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How to get Unix sockets open on Linux

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How to get Unix sockets open on Linux from the console.
Introduction to Unix sockets.

Unix sockets are one of the forms of communication between processes that are most used in Linux. For this reason, it is often very useful when making diagnostics to obtain open or listening Unix sockets .

image

This will be the objective of this tutorial, which explains how to obtain a list with all open Unix sockets and for this we will only need access to the Linux terminal.

How to get Unix sockets open on Linux from the console.

I know that many of you already know in depth the Unix sockets and that you are only interested in how to list them. Therefore, I show you in the first place the command that we will use and then we will make a brief explanation of different concepts for those who are new in the world of Unix sockets.

Command to list the Unix sockets open in the system:
netstat -xl

zeokat@ubuntu:~$ sudo netstat -xl
Active UNIX domain sockets (only servers)
Proto RefCnt Flags       Type       State         I-Node   Path
unix  2      [ ACC ]     STREAM     LISTENING     16209    public/flush
unix  2      [ ACC ]     STREAM     LISTENING     10818    /run/systemd/private
unix  2      [ ACC ]     STREAM     LISTENING     10822    /run/systemd/fsck.progress
unix  2      [ ACC ]     STREAM     LISTENING     16436    /run/user/1000/systemd/private
unix  2      [ ACC ]     SEQPACKET  LISTENING     10833    /run/udev/control
unix  2      [ ACC ]     STREAM     LISTENING     16218    private/smtp
unix  2      [ ACC ]     STREAM     LISTENING     10836    /run/systemd/journal/stdout
unix  2      [ ACC ]     STREAM     LISTENING     16221    private/relay
unix  2      [ ACC ]     STREAM     LISTENING     16224    public/showq
unix  2      [ ACC ]     STREAM     LISTENING     16184    public/cleanup
unix  2      [ ACC ]     STREAM     LISTENING     16180    public/pickup
unix  2      [ ACC ]     STREAM     LISTENING     16187    public/qmgr
unix  2      [ ACC ]     STREAM     LISTENING     15597    /var/run/mysqld/mysqld.sock
unix  2      [ ACC ]     STREAM     LISTENING     16251    private/maildrop
unix  2      [ ACC ]     STREAM     LISTENING     16254    private/uucp
unix  2      [ ACC ]     STREAM     LISTENING     16257    private/ifmail
unix  2      [ ACC ]     STREAM     LISTENING     16260    private/bsmtp
unix  2      [ ACC ]     STREAM     LISTENING     16263    private/scalemail-backend
unix  2      [ ACC ]     STREAM     LISTENING     13362    /run/uuidd/request
unix  2      [ ACC ]     STREAM     LISTENING     13363    /var/run/dbus/system_bus_socket
unix  2      [ ACC ]     STREAM     LISTENING     16266    private/mailman

The explanation of the command is very simple, we use the netstat tool with the following options:

  • x : Filter the list to show only Unix sockets.
  • l : shows only the sockets that are listening, also called servers.

Sometimes it is interesting to know PID of the process or program that owns the socket , for this we just have to add the -p option, so that the command would be:
sudo netstat -xlp

zeokat@ubuntu:~$ sudo netstat -xlp
Active UNIX domain sockets (only servers)
Proto RefCnt Flags       Type       State         I-Node   PID/Program name    Path
unix  2      [ ACC ]     STREAM     LISTENING     16209    1224/master         public/flush
unix  2      [ ACC ]     STREAM     LISTENING     10818    1/init              /run/systemd/private
unix  2      [ ACC ]     STREAM     LISTENING     10822    1/init              /run/systemd/fsck.progress
unix  2      [ ACC ]     STREAM     LISTENING     16436    1233/systemd        /run/user/1000/systemd/private
unix  2      [ ACC ]     SEQPACKET  LISTENING     10833    1/init              /run/udev/control
unix  2      [ ACC ]     STREAM     LISTENING     16218    1224/master         private/smtp
unix  2      [ ACC ]     STREAM     LISTENING     10836    1/init              /run/systemd/journal/stdout
unix  2      [ ACC ]     STREAM     LISTENING     16221    1224/master         private/relay
unix  2      [ ACC ]     STREAM     LISTENING     16224    1224/master         public/showq
unix  2      [ ACC ]     STREAM     LISTENING     16184    1224/master         public/cleanup
unix  2      [ ACC ]     STREAM     LISTENING     16180    1224/master         public/pickup
unix  2      [ ACC ]     STREAM     LISTENING     16187    1224/master         public/qmgr
unix  2      [ ACC ]     STREAM     LISTENING     15597    1031/mysqld         /var/run/mysqld/mysqld.sock
unix  2      [ ACC ]     STREAM     LISTENING     16251    1224/master         private/maildrop
unix  2      [ ACC ]     STREAM     LISTENING     16254    1224/master         private/uucp
unix  2      [ ACC ]     STREAM     LISTENING     16257    1224/master         private/ifmail
unix  2      [ ACC ]     STREAM     LISTENING     16260    1224/master         private/bsmtp
unix  2      [ ACC ]     STREAM     LISTENING     16263    1224/master         private/scalemail-backend
unix  2      [ ACC ]     STREAM     LISTENING     13362    1/init              /run/uuidd/request
unix  2      [ ACC ]     STREAM     LISTENING     13363    1/init              /var/run/dbus/system_bus_socket
unix  2      [ ACC ]     STREAM     LISTENING     16266    1224/master         private/mailman

In this case I used sudo to execute the command, but surely we do not have sufficient permissions to see all the PIDs.

Introduction to Unix sockets.

To get a little deeper into the world of Unix sockets , let's try to make a simple and affordable explanation. For this, we must start by explaining what a socket is and we could define it as an abstract method that allows data flow . Generally the concept of socket is associated with that of a plug that contacts two programs.

Now we can define as a Unix socket a method that allows communication between two processes of the same equipment . In other words, it allows the flow of data between the two processes in an orderly and reliable way within the operating system.

Communication can be bidirectional when so-called stream sockets are used and unidirectional when datagram sockets are used.

A very important peculiarity of UNIX sockets is that they use the file system as a namespace address , so a UNIX socket is identified by its path name. This allows us to use the file permission control system and use them in conjunction with the socket to set permissions.

Finally, I propose as an example of a UNIX socket when we configure a Nginx or Apache server with PHP-FPM , where we usually use a socket for the server and the PHP processing module to communicate.


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