1 This file contains material useful for client writers.
6 The protocol between Empire client and server is plain text. It is
7 simple enough that you could play using nothing more than telnet.
10 A session uses either ASCII or UTF-8, controlled by session option
11 utf-8. See below for session options. The session always starts in
14 Client-server communication is line-oriented.
16 The server sends lines of output. Each output line starts with an
17 identification string, followed by a space, then arbitrary text, then
20 Identification strings encode small integers called output IDs as base
21 36 numbers. Characters '0' to '9' represent digits 0 to 9, and 'a' to
22 'z' represent 10..35, as do 'A' to 'Z'. Symbolic names for IDs are
25 empire-client versions before version 4.3.11 parse large output IDs
26 incorrectly. Such IDs do not currently occur.
28 Clients shall be able to safely handle output lines of arbitrary
29 length. Naturally, a client may not be able to handle a sufficiently
30 large line without loss of functionality, but safety must be ensured.
32 When using ASCII, the most significant bit in text characters
33 indicates highlighting.
35 The following control characters have a defined meaning:
37 ASCII name decimal meaning
38 ----------------------------------------------------
40 horizontal tab 9 move to next tab stop(1)
41 line feed 10 end of line
42 shift out 14 begin highlighting(2)
43 shift in 15 end highlighting(2)
45 (1) Tab stops are every eight columns.
46 (2) Only if session uses UTF-8
48 Other ASCII control characters should not occur and may be ignored by
49 clients. Likewise, overlong or malformed UTF-8 sequences should not
50 occur and may be ignored.
52 The server prompts for input. Each prompt `consumes' one line of
53 input (except for C_EXECUTE, and when a command is aborted, as
54 described below). Input lines are arbitrary text, terminated by line
55 feed, which is optionally preceded by carriage return (decimal 13).
56 Lines should not contain ASCII control characters other than
57 horizontal tab. Clients should not send overlong or malformed UTF-8
60 A client is called synchronous if it waits for a prompt before it
61 sends another line of input. Else it is called asynchronous.
63 Asynchronous clients must take care to avoid blocking on sending
64 input. If the client process blocks that way, it can't receive server
65 output until the server reads more input. That may never happen,
66 because the server may well block on output, which then deadlocks the
69 An Empire session consists of three phases: login, playing, and
70 shutdown. empire-client is synchronous in the login phase, and
71 asynchronous afterwards. Versions before 4.3.11 could deadlock as
77 In the login phase, the server prompts for login commands.
79 The server starts with a C_INIT prompt. The login phase ends when the
80 server sends another C_INIT prompt, which starts the playing phase, or
81 when it enters the shutdown phase, as described below
83 The server replies to a login command with another prompt. Except as
84 noted below, the server replies C_BADCMD for syntax errors, C_CMDERR
85 for other errors, and C_CMDOK on success. In any case, the prompt
86 text contains further information intended for humans.
92 Identify the client. This is optional. If given, version
93 information should be included.
97 Set country name to COUNTRY.
101 If another connection for this country is in the playing phase,
102 force it into the shutdown phase, else do nothing. Country must be
105 Reply is C_EXIT regardless of success.
107 * options OPTION[=VALUE]...
109 Negotiate session options. Each argument requests setting OPTION to
110 VALUE. The string VALUE is interpreted in an option-specific way.
111 The form without the `=' sets it to an option-specific implied
114 The server accepts the request by sending C_CMDOK. If the server
115 encounters an unknown option, it rejects the request by sending
116 C_BADCMD. It rejects unsupported values by sending C_CMDERR. It
117 may or may not process valid parts of rejected requests.
119 If there are no arguments, the server lists its options. For each
120 option, it sends a C_DATA line with OPTION=VALUE as text, and
121 finally a C_CMDOK. If it supports no options at all, it may reply
122 with C_BADCMD instead.
124 See below for supported session options.
128 Authenticate. Country name must be set already.
130 * play [USER COUNTRY PASSWORD]
134 If no arguments are given, the country must be authenticated
137 Else, argument USER sets the user name, COUNTRY sets the country
138 name, and PASSWORD authenticates.
140 Some error conditions result in a C_EXIT reply. Clients should
141 treat it just like C_CMDERR.
143 On success, the server sends C_INIT. The text is the protocol
144 version number in decimal, currently 2. The client should terminate
145 on protocol versions it doesn't know how to handle.
147 The protocol version is not expected to change often. In fact, it
148 hasn't changed since the oldest known versions of Empire.
150 Unlike the first C_INIT, the second one is not a prompt, i.e the
151 server does not consume a line of input for it.
153 The session then proceeds to the playing phase.
157 Terminate the session. The server replies with C_EXIT and enters
162 List all countries that are still in sanctuary. The output consists
163 of a number of C_DATA lines with human-readable text.
165 This command is only recognized if option BLITZ is enabled.
169 Set the user name. This is optional and defaults to "".
171 If the login phase takes more than login_grace_time (default 120s),
172 the server enters the shutdown phase.
177 In the playing phase, the server sends data, control and prompt lines.
179 Clients signal `EOF' by sending a line "ctld\n", and `interrupt' by
180 sending "aborted\n". The server treats these conditions specially, as
181 described below. Anything else is either a command or input for a
182 command, depending on how the server prompts for the line.
184 empire-client signals `EOF' when it encounters an end-of-file
185 condition while reading player input. It signals `interrupt' when it
186 catches SIGINT, which is normally triggered by Ctrl-C.
188 The following IDs occur:
190 * Command prompt C_PROMPT
192 The server consumes a line of input. On EOF, the server enters the
193 shutdown phase. Interrupt is ignored. Anything else is interpreted
196 Text is minutes used, space, BTUs left. Both numbers are in
197 decimal. Clients shall ignore additional text separated by another
200 empire-client prints this prompt using format "[%d:%d] Command : ".
201 Clients with a tty-like user interface are advised to use a similar
202 format, to minimize differences to the examples in info.
204 * Argument prompt C_FLUSH
206 The server consumes a line of input and passes it to the currently
209 The server aborts the command on interrupt and EOF. Any argument
210 prompts it may send before the next command prompt do not consume
211 input. On EOF, the server then enters the shutdown phase (but see
212 C_EXECUTE for an exception).
214 If an update runs while the server waits for the line of input to
215 arrive, the current command is aborted. Whether the server consumes
216 a line of input for this argument prompt is unpredictable. Any
217 argument prompts it may send before the next command prompt do not
220 Text is a human-readable prompt supplied by the command.
222 empire-client prints the text verbatim.
226 Text is human-readable server output.
228 empire-client prints the text verbatim.
232 Request execution of a batch file. The text is whatever was on the
233 line of input after the execute command. Its syntax and semantics
234 are left to the client.
236 empire-client interprets the first word (sequence of non-space
237 characters) in the text as file name, and sends the contents of that
240 The security considerations on C_PIPE (below) apply to C_EXECUTE as
243 Note that servers before 4.3.11 sent a copy of the execute command's
244 first argument as text. This made it hard for clients to ensure
245 that the text is identical to what was sent, because the server
246 strips funny characters and interprets and strips '"' characters
247 when splitting input lines into command and arguments.
249 empire-client gets confused when old servers mangle the text that
252 The client shall mark the end of the batch file by signalling EOF as
253 described above. This does not terminate the session. It may
254 signal interrupt if it is unable or unwilling to send the complete
257 While executing the batch file, the server rejects redirections and
258 execute commands, and sends no C_PROMPT command prompts. It still
259 sends C_FLUSH argument prompts.
261 Protocol flaw: not sending C_PROMPT here screws up redirections:
262 they'd apply until the next C_PROMPT, i.e. from start of redirected
263 command until end of containing batch file. That's why the server
264 rejects redirections in batch files.
266 Certain bad failures make the server ignore the rest of the batch
267 file. This feature is too hard to predict to be really useful.
269 Protocol flaw: strictly asynchronous clients cannot support
270 C_EXECUTE correctly. By the time C_EXECUTE arrives, the client may
271 have sent more input already. That input `overtakes' the contents
272 of the batch file in the input stream, and is interpreted as part of
273 the batch file. Because this is almost certain to happen when the
274 execute comes from a batch file, the server rejects execute commands
277 empire-client has this problem.
279 Clients are not required to support C_EXECUTE. Clients are
280 encouraged to offer alternative means for scripting.
284 Asynchronous message. The client should display the text
287 empire-client prints the text verbatim, prepended by a line feed.
288 This is clearly sub-optimal, because it can be inserted in the
289 middle of user input. Clients wishing to display asynchronous
290 messages together with normal I/O should insert them before the
293 Although asynchronous messages can be switched off with the toggle
294 command, client support for C_FLASH is not really optional, because
295 a C_FLASH could arrive before the client manages to switch them off.
296 And if the client lets the user send arbitrary commands, the user
297 can switch it back on at any time. A session option controlling
298 C_FLASH would make more sense. Since all popular clients support
299 C_FLASH, it's not worth the trouble.
303 Notification that the number of unread telegrams changed. The text
304 is the notification in human-readable form.
306 empire-client prints the last received C_INFORM text right before
307 each prompt. It also repeats the last prompt when a C_INFORM
308 arrives. This is sub-optimal just like its treatment of C_FLASH.
310 The user can switch these off with the toggle command. Client
311 support is not really optional for the same reasons as for C_FLASH.
315 When a command is redirected to a pipeline, its output is preceded
316 by a C_PIPE line. The text is a copy of the redirection, starting
317 with '|'. Syntax and semantics of the text after the '|' are left
320 empire-client executes text after '|' as shell command, with
321 standard input connected to the Empire command's output.
323 The redirection applies to a single command, i.e. until the next
326 For obvious security reasons, clients supporting pipes shall ensure
327 that the text is identical to whatever was sent to the server. Note
328 that the server recognizes redirection only in command lines, not
329 argument lines. Asynchronous clients cannot distinguish the two.
331 empire-client prepares for redirections being recognized in any
332 line, and copes with only some of them being actually recognized.
336 When a command is redirected to a file, its output is preceded by a
337 C_REDIR line. The text is a copy of the redirection, starting with
338 '>', optionally followed by '>' or '!'. Syntax and semantics of the
339 remaining text are left to the client.
341 empire-client interprets the first word (sequence of non-space
342 characters) in the remaining text as file name, and redirects the
343 command's output to that file. The use of the first word is
344 unfortunate, as it arbitrarily limits the user's choice of file
345 names. If the text starts with '>!', it silently overwrites any
346 existing file, with '>>' it appends to the file, and with just '>'
347 it refuses to overwrite an existing file.
349 The redirection applies to a single command, i.e. until the next
352 The security considerations on C_PIPE apply to C_REDIR as well.
356 Other IDs do not occur currently. Clients shall deal gracefully
357 with IDs they don't know.
359 empire-client treats unknown IDs like C_DATA. Versions before
360 4.3.11 prepend "Aborted\n" to C_ABORT lines, and "Error; " to
361 C_CMDERR and C_BADCMD lines for historical reasons.
366 In the shutdown phase, the server first sends a C_EXIT message. Its
367 text is a human-readable farewell.
369 empire-client prints this text prepended with "Exit: ".
371 The server then waits for the output queue to drain, then shuts down
372 the output direction of the connection. This makes the client detect
373 an end-of-file condition after it received all output. The client
374 should then close its end of the connection. The server continues to
375 read and ignore input until it detects an end-of-file condition. It
376 then closes its end of the connection, and the session terminates.
378 If the server closed its end of the connection without waiting for the
379 client to close the other end first, asynchronous clients could lose
380 output: an attempt to send input after the server closed its end would
381 fail with a connection reset error even when some output is still in
382 flight. Versions before 4.3.30 could misbehave like that in certain
385 If the shutdown phase doesn't complete within login_grace_time
386 (default 120s), the server closes the connection immediately. Output
392 Session options control client preferences. They are not persistent;
393 each session starts with the same default session options.
395 The only session option so far is utf-8. It controls whether to use
396 ASCII (disabled) or UTF-8 (enabled). Initial value is disabled.
397 Setting it to 0 disables, setting it to 1 enables, and the implied
400 Session options should not be confused with command toggle, which
401 controls player preferences. Clients should leave those accessible to
405 Commands Useful for Clients
406 ===========================
411 A number of commands are available for clients that wish to maintain
412 their own game state. These commands are called dumps.
414 dump - Dump sector information
415 ldump - Dump land unit information
416 lost - Report lost items
417 ndump - Dump nuclear stockpile information
418 pdump - Dump plane information
419 sdump - Dump ship information
421 See the various info pages on these for complete documentation on how
422 they work and how you can use them to help improve your clients.
424 Each of the above commands prints a timestamp, which is a decimal
425 number. This together with the timestamp selector enables you to dump
426 incrementally, i.e. retrieve only what changed since another dump.
427 For instance, if `dump *' printed timestamp 855544597, `dump *
428 ?timestamp>855544596' dumps everything changed since.
430 Note that the condition compares with the timestamp value minus one.
431 That's because timestamps have a noticeable granularity: things may
432 change between a dump and the next timestamp increase.
434 Timestamp values are currently seconds since the epoch, but this might
435 change, and clients are advised not to rely on it.
440 Traditional dumps have a number of shortcomings. They cover only the
441 most important part of the game state (sectors, ships, planes, land
442 units, nukes), but not game configuration, loans, news, and so forth.
443 They are not quite complete even for what they attempt to cover.
444 Finally, their output is harder to parse than necessary.
446 The new `xdump' command is designed to overcome these deficiencies.
447 See doc/xdump for the full story.
450 Advice on parsing human-readable command output
451 ===============================================