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Linux / UNIX Find Out What Program / Service is Listening on a Specific TCP Port

How do I find out which service is listening on a specific port? How do I find out what program is listening on a specific TCP Port?

Under Linux and UNIX you can use any one of the following command to get listing on a specific TCP port:

  • lsof : list open files including ports.
  • netstat : The netstat command symbolically displays the contents of various network-related data and information.

lsof command example
Type the following command to see IPv4 port(s), enter:
# lsof -Pnl +M -i4

Type the following command to see IPv6 listing port(s), enter:
# lsof -Pnl +M -i6

Sample output:

COMMAND    PID     USER   FD   TYPE DEVICE SIZE NODE NAME
gweather- 6591 1000 17u IPv4 106812 TCP 192.168.1.100:57179->140.90.128.70:80 (ESTABLISHED)
firefox-b 6613 1000 29u IPv4 106268 TCP 127.0.0.1:60439->127.0.0.1:3128 (ESTABLISHED)
firefox-b 6613 1000 31u IPv4 106321 TCP 127.0.0.1:60440->127.0.0.1:3128 (ESTABLISHED)
firefox-b 6613 1000 44u IPv4 106325 TCP 127.0.0.1:60441->127.0.0.1:3128 (ESTABLISHED)
firefox-b 6613 1000 50u IPv4 106201 TCP 127.0.0.1:60437->127.0.0.1:3128 (ESTABLISHED)
deluge 6908 1000 8u IPv4 23179 TCP *:6881 (LISTEN)
deluge 6908 1000 30u IPv4 23185 UDP *:6881
deluge 6908 1000 45u IPv4 106740 TCP 192.168.1.100:50584->217.169.223.161:38406 (SYN_SENT)
deluge 6908 1000 57u IPv4 70529 TCP 192.168.1.100:57325->24.67.82.222:21250 (ESTABLISHED)
deluge 6908 1000 58u IPv4 106105 TCP 192.168.1.100:38073->24.16.233.1:48479 (ESTABLISHED)
..........
......
ssh 6917 1000 3u IPv4 23430 TCP 10.1.11.3:42658->10.10.29.66:22 (ESTABLISHED)

First column COMMAND – gives out information about program name. Please see output header for details. For example, gweather* command gets the weather report weather information from the U.S National Weather Service (NWS) servers (140.90.128.70), including the Interactive Weather Information Network (IWIN) and other weather services.
Where,

  1. -P : This option inhibits the conversion of port numbers to port names for network files. Inhibiting the conver-
    sion may make lsof run a little faster. It is also useful when port name lookup is not working properly.
  2. -n : This option inhibits the conversion of network numbers to host names for network files. Inhibiting conversion may make lsof run faster. It is also useful when host name lookup is not working properly.
  3. -l : This option inhibits the conversion of user ID numbers to login names. It is also useful when login name lookup is working improperly or slowly.
  4. +M : Enables the reporting of portmapper registrations for local TCP and UDP ports.
  5. -i4 : IPv4 listing only
  6. -i6 : IPv6 listing only

netstat command example

Type the command as follows:
# netstat -tulpn

OR
# netstat -npl

Output:

Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp 0 0 0.0.0.0:6881 0.0.0.0:* LISTEN 6908/python
tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN 5562/cupsd
tcp 0 0 127.0.0.1:3128 0.0.0.0:* LISTEN 6278/(squid)
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN 5854/exim4
udp 0 0 0.0.0.0:32769 0.0.0.0:* 6278/(squid)
udp 0 0 0.0.0.0:3130 0.0.0.0:* 6278/(squid)
udp 0 0 0.0.0.0:68 0.0.0.0:* 4583/dhclient3
udp 0 0 0.0.0.0:6881 0.0.0.0:* 6908/python

Last column PID/Program name gives out information regarding program name and port.
Where,

  • -t : TCP port
  • -u : UDP port
  • -l : Show only listening sockets.
  • -p : Show the PID and name of the program to which each socket / port belongs
  • -n : No DNS lookup (speed up operation)

/etc/services file

/etc/services is a plain ASCII file providing a mapping between friendly textual names for internet services, and their underlying assigned port numbers and protocol types. Every networking program should look into this file to get the port number (and protocol) for its service. You can view this file with the help of cat or less command:
$ cat /etc/services
$ grep 110 /etc/services
$ less /etc/services

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