This document discusses various techniques for allowing peer-to-peer communication between hosts located behind Network Address Translation (NAT) devices, including NAT traversal using UDP hole punching, TCP hole punching, relaying, connection reversal, and the TURN protocol. It also covers proxy protocols like SOCKS that can be used to traverse NATs, as well as the UPnP standard for automatic port forwarding configuration.

1. P2P and NAT How to traverse NAT Davide Carboni © 2005-2006

2. License Attribution-ShareAlike 2.5 You are free: to copy, distribute, display, and perform the work to make derivative works to make commercial use of the work Under the following conditions: Attribution . You must give the original author credit. Share Alike . If you alter, transform, or build upon this work, you may distribute the resulting work only under a licence identical to this one. For any reuse or distribution, you must make clear to others the licence terms of this work. Any of these conditions can be waived if you get permission from the copyright holder. Your fair use and other rights are in no way affected by the above. This is a human-readable summary of the Legal Code (the full licence ) . Disclaimer

3. The problem The large deployment of NAT builds a barrier to the development of peer-to-peer networks. Host behind a NAT/Firewall are only authorized to initiate outgoing traffic through a limited set of ports (UDP/TCP) Host behind a NAT/Firewall are never authorized to receive incoming TCP or UDP traffic initiated by a foreign host

4. Firewall A Firewall is a system that filters TCP/IP UDP/IP packet according to rules It can be a software running in the user machine or in a network router Rules

5. Firewall Rules router (Global IP addresses)

6. NAT the process of network address translation ( NAT , also known as network masquerading or IP-masquerading ) involves re-writing the source and/or destination addresses of IP packets as they pass through a router or firewall .

7. Why NAT is so popular IPv4 address shortage standard feature in routers for home and small-office Internet connections can enhance the reliability of local systems by stopping worms and enhance privacy by discouraging scans

8. Simple NAT NAT (Private IP addresses) (Public IP addresses) Main Internet (Public IP addresses)

9. Multiple NAT ISP NAT (Private IP addresses) (Public IP addresses) Main Internet ISP network Home NAT Home network 10.0.0.12 192.168.2.12 192.168.2.99 156.148.70.32

10. NAT Mappings (192.168.2.2) (1.1.1.4) (1.1.1.5) 192.168.2.2:4445 <-> 1.1.1.5:10100 S=192.168.2.2:4445 D=1.1.1.4:7777 datagram S=1.1.1.5:10100 D=1.1.1.4:7777 datagram A

11. Traversing a NAT that does not collaborate

12. Relaying NAT Main Internet Local network NAT Local network 10.0.0.12 192.168.2.99 Relay S host A host B 1 2

13. Connection reversal NAT Main Internet Local network 1.1.1.4 192.168.2.99 rendezvous S host A host B 1 2 3

14. TURN protocol TURN is a protocol for UDP/TCP relaying behind a NAT Unlike STUN there is no hole punching and data are bounced to a public server called the TURN server. TURN is the last resource. For instance behind a symmetric NAT

15. Role in TURN A TURN client is an entity that generates TURN requests A TURN Server is an entity that receives TURN requests, and sends TURN responses. The server is a data relay, receiving data on the address it provides to clients, and forwarding them to the clients

16. NAT policies Full cone NAT Restricted cone NAT Port restricted cone NAT Symmetric NAT

17. UDP Hole Punching Hole punching is a tecnique to allow traffic from/to a host behind a firewall/NAT without the collaboration of the NAT itself The simplest way is to use UDP packets

18. Full cone Host A Host C Full cone Host B (192.168.2.2) (1.1.1.4) (192.168.2.1) (1.1.1.5) (1.1.1.6) Packet(S=192.168.2.2:4445, D=1.1.1.5:7777) Packet(S=1.1.1.4:10100, D=1.1.1.5:7777) Packet(S=1.1.1.5:4321, D=1.1.1.4:10100) Packet(S=1.1.1.5:4321, D=192.168.2.2:4445) Packet(S=1.1.1.6:1234, D=1.1.1.4:10100) Packet(S=1.1.1.6:1234, D=192.168.2.2:4445)

19. Full cone mapping and policy Mapping 192.168.2.2:4445 <-> 1.1.1.4:10100 Policy ALLOW ALL TO 1.1.1.4:10100

20. Holes in Full Cone NAT rendezvous host A host B 1 2 3 4 5

21. Restricted cone Host A Host C Restricted cone Host B (192.168.2.2) (1.1.1.4) (192.168.2.1) (1.1.1.5) (1.1.1.6) Packet(S=192.168.2.2:4445, D=1.1.1.5:7777) Packet(S=1.1.1.4:10100, D=1.1.1.5:7777) Packet(S=1.1.1.5:4321, D=1.1.1.4:10100) Packet(S=1.1.1.5:4321, D=192.168.2.2:4445) Packet(S=1.1.1.6:1234, D=1.1.1.4:10100) X Packet(S=192.168.2.2:4445, D=1.1.1.6:7777) Packet(S=1.1.1.4:10100, D=1.1.1.6:7777) Packet(S=1.1.1.6:4321, D=1.1.1.4:10100) Packet(S=1.1.1.6:4321, D=192.168.2.2:4445)

22. Restricted cone mapping and policy Mapping 192.168.2.2:4445 <-> 1.1.1.4:10100 Policy ALLOW 1.1.1.5 TO 1.1.1.4:10100 ALLOW 1.1.1.6 TO 1.1.1.4:10100

23. Holes in Restricted Cone NAT rendezvous host A host B 1 2 3 5 4 6

24. Port restricted cone Host A Host C Port - restr cone Host B (192.168.2.2) (1.1.1.4) (192.168.2.1) (1.1.1.5) (1.1.1.6) Packet(S=192.168.2.2:4445, D=1.1.1.5:7777) Packet(S=1.1.1.4:10100, D=1.1.1.5:7777) Packet(S=1.1.1.5:4321, D=1.1.1.4:10100) Packet(S=1.1.1.5:7777, D=192.168.2.2:4445) X Packet(S=1.1.1.5:7777, D=1.1.1.4:10100)

25. Port restricted cone mapping and policy Mapping 192.168.2.2:4445 <-> 1.1.1.4:10100 Policy ALLOW 1.1.1.5:7777 TO 1.1.1.4:10100 ALLOW 1.1.1.6:7777 TO 1.1.1.4:10100

26. Holes in Port restricted Cone NAT rendezvous host A host B 1 2 3 5 4 6

27. Symmetric NAT Host A Host C symmetric Host B (192.168.2.2) (1.1.1.4) (192.168.2.1) (1.1.1.5) (1.1.1.6) Packet(S=192.168.2.2:4445, D=1.1.1.5:7777) Packet(S=1.1.1.4:10100, D=1.1.1.5:7777) Packet(S=1.1.1.5:7777, D=192.168.2.2:4445) Packet(S=1.1.1.5:7777, D=1.1.1.4:10100) Packet(S=192.168.2.2:4445, D=1.1.1.6:7777) Packet(S=1.1.1.4:10179, D=1.1.1.6:7777) Packet(S=1.1.1.6:7777, D=192.168.2.2:4445) Packet(S=1.1.1.6:7777, D=1.1.1.4:10179) Packet(S=1.1.1.6:7777, D=1.1.1.4:10100) X

28. Symmetric mapping and policy Mapping 192.168.2.2:4445 <-> 1.1.1.4:10100 192.168.2.2:4445 <-> 1.1.1.4:10179 Policy ALLOW 1.1.1.5:7777 TO 1.1.1.4:10100 ALLOW 1.1.1.6:7777 TO 1.1.1.4:10179

29. Holes in Symmetric

30. STUN protocol protocol to discover the presence and types of NAT and firewalls between them and the public Internet STUN allows applications to determine the public IP addresses allocated to them by the NAT

31. STUN protocol STUN is specified in RFC 3489 and defines the operations and the message format needed to understand the type of NAT

32. TCP Hole Punching TCP connections between hosts behind NATs is slightly more complex than for UDP Berkeley sockets allows a TCP socket to initiate an outgoing or to listen for incoming connections but not both .

33. TCP Hole punching we need to use a single local TCP port to listen for incoming TCP connections and to initiate multiple outgoing TCP connections concurrently to bind multiple sockets to the same local endpoint BSD systems have introduced a SO_REUSEADDR and SO_REUSEPORT

34. TCP Hole punching NAT Main Internet Local network NAT Local network 10.0.0.12 192.168.2.99 rendezvous S host A host B 1.1.1.4 1.1.1.5 1.1.1.6

35. TCP Hole punching NAT Main Internet Local network NAT Local network rendezvous S host A host B 1.1.1.4:1234 1.1.1.5:4444 1.1.1.6

36. STUNT Simple Traversal of UDP Through NATs and TCP too (STUNT), which extends STUN to include TCP functionality A JAVA implementation of STUNT is available See http://nutss.gforge.cis.cornell.edu/stunt.php

37. Traversing a NAT that collaborates

38. Socks SOCKS is a client server protocol that allows a client behind a firewall to use a server in the public Internet to relay traffic Two operations: CONNECT and BIND It is widely adopted, for instance Mozilla can be configured to use SOCKS Two versions. SOCKS4 and SOCKS5

39. SOCKS CONNECT NAT Socks proxy host A server S 1. CONNECT 2. connect()

40. SOCKS BIND NAT Socks proxy host A listening on 4445 server S 1. BIND (localport=4445, S) 3. connect(33102) 2. Ok. Port=33102

41. SOCKS and Java SocketAddress addr = new InetSocketAddress(&quot; socks.mydomain.com &quot;, 1080); Proxy proxy = new Proxy(Proxy.Type.SOCKS, addr); URL url = new URL(&quot; ftp://ftp.gnu.org/README &quot;); URLConnection conn = url.openConnection(proxy);

42. SOCKS4 and SOCKS5 SOCKS4 doesn't support authentication while SOCKS5 has the built-in mechanism to support a variety of authentications methods. SOCKS4 doesn't support UDP proxy while SOCKS5 does. SOCKS4 clients require full support of DNS while SOCKS5 clients can rely on SOCKS5 server to perform the DNS lookup.

43. UPnP NAT Traversal Internet Gateway Device ( IGD ) protocol[1] is defined by UPnP It is implemented in some internet routers . It allows applications to automatically configure NAT routing. IGD makes it easy to do the following: Learn the public (external) IP address Enumerate existing port mappings Add and remove port mappings Assign lease times to mappings

44. UPnP API provided by COM IStaticPortMapping::get_ExternalIPAddress() IStaticPortMapping::get_ExternalPort() IStaticPortMapping::get_InternalPort() IStaticPortMapping::get_Protocol() IStaticPortMapping::get_InternalClient() IStaticPortMapping::get_Enabled() IStaticPortMapping::get_Description()

45. UPnP Port Forward

46. Issues with UPnP Oppents to IGD see a significant security risk UPnP allows any program, even malicious programs, to create a port mapping through the router. with UPnP, the port mapping can be created even without any knowledge of the administrative password to the router

47. References Peer-to-Peer Communication Across NAT http://www.brynosaurus.com/pub/net/p2pnat/ STUN Protocol RFC. http://www.ietf.org/rfc/rfc3489.txt TCP NAT traversal. http://nutss.gforge.cis.cornell.edu//stunt.php Traversal Using Relay NAT (TURN) IETF RFC

48. References (2) SOCKS5 IETF RFC http://www.ietf.org/rfc/rfc1928.txt SOCKS4 http://archive.socks.permeo.com/protocol/socks4.protocol Java Networking and Proxies http://java.sun.com/j2se/1.5.0/docs/guide/net/proxies.html Using UPnP for Programmatic Port Forwardings and NAT Traversal http://www.codeproject.com/internet/PortForward.asp

49. License Attribution-ShareAlike 2.5 You are free: to copy, distribute, display, and perform the work to make derivative works to make commercial use of the work Under the following conditions: Attribution . You must give the original author credit. Share Alike . If you alter, transform, or build upon this work, you may distribute the resulting work only under a licence identical to this one. For any reuse or distribution, you must make clear to others the licence terms of this work. Any of these conditions can be waived if you get permission from the copyright holder. Your fair use and other rights are in no way affected by the above. This is a human-readable summary of the Legal Code (the full licence ) . Disclaimer