Re: [PATCH] netlink: With no default route, pick the first interface with a route

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Jun 18, 2024 at 07:18:03PM +0200, Stefano Brivio wrote:
> While commit f919dc7a4b1c ("conf, netlink: Don't require a default
> route to start") sounded reasonable in the assumption that, if we
> don't find default routes for a given address family, we can still
> proceed by selecting an interface with any route *iff it's the only
> one for that protocol family*, Jelle reported a further issue in a
> similar setup.
> 
> There, multiple interfaces are present, and while remote container
> connectivity doesn't matter for the container, local connectivity is
> desired. There are no default routes, but those multiple interfaces
> all have non-default routes, so we should just pick one and start.
> 
> Pick the first interface reported by the kernel with any route, if
> there are no default routes. There should be no harm in doing so.
> 
> Reported-by: Jelle van der Waa <jvanderwaa@redhat.com>
> Reported-by: Martin Pitt <mpitt@redhat.com>
> Link: https://bugzilla.redhat.com/show_bug.cgi?id=2277954
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Seems reasonable until we can tackle this better.

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  netlink.c | 16 ++++++++--------
>  passt.1   |  4 ++--
>  2 files changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/netlink.c b/netlink.c
> index 0be4ea3..3aed7e5 100644
> --- a/netlink.c
> +++ b/netlink.c
> @@ -269,8 +269,7 @@ unsigned int nl_get_ext_if(int s, sa_family_t af)
>  	size_t na;
>  
>  	/* Look for an interface with a default route first, failing that, look
> -	 * for any interface with a route, and pick it only if it's the only
> -	 * interface with a route.
> +	 * for any interface with a route, and pick the first one, if any.
>  	 */
>  	seq = nl_send(s, &req, RTM_GETROUTE, NLM_F_DUMP, sizeof(req));
>  	nl_foreach_oftype(nh, status, s, buf, seq, RTM_NEWROUTE) {
> @@ -324,18 +323,19 @@ unsigned int nl_get_ext_if(int s, sa_family_t af)
>  		warn("netlink: RTM_GETROUTE failed: %s", strerror(-status));
>  
>  	if (defifi) {
> -		if (ndef > 1)
> +		if (ndef > 1) {
>  			info("Multiple default %s routes, picked first",
>  			     af_name(af));
> +		}
>  		return defifi;
>  	}
>  
>  	if (anyifi) {
> -		if (nany == 1)
> -			return anyifi;
> -
> -		info("Multiple interfaces with %s routes, use -i to select one",
> -		     af_name(af));
> +		if (nany > 1) {
> +			info("Multiple interfaces with %s routes, picked first",
> +			     af_name(af));
> +		}
> +		return anyifi;
>  	}
>  
>  	if (!nany)
> diff --git a/passt.1 b/passt.1
> index 6dfa670..6ee1e2e 100644
> --- a/passt.1
> +++ b/passt.1
> @@ -152,8 +152,8 @@ This option can be specified zero (for defaults) to two times (once for IPv4,
>  once for IPv6).
>  By default, assigned IPv4 and IPv6 addresses are taken from the host interfaces
>  with the first default route, if any, for the corresponding IP version. If no
> -default routes are available and there is just one interface with any route,
> -that interface will be chosen instead.
> +default routes are available and there is any interface with any route for a
> +given IP version, the first of these interfaces will be chosen instead.
>  
>  .TP
>  .BR \-n ", " \-\-netmask " " \fImask

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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