Re: [PATCH] netlink: Skip loopback interface while looking for a template

[Paul Holzinger <pholzing@redhat.com>]

On 23/01/2025 09:05, Stefano Brivio wrote:
> 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.

I think hard coding 1 is fine but I think there is also the IFF_LOOPBACK 
flag that could be used instead.

 From strace:
ifi_index=if_nametoindex("lo"), 
ifi_flags=IFF_UP|IFF_LOOPBACK|IFF_RUNNING|IFF_LOWER_UP

>
> Link: https://www.reddit.com/r/podman/comments/1i6pj7u/starting_pod_without_external_network/
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> ---
>   netlink.c | 4 ++++
>   1 file changed, 4 insertions(+)
>
> 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))

-- 
Paul Holzinger