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

On 23/01/2025 12:53, Stefano Brivio wrote:

On Thu, 23 Jan 2025 12:49:06 +0100 Paul Holzinger wrote:

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

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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.

Ah yes totally overlooked the fact the ip route does the RTM_GETLINK call there. -- Paul Holzinger