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NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.

5.2. Securing Portmap

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.

Note Note
 

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.

5.2.1. Protect portmap With TCP Wrappers

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.

5.2.2. Protect portmap With IPTables

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
Tip Tip
 

Refer to Chapter 7 Firewalls for more information about implementing firewalls with IPTables commands.

 
 
  Published under the terms of the GNU General Public License Design by Interspire