netlink: Skip loopback interface while looking for a template

There might be reasons to have routes on the loopback interface, for
example Any-IP/AnyIP routes as implemented by Linux kernel commit
ab79ad14a2d5 ("ipv6: Implement Any-IP support for IPv6.").

If we use the loopback interface as a template, though, we'll pick
'lo' (typically) as interface name for our tap interface, but we'll
already have an interface called 'lo' in the target namespace, and as
we TUNSETIFF on it, we'll fail with EINVAL, because it's not a tap
interface.

Skip the loopback interface while looking for a template interface or,
more accurately, skip the interface with index 1.

Strictly speaking, we should fetch interface flags via RTM_GETLINK
instead, and check for IFF_LOOPBACK, but interleaving that request
while we're iterating over routes is unnecessarily complicated.

Link: https://www.reddit.com/r/podman/comments/1i6pj7u/starting_pod_without_external_network/
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

1 files changed, 4 insertions, 0 deletions

diff --git a/netlink.c b/netlink.c
index 0407692..37d8b5b 100644
--- a/netlink.c
+++ b/netlink.c
@@ -297,6 +297,10 @@ unsigned int nl_get_ext_if(int s, sa_family_t af)
 		if (!thisifi)
 			continue; /* No interface for this route */
 
+		/* Skip 'lo': we should test IFF_LOOPBACK, but keep it simple */
+		if (thisifi == 1)
+			continue;
+
 		/* Skip routes to link-local addresses */
 		if (af == AF_INET && dst &&
 		    IN4_IS_PREFIX_LINKLOCAL(dst, rtm->rtm_dst_len))