From 7d44964dcd37a32ef56fedae49d1631c4418b6ab Mon Sep 17 00:00:00 2001
From: Shivaram Lingamneni <slingamn@cs.stanford.edu>
Date: Mon, 9 Dec 2019 22:47:59 -0500
Subject: [PATCH] fix #641

Track the address family of the last failed connection attempt
(either immediately or during TLS handshake), then disprefer
that address family during reconnection.
---
 src/core/server-connect-rec.h |  2 ++
 src/core/servers-reconnect.c  |  1 +
 src/core/servers.c            | 15 ++++++++++++---
 3 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/src/core/server-connect-rec.h b/src/core/server-connect-rec.h
index fa348769..2f68bd04 100644
--- a/src/core/server-connect-rec.h
+++ b/src/core/server-connect-rec.h
@@ -11,6 +11,7 @@ int proxy_port;
 char *proxy_string, *proxy_string_after, *proxy_password;
 
 unsigned short family; /* 0 = don't care, AF_INET or AF_INET6 */
+unsigned short chosen_family; /* family actually chosen during name resolution */
 char *tag; /* try to keep this tag when connected to server */
 char *address;
 int port;
@@ -43,5 +44,6 @@ unsigned int unix_socket:1; /* Connect using named unix socket */
 unsigned int use_tls:1; /* this connection uses TLS */
 unsigned int tls_verify:1;
 unsigned int no_connect:1; /* don't connect() at all, it's done by plugin */
+unsigned short last_failed_family; /* #641: if we failed to connect to ipv6, try ipv4 and vice versa */
 char *channels;
 char *away_reason;
diff --git a/src/core/servers-reconnect.c b/src/core/servers-reconnect.c
index 2cbe60b4..0cfac2ec 100644
--- a/src/core/servers-reconnect.c
+++ b/src/core/servers-reconnect.c
@@ -168,6 +168,7 @@ server_connect_copy_skeleton(SERVER_CONNECT_REC *src, int connect_info)
         server_connect_ref(dest);
 	dest->type = module_get_uniq_id("SERVER CONNECT", 0);
 	dest->reconnection = src->reconnection;
+	dest->last_failed_family = src->last_failed_family;
 	dest->proxy = g_strdup(src->proxy);
         dest->proxy_port = src->proxy_port;
 	dest->proxy_string = g_strdup(src->proxy_string);
diff --git a/src/core/servers.c b/src/core/servers.c
index 20452f8c..27f788bb 100644
--- a/src/core/servers.c
+++ b/src/core/servers.c
@@ -156,6 +156,7 @@ static void server_connect_callback_init(SERVER_REC *server, GIOChannel *handle)
 	error = net_geterror(handle);
 	if (error != 0) {
 		server->connection_lost = TRUE;
+		server->connrec->last_failed_family = server->connrec->chosen_family;
 		server_connect_failed(server, g_strerror(error));
 		return;
 	}
@@ -176,6 +177,7 @@ static void server_connect_callback_init_ssl(SERVER_REC *server, GIOChannel *han
 	error = irssi_ssl_handshake(handle);
 	if (error == -1) {
 		server->connection_lost = TRUE;
+		server->connrec->last_failed_family = server->connrec->chosen_family;
 		server_connect_failed(server, NULL);
 		return;
 	}
@@ -216,6 +218,7 @@ static void server_real_connect(SERVER_REC *server, IPADDR *ip,
 		return;
 
 	if (ip != NULL) {
+		server->connrec->chosen_family = ip->family;
 		own_ip = IPADDR_IS_V6(ip) ? server->connrec->own_ip6 : server->connrec->own_ip4;
 		port = server->connrec->proxy != NULL ?
 			server->connrec->proxy_port : server->connrec->port;
@@ -241,9 +244,11 @@ static void server_real_connect(SERVER_REC *server, IPADDR *ip,
 			server->no_reconnect = TRUE;
 
 		server->connection_lost = TRUE;
+		server->connrec->last_failed_family = ip->family;
 		server_connect_failed(server, errmsg2 ? errmsg2 : errmsg);
 		g_free(errmsg2);
 	} else {
+		server->connrec->last_failed_family = 0;
 		server->handle = net_sendbuffer_create(handle, 0);
 		if (server->connrec->use_tls)
 			server_connect_callback_init_ssl(server, handle);
@@ -289,11 +294,15 @@ static void server_connect_callback_readpipe(SERVER_REC *server)
 		ip = iprec.ip6.family == 0 ? NULL : &iprec.ip6;
 		servername = iprec.host6;
 	} else {
-		/* pick the one that was found, or if both do it like
-		   /SET resolve_prefer_ipv6 says. */
+		/* pick the one that was found. if both were found:
+		   1. disprefer the last one that failed
+		   2. prefer ipv4 over ipv6 unless resolve_prefer_ipv6 is set
+		*/
 		if (iprec.ip4.family == 0 ||
 		    (iprec.ip6.family != 0 &&
-		     settings_get_bool("resolve_prefer_ipv6"))) {
+		     (server->connrec->last_failed_family == AF_INET ||
+		      (settings_get_bool("resolve_prefer_ipv6") &&
+		       server->connrec->last_failed_family != AF_INET6)))) {
 			ip = &iprec.ip6;
 			servername = iprec.host6;
 		} else {