The portmap service is a dynamic port assignment daemon for RPC services such as NIS and NFS. It has weak authentication mechanisms and has the ability to assign a wide range of ports for the services it controls. For these reasons, it is difficult to secure.
If running RPC services, follow these basic rules.
It is important to use TCP wrappers to limit which networks or hosts have access to the portmap service since it has no built-in form of authentication.
Further, use only IP addresses when limiting access to the service. Avoid using hostnames, as they can be forged via DNS poisoning and other methods.
To further restrict access to the portmap service, it is a good idea to add IPTables rules to the server restricting access to specific networks.
Below is are two example IPTables commands that allow TCP connections to the portmap service (listening on port 111) from the 192.168.0/24 network and from the localhost (which is necessary for the sgi_fam service used by Nautilus). All other packets are dropped.
iptables -A INPUT -p tcp -s! 192.168.0.0/24 --dport 111 -j DROP iptables -A INPUT -p tcp -s 127.0.0.1 --dport 111 -j ACCEPT
To similarly limit UDP traffic, use the following command.
iptables -A INPUT -p udp -s! 192.168.0.0/24 --dport 111 -j DROP
Refer to Chapter 7 Firewalls for more information about implementing firewalls with IPTables commands.