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

On Thu, 23 Jan 2025 12:49:06 +0100
Paul Holzinger <pholzing(a)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