From 4c9ec30abbcac635a989a1929a4fcb10d6d4bb9e Mon Sep 17 00:00:00 2001 From: Stefano Brivio Date: Thu, 23 Jul 2026 00:14:39 +0200 Subject: conf, fwd: Prefer same-scope address as inbound source address from host 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 Link: https://bugs.passt.top/show_bug.cgi?id=217 Signed-off-by: Stefano Brivio Reviewed-by: David Gibson --- 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; -- cgit v1.2.3