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P2P and NAT
How to traverse NATDavide Carboni © 2005-2006
LicenseAttribution-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
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.
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
Simple NAT
NAT
(Private IP addresses)
(Public IP addresses)
Main Internet
(Public IP addresses)
Multiple NAT
ISPNAT
(Private IP addresses)
(Public IP addresses)
Main Internet
ISP network
HomeNAT
Home network
10.0.0.12
192.168.2.12
192.168.2.99
156.148.70.32
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=192.168.2.2:4445D=1.1.1.4:7777
datagram S=1.1.1.5:10100D=1.1.1.4:7777
datagramA
Traversing a NAT that does not collaborate
Relaying
NAT
Main Internet
Local network
NAT
Local network
10.0.0.12
192.168.2.99
Relay S
host A
host B
12
Connection reversal
NAT
Main Internet
Local network
1.1.1.4
192.168.2.99
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 A Host CFull 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)
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
Holes in Full Cone
NAT
rendezvous
host A
host B
1
2
3
4
5
Restricted cone
Host A Host CRestricted
coneHost 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)
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
Holes in Restricted Cone
NAT
rendezvous
host A
host B
1
2
35
4 6
Port restricted cone
Host A Host CPort - restr
coneHost 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)
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
Holes in Restricted Cone
NAT
rendezvous
host A
host B
1
2
35
4 6
Symmetric NAT
Host A Host Csymmetric 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
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
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
NATLocal network
10.0.0.12
192.168.2.99
rendezvous S
host A
host B
1.1.1.41.1.1.5
1.1.1.6
TCP Hole punching
NAT
Main Internet
Local network
NAT
Local network
rendezvous S
host A
host B1.1.1.4:1234
1.1.1.5:4444
1.1.1.6
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
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);
Proxy proxy = new Proxy(Proxy.Type.SOCKS, addr);
URL url = new URL("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. 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 COMIStaticPortMapping::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 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
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 Traversalhttp://www.codeproject.com/internet/PortForward.asp
LicenseAttribution-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
