Abstract—Traditional NAT traversal methods require the helpof a third party for signalling. This paper investigates a newautonomous method for establishing connections to peers behindNAT. The proposed method for autonomous NAT traversaluses fake ICMP messages to initially contact the NATed peer.This paper presents how the method is supposed to work intheory, discusses some possible variations, introduces variousconcrete implementations of the proposed approach and evaluatesempirical results of a measurement study designed to evaluatethe efficacy of the idea in practice.
A large fraction of the hosts in a typical peer-to-peer network are in home networks. Most home networks use networkaddress translation (NAT)  to facilitate multiple computerssharing a single global public IP address, to enhance securityor simply because the provider’s hardware often defaults tothis configuration. Recent studies have reported that up to 70%of users access P2P networks from behind a NAT system .This creates a well-known problem for peer-to-peer networkssince it is not trivial to initiate a connection to a peer behindNAT. For this paper, we will use the term server to refer to apeer behind NAT and the term client for any other peer tryingto initiate a connection to the server.
Unless configured otherwise (protocols such as the InternetGateway Device Protocol  are counted as configurationin this context), almost all NAT implementations refuse toforward inbound traffic that does not correspond to a recentmatching outbound request. This is not primarily an implementation issue: if there are multiple hosts in the private network,the NAT is likely unable to tell which host is the intended recipient. Configuration of the NAT is not always an alternative;problems range from end-user convenience and capabilities ofthe specific NAT implementation to administrative policies thatmay prohibit changes to the NAT configuration (for example,due to security concerns).
Since NAT systems prohibit inbound requests that do notmatch a previous outbound request, all existing NAT traversaltechniques (aside from those changing the configuration of theNAT system) that we are aware of require some amount ofactive facilitation by a third party , . The basic approachin most of these cases is that the server in the private networkbehind the NAT is notified by the third party that the clientwould like to establish a connection. The server then initiatesthe connection to the client. This requires that the servermaintains a connection to a third party, that the client is able to locate the responsible third party and that the third partyacts according to a specific protocol.
The goal of this paper is autonomous NAT traversal,meaning NAT traversal without a third party. Using thirdparties increases the complexity of the software and potentiallyintroduces new vulnerabilities. For example, if anonymizingpeer-to-peer networks (such as GNUnet  or Tor ) usedthird parties for NAT traversal, an attacker may be able tomonitor connections or even traffic volumes of peers behindNATs which in turn might enable deanonymization attacks ,. Another problem is that the decrease in available globallyroutable IPv4 addresses  will in the near future sharplyreduce the fraction of hosts that would be able to facilitateNAT traversal.
II. TECHNICAL APPROACH
The proposed technique assumes that the client has somehow learned the current external (globally routable) IP addressof the server’s NAT. This could be due to a previous connectionbetween the two systems or a third party having provided theIP address in a previous exchange. Note that we specificallyassume that no third party is available at the time when theclient attempts to connect to the server behind the NAT.
The first goal of the presented NAT traversal method is tocommunicate the public IP address of a client that wants toconnect to the server behind the NAT. After the server is awareof the IP address of the client, it connects to the client (similarto NAT traversal methods that involve a third party).
The key idea for enabling the server to learn the client’sIP address is for the server to periodically send a message toa fixed, known IP address. The simplest approach uses ICMPECHO REQUEST messages to an unallocated IP address, suchas 126.96.36.199. Since 188.8.131.52 is not allocated, the ICMP REQUESTwill will not be routed by routers without a default route;ICMP DESTINATION UNREACHABLE messages that maybe created by those routers can just be ignored by the server.
As a result of the messages sent to 184.108.40.206, the NATwill enable routing of replies in response to this request.The connecting client will then fake such a reply. Specifically, the client will transmit an ICMP message indicatingTTL_EXPIRED (Figure 1). Such a message could legitimatelybe transmitted by any Internet router and the sender addresswould not be expected to match the server’s target IP.
The server listens for (fake) ICMP replies and upon receiptinitiates a connection to the sender IP specified in the ICMP reply. If the client is using a globally routable IP address, thisis entirely unproblematic and both TCP or UDP can be usedto establish a bi-directional connection if the client listens ona pre-agreed port. In cases where there is no pre-agreed port, aport number can in most cases be communicated as part of thepayload of the ICMP ECHO RESPONSE, which is typicallynot checked against the payload of the corresponding ICMPECHO REQUEST by NAT implementations.
(Fig. 1. This figure diagrams the process of sending and receiving the fake ICMP messages for the server and client. In step 1, the server sends a fake ICMPrequest to 220.127.116.11 and in step 2 the client sends the matching reply. Note that this is a fake reply since the client never receives the ICMP request sent to18.104.22.168 by the server. The important information contained in the actual packets is displayed for each step. The blue (solid) line shows the ICMP request pathand the dashed (green) line shows the ICMP reply path.)
A. NAT-to-NAT CommunicationFurther complications arise if both the client and the serverare behind NAT. In this case, often the client will be unableto transmit a fake ICMP response to the server due to restrictions imposed by the NAT implementation of the client. Onepossible idea for circumventing this problem is for the clientto send the same message that the server is sending exceptwith TTL 1 to its NAT. If the NAT accepts the packet despitethe forged sender IP address it might theoretically generate thedesired ICMP response and forward it to the external network.However, in practice we did not find NATs where generatingthe necessary ICMP message using a TTL of 1 works.
Even if the client is able to transmit the fake ICMP response,the next step; in which both the client and server are awareof the others IP address and now intend to establish a TCP orUDP connection can still be complicated. The reason is thatNAT systems can change the source port numbers of outboundmessages. Without a third party, both client and server wouldhave to guess matching source and destination port numbersas chosen (possibly at random) by their respective NAT implementations. Depending on the type of the NAT implementations (Full cone, restricted cone, port-restricted, symmetric),finding the correct port may take several messages. Client and server can reduce the total number of messages required bytransmitting and listening on multiple ports in this phase.
B. Using UDP packets instead of ICMP ECHO REQUESTsA possible alternative to having the sender transmit ICMPECHO REQUESTs to a fixed, known IP address is havingthe sender transmit UDP packets to a fixed, known IP address and port. In this case, the client would again forge anICMP TTL EXPIRED message, only this time using the UDPformat. The main disadvantage of this variation is that thesender has to guess the external UDP sender port number whenfaking the ICMP response. Since some NAT implementationsrandomly change those port numbers, the server might haveto send UDP packets using multiple sender ports in order togive the client a sufficient chance at guessing correctly.
The main advantage of this technique is that the server nolonger needs to send using RAW sockets, which may reducethe privileges required for the server. Note that the server stillneeds to be able to listen for the ICMP reply, which requiresRAW sockets on Linux. In the case of a full-cone NAT, usingUDP packets instead of ICMP ECHO REQUESTs also hasthe advantage of establishing a port mapping which can thenbe used as an alternative method for contacting the peer.
Another difference between the two approaches is thepossible payload that can be embedded in the response. WithICMP ECHO REQUESTs, the payload can be as big as thepacket size permits and is hence only limited by the MTU ofthe respective physical network. Well-formed ICMP UDP TTLexceeded replies on the other hand can only contain 32 bits ofpayload: the ICMP TTL EXCEEDED response contains thefirst 64 bits of the payload of the original IP packet. In those64 bits, the 16-bit UDP checksum field and the 16-bit UDP packet length are unverifiable (for NAT’s that do not trackextensive information about outgoing UDP packets) and canhence be used to transmit 32 bits of information to the server(in addition to the sender’s IP address). With our approach,either of these payload sizes is enough as we only transmit aport number in addition to the IP address.
This section summarizes the three implementations of theproposed method that we have done so far. All of the presentedimplementations are freely available from the web pages of therespective projects.
A. Implementation in NAT-Tester FrameworkOur implementation in the NAT-Tester framework was usedto gather the data for this paper. It transmits the various packettypes (with or without payload) using raw sockets and useslibpcap to determine which messages were forwarded bythe NAT. The client is currently available for W32 and Linuxand must be run with administrator rights. This implementation is useful for researchers interested in exploring the variousvariations of this and other NAT traversal methods.
B. Implementation in pwnat toolThe pwnat tool1is a GNU/Linux-only stand-alone implementation of autonomous NAT traversal. After contacting theserver behind the NAT, it establishes a channel with TCPsemantics using UDP packets. It supports both client andserver behind NAT (if one of the NATs allows the fake ICMPmessages to be transmitted). This implementation targets endusers.
C. Implementation in the GNUnet FrameworkFinally, we have created a re-usable implementation ofthe presented ICMP-based NAT traversal method in GNUnet,GNU’s framework for secure peer-to-peer networking .Since the use of ICMP requires the use of non-portable andoften privileged system calls, this implementation is split intothree main components:
This component is a small program that provides thecore ICMP-related functionality for the server. The code periodically generates the ICMP ECHO REQUEST message and also listens for incoming ICMPTTL EXCEEDED responses. If such a response isreceived, it simply prints the IP address of the senderto stdout. If the ICMP also contains a 16-bytepayload, it is interpreted as a port number and alsoprinted.
This component is a small binary which simply sendsa single (fake) ICMP message to the IP address specified at the command line. An additional argumentcan be given which will be interpreted as a portnumber to be transmitted in the payload of the fakeICMP response message.
This component implements a GNUnet transport plugin  and is thus specific to the GNUnet peerto-peer framework. Depending on how the peer isconfigured, it controls ICMP servers or clients andultimately establishes connections between peers.
Splitting the implementation into these three componentshas the advantage of minimizing the amount of code thatmust run with super-user privileges on POSIX systems (byinstalling the ICMP server and client with the SUID bit set).Furthermore, since the ICMP code is platform-specific, thismakes it easier to manage this platform-specific part of thecode. Finally, this split makes it easy to share the platformspecific but peer-to-peer network agnostic ICMP code sothat it can be used with other peer-to-peer applications. Thisimplementation is suitable as a starting point for developersof P2P networks.
IV. EXPERIMENTAL RESULTS
We have evaluated the proposed autonomous NAT traversaltechniques on a large number of NAT implementations usingour NAT-Tester framework , . The framework consistsof a public client that volunteers download and execute.The client then performs various tests against the local NATimplementation and reports the results back to the NAT-Testerserver. This enables us to evaluate NAT traversal strategiesagainst a wide range of NAT implementations. Detailed resultsare made public on the NAT-Tester web page.2In this section we will summarize the results based on the data available sofar.
Table I summarizes which fractions of the NAT implementations evaluated so far support the proposed method forautonomous NAT traversal. We distinguish between behaviorrelevant for using autonomous NAT traversal from the pointof view of both clients and servers behind NAT. We considertwo cases: the case where the server uses ICMP ECHO REQUESTs and the case where the server transmits UDP packets.We also consider the extend of UDP port randomization whichdetermines how efficient the second stage in the case of NATto-NAT communication would be. NAT implementations arecategorized into the typical four types (full cone, restrictedcone, port-restricted, symmetric) in cases where NAT-Testeris able to determine the type. NAT implementations that donot seem to fall into any of these categories are only includedin the total.
The data shows that in virtually all cases NATs forwardthe faked ICMP messages for UDP (UDP-Server), but onlyin about half the cases for ICMP ECHO REQUESTs (EchoServer). Furthermore, a significant majority of all NATs alsopreserve the source port (when possible), so the additionalrequirement of guessing the port for faking the ICMP responsefor a UDP message does not change the overall cost ofthe approach. Finally, NATs virtually always prevent theirclients from transmitting the fake ICMP messages used byour clients (Echo-Client, ICMP-UDP-Client). Based on whatwe have seen from inspecting NAT configurations directly, thereason seems to be that NAT rules typically only allow ICMPpackets for the states “NEW” and “ESTABLISHED” in thestate machine  — and the fake response falls into neithercategory.
The proposed method of autonomous NAT traversal workswell in the case of an unrestricted client attempting to initiate aconnection to a server behind NAT. Here, in virtually all casesa single ICMP message by the client would be followed bytraditional connection reversal  which then reliably createsa UDP or TCP connection. In other words, there is no needfor third parties to help initiate connections to NATed serversin this case.
On the other hand, if both systems are behind NAT, theproposed method rarely works and a third party is required.Assuming 70% of the peers in a network are behind NAT, thismeans that roughly 50% of all possible connections can beestablished using autonomous NAT traversal. However, evenin the case where both systems are behind NAT a possibleadvantage of the proposed method remains; it is easy tocreate a simple, generic and fully stateless service that receivesrequests from NATed peers and generates fake ICMP replies tonotify the server behind NAT. In this case, the payload of theICMP reply would need to contain the original IP address (andlikely source port number) of the client since the IP header ofthe faked ICMP response would now contain the IP addressof the service.
EXPERIMENTAL EVALUATION OF AUTONOMOUS NAT TRAVERSAL. “ECHO-SERVER” LISTS THE NUMBER OF NAT IMPLEMENTATIONS THAT ALLOWS(FAKED) ICMP TTL EXPIRED REPLIES TO TRAVERSE THE NAT IN RESPONSE TO ICMP ECHO REQUEST MESSAGES. “ECHO-CLIENT” LISTS THENUMBER OF NAT IMPLEMENTATIONS THAT ALLOW CLIENTS TO TRANSMIT (FAKED) ICMP TTL EXPIRED MESSAGES. “UDP-SERVER” AND“ICMP-UDP-CLIENT” GIVE THE SAME NUMBERS WHEN USING UDP PACKETS INSTEAD OF ICMP ECHO REQUESTS. “PRESERVES PORTS”INDICATES THE NUMBER OF IMPLEMENTATIONS THAT PRESERVE THE SENDER’S LOCAL PORT AS THE EXTERNAL PORT IF POSSIBLE. “ANY SERVER”LISTS THE NUMBER OF NATS WHERE EITHER THE ECHO-SERVER OR THE UDP-SERVER WORK. FINALLY, “TWO-MESSAGE SUCCESS” LISTS THENUMBER OF NATS WHERE AUTONOMOUS NAT TRAVERSAL (AS A SERVER) SUCCEEDS EITHER WITH ECHO-SERVER OR WITH UDP WITH PORTPRESERVATION AND HENCE ONLY TWO MESSAGES ARE NECESSARY TO REACH THE SERVER.
Fake replies can enable autonomous NAT traversal in anumber of cases. As with most NAT traversal techniques,this approach does not work for all installations. What isunusual about the presented method is that it works extremelywell if only one peer is behind NAT and virtually neverif both peers are behind NAT. Systems that require highNAT traversal success rates typically implement a number oftraversal techniques and the presented approach extends theset of available methods by one that, if applicable, is cheaperand simpler than most of the existing techniques.
The authors thank the many volunteers that have downloaded and executed the NAT-Tester and helped us gatherdata to evaluate the method. This work was funded in partby the Deutsche Forschungsgemeinschaft (DFG) under ENPGR 3688/1-1.