Viewing Listening Ports on any Platform (That I Use Regularly)

For much of my career, all I needed to remember to view listening ports on any local machine was good, old netstat -na. Developing on MacOS and working with Linux Docker containers, however, require additional tricks. Here are the tools I use to quickly check ports on various platforms.

1. Windows: netstat

> netstat -na

Active Connections

  Proto  Local Address          Foreign Address        State
  TCP    0.0.0.0:22             0.0.0.0:0              LISTENING
  TCP    0.0.0.0:80             0.0.0.0:0              LISTENING
  ...
  UDP    0.0.0.0:53             *:*
  ...

2. Desktop/server Linux: also netstat

> netstat -na
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address           Foreign Address         State
tcp        0      0 127.0.0.1:5001          0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:5000            0.0.0.0:*               LISTEN
udp        0      0 127.0.0.1:323           0.0.0.0:*
udp6       0      0 ::1:323                 :::*
...

3. Linux containers: netstat without netstat

Has netstat?

>docker run -it ubuntu:latest bash
root@eedceb5073f6:/# netstat -na
bash: netstat: command not found

No. Ugh. And I can never remember the name of the package to install (Shawn, it's net-tools). One day I gave up and found this script from the post netstat-without-netstat. It works. Now I can find it here.

> awk 'function hextodec(str,ret,n,i,k,c){
    ret = 0
    n = length(str)
    for (i = 1; i <= n; i++) {
        c = tolower(substr(str, i, 1))
        k = index("123456789abcdef", c)
        ret = ret * 16 + k
    }
    return ret
}
function getIP(str,ret){
    ret=hextodec(substr(str,index(str,":")-2,2));
    for (i=5; i>0; i-=2) {
        ret = ret"."hextodec(substr(str,i,2))
    }
    ret = ret":"hextodec(substr(str,index(str,":")+1,4))
    return ret
}
NR > 1 {{if(NR==2)print "\nLocal - Remote";local=getIP($2);
   remote=getIP($3)}{print local" - "remote}}' /proc/net/tcp

Local - Remote
127.0.0.1:5001 - 0.0.0.0:0
0.0.0.0:5000 - 0.0.0.0:0

4. MacOS: lsof wrapper

I'm also not going to remember lsof arguments. I put these functions in .zshenv. Call with no args to see all the listening ports. The optional argument is a pattern to grep, but I usually end up grepping the full output myself.

listening_tcp() {
    if [ $# -eq 0 ]; then
        sudo lsof -iTCP -sTCP:LISTEN -n -P
    elif [ $# -eq 1 ]; then
        sudo lsof -iTCP -sTCP:LISTEN -n -P | grep -i --color $1
    else
        echo "Usage: listening [pattern]"
    fi
}

listening_udp() {
    if [ $# -eq 0 ]; then
        sudo lsof -iUDP -n -P
    elif [ $# -eq 1 ]; then
        sudo lsof -iUDP -n -P | grep -i --color $1
    else
        echo "Usage: listening [pattern]"
    fi
}