dtrace to trace incoming connection not suceeding ?

From: Kurt Jaeger <pi_at_freebsd.org>
Date: Fri, 12 Nov 2021 13:09:26 UTC

I'm trying to investigate tcp-179 connection issues with the
local frr setup. See below for more background.

The question is: What can I do to find the cause of the failing
connection ? Is there a way to trace the incoming packet to see
if it ever ends up at bgpd process ?

Background: a fbsd 13.0p4 amd64 box, with a frr-7.4_4 (or 7.5.1_3)
fails to act on incoming ipv4 tcp 179 connections.

The bgpd daemon of frr listens on port 179:

frr      bgpd       96061 20 tcp6   *:179                 *:*
frr      bgpd       96061 21 tcp4   *:179                 *:*

Incoming packets can be seen with tcpdump on the dest host:

# tcpdump -i vlan500 -n -s 1500 port 179 and host <source-host>
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on vlan500, link-type EN10MB (Ethernet), capture size 1500 bytes
13:53:13.795162 IP <source>.12274 > <dest>.179: Flags [S], seq 62227917, win 65535, options [mss 1460,nop,wscale 9,sackOK,TS val 2870882787 ecr 0], length 0

But: No connection is established 8-(

The basic ipfw firewall is active, but

pi@FreeBSD.org         +49 171 3101372                  Now what ?