Separately locate external interfaces for IPv4 and IPv6

Now that the back end allows passt/pasta to use different external
interfaces for IPv4 and IPv6, use that to do the right thing in the case
that the host has IPv4 and IPv6 connectivity via different interfaces.
If the user hasn't explicitly chosen an interface, separately search for
a suitable external interface for each protocol.

As a bonus, this substantially simplifies the external interface probe.  It
also eliminates a subtle confusing case where in some circumstances we
would pick the first interface in interface index order, and sometimes in
order of routes returned from netlink.  On some network configurations that
could cause tests to fail, because the logic in the tests was subtly
different (it always used route order).

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

1 files changed, 17 insertions, 2 deletions

diff --git a/conf.c b/conf.c
index 943526d..cc08de3 100644
--- a/conf.c
+++ b/conf.c
@@ -630,8 +630,23 @@ static void conf_ip(struct ctx *c)
 		v4 = v6		= IP_VERSION_PROBE;
 	}
 
-	if (!c->ifi4 && !c->ifi6)
-		c->ifi4 = c->ifi6 = nl_get_ext_if(&v4, &v6);
+	if (v4 != IP_VERSION_DISABLED) {
+		if (!c->ifi4)
+			c->ifi4 = nl_get_ext_if(AF_INET);
+		if (!c->ifi4) {
+			warn("No external routable interface for IPv4");
+			v4 = IP_VERSION_DISABLED;
+		}
+	}
+
+	if (v6 != IP_VERSION_DISABLED) {
+		if (!c->ifi6)
+			c->ifi6 = nl_get_ext_if(AF_INET6);
+		if (!c->ifi6) {
+			warn("No external routable interface for IPv6");
+			v6 = IP_VERSION_DISABLED;
+		}
+	}
 
 	if (v4 != IP_VERSION_DISABLED) {
 		if (!c->gw4)