P2P and NAT How to traverse NAT Davide Carboni ©

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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

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

Firewall Rules router (Global IP addresses)

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. addressesIPpacketsrouterfirewall

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

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

Multiple NAT ISP NAT (Private IP addresses) (Public IP addresses) Main Internet ISP network Home NAT Home network

NAT Mappings ( ) ( ) ( ) : :10100 S= :4445 D= :7777 datagram S= :10100 D= :7777 datagram A

Traversing a NAT that does not collaborate

Relaying NAT Main Internet Local network NAT Local network Relay S host A host B 1 2

Connection reversal NAT Main Internet Local network rendezvous S host A host B 1 2 3

NAT policies Full cone NAT is NAT where all requests from the same internal IP address and port are mapped to the same public IP address and port. Once a mapping is created, all incoming traffic to the public address is routed to the internal host without checking the address of the remote host. A restricted cone NAT: like full cone all requests from the same internal IP address and port are mapped to the same public IP address and port. Unlike a full cone NAT, a remote host (with IP address X) can send a packet to the internal host only if the internal host had previously sent a packet to IP address X. A port restricted cone NAT is like a restricted cone NAT, but the restriction includes port numbers. Specifically, an external host can send a packet, with source IP address X and source port P, to the internal host only if the internal host had previously sent a packet to IP address X and port P. A symmetric NAT is a NAT where all requests from the same internal IP address and port to a specific destination IP address and port are mapped to the same external source IP address and port. If the same internal host sends a packet with the same source address and port to a different destination, a different mapping is used. Furthermore, only the external host that receives a packet can send a UDP packet back to the internal host

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

Full cone Host AHost C Full cone Host B ( ) ( )( )( )( ) Packet(S= :4445, D= :7777) Packet(S= :10100, D= :7777) Packet(S= :4321, D= :10100) Packet(S= :4321, D= :4445) Packet(S= :1234, D= :10100) Packet(S= :1234, D= :4445)

Full cone mapping and policy Mapping  : :10100 Policy  ALLOW ALL TO :10100

Holes in Full Cone NAT rendezvous host A host B

Restricted cone Host AHost C Restricted cone Host B ( ) ( )( )( )( ) Packet(S= :4445, D= :7777) Packet(S= :10100, D= :7777) Packet(S= :4321, D= :10100) Packet(S= :4321, D= :4445) Packet(S= :1234, D= :10100) X Packet(S= :4445, D= :7777) Packet(S= :10100, D= :7777) Packet(S= :4321, D= :10100) Packet(S= :4321, D= :4445)

Restricted cone mapping and policy Mapping  : :10100 Policy  ALLOW TO :10100  ALLOW TO :10100

Holes in Restricted Cone NAT rendezvous host A host B

Port restricted cone Host AHost C Port - restr cone Host B ( ) ( )( )( )( ) Packet(S= :4445, D= :7777) Packet(S= :10100, D= :7777) Packet(S= :4321, D= :10100) Packet(S= :7777, D= :4445) X Packet(S= :7777, D= :10100)

Port restricted cone mapping and policy Mapping  : :10100 Policy  ALLOW :7777 TO :10100  ALLOW :7777 TO :10100

Holes in Restricted Cone NAT rendezvous host A host B

Symmetric NAT Host AHost CsymmetricHost B ( ) ( )( )( )( ) Packet(S= :4445, D= :7777) Packet(S= :10100, D= :7777) Packet(S= :7777, D= :4445) Packet(S= :7777, D= :10100) Packet(S= :4445, D= :7777) Packet(S= :10179, D= :7777) Packet(S= :7777, D= :4445) Packet(S= :7777, D= :10179) Packet(S= :7777, D= :10100) X

Symmetric mapping and policy Mapping  : :10100  : :10179 Policy  ALLOW :7777 TO :10100  ALLOW :7777 TO :10179

Holes in Symmetric NATs The only way to traverse this NAT is by Connection Reversal or Relaying.

STUN protocol (to simplify hole punching) 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

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

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

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

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.

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

TCP Hole punching NAT Main Internet Local network NAT Local network rendezvous S host A host B

TCP Hole punching NAT Main Internet Local network NAT Local network rendezvous S host A host B : :

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

Traversing a NAT that collaborates

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

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

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

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

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.

UPnP NAT Traversal Internet Gateway Device (IGD) protocol[1] is defined by UPnP It is implemented in some internet routers.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

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()

UPnP Port Forward

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

References Peer-to-Peer Communication Across NAT STUN Protocol RFC. TCP NAT traversal. Traversal Using Relay NAT (TURN) IETF RFC

References (2) SOCKS5 IETF RFC SOCKS4 Java Networking and Proxies Using UPnP for Programmatic Port Forwardings and NAT Traversal

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).Legal Code (the full licence) Disclaimer