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searchaux" style="display:none">Computer networking technique</div> <style data-mw-deduplicate="TemplateStyles:r1251242444">.mw-parser-output .ambox{border:1px solid #a2a9b1;border-left:10px solid #36c;background-color:#fbfbfb;box-sizing:border-box}.mw-parser-output .ambox+link+.ambox,.mw-parser-output .ambox+link+style+.ambox,.mw-parser-output .ambox+link+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+style+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+link+.ambox{margin-top:-1px}html body.mediawiki .mw-parser-output .ambox.mbox-small-left{margin:4px 1em 4px 0;overflow:hidden;width:238px;border-collapse:collapse;font-size:88%;line-height:1.25em}.mw-parser-output .ambox-speedy{border-left:10px solid #b32424;background-color:#fee7e6}.mw-parser-output .ambox-delete{border-left:10px solid #b32424}.mw-parser-output .ambox-content{border-left:10px solid #f28500}.mw-parser-output .ambox-style{border-left:10px solid #fc3}.mw-parser-output .ambox-move{border-left:10px solid #9932cc}.mw-parser-output .ambox-protection{border-left:10px solid #a2a9b1}.mw-parser-output .ambox .mbox-text{border:none;padding:0.25em 0.5em;width:100%}.mw-parser-output .ambox .mbox-image{border:none;padding:2px 0 2px 0.5em;text-align:center}.mw-parser-output .ambox .mbox-imageright{border:none;padding:2px 0.5em 2px 0;text-align:center}.mw-parser-output .ambox .mbox-empty-cell{border:none;padding:0;width:1px}.mw-parser-output .ambox .mbox-image-div{width:52px}@media(min-width:720px){.mw-parser-output .ambox{margin:0 10%}}@media print{body.ns-0 .mw-parser-output .ambox{display:none!important}}</style><table class="box-More_citations_needed plainlinks metadata ambox ambox-content ambox-Refimprove" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><a href="/wiki/File:Question_book-new.svg" class="mw-file-description"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/50px-Question_book-new.svg.png" decoding="async" width="50" height="39" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/75px-Question_book-new.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/100px-Question_book-new.svg.png 2x" data-file-width="512" data-file-height="399" /></a></span></div></td><td class="mbox-text"><div class="mbox-text-span">This article <b>needs additional citations for <a href="/wiki/Wikipedia:Verifiability" title="Wikipedia:Verifiability">verification</a></b>.<span class="hide-when-compact"> Please help <a href="/wiki/Special:EditPage/Hole_punching_(networking)" title="Special:EditPage/Hole punching (networking)">improve this article</a> by <a href="/wiki/Help:Referencing_for_beginners" title="Help:Referencing for beginners">adding citations to reliable sources</a>. Unsourced material may be challenged and removed.<br /><small><span class="plainlinks"><i>Find sources:</i> <a rel="nofollow" class="external text" href="https://www.google.com/search?as_eq=wikipedia&q=%22Hole+punching%22+networking">"Hole punching" networking</a> – <a rel="nofollow" class="external text" href="https://www.google.com/search?tbm=nws&q=%22Hole+punching%22+networking+-wikipedia&tbs=ar:1">news</a> <b>·</b> <a rel="nofollow" class="external text" href="https://www.google.com/search?&q=%22Hole+punching%22+networking&tbs=bkt:s&tbm=bks">newspapers</a> <b>·</b> <a rel="nofollow" class="external text" href="https://www.google.com/search?tbs=bks:1&q=%22Hole+punching%22+networking+-wikipedia">books</a> <b>·</b> <a rel="nofollow" class="external text" href="https://scholar.google.com/scholar?q=%22Hole+punching%22+networking">scholar</a> <b>·</b> <a rel="nofollow" class="external text" href="https://www.jstor.org/action/doBasicSearch?Query=%22Hole+punching%22+networking&acc=on&wc=on">JSTOR</a></span></small></span> <span class="date-container"><i>(<span class="date">May 2022</span>)</i></span><span class="hide-when-compact"><i> (<small><a href="/wiki/Help:Maintenance_template_removal" title="Help:Maintenance template removal">Learn how and when to remove this message</a></small>)</i></span></div></td></tr></tbody></table> <p><b>Hole punching</b> (or sometimes <b>punch-through</b>) is a technique in <a href="/wiki/Computer_networking" class="mw-redirect" title="Computer networking">computer networking</a> for establishing a direct connection between two parties in which one or both are behind <a href="/wiki/Firewall_(networking)" class="mw-redirect" title="Firewall (networking)">firewalls</a> or behind <a href="/wiki/Router_(computing)" title="Router (computing)">routers</a> that use <a href="/wiki/Network_address_translation" title="Network address translation">network address translation</a> (NAT). To punch a hole, each client connects to an unrestricted third-party server that temporarily stores external and internal <a href="/wiki/Network_address" title="Network address">address</a> and <a href="/wiki/Port_(computer_networking)" title="Port (computer networking)">port</a> information for each client. The server then relays each client's information to the other, and using that information each client tries to establish direct connection; as a result of the connections using valid port numbers, restrictive firewalls or routers accept and forward the incoming packets on each side. </p><p>Hole punching does not require any knowledge of the network topology to function. <a href="/wiki/ICMP_hole_punching" title="ICMP hole punching">ICMP hole punching</a>, <a href="/wiki/UDP_hole_punching" title="UDP hole punching">UDP hole punching</a> and <a href="/wiki/TCP_hole_punching" title="TCP hole punching">TCP hole punching</a> respectively use <a href="/wiki/Internet_Control_Message_Protocol" title="Internet Control Message Protocol">Internet Control Message</a>, <a href="/wiki/User_Datagram_Protocol" title="User Datagram Protocol">User Datagram</a> and <a href="/wiki/Transmission_Control_Protocol" title="Transmission Control Protocol">Transmission Control Protocols</a>. </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Overview">Overview</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hole_punching_(networking)&action=edit&section=1" title="Edit section: Overview"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Networked devices with public or globally accessible <a href="/wiki/IP_address" title="IP address">IP addresses</a> can create connections between one another easily. Clients with private addresses may also easily connect to public servers, as long as the client behind a router or firewall initiates the connection. However, hole punching (or some other form of <a href="/wiki/NAT_traversal" title="NAT traversal">NAT traversal</a>) is required to establish a direct connection between two clients that both reside behind different firewalls or routers that use <a href="/wiki/Network_address_translation" title="Network address translation">network address translation (NAT)</a>. </p><p>Both clients initiate a connection to an unrestricted server, which notes endpoint and session information including public IP and port along with private IP and port. The firewalls also note the endpoints in order to allow responses from the server to pass back through. The server then sends each client's endpoint and session information to the other client, or peer. Each client tries to connect to its peer through the specified IP address and port that the peer's firewall has opened for the server. The new connection attempt punches a hole in the client's firewall as the endpoint now becomes open to receive a response from its peer. Depending on network conditions, one or both clients might receive a connection request. Successful exchange of an <a href="/wiki/Cryptographic_nonce" title="Cryptographic nonce">authentication nonce</a> between both clients indicates the completion of a hole punching procedure.<sup id="cite_ref-Ford_1-0" class="reference"><a href="#cite_note-Ford-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Examples">Examples</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hole_punching_(networking)&action=edit&section=2" title="Edit section: Examples"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p><a href="/wiki/Voice_over_IP" title="Voice over IP">VoIP</a> products, online gaming applications, and <a href="/wiki/Peer-to-peer" title="Peer-to-peer">P2P</a> networking software all use hole punching. </p> <ul><li>Telephony software <a href="/wiki/Skype" title="Skype">Skype</a> uses hole punching to allow users to communicate with one or more users audibly.<sup id="cite_ref-Schmidt_2-0" class="reference"><a href="#cite_note-Schmidt-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup></li> <li>Fast-paced online multi-player games may use a hole punching technique or require users to create a permanent <a href="/wiki/Firewall_pinhole" title="Firewall pinhole">firewall pinhole</a> in order to reduce network latency.</li> <li><a href="/wiki/Virtual_private_network" title="Virtual private network">VPN</a> applications such as <a href="/wiki/Hamachi_(software)" class="mw-redirect" title="Hamachi (software)">Hamachi</a>, <a href="/wiki/ZeroTier" title="ZeroTier">ZeroTier</a>, and <a href="/wiki/Tailscale" title="Tailscale">Tailscale</a> utilize hole punching to allow users to connect directly to subscribed devices behind firewalls.</li> <li>Decentralized <a href="/wiki/Peer-to-peer_file_sharing" title="Peer-to-peer file sharing">peer-to-peer file sharing</a> software relies on hole punching for file distribution.</li></ul> <div class="mw-heading mw-heading2"><h2 id="Requirements">Requirements</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hole_punching_(networking)&action=edit&section=3" title="Edit section: Requirements"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Reliable hole punching requires consistent endpoint translation, and for multiple levels of NATs, hairpin translation. </p><p>When an outbound connection from a private endpoint passes through a firewall, it receives a public endpoint (public IP address and port number), and the firewall translates traffic between them. Until the connection is closed, the client and server communicate through the public endpoint, and the firewall directs traffic appropriately. Consistent endpoint translation reuses the same public endpoint for a given private endpoint, instead of allocating a new public endpoint for every new connection. </p><p>Hairpin translation creates a loopback connection between two of its own private endpoints when it recognizes that the destination endpoint is itself. This functionality is necessary for hole punching only when used within a multiple-layered NAT.<sup id="cite_ref-Ford_1-1" class="reference"><a href="#cite_note-Ford-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hole_punching_(networking)&action=edit&section=4" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Port_Control_Protocol" title="Port Control Protocol">Port Control Protocol</a> (PCP)</li> <li><a href="/wiki/NAT_Port_Mapping_Protocol" title="NAT Port Mapping Protocol">NAT Port Mapping Protocol</a> (NAT-PMP)</li> <li><a href="/wiki/Internet_Gateway_Device_Protocol" title="Internet Gateway Device Protocol">Internet Gateway Device Protocol</a> (UPnP IGD)</li> <li><a href="/wiki/Port_knocking" title="Port knocking">Port knocking</a></li> <li><a href="/wiki/Session_Initiation_Protocol" title="Session Initiation Protocol">Session Initiation Protocol</a></li> <li><a href="/wiki/STUN" title="STUN">STUN</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hole_punching_(networking)&action=edit&section=5" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap"><ol class="references"> <li id="cite_note-Ford-1"><span class="mw-cite-backlink">^ <a href="#cite_ref-Ford_1-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Ford_1-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text">Ford, Bryan; Srisuresh, Pyda; Kegel, Dan (2005), <a rel="nofollow" class="external text" href="http://www.brynosaurus.com/pub/net/p2pnat/">Peer-to-Peer Communication Across Network Address Translators</a></span> </li> <li id="cite_note-Schmidt-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-Schmidt_2-0">^</a></b></span> <span class="reference-text">Schmidt, Jürgen (2006), <a rel="nofollow" class="external text" href="https://web.archive.org/web/20110810233331/http://www.h-online.com/security/features/How-Skype-Co-get-round-firewalls-747197.html">The hole trick</a></span> </li> </ol></div></div> <div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hole_punching_(networking)&action=edit&section=6" title="Edit section: External links"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a rel="nofollow" class="external text" href="https://tailscale.com/blog/how-nat-traversal-works/">How NAT traversal works</a></li></ul>    </div><noscript><img src="https://login.wikimedia.org/wiki/Special:CentralAutoLogin/start?type=1x1" alt="" width="1" height="1" style="border: none; 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