D. J. Bernstein
Internet publication
FTP: File Transfer Protocol

Greetings

The server's initial response to the client must use code 220 or 421.

RFC 959 permits the server to send a mark before its greeting:

     120 Give me 37 seconds...
     220 Features: p .

However, servers must not do this. The BSD ftp program will treat the 120 as if it were a 220, causing a disastrous loss of synchronization with the server.

220 greetings

220 accepts the connection:

     220 Features: p .

RFC 959 does not impose any requirements on the text after 220. Most servers use the text for a human-readable operating system name. Some servers use multiple-line responses for human-readable announcements:

     220-See ir.html if you're looking for IR information.
     220-See index.html for everything else.
     220-
     220 Features: a p .

The string 220 Features: on the last line of the greeting indicates an experimental greeting format, designed to help speed up clients. The rest of the line is a series of feature strings, in any order, each preceded by one space, terminated by a space and a dot at the end of the line.

Feature strings do not contain spaces. Feature strings have the general format xy, where x is one of the following strings:

p: The server supports pipelining. The server must use an empty string for y.
a: The server asks the client not to bother sending USER or PASS if the USER parameter is anonymous. The server assumes the anonymous username by default; it will provide the same files without a USER request from the client. The server must use an empty string for y.

New feature strings will be allocated upon request, although not necessarily with the requested names.

Note that greeting lines are case-sensitive: servers must use lowercase and uppercase exactly as shown here.

421 greetings

421 temporarily rejects the connection:

     421-ftp.heaven.af.mil is too busy to provide files right now.
     421 Please come back in 2040 seconds.

The server closes the connection without reading any requests from the client.