TCP half-open: Difference between revisions

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TCP connections are referred to as ''half-open'' when the station at one end of the TCP connection has crashed, or otherwise removed the socket without formally notifying the other end of the TCP connection.

The term '''half-open''' refers to [[Transmission Control Protocol|TCP]] connections whose state is out of synchronization between the two communicating hosts, possibly due to a crash of one side. A connection which is in the process of being established is also known as '''embryonic connection'''. The lack of synchronization could be due to [[SYN flood|malicious intent]].

Due to the stateless nature of the Transfer Control Protocol, it is possible that if the end of the connection on which the socket still exists is not expecting a response, then the socket may persist indefinitely in this state, known as half-open.

== RFC 793 ==

Note: A common misconception is that the term TCP half-open applies to an partially completed TCP handshake state. A better term for this state is an embryonic state.

According to [http://www.ietf.org/rfc/rfc0793.txt RFC 793], a TCP connectionsconnection areis referred to as ''half-open'' when the stationhost at one end of thethat TCP connection has crashed, or has otherwise removed the socket without formally notifying the other end. of If the TCPremaining end is idle, the connection may remain in the half-open state for unbounded periods of time.

== Stateful Firewall Timeout ==

Another circumstance that can lead to half-open connections is if a [[stateful firewall]] times out a connection that is idle for too long. In this case, the firewall clears its internal state, and if either side of the connection sends a packet, the firewall will drop the packet. This will often result in a half-open connection as the two sides of the connection can end up with inconsistent connection states.

== Embryonic connection ==

The term ''half-open connection'' can also be used to describe an '''embryonic connection''', i.e. a [[Transmission Control Protocol|TCP]] connection that is in the process of being established.

[[Transmission Control Protocol|TCP]] has a [[Three-way handshake|three state system]] for opening a connection. First, the originating endpoint (A) sends a [[SYN (TCP)|SYN packet]] to the destination (B). A is now in an embryonic state (specifically, SYN_SENT), and awaiting a response. B now updates its kernel information to indicate the incoming connection from A, and sends out a request to open a channel back (the [[SYN/ACK]] packet).

At this point, B is also in an embryonic state (specifically, SYN_RCVD). Note that B was put into this state by another machine, outside of B's control.

Under normal circumstances (see [[denial-of-service attack]] for deliberate failure cases), A will receive the SYN/ACK from B, update its tables (which now have enough information for A to both send and receive), and send a final ACK back to B.

Once B receives this final ACK, it also has sufficient information for two-way communication, and the connection is fully open. Both endpoints are now in an established state.

== See also ==

* [[SYN flood]]

* [[SYN cookies]]

* [[Stateful firewall]]

== References ==

{{refbegin}}

*Twingate. (n.d.). ''What is a TCP Half Open Scan?''. Retrieved May 2, 2025, from [https://www.twingate.com/blog/glossary/tcp-half-open-scan](https://www.twingate.com/blog/glossary/tcp-half-open-scan)

*Palo Alto Networks. (n.d.). ''TCP Half Closed and TCP Time Wait Timers''. Retrieved May 2, 2025, from [https://docs.paloaltonetworks.com/pan-os/10-1/pan-os-networking-admin/session-settings-and-timeouts/tcp/tcp-half-closed-and-tcp-time-wait-timers](https://docs.paloaltonetworks.com/pan-os/10-1/pan-os-networking-admin/session-settings-and-timeouts/tcp/tcp-half-closed-and-tcp-time-wait-timers)

*Sanchit Gurukul. (n.d.). ''Understanding TCP Half-Open Connections''. Retrieved May 2, 2025, from [https://sanchitgurukul.com/understanding-tcp-half-open-connections](https://sanchitgurukul.com/understanding-tcp-half-open-connections)

[[Category:Transmission Control Protocol|Half-Open]]

{{refend}}

== External links ==

* [http://http://www.ietf.org/rfc/rfc0793.txt TransferTransmission Control Protocol DARPA Internet Program Protocol Specification]