Open port: Difference between revisions

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[[File:Open ports.png|frame|Listing open TCP ports that are listening on the local machine.]]

In security parlance, the term '''open port''' is used to mean a [[InternetTransmission protocolControl suiteProtocol|TCP/IP]] or [[TCPUser andDatagram Protocol|UDP]] [[port (computer networking)|port]] number]] that is configured to accept [[packet (information technology)|packet]]s. In contrast, a port which rejects connections or ignores all packets directed at it is commonly referred to ascalled a "'''closed port"'''.<ref>[httphttps://www.pcmag.com/encyclopedia_term/0,2542,t=open+port&i=48464,00.asp Pcmag.com encyclopedia term]</ref>

Ports are an integral part of the Internet's [[Internet socket|communication model]] — they are the channel through which applications on the [[client (computing)|client computer]] can reach the software on the [[server (computing)|server]]. Services, such as [[web page]]s or [[ftp server|FTP]], require their respective ports to be "open" on the server in order to be publicly reachable.

The above use of the terms "open" and "closed" can sometimes be misleading, though; it blurs the distinction between a given port being reachable (unfiltered) and whether or not there is an application actually listening on that port. Technically, a given port being "open" (in this context, reachable) is not enough for a communication channel to be established. There needs to be an application (service) listening on that port, accepting the incoming packets and processing them. If there is no application listening on a port, incoming packets to that port will simply be rejected by the computer's operating system.

Ports can be "closed" (in this context, filtered) through the use of a [[Firewall (computing)|firewall]]. The firewall will filter incoming packets, only letting through those packets for which it has been configured. Packets directed at a port which the firewall is configured to "close" will simply be dropped in transit, as though they never existed.

Some malicious software acts as a service, waiting for connections from a remote attacker in order to give himthem information or control over the machine. It is common security practice to close unused ports in personal computers, so as to block public access to any services which might be running on the computer without the user's knowledge, whether due to legitimate services being misconfigured, or the presence of malicious software.

Malicious ("[[hackerblack hat (computer security)|Maliciousblack hackershat]]") [[hacker (orcomputer crackerssecurity)|hackers]] commonly use [[port scanning]] software to find which ports are "open" (unfiltered) in a given computer, and whether or not an actual service is listening on that port. They can then attempt to exploit potential [[vulnerability (computing)|vulnerabilities]] in any services they find.

== Example ==

<syntaxhighlight lang="console">

alice@wikipedia:~$ netstat --tcp --listening --numeric

Active Internet connections (only servers)

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN

tcp 0 0 127.0.0.1:5432 0.0.0.0:* LISTEN

tcp 0 0 127.0.0.53:53 0.0.0.0:* LISTEN

tcp6 0 0 ::1:631 :::* LISTEN

</syntaxhighlight>

== See also ==

* [[Port scanning]]

* [[Nmap]]
* [[Computer security]]

* [[List of TCP and UDP port numbers]]

*[[Computer insecurity]]

*[[List of TCP and UDP port numbers]]

== References ==

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== External links ==

* [http://www.ports-servicesemsisoft.com/en/kb/articles/tec021114/ How Searchablecan PortsI &close Servicesa Databaseport?]

*[http://www.emsisoft.com/en/kb/articles/tec021114/ How can I close a port?]

[[Category:Internet protocols]]

[[Category:TCP/IP]]

[[Category:Internet protocols]]

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