In this article, we will briefly talk about ports in computer networking and move to how you can list all open ports in Linux.<\/p>\n
In computer networking, and more definitely in software terms, a port is a logical entity which acts as a endpoint of communication to\u00a0identify a given application or process on an Linux<\/a>\u00a0operating system. It is a 16-bit number (0<\/strong>\u00a0to\u00a065535<\/strong>) which differentiates one application from another on end systems.<\/p>\n The two most popular Internet transport protocols,\u00a0Transmission Control Protocol<\/strong>\u00a0(TCP<\/strong>) and the\u00a0User Datagram Protocol<\/strong>\u00a0(UDP<\/strong>) and other less known protocols use port numbers for communication sessions (source and destination port numbers in conjunction with the source and destination IP addresses).<\/p>\n In addition, a combination of an IP address, port and protocol such as\u00a0TCP\/UDP<\/strong>\u00a0is known as a socket, and every service must have a unique socket.<\/p>\n Below are the different categories of ports:<\/p>\n You can view a list of different applications and port\/protocol combination in\u00a0 To list all open ports or currently running ports including\u00a0TCP<\/strong>\u00a0and\u00a0UDP<\/strong>\u00a0in Linux, we will use\u00a0netstat<\/a>, is a powerful tool for monitoring network connections and statistics.<\/p>\n Where,<\/p>\n You can also use\u00a0ss<\/strong>\u00a0command, a well known useful utility for examining sockets in a Linux system. Run the command below to list all your open TCP and UCP ports:<\/p>\n Make it a point to read through the man pages of the commands above for more usage information.<\/p>\n In summary, understanding the concept of ports in computer networking is very vital for system and network administrators. You can as well go through this\u00a0netstat guide<\/a>\u00a0with simple, precise and well explained examples.<\/p>\n Last but not least, get in touch with us by sharing other methods for listing open ports in Linux or asking a question via the response form below.<\/p>\n\n
\/etc\/services<\/code>\u00a0file in Linux using\u00a0cat command<\/a>:<\/p>\n$ cat \/etc\/services \r\nOR\r\n$ cat \/etc\/services | less\r\n<\/pre>\n
# \/etc\/services:\r\n# $Id: services,v 1.48 2009\/11\/11 14:32:31 ovasik Exp $\r\n#\r\n# Network services, Internet style\r\n# IANA services version: last updated 2009-11-10\r\n#\r\n# Note that it is presently the policy of IANA to assign a single well-known\r\n# port number for both TCP and UDP; hence, most entries here have two entries\r\n# even if the protocol doesn't support UDP operations.\r\n# Updated from RFC 1700, ``Assigned Numbers'' (October 1994). Not all ports\r\n# are included, only the more common ones.\r\n#\r\n# The latest IANA port assignments can be gotten from\r\n# http:\/\/www.iana.org\/assignments\/port-numbers\r\n# The Well Known Ports are those from 0 through 1023.\r\n# The Registered Ports are those from 1024 through 49151\r\n# The Dynamic and\/or Private Ports are those from 49152 through 65535\r\n#\r\n# Each line describes one service, and is of the form:\r\n#\r\n# service-name port\/protocol [aliases ...] [# comment]\r\n\r\ntcpmux 1\/tcp # TCP port service multiplexer\r\ntcpmux 1\/udp # TCP port service multiplexer\r\nrje 5\/tcp # Remote Job Entry\r\nrje 5\/udp # Remote Job Entry\r\necho 7\/tcp\r\necho 7\/udp\r\ndiscard 9\/tcp sink null\r\ndiscard 9\/udp sink null\r\nsystat 11\/tcp users\r\nsystat 11\/udp users\r\ndaytime 13\/tcp\r\ndaytime 13\/udp\r\nqotd 17\/tcp quote\r\nqotd 17\/udp quote\r\nmsp 18\/tcp # message send protocol\r\nmsp 18\/udp # message send protocol\r\nchargen 19\/tcp ttytst source\r\nchargen 19\/udp ttytst source\r\nftp-data 20\/tcp\r\nftp-data 20\/udp\r\n# 21 is registered to ftp, but also used by fsp\r\nftp 21\/tcp\r\nftp 21\/udp fsp fspd\r\nssh 22\/tcp # The Secure Shell (SSH) Protocol\r\nssh 22\/udp # The Secure Shell (SSH) Protocol\r\ntelnet 23\/tcp\r\ntelnet 23\/udp\r\n<\/pre>\n
$ netstat -lntu<\/strong>\r\n\r\nProto Recv-Q Send-Q Local Address Foreign Address State \r\ntcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN \r\ntcp 0 0 0.0.0.0:3306 0.0.0.0:* LISTEN \r\ntcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN \r\ntcp 0 0 :::22 :::* LISTEN \r\ntcp 0 0 :::80 :::* LISTEN \r\ntcp 0 0 :::25 :::* LISTEN \r\nudp 0 0 0.0.0.0:68 0.0.0.0:* \r\n<\/pre>\n
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-l<\/code>\u00a0\u2013 prints only listening sockets<\/li>\n-n<\/code>\u00a0\u2013 shows port number<\/li>\n-t<\/code>\u00a0\u2013 enables listing of tcp ports<\/li>\n-u<\/code>\u00a0\u2013 enables listing of udp ports<\/li>\n<\/ol>\n$ ss -lntu<\/strong>\r\n\r\nNetid State Recv-Q Send-Q Local Address:Port Peer Address:Port \r\nudp UNCONN 0 0 *:68 *:* \r\ntcp LISTEN 0 128 :::22 :::* \r\ntcp LISTEN 0 128 *:22 *:* \r\ntcp LISTEN 0 50 *:3306 *:* \r\ntcp LISTEN 0 128 :::80 ::* \r\ntcp LISTEN 0 100 :::25 :::* \r\ntcp LISTEN 0 100 *:25 \r\n<\/pre>\n