Set socket TCP keepalive option on incoming connections

If a connected host disappears without our knowledge, as can happen over
wireless or a hibernating machine, we continue to hold the port open waiting
for messages. Because we never try to send anything down this now-broken
pipe, the connection will sit idle taking up a slot in our allowed incoming
connections list.

If enough of these happen, an unintended Denial of Service takes place,
where all connection slots are filled with now-broken, never ending
connections. Setting the TCP keepalive option at least allows these to time
out after the default two hours, which is sufficient in the non-malicious
case.

Signed-off-by: Dan McGee <dan@archlinux.org>

src/server_socket.c

@@ -160,12 +160,16 @@ server_socket_in_event(G_GNUC_UNUSED GIOChannel *source,
 	size_t address_length = sizeof(address);
 	int fd = accept_cloexec_nonblock(s->fd, (struct sockaddr*)&address,
 					 &address_length);
-	if (fd >= 0)
+	if (fd >= 0) {
+		if (socket_keepalive(fd))
+			g_warning("Could not set TCP keepalive option: %s",
+				  g_strerror(errno));
 		s->parent->callback(fd, (const struct sockaddr*)&address,
 				    address_length, get_remote_uid(fd),
 				    s->parent->callback_ctx);
-	else
+	} else {
 		g_warning("accept() failed: %s", g_strerror(errno));
+	}
 
 	return true;
 }

src/socket_util.c

@@ -148,3 +148,18 @@ socket_bind_listen(int domain, int type, int protocol,
 
 	return fd;
 }
+
+int
+socket_keepalive(int fd)
+{
+	const int reuse = 1;
+
+#ifdef WIN32
+	const char *optval = (const char *)&reuse;
+#else
+	const void *optval = &reuse;
+#endif
+
+	return setsockopt(fd, SOL_SOCKET, SO_KEEPALIVE,
+			optval, sizeof(reuse));
+}

src/socket_util.h

@@ -63,4 +63,7 @@ socket_bind_listen(int domain, int type, int protocol,
 		   int backlog,
 		   GError **error);
 
+int
+socket_keepalive(int fd);
+
 #endif