Open Port
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In security parlance, the term open port is used to mean a TCP or UDP
port number In computer networking, a port is a communication endpoint. At the software level within an operating system, a port is a logical construct that identifies a specific process or a type of network service. A port is uniquely identified by a numbe ...
that is configured to accept
packet Packet may refer to: * A small container or pouch ** Packet (container), a small single use container ** Cigarette packet ** Sugar packet * Network packet, a formatted unit of data carried by a packet-mode computer network * Packet radio, a form ...
s. In contrast, a port which rejects connections or ignores all packets directed at it is called a closed port.Pcmag.com encyclopedia term
/ref> Ports are an integral part of the Internet's communication model — they are the channel through which applications on the
client computer is a computer that gets information from another computer called server in the context of client–server model of computer networks. The server is often (but not always) on another computer system, in which case the client accesses the service ...
can reach the software on the
server Server may refer to: Computing *Server (computing), a computer program or a device that provides requested information for other programs or devices, called clients. Role * Waiting staff, those who work at a restaurant or a bar attending custome ...
. Services, such as
web page A web page (or webpage) is a World Wide Web, Web document that is accessed in a web browser. A website typically consists of many web pages hyperlink, linked together under a common domain name. The term "web page" is therefore a metaphor of pap ...
s or
FTP The File Transfer Protocol (FTP) is a standard communication protocol used for the transfer of computer files from a server to a client on a computer network. FTP is built on a client–server model architecture using separate control and dat ...
, 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 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 Firewall may refer to: * Firewall (computing), a technological barrier designed to prevent unauthorized or unwanted communications between computer networks or hosts * Firewall (construction), a barrier inside a building, designed to limit the spre ...
. 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 them 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 (" black hat")
hackers A hacker is a person skilled in information technology who achieves goals and solves problems by non-standard means. The term has become associated in popular culture with a security hackersomeone with knowledge of bugs or exploits to break ...
commonly use
port scanning A port scanner is an application designed to probe a server or host for open ports. Such an application may be used by administrators to verify security policies of their networks and by attackers to identify network services running on a host and ...
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
vulnerabilities Vulnerability refers to "the quality or state of being exposed to the possibility of being attacked or harmed, either physically or emotionally." The understanding of social and environmental vulnerability, as a methodological approach, involves ...
in any services they find.


Example

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


See also

*
Port scanning A port scanner is an application designed to probe a server or host for open ports. Such an application may be used by administrators to verify security policies of their networks and by attackers to identify network services running on a host and ...
*
Nmap Nmap (Network Mapper) is a network scanner created by Gordon Lyon (also known by his pseudonym ''Fyodor Vaskovich''). Nmap is used to discover hosts and services on a computer network by sending packets and analyzing the responses. Nmap pro ...
*
Computer security Computer security (also cybersecurity, digital security, or information technology (IT) security) is a subdiscipline within the field of information security. It consists of the protection of computer software, systems and computer network, n ...
*
List of TCP and UDP port numbers This is a list of TCP and UDP port numbers used by protocols for operation of network applications. The Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) only need one Port (computer networking), port for Duplex (telecommuni ...


References

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External links


How can I close a port?
Internet protocols