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NAT Hole Punching in Computer Network
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Easy Last Updated :
05 Jul, 2022
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The main restriction or disadvantage for a Device under a NAT is that it cannot accept outbound connections. And if any such attempt is made a NAT makes sure that the message reaches the trash can. This restriction is overcome a method known as Hole Punching . In this method a node outside a NAT, with a publicly reachable Static IP is used, It is called a Rendezvous server. It is used to establish a connection between two nodes which are either both behind different NATs or both behind same NAT or one behind a NAT and other not behind a NAT or nodes under multiple NATs.
Since most computers in the world are behind some kind of NAT this method is widely used in P2P (Peer to Peer) architectures . The establishment of a P2P network requires direct connection between any two nodes. Hole Punching is the most important concept one has to learn when creating a P2P network. Network firewalls can be bypassed using this concept. Hole Punching is very safe as Connection must be initiated from both ends, hence consent of both users is required.
The detailed explanation of NAT hole punching using Transmission Control Protocol (TCP) , User Datagram Protocol (UDP) , and for Internet Control Message Protocol (ICMP) . This article explains the basic concepts of hole punching.
We will discuss how to establish connection between two node using NAT hole punching for the below cases.
The basic story of a NAT Hole Punch is that, when both the Nodes who want to connect to each other send a message to the server. Server responds to both nodes with each other’s IP address and Ports (Endpoints). Both Nodes try to establish a connection with each other by sending a message to each other, The messages are discarded by the receiver side NATs but the sender side NATs note down the address to which message is sent and any incoming messages from that address will be considered as a reply to this message and will be routed to the source endpoint. Both Nodes again try connecting to each other by sending a message. But now the messages are received successfully as the NAT thinks that this message is a reply to the previous(failed) message.BOOM! a connection is established and a HOLE IS PUNCHED .
The above given NAT hole punching method can be used for any of the cases, either both nodes are behind the same NAT or both nodes are behind the different NATs or even if both nodes are behind multiple levels of NATs.
Maybe the only disadvantage of NAT hole punching is that, a publicly reachable server with static ip is always required. An architecture called pwnat was proposed to overcome this, but pwnat only works when either one of the two nodes is not behind a NAT.
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Apr 30, 2007 12:00 am PST

It is crucial today to prevent vulnerabilities across the enterprise and remove security holes in your desktops, laptops and servers. Knowing what they are, where they are on your network, and how to remove them is more important than sniffing packets and listening for burglars.
Exploiters on the Internet have caused billions of dollars in damages. These exploiters are intelligent cyber terrorists, criminals and hackers who have a plethora of tools available in their war chests ranging from spyware, rootkits, Trojans, viruses, worms, bots, and zombies to various other blended threats.
Exploits can be grown and harvested the same day a security hole is announced - in so-called "zero-day attacks" - so they are getting much harder to stop. Open source malware code, freely available on the Internet, is enabling this phenomenon and cannot be reversed. Although the number and types of exploits "in the wild" continues to rise exponentially, there are fewer than a dozen core methodologies used for their execution and proliferation. Most exploits can be removed, but some exist indefinitely and can only be destroyed or removed by loss of data - you've probably heard of these "rootkits." Most exploits will re-infect a host if a security hole, also known as the Common Vulnerability and Exposure (CVE) , is not removed.
Many exploiters are doing it for profit. Just take a look here and you'll see where the $10 billion in identity theft last year occurred the most.
Not all exploits are created equal. Most are evolutionary improvements on existing exploits. What’s very interesting is that the average exploit currently has a dozen names. With the advent of the Common Malware Enumeration (CME) standard, there will be one shared, neutral indexing capability for malware but that will take years - probably more than five years, like the CVE standard which is still just starting to catch on, since its inception in 1999 by Mitre, now funded by the U.S. Department of Homeland Security.
It is crucial today to prevent vulnerabilities across the enterprise and remove these CVEs - these security holes in your desktops, laptops and servers. Knowing what they are, where they are on your network, and how to remove them is more important than sniffing packets and listening for burglars.
According to USCERT, 95% of downtime and IT related compliance issues are a direct result of an exploit against a CVE. Your firewall, IDS, IPS, anti-virus software and other countermeasures don’t look for or show you how to remove your CVEs. So you are really only 5% secure.
You probably never heard of a CVE, but you do know about Blaster, Msblast, LovSAN and the Nachi and Welchia worms which caused massive downtime and financial losses. They all exploited one CVE - one minor hole. It was a software flaw known as a buffer overflow in a certain service called the “DCOM interface for RPC” running in most Microsoft Windows operating systems which allowed hackers to send these exploits out and take advantage of most of the Windows systems that had this flaw.
Just take a look at the U.S. National Vulnerability Database powered by CVE and you can search for CVEs that you might have in your own systems. If you just purchased a Cisco router or a Linksys wireless router or Solaris version 9 running Netscape Enterprise, or anything else that plugs into a network, type in the name of the system in the NVD and see how many CVEs (vulnerabilities) you find.
Want to see the top 20 exploited vulnerabilities? Visit SANS and you’ll see 10 vulnerabilities in Windows and 10 in Unix/Linux systems. If you have one of these holes, close it as quick as you can or you can expect to be taken advantage of when you least expect it.
Hackers, viruses and worms cause billions in damages by using CVEs against us, and the damages are growing annually. How many CVEs do you have in your network? Is your computer network taking you out of compliance? Knowing if you have any CVEs is the only way to find out and is considered due diligence. Removing critical CVEs is considered due care. Frequent and consistently scheduled security audits for CVEs and their removal is the only prudent thing to do as a proactive information security manager.
Now is the time to find and fix your CVEs so you can be more productive and suffer less downtime and successful hacker attacks. If you remove all of your CVEs you’ll be as close to 100% secure as possible. This will make the exploiters less successful in breaking into or taking control of your computers, and they’ll have to start looking elsewhere. Remember, if you harden your network assets before an attack, you'll most likely reduce your risk of downtime and data theft dramatically. Go here today and start working on removing your CVEs before the next attack.
Gary Miliefsky is founder and CTO of NetClarity, Inc. He servers as an advisor to MITRE Corp. and is a member of the National Information Security Group’s Board of Directors. He received his undergraduate degree from UMASS Lowell in Computer Science and subsequently earned certification as a CISSP. Miliefsky holds more than a dozen published and pending patents on information security.
See our video interview with Miliefsky, where he discusses his company's security technology.
Copyright © 2007 IDG Communications, Inc.
Copyright © 2022 IDG Communications, Inc.

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Note: This article focuses on how to share files or folders over a Local Area Network (or LAN), such as connected computers within your home or workplace. If you are trying to find out how to share a file over the Internet, for example with a friend or family member, OneDrive provides easy ways to accomplish this. For more details, see Share OneDrive files and folders .
Here are some answers to common questions about changes to file and folder sharing in Windows 11:
To share a file or folder over a network in File Explorer, do the following:
Right-click (or long-press) a file, and then select Show more options > Give access to > Specific people .
Select a user on the network to share the file with, or select Everyone to give all network users access to the file.
If you select multiple files at once, you can share them all in the same way. It works for folders, too—share a folder, and all files in it will be shared.
Right-click or press a file or folder, then select Show more options > Give access to > Remove access .
File Explorer shows the Remove access option for all files, even
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