Localization - agnostic output for netcat

Question

I would like to use netcat to test if some ports are listening or not but, since the command will be executed in machines that will have languages different than English, it's not a great idea to grep its output based on the word LISTENING (for example, I saw on a German pc the word ABHOREN).

I tried to look at the manpages for netcat, but I can't seem to find a solution for that, so I was wondering if anybody ever managed to translate the status of the port into a numerical format, rather than a string.

Thanks in advance :)

score 2 · Answer 1 · edited Jan 19 '17 at 18:51

2

You can export the language for netcat before running it (or even for your whole script):

LANG=C netcat ...

edited Jan 19 '17 at 18:51

sebix

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answered Jan 19 '17 at 16:34

DusteD

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score 2 · Accepted Answer · answered Jan 19 '17 at 16:35

If you say LISTENING, I guess that you actually want to netstat (not netcat). Most modern systems understand netstat -lnt which only lists LISTENING tcp sockets, nothing else. Beware on minimal RHEL7 install there is no netstat - it is deprecated in favor of ss (same flags ss -lnt but slightly different output format).

In a general case the answer would be to set locale in the environment properly, this way all processes will talk to you in English (or better, C).

yes indeed, I was referring to `netstat` but typed it wrong. Too many hours on the terminal :) — Polentino, Jan 20 '17 at 08:37

score 1 · Answer 3 · edited May 23 '17 at 12:41

Like netstat you can ask the kernel directly. Its info comes from /proc/net/tcp and /proc/net/tcp6. Both files are formatted in the same way and are slightly (cough) harder to read that the netstat output, but are language independent

This is a chopped and slightly masked sample output:

$ cat /proc/net/tcp
  sl  local_address rem_address   st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode
   0: 00000000:0385 00000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 5126 1 f4807080 100 0 0 10 -1
   1: 00000000:2328 00000000:0000 0A 00000000:00000000 00:00000000 00000000   113        0 5264 1 f62ff540 100 0 0 10 -1
   2: 00000000:008B 00000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 4535 1 f5bf1540 100 0 0 10 -1
   9: 0200000A:08AE xxxxxxxx:D512 01 00000030:00000000 01:00000018 00000000     0        0 759198 4 f63e95c0 24 4 3 10 19
  10: 0200000A:0D9B xxxxxxxx:7582 01 00000000:00000000 00:00000000 00000000   113        0 634192 1 f63e90c0 21 4 22 5 3

The key fields you're looking at at the hex following the : under local_address (Local port) and the st column (Status).

The status columns are referred to in the kernel source ./include/net/tcp_states.h, but helpfully someone has asked the question over on StackExchange: https://stackoverflow.com/questions/5992211/list-of-possible-internal-socket-statuses-from-proc

I'll duplicate here for completeness:

enum {
    TCP_ESTABLISHED = 1,
    TCP_SYN_SENT,
    TCP_SYN_RECV,
    TCP_FIN_WAIT1,
    TCP_FIN_WAIT2,
    TCP_TIME_WAIT,
    TCP_CLOSE,
    TCP_CLOSE_WAIT,
    TCP_LAST_ACK,
    TCP_LISTEN,
    TCP_CLOSING,    /* Now a valid state */

    TCP_MAX_STATES  /* Leave at the end! */
};

As one of the comments states, this is an enum so the states are numbered sequentially from 1. Mostly you're looking for 0A (LISTENING) and 01 (ESTABLISHED)

NB: To find out where netstat was getting this info from, I ran strace -e open,write netstat -an.

There's a deeper dive into the field headers, here: http://www.onlamp.com/pub/a/linux/2000/11/16/LinuxAdmin.html

OP didn't say "Linux" and other Unix-like don't have `/proc/net/` — kubanczyk, Jan 20 '17 at 18:12

Localization - agnostic output for netcat

3 Answers3