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diff --git a/listening.1 b/listening.1 new file mode 100644 index 0000000..54c9f5f --- /dev/null +++ b/listening.1 @@ -0,0 +1,88 @@ +.Dd August 8, 2023 +.Dt LISTENING 1 +.Os +.Sh NAME +.Nm listening +.Nd check if a TCP server is listening +.Sh SYNOPSIS +.Nm +.Oo Fl t Ar connect-timeout Oc +.Oo Fl w Ar wait-timeout Oc +.Oo Ar host Ns \&: Oc Ns Ar port +.Sh DESCRIPTION +The +.Nm +utility performs a TCP scan against the given +.Ar host +.Po defaulting to localhost Pc +and +.Ar port . +.Pp +This can be used to detect if a slowly starting service is ready to +accept connections. +.Pp +The options are as follows: +.Bl -tag -width Ds +.It Fl t Ar connect-timeout +Wait at most +.Ar connect-timeout +seconds per connection attempt +.Po default: 0.2s, decimal fractions are allowed Pc . +.It Fl w Ar wait-timeout +Wait at most +.Ar wait-timeout +seconds total +.Po decimal fractions are allowed Pc , +and keep trying to connecting when connection has been refused. +.El +.Sh DETAILS +.Nm +implements a TCP SYN scan (half-open scan), +which has several benefits: +.Bl -bullet +.It +As the target program does not +.Xr accept 2 +the connection, there's no trace of testing. +.It +It's possible to do in unprivileged Linux userspace, +thanks to +.Dv TCP_QUICKACK +and +.Dv SO_LINGER +.Po but also Linux specific Pc . +.El +.Pp +Note that firewalls may block this kind of scan, +so for reliable results +.Nm +should be used on localhost only or within a DMZ. +.Sh EXIT STATUS +.Nm +returns one of the following status codes: +.Pp +.Bl -tag -compact -width Ds +.It 0 +when the port is up +.It 1 +when the port refuses connection +.It 2 +when timeout was reached +.It 99 +if some other error occurred +.El +.Sh SEE ALSO +.Xr nc 1 , +.Xr nmap 1 +.Sh AUTHORS +.An Leah Neukirchen Aq Mt leah@vuxu.org +.Sh LICENSE +.Nm +is in the public domain. +.Pp +To the extent possible under law, +the creator of this work +has waived all copyright and related or +neighboring rights to this work. +.Pp +.Lk http://creativecommons.org/publicdomain/zero/1.0/ |