The Portmapper (portmap, rpcbind) is required for mapping RPC requests (remote procedure calls) to a network service. It is needed e. g. for mounting network shares using the Network File System (NFS) in local networks.

Problem

Openly accessible Portmapper services can be abused for DDoS reflection attacks against third parties.

Verification

In this section, we show how to check a host for an openly accessible service. All tests are performed using tools commonly included with standard Linux/Unix distributions. To verify the service is openly accessible from the Internet, the test should not be run on the host itself or the local network but instead from a different node on the Internet, for example a host on a cable/DSL line. In all examples, replace 192.168.45.67 with the IP address of the host to check.

To check if a Portmapper service is openly accessible from the Internet, the tool 'rpcinfo' can be used:

$ rpcinfo -T udp -p 192.168.45.67

An openly accessible Portmapper service will return a response like this:

program vers proto   port  service

100000    4   tcp    111  portmapper

100000    3   tcp    111  portmapper

100000    2   tcp    111  portmapper

100000    4   udp    111  portmapper

100000    3   udp    111  portmapper

100000    2   udp    111  portmapper

100024    1   udp  48035  status

100024    1   tcp  52605  status

Otherwise, rpcinfo will run into a timeout:

rpcinfo: can't contact portmapper: RPC: Remote system error - Connection timed out

Solution

If the Portmapper service (portmap, rpcbind) is not required, disable or deinstall it. Otherwise, restrict access to trusted clients, for example by blocking incoming connections to port 111/tcp and 111/udp on the firewall.

On Linux systems, the portmapper service can usually be deactivated with the following commands:

# systemctl stop rpcbind.service rpcbind.socket

# systemctl disable rpcbind.service rpcbind.socket

Further information

• Wikipedia: Portmap
• Securing Debian Manual: Securing RPC services
• Lumen: A New DDoS Reflection Attack: Portmapper