fetch http://localhost:6666 hangs



Hello!

I just noticed, that on my recent "6.1-STABLE #4: Thu Jun 8" amd64 system
attempts to connect to a bogus port (like 6666) hang instead of failing
with "Connection refused" immediately, as they on other systems.

Why would this be? There is nothing listening: ``netstat -n | grep 6666'' is
empty. The ipfw rules are very simple:

00200 pipe 1 ip from any to 172.21.128.43 dst-port 2049
65535 allow ip from any to any

While fetch is trying to connect, the tcpdump prints:

% tcpdump -vvv -i lo0 port 6666
tcpdump: listening on lo0, link-type NULL (BSD loopback), capture size 68 bytes
14:28:43.465182 IP (tos 0x0, ttl 64, id 56427, offset 0, flags [DF], proto: TCP (6), length: 64) localhost.52326 > localhost.6666: S, cksum 0x0558 (correct), 583002422:583002422(0) win 65535 <mss 16344,nop,wscale 1,nop,nop,timestamp 1610196770 0,sackOK,eol>
14:28:46.464121 IP (tos 0x0, ttl 64, id 56491, offset 0, flags [DF], proto: TCP (6), length: 64) localhost.52326 > localhost.6666: S, cksum 0xf99f (correct), 583002422:583002422(0) win 65535 <mss 16344,nop,wscale 1,nop,nop,timestamp 1610199770 0,sackOK,eol>
14:28:49.663980 IP (tos 0x0, ttl 64, id 56530, offset 0, flags [DF], proto: TCP (6), length: 64) localhost.52326 > localhost.6666: S, cksum 0xed1f (correct), 583002422:583002422(0) win 65535 <mss 16344,nop,wscale 1,nop,nop,timestamp 1610202970 0,sackOK,eol>
14:28:52.863836 IP (tos 0x0, ttl 64, id 56559, offset 0, flags [DF], proto: TCP (6), length: 48) localhost.52326 > localhost.6666: S, cksum 0x5993 (correct), 583002422:583002422(0) win 65535 <mss 16344,sackOK,eol>
[...]

Meanwhile, a "healthy" system prints:

% tcpdump -vvv -i lo0 port 6666
tcpdump: listening on lo0, link-type NULL (BSD loopback), capture size 68 bytes
14:29:55.716350 IP (tos 0x0, ttl 64, id 7958, offset 0, flags [DF], proto: TCP (6), length: 64) localhost.49238 > localhost.6666: S, cksum 0x7d0e (correct), 3929347125:3929347125(0) win 65535 <mss 16344,nop,wscale 1,nop,nop,timestamp 1704674149 0,sackOK,eol>
14:29:55.718358 IP (tos 0x0, ttl 64, id 7959, offset 0, flags [DF], proto: TCP (6), length: 40) localhost.6666 > localhost.49238: R, cksum 0xd901 (correct), 0:0(0) ack 3929347126 win 0

Any clues? Thanks!

-mi
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