Advertising services on certain ports

[Jesse Manibusan]

Does anyone know how to advertise a certain service on a port and not on
others?  For example, say I want to have DHCP running on my firewall for my
internal network but don't want it to accept DHCP requests coming from the
external ethernet port.  Is there any way to do this other than to create
rules in the iptables/ipchains setup?

Also, what is the smallest size Linux distribution I can use for a firewall
setup?  I currently only have 2 spare HD's to use, one is an 800MB and the
other is a 20GB.  2 lousy choices since the 20 GB is too large to waste on a
firewall that won't do anything except masquerading and the other is too
small too be able to accept the most current versions of Redhat or Mandrake.

Are the "firewall on a floppy" setups pretty secure?  Any place where I can
get current information, not some how-to that has all sorts of out-of-date
warnings and disclaimers on it? 



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