When I connect to various VPN servers (Windows PPTP, Cisco IPSEC) the hostname of my laptop gets updated.
Is there a setting somewhere that will stop this from happening?
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The VPN's DHCP server probably assigns hostname together with the IP address and the computer uses it instead of what you've set during setup and what appears in System Preferences -> Sharing -> Computer name.
To set a permanent hostname do
sudo scutil --set HostName DesiredHostname in Terminal.app (there are also ComputerName and LocalHostName that can be changed there).
Restart may be required.
This question is a manifestation of the same situation described in question Why is my host name wrong at the Terminal prompt when connected to a public WiFi network?, and the answer https://apple.stackexchange.com/a/127405/17600 explains the cause and the solution. Copying from that answer:
Type in Terminal:
scutil --get HostName
If there's no HostName available, what you see is probably coming from the DNS or DHCP server.
Set your HostName with:
sudo scutil --set HostName 'yourHostName'
That should do it.