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/>