BGP neighbor tcp-mss doesn't work when neighbor in passive mode

[forsunwell]

• OS Version: ubuntu20.04
• Kernel: Linux 4.19
• FRR Version: 9.0
• When config bgp neighbor tcp-mss in neighbor passive mode:

Configuration like this:
router bgp 3
 neighbor 169.254.0.2 remote-as 2
 neighbor 169.254.0.2 tcp-mss 1000
 neighbor 169.254.0.2 passive

test_frr# show bgp neighbors 169.254.0.2
BGP neighbor is 169.254.0.2, remote AS 2, local AS 3, external link
Hostname: frr_1
  BGP version 4, remote router ID 10.222.40.1, local router ID 26.17.9.77
  BGP state = Established, up for 00:23:12
  Last read 00:00:12, Last write 00:00:12
  Hold time is 180, keepalive interval is 60 seconds
  Configured tcp-mss is 1000, synced tcp-mss is 1448
  Neighbor capabilities:
    4 Byte AS: advertised and received
    AddPath:
      IPv4 Unicast: RX advertised IPv4 Unicast and received
    Route refresh: advertised and received(old & new)
    Enhanced Route Refresh: advertised and received
    Address Family IPv4 Unicast: advertised and received
    Hostname Capability: advertised (name: test_frr,domain name: n/a) received (name: frr_1,domain name: n/a)
    Graceful Restart Capability: advertised and received
      Remote Restart timer is 120 seconds
      Address families by peer:
        none
......

Configured tcp-mss is 1000, but synced tcp-mss is 1448.

[forsunwell]

@ton31337
The pull request https://github.com/FRRouting/frr/pull/14423 not solve this problem.
The tcp-mss is incorrect because tcp-mss of listen socket is 1500 by default, and the remote tcp-mss is 1500 too, so tcp-mss of the neighbor seted to 1500 in bgp_accept function, the three-way handshake was over, have no way to modify the tcp-mss.

[ton31337]

Thanks, I'll try to verify this in my own lab.

[ton31337]

@forsunwell could you test this patch #14436?

[forsunwell]

@forsunwell could you test this patch #14436?

Hi, @ton31337 , I test this patch, there may be some problems in the code, i reply it on the pull request #14436 .

[ton31337]

You mean reusing same value for other peers? I don't know a proper way to set this for passive peers except this. If you have some ideas, go ahead or even better - open a PR.

[forsunwell]

You mean reusing same value for other peers? I don't know a proper way to set this for passive peers except this. If you have some ideas, go ahead or even better - open a PR.

I try to fix it in pull request #14449 .