Thanks for the reply, Ray.
I couldn't connect to the link you gave,
www.ibiblio.org could not be resolved here, so here is some additional info.
My application is that the SME server/gateway is in a building with some staff computers and some spare RJ45 sockets that exist in offices that clients hire by the week.
We provide a separate Wi-Fi network with a separate dsl connection for our visiting clients to use for internet browsing, but some of these clients plug into our main network instead.
If I could configure the SME to require a logon before it offered a gateway, then I think these clients would soon get bored and assume the RJ45 sockets don't work.
You suggest filtering by ip address, but I cannot predict the ip's that these clients' machines will use - either because of DHCP or because they are free to choose any ip they fancy.
Is DansGuardian still your recommendation for this scenario, or is there another solution?
Many thanks
David