Where are netstat's -t and -u flags documented?

Question

I use the following command to find open ports on my servers:

$ netstat -ntulp

I've probably found that on some random website over the years. However, I cannot find where -t or -u are documented. It certainly isn't in the man page. The man page does allude the their existence, and possibly hints that they are synonyms for -tcp and -udp (also undocumented, though easy enough to infer meaning for) respectively.

Though it seems that -t and -u are synonyms for -tcp and -udp, where is this documented?

Answer 1

The manpage and netstat --help both say [--tcp|-t] [--udp|-u] in the synopsis. That's more than a hint - this syntax pretty clearly states that -t is the same as --tcp and that -u is the same as --udp. You're right though that the manpage doesn't document --tcp and --udp.

netstat --help shows that --tcp and --udp are two of the several socket selectors that you can use, see the <Socket> line:

host ~ # netstat --help
usage: netstat [-vWeenNcCF] [<Af>] -r         netstat {-V|--version|-h|--help}
       netstat [-vWnNcaeol] [<Socket> ...]
       netstat { [-vWeenNac] -i | [-cWnNe] -M | -s }

        -r, --route              display routing table
        -i, --interfaces         display interface table
        -g, --groups             display multicast group memberships
        -s, --statistics         display networking statistics (like SNMP)
        -M, --masquerade         display masqueraded connections

        -v, --verbose            be verbose
        -W, --wide               don't truncate IP addresses
        -n, --numeric            don't resolve names
        --numeric-hosts          don't resolve host names
        --numeric-ports          don't resolve port names
        --numeric-users          don't resolve user names
        -N, --symbolic           resolve hardware names
        -e, --extend             display other/more information
        -p, --programs           display PID/Program name for sockets
        -c, --continuous         continuous listing

        -l, --listening          display listening server sockets
        -a, --all, --listening   display all sockets (default: connected)
        -o, --timers             display timers
        -F, --fib                display Forwarding Information Base (default)
        -C, --cache              display routing cache instead of FIB

  <Socket>={-t|--tcp} {-u|--udp} {-w|--raw} {-x|--unix} --ax25 --ipx --netrom
  <AF>=Use '-6|-4' or '-A <af>' or '--<af>'; default: inet
  List of possible address families (which support routing):
    inet (DARPA Internet) inet6 (IPv6) ax25 (AMPR AX.25) 
    netrom (AMPR NET/ROM) ipx (Novell IPX) ddp (Appletalk DDP) 
    x25 (CCITT X.25) 

Answer 2

The man pages are your friends for info such as this:

$ man netstat
...
SYNOPSIS
   netstat   [address_family_options]   [--tcp|-t]   [--udp|-u]  ...

Notice in the usage that those switches are the short forms of the GNU style switches --tcp and --udp.