Source Dedicated Server Issues

[Ms. Mudpie]

I've been trying to set up a dedicated server for Team Fortress 2 using the Source Multiplayer Dedicated Server tool. I have no problems starting it up, but when I, or anyone else, for that matter, attempt to join, I and anyone else is immediately kicked for the reason "STEAM validation rejected". I tried making a server for Counter-Strike: Source. Same problem. I would also try making one for Half-Life 2 Deathmatch but I've never even touched the bloody game.

Here are the settings I'm using for the server:

Game- Team Fortress 2
Server name- Sir Raffi's Server
Map- cp_dustbowl (this is blanked out)
Network- Internet
Max Players- 24
UDP Port- 27015
RCON Password- (am I really that stupid?)
Secure (Valve Anti-Cheat)- Disabled

I have a feeling it might have something to do with the UDP port, since I have absolutely no clue what I am doing when I'm setting out to open ports (and it's all blocked by a password anyway). I have tried changing the Network type to LAN, to no avail, and I would change the UDP port if I knew what port to change it to, so now I'm a little stuck. And yes, I have tried enabling VAC.


Things I've tried:
Re-installing the games in question
Re-installing SMDS
Creating steam_appid.txt and adding '440' to it
Changing network type
Enabling/Disabling VAC
Deleting clientregistry.blob

And I've tried the following:

Try to add -secure in your startup line.

So far, nothing's worked. Suggestions anyone?