Re: [PATCH v2 2/2] conf, fwd: Prefer same-scope address as inbound source address from host

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

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On Thu, Jul 23, 2026 at 05:16:03PM +0200, Stefano Brivio wrote:
> We might have situations, such as the one described in
> https://bugs.passt.top/show_bug.cgi?id=217, where using a link-local
> address as source in a given namespace doesn't guarantee that we can
> reach the intended destination, because, for instance, the inbound
> traffic we forward is in turn forwarded to a different interface, such
> as a bridge.
> 
> In that case, the assumption from 9618d247006a ("ndp, dhcpv6, tcp,
> udp: Always use link-local as source if gateway isn't") isn't a safe
> one: the user might have specified a valid gateway address, matching
> the scope of the destination address, but we won't use it as address
> of last resort, and prefer a link-local address with a mismatch in
> scope instead.
> 
> So, if the user specifies a given default gateway address for IPv6,
> note that as 'our_tap_addr', like we would do with with IPv4, and
> stick to Rule 2 of RFC 6724, Section 5, when selecting a source
> address, by preferring an address with the same scope, if available.
> 
> Note that this preference will be applied only if, at the point of
> the source address selection, the destination address is already
> known, which, in general, only happens if there's an explicit mapping
> rule specifying the target destination address.
> 
> Otherwise, if there's no explicit rule picking the destination
> address, we select the destination address after selecting the source
> address. This should probably be improved at some point, to select
> matching address pairs (when a compatible one is available) instead of
> sequentially picking source and destination addresses. That appears to
> be beyond the scope of this patch, though.
> 
> Reported-by: Paul Holzinger <pholzing@redhat.com>
> Link: https://bugs.passt.top/show_bug.cgi?id=217
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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

> ---
> v2: Use the address passed as --gateway as source for inbound NAT from
> the host regardless of local mode, see:
> 
>   https://bugs.passt.top/show_bug.cgi?id=217#c5
> 
>  conf.c  | 8 ++++++--
>  fwd.c   | 6 +++++-
>  passt.h | 2 ++
>  3 files changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/conf.c b/conf.c
> index 8205dbf..faf2681 100644
> --- a/conf.c
> +++ b/conf.c
> @@ -490,6 +490,8 @@ static unsigned int conf_ip6(unsigned int ifi, struct ip6_ctx *ip6)
>  
>  	if (IN6_IS_ADDR_LINKLOCAL(&ip6->guest_gw))
>  		ip6->our_tap_ll = ip6->guest_gw;
> +	else
> +		ip6->our_tap_addr = ip6->guest_gw;
>  
>  	if (IN6_IS_ADDR_UNSPECIFIED(&ip6->addr) ||
>  	    IN6_IS_ADDR_UNSPECIFIED(&ip6->our_tap_ll))
> @@ -507,10 +509,12 @@ static void conf_ip6_local(struct ip6_ctx *ip6)
>  	if (IN6_IS_ADDR_UNSPECIFIED(&ip6->guest_gw))
>  		ip6->guest_gw = IP6_LL_GUEST_GW;
>  
> -	if (IN6_IS_ADDR_LINKLOCAL(&ip6->guest_gw))
> +	if (IN6_IS_ADDR_LINKLOCAL(&ip6->guest_gw)) {
>  		ip6->our_tap_ll = ip6->guest_gw;
> -	else
> +	} else {
> +		ip6->our_tap_addr = ip6->guest_gw;
>  		ip6->our_tap_ll = IP6_LL_GUEST_GW;
> +	}
>  
>  	ip6->no_copy_addrs = ip6->no_copy_routes = true;
>  }
> diff --git a/fwd.c b/fwd.c
> index 7152169..4ba0af3 100644
> --- a/fwd.c
> +++ b/fwd.c
> @@ -1090,7 +1090,11 @@ uint8_t fwd_nat_from_host(const struct ctx *c,
>  				return PIF_NONE;
>  			tgt->oaddr = inany_from_v4(c->ip4.our_tap_addr);
>  		} else {
> -			tgt->oaddr.a6 = c->ip6.our_tap_ll;
> +			if (inany_is_linklocal6(&tgt->eaddr) ||
> +			    IN6_IS_ADDR_UNSPECIFIED(&c->ip6.our_tap_addr))
> +				tgt->oaddr.a6 = c->ip6.our_tap_ll;
> +			else
> +				tgt->oaddr.a6 = c->ip6.our_tap_addr;
>  		}
>  	}
>  	tgt->oport = ini->eport;
> diff --git a/passt.h b/passt.h
> index a61baca..51ccd4f 100644
> --- a/passt.h
> +++ b/passt.h
> @@ -121,6 +121,7 @@ struct ip4_ctx {
>   * @dns:		DNS addresses for DHCPv6 and NDP
>   * @dns_match:		Forward DNS query if sent to this address
>   * @our_tap_ll:		Link-local IPv6 address for passt's use on tap
> + * @our_tap_addr:	Non-LL IPv6 address for passt's use on tap (if any)
>   * @dns_host:		Use this DNS on the host for forwarding
>   * @addr_out:		Optional source address for outbound traffic
>   * @ifname_out:		Optional interface name to bind outbound sockets to
> @@ -140,6 +141,7 @@ struct ip6_ctx {
>  	struct in6_addr dns[MAXNS];
>  	struct in6_addr dns_match;
>  	struct in6_addr our_tap_ll;
> +	struct in6_addr our_tap_addr;
>  
>  	/* PIF_HOST addresses */
>  	struct in6_addr dns_host;
> -- 
> 2.43.0
> 

-- 
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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