[ipv6] Perform SLAAC only during autoconfiguration

We currently perform IPv6 stateless address autoconfiguration (SLAAC)
in response to any router advertisement with the relevant flags set.
This can result in the local IPv6 source address changing midway
through a TCP connection, since our connections bind only to a local
port number and do not store a local network address.

In addition, this behaviour for SLAAC is inconsistent with that for
DHCPv4 and stateful DHCPv6, both of which will be performed only as a
result of an explicit autoconfiguration action (e.g. via the default
autoboot sequence, or the "ifconf" command).

Fix by ignoring router advertisements arriving outside the context of
an ongoing autoconfiguration attempt.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/ndp.c

  *
  */
 
+static struct ipv6conf * ipv6conf_demux ( struct net_device *netdev );
 static int
 ipv6conf_rx_router_advertisement ( struct net_device *netdev,
 				   struct ndp_router_advertisement_header *radv,
 	struct ndp_prefix_information_option *prefix_opt = &option->prefix;
 	struct in6_addr *router = &sin6_src->sin6_addr;
 	struct in6_addr address;
+	struct ipv6conf *ipv6conf;
 	int prefix_len;
 	int rc;
 
 		       "short at %zd bytes\n", netdev->name, len );
 		return -EINVAL;
 	}
+
+	/* Identify IPv6 configurator, if any */
+	ipv6conf = ipv6conf_demux ( netdev );
 	DBGC ( netdev, "NDP %s found %sdefault router %s ",
 	       netdev->name, ( radv->lifetime ? "" : "non-" ),
 	       inet6_ntoa ( &sin6_src->sin6_addr ) );
-	DBGC ( netdev, "for %s-link %sautonomous prefix %s/%d\n",
+	DBGC ( netdev, "for %s-link %sautonomous prefix %s/%d%s\n",
 	       ( ( prefix_opt->flags & NDP_PREFIX_ON_LINK ) ? "on" : "off" ),
 	       ( ( prefix_opt->flags & NDP_PREFIX_AUTONOMOUS ) ? "" : "non-" ),
 	       inet6_ntoa ( &prefix_opt->prefix ),
-	       prefix_opt->prefix_len );
+	       prefix_opt->prefix_len, ( ipv6conf ? "" : " (ignored)" ) );
+
+	/* Do nothing unless IPv6 autoconfiguration is in progress */
+	if ( ! ipv6conf )
+		return 0;
 
 	/* Ignore off-link prefixes */
 	if ( ! ( prefix_opt->flags & NDP_PREFIX_ON_LINK ) )
 
 	/* Identify IPv6 configurator, if any */
 	ipv6conf = ipv6conf_demux ( netdev );
-	if ( ! ipv6conf ) {
-		/* Not an error; router advertisements are processed
-		 * as a background activity even when no explicit
-		 * autoconfiguration is taking place.
-		 */
+
+	/* Do nothing unless IPv6 autoconfiguration is in progress */
+	if ( ! ipv6conf )
 		return 0;
-	}
 
 	/* If this is not the first solicited router advertisement, ignore it */
 	if ( ! timer_running ( &ipv6conf->timer ) )