Selecting either maximum or medium security disables ports on which ypbind
listens. If NIS is enabled (and I assume similarly LDAP, kerberos) in
these cases, ypbind fails to attach to a NIS server.
The ipchains rules need to take into account use of NIS, LDAP, and
The documentation on the firewall screen makes it pretty clear what is not going
to work if you choose a security level.
If you are using NIS then you had better be on a secure network, making setting
up a firewall a moot issue.