1 hack.txt for Citadel/UX
2 (some of this stuff is *very* out of date.)
4 Much of this document is borrowed from the original hack.doc from
5 Citadel-CP/M and Citadel-86, because many of the concepts are the same. Hats
6 off to whoever wrote the original, for a fine document that inspired the
7 implementation of Citadel for Unix.
9 Note that this document is really out of date. It doesn't cover anything
10 about the threaded server architecture or any of the network stuff. What is
11 covered here is the basic architecture of the databases.
13 But enough of the preamble. Here's how Citadel/UX works :)
15 Here are the major databases to be discussed:
17 msgmain The big circular file that contains message text
18 quickroom Contains room info such as room names, stats, etc.
19 fullroom One fullrm file per room: message numbers and pointers.
20 usersupp Contains info for each user on the system.
22 The fundamental structure of the system differs greatly from the way
23 Citadels used to work. Citadel now depends on a record manager or database
24 manager of some sort. Thanks to the API which is in place for connecting to
25 a data store, any record manager may be used as long as it supports the
26 storage and retrieval of large binary objects (blobs) indexed by unique keys.
27 Please see database.c for more information on data store primitives.
29 The message base (MSGMAIN) is a big file of messages indexed by the message
30 number. Messages are numbered consecutively and start with an FF (hex)
31 byte. Except for this FF start-of-message byte, all bytes in the message
32 file have the high bit set to 0. This means that in principle it is
33 trivial to scan through the message file and locate message N if it
34 exists, or return error. (Complexities, as usual, crop up when we
35 try for efficiency...)
37 Each room is basically just a list of message numbers. Each time
38 we enter a new message in a room, its message number is appended to the end
39 of the list. If an old message is to be expired, we must delete it from the
40 message base. Reading a room is just a matter of looking up the messages
41 one by one and sending them to the client for display, printing, or whatever.
43 Implementing the "new message" function is also trivial in principle:
44 we just keep track, for each caller in the userlog, of the highest-numbered
45 message which existed on the *last* call. (Remember, message numbers are
46 simply assigned sequentially each time a message is created. This
47 sequence is global to the entire system, not local within a room.) If
48 we ignore all message-numbers in the room less than this, only new messages
49 will be printed. Voila!
52 Message format on disk (MSGMAIN)
54 As discussed above, each message begins with an FF byte.
56 The next byte denotes whether this is an anonymous message. The codes
57 available are MES_NORMAL, MES_ANON, or MES_AN2 (defined in citadel.h).
59 The third byte is a "message type" code. The following codes are defined:
60 0 - "Traditional" Citadel format. Message is to be displayed "formatted."
61 1 - Plain pre-formatted ASCII text (otherwise known as text/plain)
62 4 - MIME formatted message. The text of the message which follows is
63 expected to begin with a "Content-type:" header.
65 After these three opening bytes, the remainder of
66 the message consists of a sequence of character strings. Each string
67 begins with a type byte indicating the meaning of the string and is
68 ended with a null. All strings are printable ASCII: in particular,
69 all numbers are in ASCII rather than binary. This is for simplicity,
70 both in implementing the system and in implementing other code to
71 work with the system. For instance, a database driven off Citadel archives
72 can do wildcard matching without worrying about unpacking binary data such
73 as message ID's first. To provide later downward compatability
74 all software should be written to IGNORE fields not currently defined.
76 The type bytes currently defined are:
78 BYTE Mnemonic Comments
80 A Author Name of originator of message.
81 D Destination Contains name of the system this message should
82 be sent to, for mail routing (private mail only).
83 E Extended ID A persistent alphanumeric Message ID used for
84 network replication. When a message arrives that
85 contains an Extended ID, any existing messages which
86 contain the same Extended ID and are *older* than this
87 message should be deleted. If there exist any messages
88 with the same Extended ID that are *newer*, then this
89 message should be dropped.
90 F rFc822 address For Internet mail, this is the delivery address of the
92 H HumanNodeName Human-readable name of system message originated on.
93 I Original ID A 32-bit integer containing the message ID on the
94 system the message *originated* on.
95 M Message Text Normal ASCII, newlines seperated by CR's or LF's,
96 null terminated as always.
97 N Nodename Contains node name of system message originated on.
98 O Room Room of origin.
99 P Path Complete path of message, as in the UseNet news
100 standard. A user should be able to send Internet mail
101 to this path. (Note that your system name will not be
102 tacked onto this until you're sending the message to
104 R Recipient Only present in Mail messages.
105 S Special field Only meaningful for messages being spooled over a
106 network. Usually means that the message isn't really
107 a message, but rather some other network function:
108 -> "S" followed by "FILE" (followed by a null, of
109 course) means that the message text is actually an
110 IGnet/Open file transfer.
111 -> "S" followed by "CANCEL" means that this message
112 should be deleted from the local message base once
113 it has been replicated to all network systems.
114 T Date/Time A 32-bit integer containing the date and time of
115 the message in standard UNIX format (the number
116 of seconds since January 1, 1970 GMT).
117 U Subject Optional. Developers may choose whether they wish to
118 generate or display subject fields. Citadel/UX does
119 not generate them, but it does print them when found.
120 0 Error This field is typically never found in a message on
121 disk or in transit. Message scanning modules are
122 expected to fill in this field when rejecting a message
123 with an explanation as to what happened (virus found,
124 message looks like spam, etc.)
128 Let <FF> be a 0xFF byte, and <0> be a null (0x00) byte. Then a message
131 Apr 12, 1988 23:16 From Test User In Network Test> @lifesys (Life BBS)
134 might be stored as...
135 <FF><40><0>I12345<0>Pneighbor!lifesys!test_user<0>T576918988<0> (continued)
136 -----------|Mesg ID#|--Message Path---------------|--Date------
138 AThe Test User<0>ONetwork Test<0>Nlifesys<0>HLife BBS<0>MHave a nice day!<0>
139 |-----Author-----|-Room name-----|-nodename-|Human Name-|--Message text-----
141 Weird things can happen if fields are missing, especially if you use the
142 networker. But basically, the date, author, room, and nodename may be in any
143 order. But the leading fields and the message text must remain in the same
144 place. The H field looks better when it is placed immediately after the N
149 Citadel nodes network by sharing one or more rooms. Any Citadel node
150 can choose to share messages with any other Citadel node, through the sending
151 of spool files. The sending system takes all messages it hasn't sent yet, and
152 spools them to the recieving system, which posts them in the rooms.
154 Complexities arise primarily from the possibility of densely connected
155 networks: one does not wish to accumulate multiple copies of a given
156 message, which can easily happen. Nor does one want to see old messages
157 percolating indefinitely through the system.
159 This problem is handled by keeping track of the path a message has taken over
160 the network, like the UseNet news system does. When a system sends out a
161 message, it adds its own name to the bang-path in the <P> field of the
162 message. If no path field is present, it generates one.
164 With the path present, all the networker has to do to assure that it doesn't
165 send another system a message it's already received is check the <P>ath field
166 for that system's name somewhere in the bang path. If it's present, the system
167 has already seen the message, so we don't send it. (Note that the current
168 implementation does not allow for "loops" in the network -- if you build your
169 net this way you will see lots of duplicate messages.)
171 The above discussion should make the function of the fields reasonably clear:
173 o Travelling messages need to carry original message-id, system of origin,
174 date of origin, author, and path with them, to keep reproduction and
175 cycling under control.
177 (Uncoincidentally) the format used to transmit messages for networking
178 purposes is precisely that used on disk, serialized. The current
179 distribution includes serv_network.c, which is basically a database replicator;
180 please see network.txt on its operation and functionality (if any).
184 Citadel/UX is 64-bit clean, architecture-independent, and Year 2000
185 compliant. The software should compile on any POSIX compliant system with
186 a full pthreads implementation and TCP/IP support. In the future we may
187 try to port it to non-POSIX systems as well.
189 On the client side, it's also POSIX compliant. The client even seems to
190 build ok on non-POSIX systems with porting libraries (such as Cygwin).
193 "Room" records (quickroom)
195 The rooms are basically indices into msgmain, the message database.
196 As noted in the overview, each is essentially an array of pointers into
197 the message file. The pointers consist of a 32-bit message ID number
198 (we will wrap around at 32 bits for these purposes).
200 Since messages are numbered sequentially, the
201 set of messages existing in msgmain will always form a continuous
202 sequence at any given time.
204 That should be enough background to tackle a full-scale room. From citadel.h:
207 char QRname[20]; /* Max. len is 19, plus null term */
208 char QRpasswd[10]; /* Only valid if it's a private rm */
209 long QRroomaide; /* User number of room aide */
210 long QRhighest; /* Highest message NUMBER in room */
211 long QRgen; /* Generation number of room */
212 unsigned QRflags; /* See flag values below */
213 char QRdirname[15]; /* Directory name, if applicable */
214 char QRfloor; /* (not yet implemented) */
217 #define QR_BUSY 1 /* Room is being updated, WAIT */
218 #define QR_INUSE 2 /* Set if in use, clear if avail */
219 #define QR_PRIVATE 4 /* Set for any type of private room */
220 #define QR_PASSWORDED 8 /* Set if there's a password too */
221 #define QR_GUESSNAME 16 /* Set if it's a guessname room */
222 #define QR_DIRECTORY 32 /* Directory room */
223 #define QR_UPLOAD 64 /* Allowed to upload */
224 #define QR_DOWNLOAD 128 /* Allowed to download */
225 #define QR_VISDIR 256 /* Visible directory */
226 #define QR_ANONONLY 512 /* Anonymous-Only room */
227 #define QR_ANON2 1024 /* Anonymous-Option room */
228 #define QR_NETWORK 2048 /* Shared network room */
229 #define QR_PREFONLY 4096 /* Preferred users only */
231 [Note that all components start with "QR" for quickroom, to make sure we
232 don't accidentally use an offset in the wrong structure. Be very careful
233 also to get a meaningful sequence of components --
234 some C compilers don't check this sort of stuff either.]
236 QRgen handles the problem of rooms which have died and been reborn
237 under another name. This will be clearer when we get to the userlog.
238 For now, just note that each room has a generation number which is
239 bumped by one each time it is recycled.
241 QRflags is just a bag of bits recording the status of the room. The
244 QR_BUSY This is to insure that two processes don't update the same
245 record at the same time, even though this hasn't been
247 QR_INUSE 1 if the room is valid, 0 if it is free for re-assignment.
248 QR_PRIVATE 1 if the room is not visible by default, 0 for public.
249 QR_PASSWORDED 1 if entry to the room requires a password.
250 QR_GUESSNAME 1 if the room can be reached by guessing the name.
251 QR_DIRECTORY 1 if the room is a window onto some disk/userspace, else 0.
252 QR_UPLOAD 1 if users can upload into this room, else 0.
253 QR_DOWNLOAD 1 if users can download from this room, else 0.
254 QR_VISDIR 1 if users are allowed to read the directory, else 0.
255 QR_ANONONLY 1 if all messages are to recieve the "****" anon header.
256 QR_ANON2 1 if the user will be asked if he/she wants an anon message.
257 QR_NETWORK 1 if this room is shared on a network, else 0.
258 QR_PREFONLY 1 if the room is only accessible to preferred users, else 0.
260 QRname is just an ASCII string (null-terminated, like all strings)
261 giving the name of the room.
263 QRdirname is meaningful only in QR_DIRECTORY rooms, in which case
264 it gives the directory name to window.
266 QRpasswd is the room's password, if it's a QR_PASSWORDED room. Note that
267 if QR_PASSWORDED or QR_GUESSNAME are set, you MUST also set QR_PRIVATE.
268 QR_PRIVATE by itself designates invitation-only. Do not EVER set all three
269 flags at the same time.
271 QRroomaide is the user number of the room's room-aide (or zero if the room
272 doesn't have a room aide). Note that if a user is deleted, his/her user number
273 is never used again, so you don't have to worry about a new user getting the
274 same user number and accidentally becoming a room-aide of one or more rooms.
276 The only field new to us in quickroom is QRhighest, recording the
277 most recent message in the room. When we are searching for rooms with
278 messages a given caller hasn't seen, we can check this number
279 and avoid a whole lot of extra disk accesses.
281 There used to also be a structure called "fullroom" which resided in one
282 file for each room on the system. This has been abandoned in favour of
283 "message lists" which are variable sized and simply contain zero or more
284 message numbers. The message numbers, in turn, point to messages on disk.
286 User records (usersupp)
288 This is the fun one. Get some fresh air and plug in your thinking cap
289 first. (Time, space and complexity are the eternal software rivals.
290 We've got lots of log entries times lots of messages spread over up to nnn
291 rooms to worry about, and with multitasking, disk access time is important...
292 so perforce, we opt for complexity to keep time and space in bounds.)
294 To understand what is happening in the log code takes a little persistence.
295 You also have to disentangle the different activities going on and
296 tackle them one by one.
298 o We want to remember some random things such as terminal screen
299 size, and automatically set them up for each caller at login.
301 o We want to be able to locate all new messages, and only new
302 messages, efficiently. Messages should stay new even if it
303 takes a caller a couple of calls to get around to them.
305 o We want to remember which private rooms a given caller knows
306 about, and treat them as normal rooms. This means mostly
307 automatically seeking out those with new messages. (Obviously,
308 we >don't< want to do this for unknown private rooms!) This
309 has to be secure against the periodic recycling of rooms
312 o We want to support private mail to a caller.
314 o We want to provide some protection of this information (via
315 passwords at login) and some assurance that messages are from
316 who they purport to be from (within the system -- one shouldn't
317 be able to forge messages from established users).
319 Lifting another page from citadel.h gives us:
321 struct usersupp { /* User record */
322 int USuid; /* uid account is logged in under */
323 char password[20]; /* password */
324 long lastseen[MAXROOMS]; /* Last message seen in each room */
325 char generation[MAXROOMS]; /* Generation # (for private rooms) */
326 char forget[MAXROOMS]; /* Forgotten generation number */
327 unsigned flags; /* See US_ flags below */
328 int screenwidth; /* For formatting messages */
329 int timescalled; /* Total number of logins */
330 int posted; /* Number of messages posted (ever) */
331 char fullname[26]; /* Bulletin Board name for messages */
332 char axlevel; /* Access level */
333 long usernum; /* Eternal user number */
334 long lastcall; /* Last time the user called */
337 #define US_PERM 1 /* Permanent user; don't scroll off */
338 #define US_LASTOLD 16 /* Print last old message with new */
339 #define US_EXPERT 32 /* Experienced user */
340 #define US_UNLISTED 64 /* Unlisted userlog entry */
341 #define US_NOPROMPT 128 /* Don't prompt after each message */
342 #define US_PREF 1024 /* Preferred user */
344 Looks simple enough, doesn't it? One topic at a time:
346 Random configuration parameters:
347 -screenwidth is the caller's screen width. We format all messages to this
348 width, as best we can. flags is another bit-bag, recording whether we want
349 prompts, people who want to suppress the little automatic hints all through
352 Attachments, names & numbers:
353 -USuid is the uid the account was established under. For most users it will
354 be the same as BBSUID, but it won't be for users that logged in from the shell.
355 -fullname is the user's full login name.
356 -usernum is the user's ID number. It is unique to the entire system:
357 once someone has a user number, it is never used again after the user is
358 deleted. This allows an easy way to numerically represent people.
359 -password is the user's password.
360 -axlevel is the user's access level, so we know who's an Aide, who's a problem
361 user, etc. These are defined and listed in the system.
364 -timescalled is the number of times the user has called.
365 -posted is the number of messages the user has posted, public or private.
368 -lastcall holds the date and time (standard Unix format) the user called, so
369 we can purge people who haven't called in a given amount of time.
371 Finding new messages:
372 This is the most important. Thus, it winds up being the most
373 elaborate. Conceptually, what we would like to do is mark each
374 message with a bit after our caller has read it, so we can avoid
375 printing it out again next call. Unfortunately, with lots of user
376 entries this would require adding lots of bits to each message... and
377 we'd wind up reading off disk lots of messages which would never
378 get printed. So we resort to approximation and a small table.
380 The approximation comes in doing things at the granularity of
381 rooms rather than messages. Messages in a given room are "new"
382 until we visit it, and "old" after we leave the room... whether
383 we read any of them or not. This can actually be defended: anyone
384 who passes through a room without reading the contents probably just
385 isn't interested in the topic, and would just as soon not be dragged
386 back every visit and forced to read them. Given that messages are
387 numbered sequentially, we can simply record the most recent message ID#
388 of each room as of the last time we visited it. Very simple.
390 Putting it all together, we can now compute whether a given room
391 has new messages for our current caller without going to the message base
392 index (fullroom) at all:
394 > We get the usersupp.lastseen[] for the room in question
395 > We compare this with the room's quickroom.QRhighest, which tells us
396 what the most recent message in the room is currently.
399 REMEMBERING WHICH PRIVATE ROOMS TO VISIT
401 This looks trivial at first glance -- just record one bit per room per
402 caller in the log records. The problem is that rooms get recycled
403 periodically, and we'd rather not run through all the log entries each
404 time we do it. So we adopt a kludge which should work 99% of the time.
406 As previously noted, each room has a generation number, which is bumped
407 by one each time it is recycled. As not noted, this generation number
408 runs from 0 -> 127 (and then wraps around and starts over).
409 When someone visits a room, we set usersupp.generation for the room
410 equal to that of the room. This flags the room as being available.
411 If the room gets recycled, on our next visit the two generation numbers
412 will no longer match, and the room will no longer be available -- just
413 the result we're looking for. (Naturally, if a room is public,
414 all this stuff is irrelevant.)
416 This leaves only the problem of an accidental matchup between the two
417 numbers giving someone access to a Forbidden Room. We can't eliminate
418 this danger completely, but it can be reduced to insignificance for
419 most purposes. (Just don't bet megabucks on the security of this system!)
420 Each time someone logs in, we set all "wrong" generation numbers to -1.
421 So the room must be recycled 127 times before an accidental matchup
422 can be achieved. (We do this for all rooms, INUSE or dead, public
423 or private, since any of them may be reincarnated as a Forbidden Room.)
425 Thus, for someone to accidentally be led to a Forbidden Room, they
426 must establish an account on the system, then not call until some room
427 has been recycled 127 to 128 times, which room must be
428 reincarnated as a Forbidden Room, which someone must now call back
429 (having not scrolled off the userlog in the mean time) and read new
430 messages. The last clause is about the only probable one in the sequence.
431 The danger of this is much less than the danger that someone will
432 simply guess the name of the room outright (if it's a guess-name room)
433 or some other human loophole.
437 This is exactly the opposite of private rooms. When a user chooses to
438 forget a room, we put the room's generation number in usersupp.forget for
439 that room. When doing a <K>nown rooms list or a <G>oto, any matchups cause
440 the room to be skipped. Very simple.
442 SUPPORTING PRIVATE MAIL
444 Can one have an elegant kludge? This must come pretty close.
446 Private mail is sent and recieved in the Mail> room, which otherwise
447 behaves pretty much as any other room. To make this work, we have a
448 separate Mail> room for each user behind the scenes. The actual room name
449 in the database looks like "0000001234.Mail" (where '1234' is the user
450 number) and it's flagged with the QR_MAILBOX flag. The user number is
451 stripped off by the server before the name is presented to the client.
453 This requires a little fiddling to get things just right. For example,
454 make_message() has to be kludged to ask for the name of the recipient
455 of the message whenever a message is entered in Mail>. But basically
456 it works pretty well, keeping the code and user interface simple and
460 PASSWORDS AND NAME VALIDATION
462 This has changed a couple of times over the course of Citadel's history. At
463 this point it's very simple, again due to the fact that record managers are
464 used for everything. The user file (usersupp) is indexed using the user's
465 name, converted to all lower-case. Searching for a user, then, is easy. We
466 just lowercase the name we're looking for and query the database. If no
467 match is found, it is assumed that the user does not exist.
469 This makes it difficult to forge messages from an existing user. (Fine
470 point: nonprinting characters are converted to printing characters, and
471 leading, trailing, and double blanks are deleted.)
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