Bird setting TTL to 1 at the end of a passive BGP session opening

[Rumen Telbizov]

Hello Bird users,

First time poster and new subscriber.
I noticed something strange and wanted to report it here in case this is in
fact a bug that deserves attention.

I run bird 2.0.7-4.1 on Debian 11.
I have a BGP section configured as passive that acts as a TCP health-check
endpoint.

It is as follows:
*--- cut --*
protocol bgp HEALTHCHECKv4 {
    hold time 6;
    startup hold time 20;
    connect delay time 3;
    connect retry time 6;
    error wait time 3, 12;
    passive on;

    local 100.64.0.5 as 65000;
    neighbor 100.64.0.4 as 65535;
}
*--- cut --*


What ends up happening on the wire is this:
*--- cut --*
23:15:09.443792 IP (tos 0x0, ttl 254, id 4040, offset 0, flags [DF], proto
TCP (6), length 60)
    100.64.0.4.16141 > 100.64.0.5.179: Flags [S], cksum 0xa78f (correct),
seq 723435095, win 8961, options [mss 8621,sackOK,TS val 3290475421 ecr
0,nop,wscale 0], length 0
23:15:09.443823 IP (tos 0xc0, ttl 255, id 0, offset 0, flags [DF], proto
TCP (6), length 60)
    100.64.0.5.179 > 100.64.0.4.16141: Flags [S.], cksum 0xc8b7 (incorrect
-> 0x0785), seq 124371865, ack 723435096, win 62643, options [mss
8961,sackOK,TS val 2210037294 ecr 3290475421,nop,wscale 7], length 0
23:15:09.444437 IP (tos 0x0, ttl 254, id 4041, offset 0, flags [DF], proto
TCP (6), length 52)
    100.64.0.4.16141 > 100.64.0.5.179: Flags [.], cksum 0x2550 (correct),
seq 1, ack 1, win 8961, options [nop,nop,TS val 3290475422 ecr 2210037294],
length 0
23:15:09.444471 IP (tos 0x0, ttl 254, id 4042, offset 0, flags [DF], proto
TCP (6), length 52)
    100.64.0.4.16141 > 100.64.0.5.179: Flags [F.], cksum 0x254e (correct),
seq 1, ack 1, win 8961, options [nop,nop,TS val 3290475423 ecr 2210037294],
length 0
23:15:09.444576 IP (tos 0xc0, *ttl 1*, id 55411, offset 0, flags [DF],
proto TCP (6), length 99)
    100.64.0.5.179 > 100.64.0.4.16141: Flags [P.], cksum 0xc8de (incorrect
-> 0x58b6), seq 1:48, ack 2, win 490, options [nop,nop,TS val 2210037294
ecr 3290475423], length 47: BGP
    Open Message (1), length: 47
      Version 4, my AS 65000, Holdtime 6s, ID 100.64.0.5
      Optional parameters, length: 18
        Option Capabilities Advertisement (2), length: 16
          Route Refresh (2), length: 0
          Graceful Restart (64), length: 2
        Restart Flags: [none], Restart Time 120s
        0x0000:  0078
          32-Bit AS Number (65), length: 4
         4 Byte AS 65000
        0x0000:  0000 fde8
          Enhanced Route Refresh (70), length: 0
        no decoder for Capability 70
          Long-lived Graceful Restart (71), length: 0

23:15:09.444602 IP (tos 0xc0, *ttl 1*, id 55412, offset 0, flags [DF],
proto TCP (6), length 52)
    100.64.0.5.179 > 100.64.0.4.16141: Flags [F.], cksum 0xc8af (incorrect
-> 0x4635), seq 48, ack 2, win 490, options [nop,nop,TS val 2210037294 ecr
3290475423], length 0
23:15:09.444670 IP (tos 0x0, ttl 64, id 1, offset 0, flags [none], proto
ICMP (1), length 56)
    100.64.0.4 > 100.64.0.5: ICMP time exceeded in-transit, length 36
    IP (tos 0xc0, ttl 1, id 55411, offset 0, flags [DF], proto TCP (6),
length 99)
    100.64.0.5.179 > 100.64.0.4.16141:  [|tcp]
*--- cut --*

As you can see the TTL on our packets is initially set to 255. At the end
of the connection
during the last PUSH and FIN packets all of a sudden bird sets the TTL to 1.

I have no ttl security enabled and even if I explicitly disable it the
problem persists.
A workaround that I found to work is to pass multihop 20 directive which
then changes the ttl 1 above to ttl 20
which alleviates the problem.

Let me know if you need any additional information.

Regards,
-- 
Rumen Telbizov
Site Reliability Engineer <http://telbizov.com/>
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