Port 111: RPC portmapper
Port 111 is the Unix RPC portmapper, which tells clients where services like NFS are listening.
What runs on port 111
rpcbind (formerly portmap) listens on TCP and UDP 111 and maps Sun RPC programs, most importantly NFS and its helpers, to the ports they actually use. It comes from Sun's network services of the 1980s and is still needed by NFS version 3 and older.
On a home network
NAS boxes with NFS enabled, Linux machines and some home servers listen on 111. Consumer devices do not.
If port 111 is reachable from the internet
Port 111 should never be reachable from the internet. It lists which RPC services a machine runs, which helps attackers find NFS shares, and exposed portmappers have been abused as reflectors in DDoS amplification attacks.
How to check whether it is open on your network
If NFS is enabled on your NAS, make sure no router rule forwards 111 or 2049. A LAN port scan will show 111 on machines where rpcbind runs.
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