User can specify his SSH identity with the tmate-identity global option

options-table.c

@@ -176,6 +176,11 @@ const struct options_table_entry session_options_table[] = {
 	  .default_num = 30000
 	},
 
+	{ .name = "tmate-identity",
+	  .type = OPTIONS_TABLE_STRING,
+	  .default_str = ""
+	},
+
 	{ .name = "history-limit",
 	  .type = OPTIONS_TABLE_NUMBER,
 	  .minimum = 0,

server.c

@@ -187,8 +187,7 @@ server_start(int lockfd, char *lockfile)
 		}
 	}
 
-	tmate_session_start();
+	tmate_session_init();
-
 	cmdq_continue(cfg_cmd_q);
 
 	server_add_accept(0);
@@ -200,6 +199,7 @@ server_start(int lockfd, char *lockfile)
 
 	set_signals(server_signal_callback);
 
+	tmate_session_start();
 	server_loop();
 	exit(0);
 }

tmate-session.c

@@ -92,7 +92,7 @@ static void lookup_and_connect(void)
 				&hints, dns_cb, NULL);
 }
 
-void tmate_session_start(void)
+void tmate_session_init(void)
 {
 	tmate_catch_sigsegv();
 
@@ -100,8 +100,17 @@ void tmate_session_start(void)
 	tmate_encoder_init(&tmate_session.encoder);
 	tmate_decoder_init(&tmate_session.decoder);
 
-	lookup_and_connect();
-
 	/* The header will be written as soon as the first client connects */
 	tmate_write_header();
 }
+
+void tmate_session_start(void)
+{
+	/* We split init and start because:
+	 * - We need to process the tmux config file during the connection as
+	 *   we are setting up the tmate identity.
+	 * - While we are parsing the config file, we need to be able to
+	 *   serialize it, and so we need a worker encoder.
+	 */
+	lookup_and_connect();
+}

tmate-ssh-client.c

@@ -83,8 +83,25 @@ static void connection_complete(struct tmate_ssh_client *connected_client)
 	}
 }
 
+static char *get_identity(void)
+{
+	char *identity;
+
+	identity = options_get_string(&global_s_options, "tmate-identity");
+	if (!strlen(identity))
+		return NULL;
+
+	if (strchr(identity, '/'))
+		identity = xstrdup(identity);
+	else
+		xasprintf(&identity, "%%d/%s", identity);
+
+	return identity;
+}
+
 static void on_session_event(struct tmate_ssh_client *client)
 {
+	char *identity;
 	ssh_key pubkey;
 	int key_type;
 	unsigned char *hash;
@@ -121,6 +138,11 @@ static void on_session_event(struct tmate_ssh_client *client)
 		ssh_options_set(session, SSH_OPTIONS_USER, "tmate");
 		ssh_options_set(session, SSH_OPTIONS_COMPRESSION, "yes");
 
+		if ((identity = get_identity())) {
+			ssh_options_set(session, SSH_OPTIONS_IDENTITY, identity);
+			free(identity);
+		}
+
 		client->state = SSH_CONNECT;
 		/* fall through */
 

tmate.h

@@ -144,6 +144,7 @@ struct tmate_session {
 };
 
 extern struct tmate_session tmate_session;
+extern void tmate_session_init(void);
 extern void tmate_session_start(void);
 
 /* tmate-debug.c */