struct server *s, const struct strv *addresses,
 
 	for (size_t i = 0; i < addresses->len; i++)
 	{
-		char *host, *port;
-		irc_split_host_port (addresses->vector[i], &host, &port);
-
+		const char *port = "6667",
+			*host = tokenize_host_port (addresses->vector[i], &port);
 		if (!socks_connector_add_target (connector, host, port, e))
 			return false;
 	}
@@ -7644,7 +8138,7 @@ irc_on_registered (struct server *s, const char *nickname)
 	if (command)
 	{
 		log_server_debug (s, "Executing \"#s\"", command);
-		process_input_utf8 (s->ctx, s->buffer, command, 0);
+		(void) process_input_utf8 (s->ctx, s->buffer, command, 0);
 	}
 
 	int64_t command_delay = get_config_integer (s->config, "command_delay");
@@ -8230,6 +8724,24 @@ irc_handle_rpl_isupport (struct server *s, const struct irc_message *msg)
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 
 static void
+irc_adjust_motd (char **motd)
+{
+	// Heuristic, force MOTD to be monospace in graphical frontends.
+	if (!strchr (*motd, '\x11'))
+	{
+		struct str s = str_make ();
+		str_append_c (&s, '\x11');
+		for (const char *p = *motd; *p; p++)
+		{
+			str_append_c (&s, *p);
+			if (*p == '\x0f')
+				str_append_c (&s, '\x11');
+		}
+		cstr_set (motd, str_steal (&s));
+	}
+}
+
+static void
 irc_process_numeric (struct server *s,
 	const struct irc_message *msg, unsigned long numeric)
 {
@@ -8251,6 +8763,10 @@ irc_process_numeric (struct server *s,
 		if (msg->params.len == 2)
 			irc_try_parse_welcome_for_userhost (s, msg->params.vector[1]);
 		break;
+	case IRC_RPL_MOTD:
+		if (copy.len)
+			irc_adjust_motd (&copy.vector[0]);
+		break;
 
 	case IRC_RPL_ISUPPORT:
 		irc_handle_rpl_isupport      (s, msg);                break;
@@ -9248,7 +9764,7 @@ lua_buffer_execute (lua_State *L)
 	struct lua_weak *wrapper = lua_weak_deref (L, &lua_buffer_info);
 	struct buffer *buffer = wrapper->object;
 	const char *line = lua_plugin_check_utf8 (L, 2);
-	process_input_utf8 (wrapper->plugin->ctx, buffer, line, 0);
+	(void) process_input_utf8 (wrapper->plugin->ctx, buffer, line, 0);
 	return 0;
 }
 
@@ -11304,7 +11820,7 @@ handle_command_buffer (struct handler_args *a)
 		show_buffers_list (ctx);
 	else if (!strcasecmp_ascii (action, "clear"))
 	{
-		buffer_clear (a->buffer);
+		buffer_clear (ctx, a->buffer);
 		if (a->buffer == ctx->current_buffer)
 			buffer_print_backlog (ctx, a->buffer);
 	}
@@ -12926,8 +13442,8 @@ complete_set_value_array (struct config_item *item, const char *word,
 	cstr_split (item->value.string.str, ",", false, &items);
 	for (size_t i = 0; i < items.len; i++)
 	{
-		struct str wrapped = str_make (), serialized = str_make ();
-		str_append (&wrapped, items.vector[i]);
+		struct str wrapped = str_from_cstr (items.vector[i]);
+		struct str serialized = str_make ();
 		config_item_write_string (&serialized, &wrapped);
 		str_free (&wrapped);
 
@@ -13546,6 +14062,25 @@ on_display_backlog_nowrap (int count, int key, void *user_data)
 	return display_backlog (user_data, FLUSH_OPT_NOWRAP);
 }
 
+static FILE *
+open_log_path (struct app_context *ctx, struct buffer *buffer, const char *path)
+{
+	FILE *fp = fopen (path, "rb");
+	if (!fp)
+	{
+		log_global_error (ctx,
+			"Failed to open `#l': #l", path, strerror (errno));
+		return NULL;
+	}
+
+	if (buffer->log_file)
+		// The regular flush will log any error eventually
+		(void) fflush (buffer->log_file);
+
+	set_cloexec (fileno (fp));
+	return fp;
+}
+
 static bool
 on_display_full_log (int count, int key, void *user_data)
 {
@@ -13555,20 +14090,13 @@ on_display_full_log (int count, int key, void *user_data)
 
 	struct buffer *buffer = ctx->current_buffer;
 	char *path = buffer_get_log_path (buffer);
-	FILE *full_log = fopen (path, "rb");
+	FILE *full_log = open_log_path (ctx, buffer, path);
 	if (!full_log)
 	{
-		log_global_error (ctx, "Failed to open log file for #s: #l",
-			ctx->current_buffer->name, strerror (errno));
 		free (path);
 		return false;
 	}
 
-	if (buffer->log_file)
-		// The regular flush will log any error eventually
-		(void) fflush (buffer->log_file);
-
-	set_cloexec (fileno (full_log));
 	launch_pager (ctx, fileno (full_log), buffer->name, path);
 	fclose (full_log);
 	free (path);
@@ -14601,6 +15129,177 @@ init_poller_events (struct app_context *ctx)
 	ctx->input_event.user_data = ctx;
 }
 
+// --- Relay processing --------------------------------------------------------
+
+// XXX: This could be below completion code if reset_autoaway() was higher up.
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+static void
+client_resync (struct client *c)
+{
+	LIST_FOR_EACH (struct buffer, buffer, c->ctx->buffers)
+	{
+		relay_prepare_buffer_update (c->ctx, buffer);
+		relay_send (c);
+
+		LIST_FOR_EACH (struct buffer_line, line, buffer->lines)
+		{
+			relay_prepare_buffer_line (c->ctx, buffer, line);
+			relay_send (c);
+		}
+	}
+
+	relay_prepare_buffer_activate (c->ctx, c->ctx->current_buffer);
+	relay_send (c);
+}
+
+static const char *
+client_message_buffer_name (const struct relay_command_message *m)
+{
+	switch (m->data.command)
+	{
+	case RELAY_COMMAND_BUFFER_COMPLETE:
+		return m->data.buffer_input.buffer_name.str;
+	case RELAY_COMMAND_BUFFER_INPUT:
+		return m->data.buffer_input.buffer_name.str;
+	case RELAY_COMMAND_BUFFER_ACTIVATE:
+		return m->data.buffer_activate.buffer_name.str;
+	case RELAY_COMMAND_BUFFER_LOG:
+		return m->data.buffer_log.buffer_name.str;
+	default:
+		return NULL;
+	}
+}
+
+static void
+client_process_buffer_log
+	(struct client *c, uint32_t seq, struct buffer *buffer)
+{
+	struct relay_event_message *m = relay_prepare (c->ctx);
+	struct relay_event_data_response *e = &m->data.response;
+	e->event = RELAY_EVENT_RESPONSE;
+	e->command_seq = seq;
+	e->data.command = RELAY_COMMAND_BUFFER_LOG;
+
+	char *path = buffer_get_log_path (buffer);
+	FILE *fp = open_log_path (c->ctx, buffer, path);
+	if (fp)
+	{
+		struct str log = str_make ();
+		char buf[BUFSIZ];
+		size_t len;
+		while ((len = fread (buf, 1, sizeof buf, fp)))
+			str_append_data (&log, buf, len);
+		if (ferror (fp))
+			log_global_error (c->ctx, "Failed to read `#l': #l",
+				path, strerror (errno));
+
+		// On overflow, it will later fail serialization.
+		e->data.buffer_log.log_len = MIN (UINT32_MAX, log.len);
+		e->data.buffer_log.log = (uint8_t *) str_steal (&log);
+		fclose (fp);
+	}
+
+	// XXX: We log failures to the global buffer,
+	//   so the client just receives nothing if there is no log file.
+
+	free (path);
+	relay_send (c);
+}
+
+static bool
+client_process_message (struct client *c,
+	struct msg_unpacker *r, struct relay_command_message *m)
+{
+	if (!relay_command_message_deserialize (m, r)
+	 || msg_unpacker_get_available (r))
+	{
+		print_error ("deserialization failed, killing client");
+		return false;
+	}
+
+	const char *buffer_name = client_message_buffer_name (m);
+	struct buffer *buffer = NULL;
+	if (buffer_name && !(buffer = buffer_by_name (c->ctx, buffer_name)))
+	{
+		relay_prepare_error (c->ctx, m->command_seq, "Unknown buffer");
+		relay_send (c);
+		return true;
+	}
+
+	switch (m->data.command)
+	{
+	case RELAY_COMMAND_HELLO:
+		if (m->data.hello.version != RELAY_VERSION)
+		{
+			// TODO: This should send back an error message and shut down.
+			print_error ("protocol version mismatch, killing client");
+			return false;
+		}
+		c->initialized = true;
+		client_resync (c);
+		break;
+	case RELAY_COMMAND_PING:
+		relay_prepare_ping (c->ctx);
+		relay_send (c);
+		break;
+	case RELAY_COMMAND_ACTIVE:
+		reset_autoaway (c->ctx);
+		break;
+	case RELAY_COMMAND_BUFFER_COMPLETE:
+		// TODO: Run the completion machinery.
+		relay_prepare_error (c->ctx, m->command_seq, "Not implemented");
+		relay_send (c);
+		break;
+	case RELAY_COMMAND_BUFFER_INPUT:
+		(void) process_input_utf8 (c->ctx,
+			buffer, m->data.buffer_input.text.str, 0);
+		break;
+	case RELAY_COMMAND_BUFFER_ACTIVATE:
+		buffer_activate (c->ctx, buffer);
+		break;
+	case RELAY_COMMAND_BUFFER_LOG:
+		client_process_buffer_log (c, m->command_seq, buffer);
+		break;
+	default:
+		print_warning ("unhandled client command");
+		relay_prepare_error (c->ctx, m->command_seq, "Unknown command");
+		relay_send (c);
+	}
+	return true;
+}
+
+static bool
+client_process_buffer (struct client *c)
+{
+	struct str *buf = &c->read_buffer;
+	size_t offset = 0;
+	while (true)
+	{
+		uint32_t frame_len = 0;
+		struct msg_unpacker r =
+			msg_unpacker_make (buf->str + offset, buf->len - offset);
+		if (!msg_unpacker_u32 (&r, &frame_len))
+			break;
+
+		r.len = MIN (r.len, sizeof frame_len + frame_len);
+		if (msg_unpacker_get_available (&r) < frame_len)
+			break;
+
+		struct relay_command_message m = {};
+		bool ok = client_process_message (c, &r, &m);
+		relay_command_message_free (&m);
+		if (!ok)
+			return false;
+
+		offset += r.offset;
+	}
+
+	str_remove_slice (buf, 0, offset);
+	return true;
+}
+
 // --- Tests -------------------------------------------------------------------
 
 // The application is quite monolithic and can only be partially unit-tested.
diff --git a/xD.c b/xD.c
index 45cd659..65ebe7e 100644
--- a/xD.c
+++ b/xD.c
@@ -853,8 +853,6 @@ client_send_str (struct client *c, const struct str *s)
 	str_append_data (&c->write_buffer, s->str,
 		MIN (s->len, IRC_MAX_MESSAGE_LENGTH));
 	str_append (&c->write_buffer, "\r\n");
-	// XXX: we might want to move this elsewhere, so that it doesn't get called
-	//   as often; it's going to cause a lot of syscalls with epoll.
 	client_update_poller (c, NULL);
 
 	// Technically we haven't sent it yet but that's a minor detail
@@ -3095,6 +3093,7 @@ irc_try_read (struct client *c)
 		{
 			buf->str[buf->len += n_read] = '\0';
 			// TODO: discard characters above the 512 character limit
+			// FIXME: we should probably discard the data if closing_link
 			irc_process_buffer (buf, irc_process_message, c);
 			continue;
 		}
@@ -3136,6 +3135,7 @@ irc_try_read_tls (struct client *c)
 		case SSL_ERROR_NONE:
 			buf->str[buf->len += n_read] = '\0';
 			// TODO: discard characters above the 512 character limit
+			// FIXME: we should probably discard the data if closing_link
 			irc_process_buffer (buf, irc_process_message, c);
 			continue;
 		case SSL_ERROR_ZERO_RETURN:
@@ -3421,16 +3421,10 @@ irc_try_fetch_client (struct server_context *ctx, int listen_fd)
 		if (errno == EINTR)
 			return true;
 
-		if (errno == EBADF
-		 || errno == EINVAL
-		 || errno == ENOTSOCK
-		 || errno == EOPNOTSUPP)
+		if (accept_error_is_transient (errno))
+			print_warning ("%s: %s", "accept", strerror (errno));
+		else
 			print_fatal ("%s: %s", "accept", strerror (errno));
-
-		// OS kernels may return a wide range of unforeseeable errors.
-		// Assuming that they're either transient or caused by
-		// a connection that we've just extracted from the queue.
-		print_warning ("%s: %s", "accept", strerror (errno));
 		return true;
 	}
 
@@ -3814,10 +3808,9 @@ irc_lock_pid_file (struct server_context *ctx, struct error **e)
 }
 
 static int
-irc_listen (struct addrinfo *gai_iter)
+irc_listen (struct addrinfo *ai)
 {
-	int fd = socket (gai_iter->ai_family,
-		gai_iter->ai_socktype, gai_iter->ai_protocol);
+	int fd = socket (ai->ai_family, ai->ai_socktype, ai->ai_protocol);
 	if (fd == -1)
 		return -1;
 	set_cloexec (fd);
@@ -3831,21 +3824,13 @@ irc_listen (struct addrinfo *gai_iter)
 #if defined SOL_IPV6 && defined IPV6_V6ONLY
 	// Make NULL always bind to both IPv4 and IPv6, irrespectively of the order
 	// of results; only INADDR6_ANY seems to be affected by this
-	if (gai_iter->ai_family == AF_INET6)
+	if (ai->ai_family == AF_INET6)
 		soft_assert (setsockopt (fd, SOL_IPV6, IPV6_V6ONLY,
 			&yes, sizeof yes) != -1);
 #endif
 
-	char host[NI_MAXHOST], port[NI_MAXSERV];
-	host[0] = port[0] = '\0';
-	int err = getnameinfo (gai_iter->ai_addr, gai_iter->ai_addrlen,
-		host, sizeof host, port, sizeof port,
-		NI_NUMERICHOST | NI_NUMERICSERV);
-	if (err)
-		print_debug ("%s: %s", "getnameinfo", gai_strerror (err));
-
-	char *address = format_host_port_pair (host, port);
-	if (bind (fd, gai_iter->ai_addr, gai_iter->ai_addrlen))
+	char *address = gai_reconstruct_address (ai);
+	if (bind (fd, ai->ai_addr, ai->ai_addrlen))
 		print_error ("bind to %s failed: %s", address, strerror (errno));
 	else if (listen (fd, 16 /* arbitrary number */))
 		print_error ("listen on %s failed: %s", address, strerror (errno));
@@ -3865,12 +3850,12 @@ static void
 irc_listen_resolve (struct server_context *ctx,
 	const char *host, const char *port, struct addrinfo *gai_hints)
 {
-	struct addrinfo *gai_result, *gai_iter;
+	struct addrinfo *gai_result = NULL, *gai_iter = NULL;
 	int err = getaddrinfo (host, port, gai_hints, &gai_result);
 	if (err)
 	{
 		char *address = format_host_port_pair (host, port);
-		print_error ("bind to %s failed: %s: %s",
+		print_error ("binding to %s failed: %s: %s",
 			address, "getaddrinfo", gai_strerror (err));
 		free (address);
 		return;
diff --git a/xF.c b/xF.c
new file mode 100644
index 0000000..054871d
--- /dev/null
+++ b/xF.c
@@ -0,0 +1,172 @@
+/*
+ * xF.c: a toothless IRC client frontend
+ *
+ * Copyright (c) 2022, Přemysl Eric Janouch <p@janouch.name>
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ */
+
+#include "config.h"
+#define PROGRAM_NAME "xF"
+
+#include "common.c"
+#include "xC-proto.c"
+
+#include <X11/Xatom.h>
+#include <X11/Xlib.h>
+#include <X11/keysym.h>
+#include <X11/XKBlib.h>
+#include <X11/Xft/Xft.h>
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+static struct
+{
+	bool polling;
+	struct connector connector;
+	int socket;
+}
+g;
+
+static void
+on_connector_connecting (void *user_data, const char *address)
+{
+	(void) user_data;
+	print_status ("connecting to %s...", address);
+}
+
+static void
+on_connector_error (void *user_data, const char *error)
+{
+	(void) user_data;
+	print_status ("connection failed: %s", error);
+}
+
+static void
+on_connector_failure (void *user_data)
+{
+	(void) user_data;
+	exit_fatal ("giving up");
+}
+
+static void
+on_connector_connected (void *user_data, int socket, const char *hostname)
+{
+	(void) user_data;
+	(void) hostname;
+	g.polling = false;
+	g.socket = socket;
+}
+
+static void
+protocol_test (const char *host, const char *port)
+{
+	struct poller poller = {};
+	poller_init (&poller);
+
+	connector_init (&g.connector, &poller);
+	g.connector.on_connecting = on_connector_connecting;
+	g.connector.on_error      = on_connector_error;
+	g.connector.on_connected  = on_connector_connected;
+	g.connector.on_failure    = on_connector_failure;
+
+	connector_add_target (&g.connector, host, port);
+
+	g.polling = true;
+	while (g.polling)
+		poller_run (&poller);
+
+	connector_free (&g.connector);
+
+	struct str s = str_make ();
+	str_pack_u32 (&s, 0);
+	struct relay_command_message m = {};
+	m.data.hello.command = RELAY_COMMAND_HELLO;
+	m.data.hello.version = RELAY_VERSION;
+	if (!relay_command_message_serialize (&m, &s))
+		exit_fatal ("serialization failed");
+
+	uint32_t len = htonl (s.len - sizeof len);
+	memcpy (s.str, &len, sizeof len);
+	if (errno = 0, write (g.socket, s.str, s.len) != (ssize_t) s.len)
+		exit_fatal ("short send or error: %s", strerror (errno));
+
+	char buf[1 << 20] = "";
+	while (errno = 0, read (g.socket, &len, sizeof len) == sizeof len)
+	{
+		len = ntohl (len);
+		if (errno = 0, read (g.socket, buf, MIN (len, sizeof buf)) != len)
+			exit_fatal ("short read or error: %s", strerror (errno));
+
+		struct msg_unpacker r = msg_unpacker_make (buf, len);
+		struct relay_event_message m = {};
+		if (!relay_event_message_deserialize (&m, &r))
+			exit_fatal ("deserialization failed");
+		if (msg_unpacker_get_available (&r))
+			exit_fatal ("trailing data");
+
+		printf ("event: %d\n", m.data.event);
+		relay_event_message_free (&m);
+	}
+	exit_fatal ("short read or error: %s", strerror (errno));
+}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+int
+main (int argc, char *argv[])
+{
+	static const struct opt opts[] =
+	{
+		{ 'h', "help", NULL, 0, "display this help and exit" },
+		{ 'V', "version", NULL, 0, "output version information and exit" },
+		{ 0, NULL, NULL, 0, NULL }
+	};
+
+	struct opt_handler oh = opt_handler_make (argc, argv, opts,
+		"HOST:PORT", "X11 frontend for xC.");
+
+	int c;
+	while ((c = opt_handler_get (&oh)) != -1)
+	switch (c)
+	{
+	case 'h':
+		opt_handler_usage (&oh, stdout);
+		exit (EXIT_SUCCESS);
+	case 'V':
+		printf (PROGRAM_NAME " " PROGRAM_VERSION "\n");
+		exit (EXIT_SUCCESS);
+	default:
+		print_error ("wrong options");
+		opt_handler_usage (&oh, stderr);
+		exit (EXIT_FAILURE);
+	}
+
+	argc -= optind;
+	argv += optind;
+	if (argc != 1)
+	{
+		opt_handler_usage (&oh, stderr);
+		exit (EXIT_FAILURE);
+	}
+	opt_handler_free (&oh);
+
+	char *address = xstrdup (argv[0]);
+	const char *port = NULL, *host = tokenize_host_port (address, &port);
+	if (!port)
+		exit_fatal ("missing port number/service name");
+
+	// TODO: Actually implement an X11-based user interface.
+	protocol_test (host, port);
+	return 0;
+}
diff --git a/xF.svg b/xF.svg
new file mode 100644
index 0000000..75b7156
--- /dev/null
+++ b/xF.svg
@@ -0,0 +1,36 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg version="1.1" width="48" height="48" viewBox="0 0 48 48"
+   xmlns:xlink="http://www.w3.org/1999/xlink"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:svg="http://www.w3.org/2000/svg">
+
+  <defs>
+    <linearGradient id="background" x1="0" y1="0" x2="1" y2="1">
+      <stop stop-color="#808080" offset="0" />
+      <stop stop-color="#000000" offset="1" />
+    </linearGradient>
+    <!-- librsvg screws up the filter's orientation in a weird way
+         otherwise a larger blur value would look better -->
+    <filter id="shadow" color-interpolation-filters="sRGB">
+      <feOffset dy="0.5" />
+      <feGaussianBlur stdDeviation="0.5" />
+      <feComposite in2="SourceGraphic" operator="in" />
+    </filter>
+    <clipPath id="clip">
+      <rect x="-7" y="-10" width="14" height="20" />
+    </clipPath>
+  </defs>
+
+  <circle cx="24" cy="24" r="20"
+     fill="url(#background)" stroke="#404040" stroke-width="2" />
+
+  <g transform="rotate(-45 24 24)" filter="url(#shadow)">
+    <path d="m 12,25 h 24 v 11 h -5 v -8 h -4.5 v 6 h -5 v -6 h -9.5 z"
+       fill="#ffffff" />
+    <g stroke-width="4" transform="translate(24, 16)" clip-path="url(#clip)"
+       stroke="#ffffff">
+      <line x1="-8" x2="8" y1="-5" y2="5" />
+      <line x1="-8" x2="8" y1="5" y2="-5" />
+    </g>
+  </g>
+</svg>
diff --git a/xP/.gitignore b/xP/.gitignore
new file mode 100644
index 0000000..68c09f0
--- /dev/null
+++ b/xP/.gitignore
@@ -0,0 +1,3 @@
+/xP
+/proto.go
+/public/mithril.js
diff --git a/xP/Makefile b/xP/Makefile
new file mode 100644
index 0000000..3c52146
--- /dev/null
+++ b/xP/Makefile
@@ -0,0 +1,14 @@
+.POSIX:
+.SUFFIXES:
+
+outputs = xP proto.go public/mithril.js
+all: $(outputs)
+
+xP: xP.go proto.go
+	go build -o $@
+proto.go: ../xC-gen-proto.awk ../xC-gen-proto-go.awk ../xC-proto
+	awk -f ../xC-gen-proto.awk -f ../xC-gen-proto-go.awk ../xC-proto > $@
+public/mithril.js:
+	curl -Lo $@ https://unpkg.com/mithril/mithril.js
+clean:
+	rm -f $(outputs)
diff --git a/xP/go.mod b/xP/go.mod
new file mode 100644
index 0000000..7d4369e
--- /dev/null
+++ b/xP/go.mod
@@ -0,0 +1,5 @@
+module janouch.name/xK
+
+go 1.18
+
+require golang.org/x/net v0.0.0-20210805182204-aaa1db679c0d
diff --git a/xP/go.sum b/xP/go.sum
new file mode 100644
index 0000000..cd5264f
--- /dev/null
+++ b/xP/go.sum
@@ -0,0 +1,2 @@
+golang.org/x/net v0.0.0-20210805182204-aaa1db679c0d h1:20cMwl2fHAzkJMEA+8J4JgqBQcQGzbisXo31MIeenXI=
+golang.org/x/net v0.0.0-20210805182204-aaa1db679c0d/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
diff --git a/xP/public/xP.css b/xP/public/xP.css
new file mode 100644
index 0000000..9a98c13
--- /dev/null
+++ b/xP/public/xP.css
@@ -0,0 +1,109 @@
+body {
+	margin: 0;
+	padding: 0;
+	font-family: sans-serif;
+}
+
+.xP {
+	height: 100vh;
+	display: flex;
+	flex-direction: column;
+	overflow: hidden;
+}
+
+.title, .status {
+	background: #f8f8f8;
+	border-bottom: 1px solid #ccc;
+	padding: .05rem .3rem;
+}
+
+.middle {
+	flex: auto;
+	display: flex;
+	flex-direction: row;
+	overflow: hidden;
+}
+
+.list {
+	overflow-y: auto;
+	border-right: 1px solid #ccc;
+	min-width: 10rem;
+}
+.item {
+	padding: .05rem .3rem;
+	cursor: default;
+}
+.item.active {
+	font-weight: bold;
+}
+
+/* Only Firefox currently supports align-content: safe end, thus this. */
+.buffer-container {
+	flex: auto;
+	display: flex;
+	flex-direction: column;
+	overflow: hidden;
+}
+.filler {
+	flex: auto;
+}
+.buffer {
+	display: grid;
+	grid-template-columns: max-content auto;
+	overflow-y: auto;
+}
+
+.date {
+	padding: .3rem;
+	grid-column: span 2;
+	font-weight: bold;
+}
+.time {
+	padding: .1rem .3rem;
+	background: #f8f8f8;
+	color: #bbb;
+	border-right: 1px solid #ccc;
+}
+.mark {
+	padding-right: .3rem;
+	text-align: center;
+	display: inline-block;
+	min-width: 2rem;
+}
+.mark.error {
+	color: red;
+}
+.mark.join {
+	color: green;
+}
+.mark.part {
+	color: red;
+}
+.content {
+	padding: .1rem .3rem;
+	white-space: pre-wrap;
+}
+.content span.b {
+	font-weight: bold;
+}
+.content span.i {
+	font-style: italic;
+}
+.content span.u {
+	text-decoration: underline;
+}
+.content span.s {
+	text-decoration: line-through;
+}
+.content span.m {
+	font-family: monospace;
+}
+
+.status {
+	border-top: 2px solid #fff;
+}
+
+textarea {
+	padding: .05rem .3rem;
+	font-family: inherit;
+}
diff --git a/xP/public/xP.js b/xP/public/xP.js
new file mode 100644
index 0000000..4eebb7a
--- /dev/null
+++ b/xP/public/xP.js
@@ -0,0 +1,188 @@
+// TODO: Probably reset state on disconnect, and indicate to user.
+let socket = new WebSocket(proxy)
+
+let commandSeq = 0
+function send(command) {
+	socket.send(JSON.stringify({commandSeq: ++commandSeq, data: command}))
+}
+
+socket.onopen = function(event) {
+	send({command: 'Hello', version: 1})
+}
+
+let buffers = new Map()
+let bufferCurrent = undefined
+
+socket.onmessage = function(event) {
+	console.log(event.data)
+
+	let e = JSON.parse(event.data).data
+	switch (e.event) {
+	case 'BufferUpdate':
+	{
+		let b = buffers.get(e.bufferName)
+		if (b === undefined) {
+			b = {lines: []}
+			buffers.set(e.bufferName, b)
+		}
+		// TODO: Update any buffer properties.
+		break
+	}
+	case 'BufferRename':
+		buffers.set(e.new, buffers.get(e.bufferName))
+		buffers.delete(e.bufferName)
+		break
+	case 'BufferRemove':
+		buffers.delete(e.bufferName)
+		break
+	case 'BufferActivate':
+		bufferCurrent = e.bufferName
+		// TODO: Somehow scroll to the end of it immediately.
+		// TODO: Focus the textarea.
+		break
+	case 'BufferLine':
+	{
+		let b = buffers.get(e.bufferName)
+		if (b !== undefined)
+			b.lines.push({when: e.when, rendition: e.rendition, items: e.items})
+		break
+	}
+	case 'BufferClear':
+	{
+		let b = buffers.get(e.bufferName)
+		if (b !== undefined)
+			b.lines.length = 0
+		break
+	}
+	}
+
+	m.redraw()
+}
+
+let BufferList = {
+	view: vnode => {
+		let items = []
+		buffers.forEach((b, name) => {
+			let attrs = {
+				onclick: e => {
+					send({command: 'BufferActivate', bufferName: name})
+				},
+			}
+			if (name == bufferCurrent)
+				attrs.class = 'active'
+			items.push(m('.item', attrs, name))
+		})
+		return m('.list', {}, items)
+	},
+}
+
+let Content = {
+	view: vnode => {
+		let line = vnode.children[0]
+		let content = []
+		switch (line.rendition) {
+		case 'Indent': content.push(m('span.mark',       {}, ''));  break
+		case 'Status': content.push(m('span.mark',       {}, '–')); break
+		case 'Error':  content.push(m('span.mark.error', {}, '⚠')); break
+		case 'Join':   content.push(m('span.mark.join',  {}, '→')); break
+		case 'Part':   content.push(m('span.mark.part',  {}, '←')); break
+		}
+
+		let classes = new Set()
+		let flip = c => {
+			if (classes.has(c))
+				classes.delete(c)
+			else
+				classes.add(c)
+		}
+		line.items.forEach(item => {
+			// TODO: Colours.
+			switch (item.kind) {
+			case 'Text':
+				// TODO: Detect and transform links.
+				content.push(m('span', {
+					class: Array.from(classes.keys()).join(' '),
+				}, item.text))
+				break
+			case 'Reset':
+				classes.clear()
+				break
+			case 'FlipBold':       flip('b'); break
+			case 'FlipItalic':     flip('i'); break
+			case 'FlipUnderline':  flip('u'); break
+			case 'FlipInverse':    flip('i'); break
+			case 'FlipCrossedOut': flip('s'); break
+			case 'FlipMonospace':  flip('m'); break
+			}
+		})
+		return m('.content', {}, content)
+	},
+}
+
+let Buffer = {
+	view: vnode => {
+		let lines = []
+		let b = buffers.get(bufferCurrent)
+		if (b === undefined)
+			return
+
+		let lastDateMark = undefined
+		b.lines.forEach(line => {
+			let date = new Date(line.when * 1000)
+			let dateMark = date.toLocaleDateString()
+			if (dateMark !== lastDateMark) {
+				lines.push(m('.date', {}, dateMark))
+				lastDateMark = dateMark
+			}
+
+			lines.push(m('.time', {}, date.toLocaleTimeString()))
+			lines.push(m(Content, {}, line))
+		})
+		return m('.buffer-container', {}, [
+			m('.filler'),
+			m('.buffer', {}, lines),
+		])
+	},
+}
+
+// TODO: This should be remembered across buffer switches,
+// and we'll probably have to intercept /all/ key presses.
+let Input = {
+	view: vnode => {
+		return m('textarea', {
+			rows: 1,
+			onkeydown: e => {
+				// TODO: And perhaps on other actions, too.
+				send({command: 'Active'})
+				if (e.keyCode !== 13)
+					return
+
+				send({
+					command: 'BufferInput',
+					bufferName: bufferCurrent,
+					text: e.currentTarget.value,
+				})
+				e.preventDefault()
+				e.currentTarget.value = ''
+			},
+		})
+	},
+}
+
+let Main = {
+	view: vnode => {
+		return m('.xP', {}, [
+			m('.title', {}, "xP"),
+			m('.middle', {}, [m(BufferList), m(Buffer)]),
+			m('.status', {}, bufferCurrent),
+			m(Input),
+		])
+	},
+}
+
+// TODO: Buffer names should work as routes.
+window.addEventListener('load', () => {
+	m.route(document.body, '/', {
+		'/': Main,
+	})
+})
diff --git a/xP/xP.example.json b/xP/xP.example.json
new file mode 100644
index 0000000..2c63c08
--- /dev/null
+++ b/xP/xP.example.json
@@ -0,0 +1,2 @@
+{
+}
diff --git a/xP/xP.go b/xP/xP.go
new file mode 100644
index 0000000..9b5df8f
--- /dev/null
+++ b/xP/xP.go
@@ -0,0 +1,186 @@
+package main
+
+import (
+	"context"
+	"encoding/binary"
+	"encoding/json"
+	"fmt"
+	"html/template"
+	"io"
+	"log"
+	"net"
+	"net/http"
+	"os"
+	"time"
+
+	"golang.org/x/net/websocket"
+)
+
+var (
+	addressBind    string
+	addressConnect string
+)
+
+func clientToRelay(
+	ctx context.Context, ws *websocket.Conn, conn net.Conn) bool {
+	var j string
+	if err := websocket.Message.Receive(ws, &j); err != nil {
+		log.Println("Command receive failed: " + err.Error())
+		return false
+	}
+
+	log.Printf("?> %s\n", j)
+
+	var m RelayCommandMessage
+	if err := json.Unmarshal([]byte(j), &m); err != nil {
+		log.Println("Command unmarshalling failed: " + err.Error())
+		return false
+	}
+
+	b, ok := m.AppendTo(make([]byte, 4))
+	if !ok {
+		log.Println("Command serialization failed")
+		return false
+	}
+	binary.BigEndian.PutUint32(b[:4], uint32(len(b)-4))
+	if _, err := conn.Write(b); err != nil {
+		log.Println("Command send failed: " + err.Error())
+		return false
+	}
+
+	log.Printf("-> %v\n", b)
+	return true
+}
+
+func relayToClient(
+	ctx context.Context, ws *websocket.Conn, conn net.Conn) bool {
+	var length uint32
+	if err := binary.Read(conn, binary.BigEndian, &length); err != nil {
+		log.Println("Event receive failed: " + err.Error())
+		return false
+	}
+	b := make([]byte, length)
+	if _, err := io.ReadFull(conn, b); err != nil {
+		log.Println("Event receive failed: " + err.Error())
+		return false
+	}
+
+	log.Printf("<? %v\n", b)
+
+	var m RelayEventMessage
+	if after, ok := m.ConsumeFrom(b); !ok {
+		log.Println("Event deserialization failed")
+		return false
+	} else if len(after) != 0 {
+		log.Println("Event deserialization failed: trailing data")
+		return false
+	}
+
+	j, err := json.Marshal(&m)
+	if err != nil {
+		log.Println("Event marshalling failed: " + err.Error())
+		return false
+	}
+	if err := websocket.Message.Send(ws, string(j)); err != nil {
+		log.Println("Event send failed: " + err.Error())
+		return false
+	}
+
+	log.Printf("<- %s\n", j)
+	return true
+}
+
+func errorToClient(ws *websocket.Conn, err error) bool {
+	j, err := json.Marshal(&RelayEventMessage{
+		EventSeq: 0,
+		Data: RelayEventData{
+			Interface: RelayEventDataError{
+				Event:      RelayEventError,
+				CommandSeq: 0,
+				Error:      err.Error(),
+			},
+		},
+	})
+	if err != nil {
+		log.Println("Event marshalling failed: " + err.Error())
+		return false
+	}
+	if err := websocket.Message.Send(ws, string(j)); err != nil {
+		log.Println("Event send failed: " + err.Error())
+		return false
+	}
+	return true
+}
+
+func handleWebSocket(ws *websocket.Conn) {
+	conn, err := net.Dial("tcp", addressConnect)
+	if err != nil {
+		errorToClient(ws, err)
+		return
+	}
+
+	// We don't need to intervene, so it's just two separate pipes so far.
+	ctx, cancel := context.WithCancel(ws.Request().Context())
+	go func() {
+		for clientToRelay(ctx, ws, conn) {
+		}
+		cancel()
+	}()
+	go func() {
+		for relayToClient(ctx, ws, conn) {
+		}
+		cancel()
+	}()
+	<-ctx.Done()
+}
+
+var staticHandler = http.FileServer(http.Dir("."))
+
+var page = template.Must(template.New("/").Parse(`<!DOCTYPE html>
+<html>
+<head>
+	<title>xP</title>
+	<meta charset="utf-8" />
+	<link rel="stylesheet" href="xP.css" />
+</head>
+<body>
+	<script src="mithril.js">
+	</script>
+	<script>
+	let proxy = '{{ . }}'
+	</script>
+	<script src="xP.js">
+	</script>
+</body>
+</html>`))
+
+func handleDefault(w http.ResponseWriter, r *http.Request) {
+	if r.URL.Path != "/" {
+		staticHandler.ServeHTTP(w, r)
+		return
+	}
+
+	wsURI := fmt.Sprintf("ws://%s/ws", r.Host)
+	if err := page.Execute(w, wsURI); err != nil {
+		log.Println("Template execution failed: " + err.Error())
+	}
+}
+
+func main() {
+	if len(os.Args) != 3 {
+		log.Fatalf("usage: %s BIND CONNECT\n", os.Args[0])
+	}
+
+	addressBind, addressConnect = os.Args[1], os.Args[2]
+
+	http.Handle("/ws", websocket.Handler(handleWebSocket))
+	http.Handle("/", http.HandlerFunc(handleDefault))
+
+	s := &http.Server{
+		Addr:           addressBind,
+		ReadTimeout:    60 * time.Second,
+		WriteTimeout:   60 * time.Second,
+		MaxHeaderBytes: 32 << 10,
+	}
+	log.Fatal(s.ListenAndServe())
+}