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[20101008] Look who's talking, or: how to find which process listens on a given socket (Update #2)
From the "learn something new every day" department: I've wondered how to find a process listening on a given socket in the past. While there is "lsof" in pkgsrc, here is a solution using NetBSD's on-board tools, netstat(1) and fstat(1).

netstat(1)'s "-A" switch can be used to show a protocol control block (PCB) associated with a socket in its output, for TCP and Unix domain sockets:

    % netstat -Aa
    Active Internet connections (including servers)
    PCB      Proto Recv-Q Send-Q  Local Address      Foreign Address    State
    c15ce1f4 tcp        0      0  10.0.0.178.ssh     mini.52788         ESTABLISHED
    c15ce5dc tcp        0      0  *.ftp              *.*                LISTEN
    c15ce7d0 tcp        0      0  *.https            *.*                LISTEN
    ...
    Active Internet6 connections (including servers)
    PCB      Proto Recv-Q Send-Q  Local Address      Foreign Address    (state)
    c15ce3e8 tcp6       0      0  *.ftp              *.*                LISTEN
    c15cedac tcp6       0      0  *.ssh              *.*                LISTEN
    Active UNIX domain sockets
    Address  Type   Recv-Q Send-Q    Inode     Conn     Refs  Nextref Addr
    c1602480 stream      0      0 cb9e6f20        0        0        0 private/scache
    ...  
Questions arising from the above output may be what process is handling the TCP/ssh connection and who is responsible for the "private/scache" Unix domain socket.

In NetBSD, a PCB identifies a certain network connection, and from that more information on the processes using that PCB can be determined. This can be done using NetBSD's fstat(1) command as follows.

The TCP ssh connection lists PCB "c15ce1f4":

    % fstat | head -1
    USER     CMD          PID   FD MOUNT       INUM MODE         SZ|DV R/W
    % fstat | grep c15ce1f4
    feyrer   sshd       23305    5* internet stream tcp c15ce1f4 10.0.0.178:22 <-> 10.0.0.2:52788
    root     sshd       26059    5* internet stream tcp c15ce1f4 10.0.0.178:22 <-> 10.0.0.2:52788
    
So it's two processes here, one SSH daemon running as root, and one under my user-id. The reason behind this is the SSH daemon's splitting of privileges across multiple processes.

The answer to who is listening on Unix domain socket "private/scache" can be found in a similar fashion:

    % fstat | grep c1602480
    root     master       511   80* unix stream c1602480
    % locate /master | grep 'master$'
    /usr/libexec/postfix/master
    
If a program "master" is not too obvious (a virus?!), looking for its place on the file system, e.g. using locate(1), may help. In this case, it shows that the socket is used by the Postfix mail server.

Update 1+2: Geert Hendricks mentioned "sockstat -l" as well... one for the category "NetBSD commands you didn't know yet" :-) Thanks Geert!

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