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Protocol name database (UNIX)
The /etc/protocols file contains information regarding the known protocols used in the DARPA Internet. For each protocol, a single line should be present with the following information:
official_protocol_name protocol_number aliases
Items are separated by any number of blanks or tabs, or both. A # in a line indicates the beginning of a comment -- any characters after a #, up to the end of the line, aren't interpreted by routines that search the file.
Protocol names may contain any printable character other than a field delimiter, newline, or comment character.
getprotoent() in the Library Reference