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.
Securing portmap only affects NFSv2 and NFSv3 implementations, since NFSv4 no longer requires it. If you plan to implement a NFSv2 or NFSv3 server, then portmap is required, and the following section applies.
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 and restrict access to specific networks.
Below 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.