What are you trying to accomplish? Do you want name resolution to work regardless of provider? There are three standard solutions to this problem:

1. Use provider nameservers. Change /etc/resolv.conf on connections.
2. Use local nameserver with forwarding. Change /etc/named.conf.
3. Use local caching nameserver. Change named.ca containing root name servers to use fake roots when disconnected. This makes lookup failures when disconnected fast.

Are you trying to solve the problem of stable names for other services? You want to use the same name (smtp.fake) in sendmail config regardless of providers. Running a local nameserver for the .fake zone will do this. But then you need to change the .fake zone file on each connection. And change the forwarding line (or root zone file) as above to make the resolution of other names work. This is doable with BIND.

Are you trying to solve the problem of stable names for other services? You want to use the same name (smtp.fake) in sendmail config regardless of providers. Running a local nameserver for the .fake zone will do this. But then you need to change the .fake zone file on each connection. And change the forwarding line (or root zone file) as above to make the resolution of other names work.

Yes, thats what I want. But I want to use nsupdate to change the .fake zone. So it's changing the forwarding line thats the problem. That was the reason for the question.