BIRD - iBGP between RS over IPv6 lead to crash (segfault)

[Javor Kliachev]

Hello, 

We're using BIRD 1.6.4 as Route Server. 

Recently we have decided to establish an internal BGP session between both RS over IPv6 

But when we added our config in BIRD then it has crashed immediately with the following error: 
[35878512.553496] bird6[2545]: segfault at 0 ip 000055f60242c331 sp 00007ffd43689460 error 4 in bird6[55f60240a000+86000] 

At same time we have several working iBGP session between RS <> Routers ( not RS ) from a long time and everything works fine with them. 

Here is the example config that crashed the system. Note: The original ASN has been replaced with 1234 in the example. 

table T1234 

protocol pipe P1234 from iBGP_PIPES { 
peer table T1234; 
export all; 
} 

protocol bgp R100 { 
description "iBGP_RS1"; 
source address 2001:67c:29f0::A:1:234:200; 
neighbor 2001:67c:29f0::A:1:234:100 as 1234; 
local as myas; 
prefer older on; 
missing lladdr ignore; 
import none; 
export none; 
passive off; 
table T1234; 
add paths on; 
direct; 
} 

Another more strange: 

Initially before the crash we have added the following config below and RS was stable but session was not able to establish. 
We have made a capture of the traffic during the attempt of session establishment and seems TCP handshake works but then not procced futher. 

Here is the config: 

template bgp iBGP { 
local as myas; 
import all; 
export all; 
import limit 21000; 
passive on; 
prefer older on; 
missing lladdr ignore; 
} 

protocol bgp R1234x100 from iBGP { 
description "iBGP_RS1"; 
neighbor 2001:67c:29f0::A:1:234:100 as 1234; 
import all; 
export all; 
passive off; 
table T1234; 
# add paths on; 
direct; 
import limit 4000000; 
} 

Here is the logs from both side: 
Jul 7 11:52:09 rs1 bird6: R1234x4: Connecting to 2001:67c:29f0:0:a:1:234:4 from local address 2001:67c:29f0:0:a:1:234:100 
2021-07-07 11:53:56 <TRACE> R1234x100: Connecting to 2001:67c:29f0:0:a:1:1234:100 from local address 2001:67c:29f0:0:a:1:234:4 

Upper logs messages are repeating several times but iBGP was not establish.No any other logs persist in the logs message regarding this session establishment attempt. 

Could someone BIRD developer to suggest some solution for fixing this issue? 
We highly appreciate any kind of support. 

I had doubt that "add paths on" command would leading to this crashes but I commented it and after reloading the system crashed again. 
At same time this setup iBGP between the same RS over IPv4 works fine. 

Thanks in advance! 

Best~ 

Javor Kliachev 
Senior Engineer IP Services 
office: +359 2 974 33 11 
mobile: +359 885 98 84 95 
[ http://www.neterra.net/ | www.neterra.net ] [ https://bg.linkedin.com/pub/javor-kliachev/11/b46/843 | 
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