I run OSX Server on a Mac mini here. It runs this blog, a few other websites and accepts mail for my family. I think it’s a great solution – easy to set up and configure and requires almost no attention. Just the sort of thing you could use at the end of your broadband connection (assuming your terms and conditions allow it).

A few days ago, however, I added another blog to the server (It’s my personal blog). Almost as soon as I did this, I started to notice things slowing down and timing out. As if my server was overloaded. Which it definitely wasn’t – load averages were trivial, there was nothing in the logs, no stray processes. Yet I’d keep getting DNS failures and various timeouts.

Suspecting something else, I took a look at my router’s logs. There was one message in particular that drew my attention. It was repeated time and time again:

10.123.4.1 exceeds the max. number of session per host!

A little bit of searching found the answer. The new blog had pushed my server beyond the default number of open NAT connections my router allowed. Setting this higher has fixed all the intermittent connection problems.

Details for Zyxel routers below the cut.
For my (elderly!) Zyxel P661, the configuration change required is quite simple. Log into your router via the web interface. Drill down through the Network settings, to the NAT page.

On the main NAT page, you will see a setting “Max NAT/Firewall Session Per User”. Don’t worry about the mention of users. It’s a mistranslation. It means device. Change that to a larger setting – I used 1024, the maximum my router would allow (it was previously set to 512) but other routers may allow larger values.

Reboot the router, and that should be all that’s required.