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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
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