As with nmap-services, nmap-rpc simply maps numbers to names. In this case, SunRPC program numbers are mapped to the program name which uses them. Example 14.3 offers a typical excerpt.

rpcbind         100000  portmap sunrpc rpcbind
rstatd          100001  rstat rup perfmeter rstat_svc
rusersd         100002  rusers
nfs             100003  nfsprog nfsd
ypserv          100004  ypprog
mountd          100005  mount showmount
rpc.operd       100080  opermsg         # Sun Online-Backup
# DMFE/DAWS (Defense Automated Warning System)
#
Gqsrv           200034  gqsrv
Ppt             200035  ppt
Pmt             200036  pmt

Nmap only cares about the first two whitespace-separated columns—the program name and number. It doesn't look at any aliases or comments that may appear beyond that. Blank lines and those starting with pound comments are permitted. This format is the same as used by /etc/rpc on Unix, so administrators may use that file instead if they desire.

nmap-rpc is only used by the RPC grinding feature of Nmap version descriptions. That feature is covered in the section called “RPC Grinding”.

Users rarely change nmap-rpc. When they do, it is usually to add a custom service or a public one that is missing from the latest nmap-rpc. In the latter case, please send a note to me at <fyodor@nmap.org> so that I can add it to the next version. As with nmap-services, some administrators strip the file down, removing obscure RPC programs to save scan time. The same warning applies: specify your stripped nmap-rpc with the --datadir option rather than installing it where it will be used implicitly.