README

LISTENING(1) General Commands Manual LISTENING(1)

NAME listening – check if a TCP server is listening

SYNOPSIS listening [-46] [-t connect-timeout] [-w wait-timeout] [host] port

DESCRIPTION The listening utility performs a TCP scan against the given host (defaulting to localhost) and port.

 This can be used to detect if a slowly starting service is ready to
 accept connections.

 The options are as follows:

 -4      Force use of IPv4.

 -6      Force use of IPv6.

 -t connect-timeout
         Wait at most connect-timeout seconds per connection attempt
         (default: 0.2s, decimal fractions are allowed).

 -w wait-timeout
         Wait at most wait-timeout seconds total (decimal fractions are
         allowed), and keep trying to connecting when connection has been
         refused (default: only try once).

DETAILS listening implements a TCP SYN scan (half-open scan), which has several benefits:

 •   As the target program does not accept(2) the connection, there's no
     trace of testing.

 •   It's possible to do in unprivileged Linux userspace, thanks to
     TCP_QUICKACK and SO_LINGER (but also Linux specific).

 Note that firewalls may block this kind of scan, so for reliable results
 listening should be used on localhost only or within a DMZ.

EXIT STATUS listening returns one of the following status codes:

 0       when the port is up
 1       when the port refuses connection
 2       when timeout was reached
 99      if some other error occurred

SEE ALSO nc(1), nmap(1)

AUTHORS Leah Neukirchen leah@vuxu.org

LICENSE listening is in the public domain.

 To the extent possible under law, the creator of this work has waived all
 copyright and related or neighboring rights to this work.

 http://creativecommons.org/publicdomain/zero/1.0/

Void Linux August 8, 2023 Void Linux