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Test on Linux (particularly: Ubuntu 20.04 LTS, kernel 5.4.0): in Python:

from socket import *
s1 = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)
s1.bind(('', 11001))
s2 = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)
s2.bind(('', 11001)) ## Here it fails: EADDRINUSE

So, at the moment s1 is bound, the port use is already registered somewhere in system.

But, unless I call listen() or connect() on the socket, it won't be shown in any registry I could find. /proc/net/tcp doesn't list it, nor netstat nor ss. lsof lists it but without the port:

python3   28296                           netch    5u     sock                0,9       0t0    4311999 protocol: TCP
python3   28296                           netch    6u     sock                0,9       0t0    4312006 protocol: TCP

Only when I call listen() on the socket, it appears in visible lists:

$ ss -at | grep 11001
LISTEN 0      128            0.0.0.0:11001                0.0.0.0:*             

TCP here is not a single case (originally I've faced it with SCTP, in the same manner).

To compare with, FreeBSD (tested 12.3) immediately lists the socket (checked using sockstat):

$ sockstat -P tcp -p 11001           
USER     COMMAND    PID   FD PROTO  LOCAL ADDRESS         FOREIGN ADDRESS      
netch    python3.8  797   4  tcp4   *:11001               *:*

Is there a tool to list presence of such sockets? Could it be done without iteration of all processes? Why the standard tools (like netstat, ss) ignore the issue?

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1 Answer 1

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Self-answer: there is a likely working recipe here, although extremely cumbersome. Thanks to @A.B for pointing in comments.

Iʼll update it if find a better solution.

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  • It was not just a comment: it was a query to mark the question as duplicate
    – A.B
    Commented Mar 21, 2023 at 8:36

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