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

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 23 Jan 2025 12:49:06 +0100
Paul Holzinger <pholzing@redhat.com> wrote:

> 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

Yeah but that's in the link flags, right? Am I missing something? Here
we're looking at routes.

Note that 'ip route show' will also fetch link attributes with
RTM_GETLINK, which is the second netlink query I wanted to avoid here.

-- 
Stefano