configure.ac

@@ -493,6 +493,9 @@ SPP_MSG="no"
 # the --with-solaris-privs option and --with-sandbox=solaris).
 SOLARIS_PRIVS="no"
 
+# Support for IPv6 source address preference selection
+IPV6_SRC_ADDR_PREF_MSG="no"
+
 # Check for some target-specific stuff
 case "$host" in
 *-*-aix*)
@@ -784,6 +787,15 @@ main() { if (NSVersionOfRunTimeLibrary("System") >= (60 << 16))
 			[Define if cmsg_type is not passed correctly])
 		;;
 	esac
+	# linux specific IPv6 sock options
+	AC_CHECK_HEADERS([linux/in6.h])
+	if test "x$ac_cv_header_linux_in6_h" = "xyes" ; then
+		AC_DEFINE([HAS_IPV6_SRC_PREF], [1],
+		    [System has IPv6 source address preference flags])
+		AC_DEFINE([LINUX_IPV6_PREF_FLAGS], [1],
+		    [Use Linux specific IPv6 options])
+		IPV6_SRC_ADDR_PREF_MSG="yes (Linux)"
+	fi
 	# tun(4) forwarding compat code
 	AC_CHECK_HEADERS([linux/if_tun.h])
 	if test "x$ac_cv_header_linux_if_tun_h" = "xyes" ; then
@@ -5099,6 +5111,7 @@ echo "           Solaris project support: $SP_MSG"
 echo "         Solaris privilege support: $SPP_MSG"
 echo "       IP address in \$DISPLAY hack: $DISPLAY_HACK_MSG"
 echo "           Translate v4 in v6 hack: $IPV4_IN6_HACK_MSG"
+echo "IPv6 src addr preference selection: $IPV6_SRC_ADDR_PREF_MSG"
 echo "                  BSD Auth support: $BSD_AUTH_MSG"
 echo "              Random number source: $RAND_MSG"
 echo "             Privsep sandbox style: $SANDBOX_STYLE"

ssh_config.5

@@ -284,6 +284,22 @@ Note that this option does not work if
 .Cm UsePrivilegedPort
 is set to
 .Dq yes .
+.Pp
+Additionally, this option may be used to control the selection of IPv6
+binding addresses on platforms that support it.
+A
+.Cm BindAddress
+of
+.Dq %public
+or
+.Dq %pub
+will cause
+.Xr ssh 1
+to use a stable public IPv6 address, while
+.Dq %temp
+or
+.Dq %tmp
+will prefer a randomised temporary address.
 .It Cm CanonicalDomains
 When
 .Cm CanonicalizeHostname

sshconnect.c

@@ -66,6 +66,10 @@
 #include "ssherr.h"
 #include "authfd.h"
 
+#ifdef LINUX_IPV6_PREF_FLAGS
+#  include <linux/in6.h>
+#endif
+
 char *client_version_string = NULL;
 char *server_version_string = NULL;
 Key *previous_host_key = NULL;
@@ -267,6 +271,51 @@ ssh_kill_proxy_command(void)
 		kill(proxy_command_pid, SIGHUP);
 }
 
+#ifdef HAS_IPV6_SRC_PREF
+enum ssh_bindpref {BINDPREF_PUB, BINDPREF_TMP};
+
+static int
+ssh_mod_ipv6bindpref(int fd, enum ssh_bindpref pref)
+{
+#  ifdef LINUX_IPV6_PREF_FLAGS
+	int val;
+	int err;
+	socklen_t len = sizeof(val);
+	int add = 0;
+	switch (pref) {
+	case BINDPREF_PUB:
+		add = IPV6_PREFER_SRC_PUBLIC;
+		break;
+	case BINDPREF_TMP:
+		add = IPV6_PREFER_SRC_TMP;
+		break;
+	default:
+		error("Unknown ssh_bindpref value");
+		return -1;
+	}
+	err = getsockopt(fd, IPPROTO_IPV6, IPV6_ADDR_PREFERENCES, &val, &len);
+	if (err < 0) {
+		error("getsockopt IPV6_ADDR_PREFERENCES: %.100s",
+		      strerror(errno));
+		return err;
+	}
+
+	val &= ~(IPV6_PREFER_SRC_PUBLIC|IPV6_PREFER_SRC_TMP|
+	         IPV6_PREFER_SRC_PUBTMP_DEFAULT);
+	val |= add;
+	err = setsockopt(fd, IPPROTO_IPV6, IPV6_ADDR_PREFERENCES, &val, len);
+	if (err < 0) {
+		error("setsockopt IPV6_ADDR_PREFERENCES: %.100s",
+		      strerror(errno));
+	}
+	return err;
+#  else
+	error("Setting IPv6 address source address not supported");
+	return -1;
+#  endif
+}
+#endif
+
 /*
  * Creates a (possibly privileged) socket for use as the ssh connection.
  */
@@ -275,6 +324,7 @@ ssh_create_socket(int privileged, struct addrinfo *ai)
 {
 	int sock, r, gaierr;
 	struct addrinfo hints, *res = NULL;
+	char *bind_address = options.bind_address;
 
 	sock = socket(ai->ai_family, ai->ai_socktype, ai->ai_protocol);
 	if (sock < 0) {
@@ -283,19 +333,42 @@ ssh_create_socket(int privileged, struct addrinfo *ai)
 	}
 	fcntl(sock, F_SETFD, FD_CLOEXEC);
 
+	/* Set IPV6_ADDR_PREFERENCES through magic bind address */
+	if (bind_address && bind_address[0] == '%') {
+#if defined(HAS_IPV6_SRC_PREF)
+		if (ai->ai_family != AF_INET6) {
+			debug("Not setting address preference on"
+			      " non-IPv6 socket");
+		} else if (strcmp(bind_address, "%pub") == 0 ||
+			   strcmp(bind_address, "%public") == 0) {
+			debug("Setting preference for public IPv6 address");
+			ssh_mod_ipv6bindpref(sock, BINDPREF_PUB);
+		} else if (strcmp(bind_address, "%tmp") == 0 ||
+			   strcmp(bind_address, "%temp") == 0) {
+			debug("Setting preference for temporary IPv6 address");
+			ssh_mod_ipv6bindpref(sock, BINDPREF_TMP);
+		} else {
+			error("Unknown bind address preference: '%s'",
+			      bind_address);
+		}
+#else
+		error("IPv6 address preference selection not supported");
+#endif
+		bind_address = NULL;
+	}
 	/* Bind the socket to an alternative local IP address */
-	if (options.bind_address == NULL && !privileged)
+	if (bind_address == NULL && !privileged)
 		return sock;
 
-	if (options.bind_address) {
+	if (bind_address) {
 		memset(&hints, 0, sizeof(hints));
 		hints.ai_family = ai->ai_family;
 		hints.ai_socktype = ai->ai_socktype;
 		hints.ai_protocol = ai->ai_protocol;
 		hints.ai_flags = AI_PASSIVE;
-		gaierr = getaddrinfo(options.bind_address, NULL, &hints, &res);
+		gaierr = getaddrinfo(bind_address, NULL, &hints, &res);
 		if (gaierr) {
-			error("getaddrinfo: %s: %s", options.bind_address,
+			error("getaddrinfo: %s: %s", bind_address,
 			    ssh_gai_strerror(gaierr));
 			close(sock);
 			return -1;
@@ -316,7 +389,7 @@ ssh_create_socket(int privileged, struct addrinfo *ai)
 		}
 	} else {
 		if (bind(sock, res->ai_addr, res->ai_addrlen) < 0) {
-			error("bind: %s: %s", options.bind_address,
+			error("bind: %s: %s", bind_address,
 			    strerror(errno));
  fail:
 			close(sock);