Following are Erik Johansen's comments on a thread titled MMOs and Firewalls :
A firewall is hardware and/or software that restricts what internet traffic can pass by it. Almost all decent firewall products today are "stateful", meaning they understand the concept of a "connection", and can associate individual data packets with this connection.
When you connect to any kind of server, you are the one initiating the connection. If the firewall permits you to make this connection, and it is a statefull firewall, it will also permit the responses from the server to return to you. This is an important distinction from the case where someone from the outside tries to connect to you, and in order for a firewall to have much practical effect it will usually deny such incoming connections.
The problem with many game servers today is that they require multiple connections. Sure, you make the first connection, which works fine, but then the game server attempts to open a new connection back to you, and since the firewall in the middle doesnt understand that this new connection is associated with the first one you started, it will deny this new connection.
Some of the better firewalls around can be taught that "if one of my protected users connect to this and that, I should expect a connection back and permit it", but most don't. Also, since this "allow incoming if this and that" functionality need to be configured by the firewall administrator for each and every way of communication different games use, it often is too much management and is simply not done.
For Darkfall, we will never require that the server is able to connect back to you. If your firewall is stateful, and allows you to connect wherever you want, you should never have problems connecting. If your firewall also limits where you may connect, you might experience problems. The Darkfall client is able to use "proxy" servers (SOCKS5 and http CONNECT), as this is a common requirement for many networks protected by firewalls.
In short: You may experience firewall-related connection problems with Darkfall,
but only if your firewall setup is a lot stricter than the average.