tcp test

start a listener and then 'suspend it' so that the connections will be accepted by the kernel but not the process:

bsmith1pts/8bsmith-y900~ nc -vvvlp 8080
Listening on any address 8080 (http-alt)
^Z
[1]  + 7303 suspended  nc -vvvlp 8080

verify its listening:

bsmith148pts/8bsmith-y900~ netstat -an | grep 8080
tcp        0      0 0.0.0.0:8080            0.0.0.0:*               LISTEN    

open some connections:
$ echo test | nc localhost 8080 &
[3] 7313
$ echo test | nc localhost 8080 &
[4] 7315

bsmith0pts/8bsmith-y900~ netstat -an | grep 8080
tcp        1      0 0.0.0.0:8080            0.0.0.0:*               LISTEN     
tcp        0      0 127.0.0.1:48736         127.0.0.1:8080          ESTABLISHED
tcp        5      0 127.0.0.1:8080          127.0.0.1:48736         ESTABLISHED

open some more (same nc & as above): after 5 we see a SYN_RECV:
which agrees with [1] and [2]

bsmith0pts/8bsmith-y900~ netstat -an | grep 8080
tcp        5      0 0.0.0.0:8080            0.0.0.0:*               LISTEN     
tcp        0      0 127.0.0.1:48816         127.0.0.1:8080          ESTABLISHED
tcp        5      0 127.0.0.1:8080          127.0.0.1:48816         ESTABLISHED
tcp        5      0 127.0.0.1:8080          127.0.0.1:48812         ESTABLISHED
tcp        0      5 127.0.0.1:48826         127.0.0.1:8080          ESTABLISHED
tcp        0      0 127.0.0.1:48812         127.0.0.1:8080          ESTABLISHED
tcp        5      0 127.0.0.1:8080          127.0.0.1:48822         ESTABLISHED
tcp        0      0 127.0.0.1:8080          127.0.0.1:48826         SYN_RECV   
tcp        0      0 127.0.0.1:48822         127.0.0.1:8080          ESTABLISHED
tcp        5      0 127.0.0.1:8080          127.0.0.1:48802         ESTABLISHED
tcp        0      0 127.0.0.1:48736         127.0.0.1:8080          ESTABLISHED
tcp        5      0 127.0.0.1:8080          127.0.0.1:48736         ESTABLISHED
tcp        0      0 127.0.0.1:48802         127.0.0.1:8080          ESTABLISHED

if we open more (more nc &) we see them go into SYN_RECV too

bsmith0pts/8bsmith-y900~ netstat -an | grep 8080
tcp        5      0 0.0.0.0:8080            0.0.0.0:*               LISTEN     
tcp        0      0 127.0.0.1:48816         127.0.0.1:8080          ESTABLISHED
tcp        5      0 127.0.0.1:8080          127.0.0.1:48816         ESTABLISHED
tcp        5      0 127.0.0.1:8080          127.0.0.1:48812         ESTABLISHED
tcp        0      5 127.0.0.1:48826         127.0.0.1:8080          ESTABLISHED
tcp        0      0 127.0.0.1:48812         127.0.0.1:8080          ESTABLISHED
tcp        5      0 127.0.0.1:8080          127.0.0.1:48822         ESTABLISHED
tcp        0      0 127.0.0.1:8080          127.0.0.1:48826         SYN_RECV   
tcp        0      0 127.0.0.1:48822         127.0.0.1:8080          ESTABLISHED
tcp        5      0 127.0.0.1:8080          127.0.0.1:48802         ESTABLISHED
tcp        0      0 127.0.0.1:48736         127.0.0.1:8080          ESTABLISHED
tcp        5      0 127.0.0.1:8080          127.0.0.1:48736         ESTABLISHED
tcp        0      0 127.0.0.1:48802         127.0.0.1:8080          ESTABLISHED
bsmith0pts/8bsmith-y900~ netstat -an | grep 8080
tcp        5      0 0.0.0.0:8080            0.0.0.0:*               LISTEN     
tcp        0      0 127.0.0.1:48816         127.0.0.1:8080          ESTABLISHED
tcp        5      0 127.0.0.1:8080          127.0.0.1:48816         ESTABLISHED
tcp        0      5 127.0.0.1:48880         127.0.0.1:8080          ESTABLISHED
tcp        5      0 127.0.0.1:8080          127.0.0.1:48812         ESTABLISHED
tcp        0      5 127.0.0.1:48826         127.0.0.1:8080          ESTABLISHED
tcp        0      0 127.0.0.1:48812         127.0.0.1:8080          ESTABLISHED
tcp        5      0 127.0.0.1:8080          127.0.0.1:48822         ESTABLISHED
tcp        0      0 127.0.0.1:8080          127.0.0.1:48826         SYN_RECV   
tcp        0      0 127.0.0.1:48822         127.0.0.1:8080          ESTABLISHED
tcp        5      0 127.0.0.1:8080          127.0.0.1:48802         ESTABLISHED
tcp        0      0 127.0.0.1:48736         127.0.0.1:8080          ESTABLISHED
tcp        0      0 127.0.0.1:8080          127.0.0.1:48880         SYN_RECV   
tcp        5      0 127.0.0.1:8080          127.0.0.1:48736         ESTABLISHED
tcp        0      0 127.0.0.1:48802         127.0.0.1:8080          ESTABLISHED

1: http://man7.org/linux/man-pages/man2/listen.2.html
2: line 656-657 https://sourceforge.net/p/netcat/code/HEAD/tree/trunk/src/network.c#l657

Hello