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The effects of this error message is that:
If you just set up your server: don't panic! It's normal to see this at first, while your server connects soon after.
You can check for connectivity by searching the console for occurrences of
VAC secure mode is activated. Afterwards, upon restarting the server you should see the following:
Connection to Steam servers successful. VAC secure mode is activated.
If after a few hours your server does not connect to Steam, here's a few things to check:
For reasons unknown, Source Dedicated Server 2007 does not seem to connect to Steam if it's running on a single-core CPU. If you're running the server in a VM, upgrading the server to 2 cores or higher should resolve the issue.
While there is not a complete and reliable list of Steam endpoints, it is known that you will need to allow the following ports for outbound connections:
TCP ports 80 and 443
TCP ports 27015-27030
UDP ports 27015-27030
It has anecdotally not been very helpful, but a last-resort option is to check the connection log for your server, which is located at
[SrcDS 2007 root]/logs/connection_log_[port].txt