sending UDP datagrams via internet [duplicate]

Question

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How does Skype work without port forwarding? (4 answers)

Closed 9 years ago.

I am a networking newbie... I have a (stupid? :) ) question regarding UDP communication.

I created two simple programs based on the boost::asio library: a server and a client. These two programs communicate via UDP sockets. Client connects to a server and starts transmitting the datagrams.

It works very well in my LAN. I use IP address to connect with the server (eg. 192.168.1.111).

What if my server application works in other LAN, and client have to connect via internet? How do I reach the server?

Only possible solutions I know are:

• port forwarding (insecure?)
• VPN (over-complicated)

Are there better solutions?

How is that possible that some applications (like Skype or LogMeIn...) works without VPN/port forwoarding, and user in one LAN can reach any computer in other LAN?

Answer 1

Are there better solutions?

Yes.

Skype uses the Hole Punching trick.

Assume A wants to contact B:

• A and B are connected to server C

• server C tells B to send dummy UDP packet to A ( to IP and port used by A for Skype connections) effectively punching a hole in it's own (B's) firewall

• this packet is discarded by A (it goes from outside of the NAT) but now A can send UDP packets to B which will let them through firewall ( B router thinks this is a response to [dummy] packet)

This article describes it with more details: How-Skype-Co-get-round-firewalls