Usually outbound connections (your friends) are allowed in most routers .. so if you know how to operate your own router what is the problem?
Since your friends would be able to connect to your hosted game.
To configure multiple clients to play from behind the same router, you may need to change one of the clients to use a different clientport number.
* Each machine must have a unique Steam account with different game licenses registered to the account.
* Each machine must have a different clientport setting.
* Each machine must have a unique Mac address.
* The router must be up-to-date.
* Your Firewall and router must have the required ports for Steam open.
The default clientport is 27005 and you can only have one game with that port active on the same lan.
Specifying a Clientport as a Launch Option
To specify a client port, please perform the following steps for each computer on the LAN
* Go to Steam's My games menu
* Right-click on the game you wish to play
* Go to Properties
* From the General tab, click on the Set launch options button
* Add a different client port number on each machine between 27005 and 27032 in the following format:
Note: Do not use 27020
* Click OK
NOTE: This solution only works if you launch the game through the My games menu.
If you have already set other launch options, leave a space between the last character of your previous launch option and the + of your clientport setting.
Specifying a Clientport in the Userconfig.cfg
The benefit of this method is that the clientport will remain set whether you are launching the game from the My games menu or from a shortcut to the game.
* Navigate to the folder that holds the config.cfg for your game
Example: The config.cfg for Counter-strike: Source can be found here:
..\Steam\steamapps\<user name>\counter-strike source\cstrike\cfg
* If you have a userconfig.cfg in that directory, open it. If you do not already have a userconfig.cfg:
1. Right-click in the folder
2. Go to: New > Text Document
3. Rename the file userconfig.cfg
4. Open the file with Notepad
* Type into the document clientport 270XX where XX are the last two digits of the clientport number. Acceptable values include 05 - 19 and 21 - 32
* Save your userconfig.cfg
* Launch your game
I'll tell you if this did help as soon as I can grab a friend for some more testing after the holidays
EDIT: I've just got someone to join a game (at last).
So, now, I'm sure that I don't need a specific listening port to host Magicka games, thanks to Steam (although a valid listening port is often required to successfully host many games or services).
Also, thank you for showing me this article about troubleshooting multiple steam client connexions with a unique public IP.