Handling IPv6 BGP routes with :: next-hop

[Alexander Shikov]

Hello!

Yesterday we noticed a strange behavior of Bird 1.6.8 with handling
one IPv6 BGP route:

>birdc6 show route for 2a09:8387:ffff:: all
BIRD 1.6.8 ready.
2a09:8387:ffff::/48 via 2001:7f8:63::f2 on bge0 [SIMTELECOM 2021-08-03 00:07:16] * (100) [AS51972i]
        Type: BGP unicast univ
        BGP.origin: IGP
        BGP.as_path: 51972
        BGP.next_hop: ::
        BGP.med: 0
        BGP.local_pref: 100
        BGP.community: (31210,31210)
        BGP.ext_community: (rt, 31210, 31210)
                   via 2001:7f8:63::69 on bge0 [DATAGROUP2 2021-07-20 16:07:45] (100) [AS51972i]
        Type: BGP unicast univ
        BGP.origin: IGP
        BGP.as_path: 3326 51972
        BGP.next_hop: 2001:7f8:63::69 fe80::2e21:7201:935e:37ce
        BGP.med: 510
        BGP.local_pref: 100
        BGP.community: (3326,3010) (3326,11001) (3326,22050) (3326,50990) (31210,31210)
        BGP.ext_community: (rt, 31210, 31210)
                   via 2001:7f8:63::60 on bge0 [HE 2021-07-22 00:53:03] (100) [AS51972i]
        Type: BGP unicast univ
        BGP.origin: IGP
        BGP.as_path: 6939 3326 51972
        BGP.next_hop: 2001:7f8:63::60 fe80::629c:9fff:fe83:8c13
        BGP.med: 0
        BGP.local_pref: 100
        BGP.community: (0,16509) (0,15169) (0,13335) (0,15133) (0,22822) (0,12989) (0,2906) (0,20940) (0,3255) (0,16265) (0,16276) (0,13188) (0,714) (0,32590) (0,48641) (0,49029) (31210,31210)
        BGP.ext_community: (rt, 31210, 31210)
                   via 2001:7f8:63::f4 on bge0 [DATAGROUP 2021-07-07 11:00:57] (100) [AS51972i]
        Type: BGP unicast univ
        BGP.origin: IGP
        BGP.as_path: 3326 51972
        BGP.next_hop: 2001:7f8:63::f4 fe80::ae4b:c8ff:fe96:67c3
        BGP.med: 510
        BGP.local_pref: 100
        BGP.community: (3326,3010) (3326,11001) (3326,22050) (3326,50990) (31210,31210)
        BGP.ext_community: (rt, 31210, 31210)


The first (best) route has next-hop '::' and routers of some of our customers
started dropping BGP sessions with 'Invalid next-hop' cause.

I dumped packets coming from customer 2001:7f8:63::f2 and it is really sending next-hop
as '::'. 

I enabled logging of 'gw' and 'from' route attributes for net 2a09:8387:ffff::/48,
and I got the following:

Aug  3 09:57:10 <daemon.info> rs1 bird6: DEBUG: gw:2001:7f8:63::f4 ---- from:2001:7f8:63::f4
Aug  3 09:57:10 <daemon.info> rs1 bird6: DEBUG: gw:2001:7f8:63::69 ---- from:2001:7f8:63::69
Aug  3 09:57:11 <daemon.info> rs1 bird6: DEBUG: gw:2001:7f8:63::f2 ---- from:2001:7f8:63::f2
Aug  3 09:57:12 <daemon.info> rs1 bird6: DEBUG: gw:2001:7f8:63::60 ---- from:2001:7f8:63::60

Thus 'gw' and 'from' variables both contain IPv6 address of the peer. 
But shouldn't 'gw' contain '::'?

Due to that the checks like
        if gw != from then
                return false;
fail.

Please advice, thanks!


-- 
Alexander Shikov
Technical Staff, Digital Telecom IX
Tel.: +380 44 201 14 07
Mob.: +380 50 410 30 57
http://dtel-ix.net/