SLIP/PPP servers?

Umm... with PPP there is no client/server. It's a symmetric protocol.
pppd can do both ends of the connection. RTFM.

With SLIP, the command you're looking for is sliplogin. Read the man page...
if it's not exactly what you're looking for you can read the source--it's
very simple cause all the real SLIP code is in the kernel--it just has to
do a few ioctls on the tty and manipulate the network interface and routing
tables.

	-Ben