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15 December 2006, 13:46

Jürgen Schmidt

The hole trick

How Skype & Co. get round firewalls

Peer-to-peer software applications are a network administrator's nightmare. In order to be able to exchange packets with their counterpart as directly as possible they use subtle tricks to punch holes in firewalls, which shouldn't actually be letting in packets from the outside world.

Increasingly, computers are positioned behind firewalls to protect systems from internet threats. Ideally, the firewall function will be performed by a router, which also translates the PC's local network address to the public IP address (Network Address Translation, or NAT). This means an attacker cannot directly adress the PC from the outside - connections have to be established from the inside.

This is of course a problem when two computers behind NAT firewalls require to talk directly to each other - if, for example, their users want to call each other using Voice over IP (VoIP). The dilemma is clear - whichever party calls the other, the recipient's firewall will decline the apparent attack and will simply discard the data packets. The telephone call doesn't happen. Or at least that's what a network administrator would expect.


But anyone who has used the popular internet telephony software Skype knows that it works as smoothly behind a NAT firewall as it does if the PC is connected directly to the internet. The reason for this is that the inventors of Skype and similar software have come up with a solution.

Naturally every firewall must also let packets through into the local network - after all the user wants to view websites, read e-mails, etc. The firewall must therefore forward the relevant data packets from outside, to the workstation computer on the LAN. However it only does so, when it is convinced that a packet represents the response to an outgoing data packet. A NAT router therefore keeps tables of which internal computer has communicated with which external computer and which ports the two have used.

The trick used by VoIP software consists of persuading the firewall that a connection has been established, to which it should allocate subsequent incoming data packets. The fact that audio data for VoIP is sent using the connectionless UDP protocol acts to Skype's advantage. In contrast to TCP, which includes additional connection information in each packet, with UDP, a firewall sees only the addresses and ports of the source and destination systems. If, for an incoming UDP packet, these match an NAT table entry, it will pass the packet on to an internal computer with a clear conscience.


The switching server, with which both ends of a call are in constant contact, plays an important role when establishing a connection using Skype. This occurs via a TCP connection, which the clients themselves establish. The Skype server therefore always knows under what address a Skype user is currently available on the internet. Where possible the actual telephone connections do not run via the Skype server; rather, the clients exchange data directly.

Let's assume that Alice wants to call her friend Bob. Her Skype client tells the Skype server that she wants to do so. The Skype server already knows a bit about Alice. From the incoming query it sees that Alice is currently registered at the IP address and a quick test reveals that her audio data always comes from UDP port 1414. The Skype server passes this information on to Bob's Skype client, which, according to its database, is currently registered at the IP address and which, by preference uses UDP port 2828.

Alice calls Bob
Step 1: Alice tries to call Bob, which signals Skype.

Bob's Skype program then punches a hole in its own network firewall: It sends a UDP packet to port 1414. This is discarded by Alice's firewall, but Bob's firewall doesn't know that. It now thinks that anything which comes from port 1414 and is addressed to Bob's IP address and port 2828 is legitimate - it must be the response to the query which has just been sent.

Hole punching
Step 2: Bob tries to reach Alice, which punches a hole through Bob's Firewall.

Now the Skype server passes Bob's coordinates on to Alice, whose Skype application attempts to contact Bob at Bob's firewall sees the recognised sender address and passes the apparent response on to Bob's PC - and his Skype phone rings.

Call established
Step 3: Alice finally reaches Bobs computer through the hole.

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