1 hack.txt for Citadel/UX
2 written by Art Cancro (ajc@uncnsrd.mt-kisco.ny.us)
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9 Note that this document is really out of date.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 B Phone number The dialup number of the system this message
82 originated on. This is optional, and is only
83 defined for helping implement C86Net gateways.
84 D Destination Contains name of the system this message should
85 be sent to, for mail routing (private mail only).
86 G Gateway domain This field is provided solely for the implementation
87 of C86Net gateways, and holds the C86Net domain of
88 the system this message originated on. Unless you're
89 implementing such a gateway, there's no need to even
90 bother with this field.
91 H HumanNodeName Human-readable name of system message originated on.
92 I Original ID A 32-bit integer containing the message ID on the
93 system the message *originated* on.
94 M Message Text Normal ASCII, newlines seperated by CR's or LF's,
95 null terminated as always.
96 N Nodename Contains node name of system message originated on.
97 O Room Room of origin.
98 P Path Complete path of message, as in the UseNet news
99 standard. A user should be able to send Internet mail
100 to this path. (Note that your system name will not be
101 tacked onto this until you're sending the message to
103 R Recipient Only present in Mail messages.
104 S Special field Only meaningful for messages being spooled over a
105 network. Usually means that the message isn't really
106 a message, but rather some other network function:
107 -> "S" followed by "FILE" (followed by a null, of
108 course) means that the message text is actually an
109 IGnet/Open file transfer.
110 T Date/Time A 32-bit integer containing the date and time of
111 the message in standard UNIX format (the number
112 of seconds since January 1, 1970 GMT).
113 U Subject Optional. Developers may choose whether they wish to
114 generate or display subject fields. Citadel/UX does
115 not generate them, but it does print them when found.
116 Z Zap (supersede) Indicates a message which this message supersedes.
117 The format is "user@node" and if the specified message
118 is found, it should be deleted.
122 Let <FF> be a 0xFF byte, and <0> be a null (0x00) byte. Then a message
125 Apr 12, 1988 23:16 From Test User In Network Test> @lifesys (Life BBS)
128 might be stored as...
129 <FF><40><0>I12345<0>Pneighbor!lifesys!test_user<0>T576918988<0> (continued)
130 -----------|Mesg ID#|--Message Path---------------|--Date------
132 AThe Test User<0>ONetwork Test<0>Nlifesys<0>HLife BBS<0>MHave a nice day!<0>
133 |-----Author-----|-Room name-----|-nodename-|Human Name-|--Message text-----
135 Weird things can happen if fields are missing, especially if you use the
136 networker. But basically, the date, author, room, and nodename may be in any
137 order. But the leading fields and the message text must remain in the same
138 place. The H field looks better when it is placed immediately after the N
143 Citadel nodes network by sharing one or more rooms. Any Citadel node
144 can choose to share messages with any other Citadel node, through the sending
145 of spool files. The sending system takes all messages it hasn't sent yet, and
146 spools them to the recieving system, which posts them in the rooms.
148 Complexities arise primarily from the possibility of densely connected
149 networks: one does not wish to accumulate multiple copies of a given
150 message, which can easily happen. Nor does one want to see old messages
151 percolating indefinitely through the system.
153 This problem is handled by keeping track of the path a message has taken over
154 the network, like the UseNet news system does. When a system sends out a
155 message, it adds its own name to the bang-path in the <P> field of the
156 message. If no path field is present, it generates one.
158 With the path present, all the networker has to do to assure that it doesn't
159 send another system a message it's already received is check the <P>ath field
160 for that system's name somewhere in the bang path. If it's present, the system
161 has already seen the message, so we don't send it. (Note that the current
162 implementation does not allow for "loops" in the network -- if you build your
163 net this way you will see lots of duplicate messages.)
165 The above discussion should make the function of the fields reasonably clear:
167 o Travelling messages need to carry original message-id, system of origin,
168 date of origin, author, and path with them, to keep reproduction and
169 cycling under control.
171 (Uncoincidentally) the format used to transmit messages for networking
172 purposes is precisely that used on disk, except that there may be any amount
173 of garbage between the null ending a message and the <FF> starting the next
174 one. This allows greater compatibility if slight problems crop up. The current
175 distribution includes netproc.c, which is basically a database replicator;
176 please see network.txt on its operation and functionality (if any).
180 At this point, all hardware-dependent stuff has been removed from the
181 system. On the server side, most of the OS-dependent stuff has been isolated
182 into the sysdep.c source module. The server should compile on any POSIX
183 compliant system with a full pthreads implementation and TCP/IP support. In
184 the future, we may try to port it to non-POSIX systems as well.
186 On the client side, it's also POSIX compliant. The client even seems to
187 build ok on non-POSIX systems with porting libraries (such as the Cygnus
191 "Room" records (quickroom)
193 The rooms are basically indices into msgmain, the message database.
194 As noted in the overview, each is essentially an array of pointers into
195 the message file. The pointers consist of a 32-bit message ID number
196 (we will wrap around at 32 bits for these purposes).
198 Since messages are numbered sequentially, the
199 set of messages existing in msgmain will always form a continuous
200 sequence at any given time.
202 That should be enough background to tackle a full-scale room. From citadel.h:
205 char QRname[20]; /* Max. len is 19, plus null term */
206 char QRpasswd[10]; /* Only valid if it's a private rm */
207 long QRroomaide; /* User number of room aide */
208 long QRhighest; /* Highest message NUMBER in room */
209 long QRgen; /* Generation number of room */
210 unsigned QRflags; /* See flag values below */
211 char QRdirname[15]; /* Directory name, if applicable */
212 char QRfloor; /* (not yet implemented) */
215 #define QR_BUSY 1 /* Room is being updated, WAIT */
216 #define QR_INUSE 2 /* Set if in use, clear if avail */
217 #define QR_PRIVATE 4 /* Set for any type of private room */
218 #define QR_PASSWORDED 8 /* Set if there's a password too */
219 #define QR_GUESSNAME 16 /* Set if it's a guessname room */
220 #define QR_DIRECTORY 32 /* Directory room */
221 #define QR_UPLOAD 64 /* Allowed to upload */
222 #define QR_DOWNLOAD 128 /* Allowed to download */
223 #define QR_VISDIR 256 /* Visible directory */
224 #define QR_ANONONLY 512 /* Anonymous-Only room */
225 #define QR_ANON2 1024 /* Anonymous-Option room */
226 #define QR_NETWORK 2048 /* Shared network room */
227 #define QR_PREFONLY 4096 /* Preferred users only */
229 [Note that all components start with "QR" for quickroom, to make sure we
230 don't accidentally use an offset in the wrong structure. Be very careful
231 also to get a meaningful sequence of components --
232 some C compilers don't check this sort of stuff either.]
234 QRgen handles the problem of rooms which have died and been reborn
235 under another name. This will be clearer when we get to the userlog.
236 For now, just note that each room has a generation number which is
237 bumped by one each time it is recycled.
239 QRflags is just a bag of bits recording the status of the room. The
242 QR_BUSY This is to insure that two processes don't update the same
243 record at the same time, even though this hasn't been
245 QR_INUSE 1 if the room is valid, 0 if it is free for re-assignment.
246 QR_PRIVATE 1 if the room is not visible by default, 0 for public.
247 QR_PASSWORDED 1 if entry to the room requires a password.
248 QR_GUESSNAME 1 if the room can be reached by guessing the name.
249 QR_DIRECTORY 1 if the room is a window onto some disk/userspace, else 0.
250 QR_UPLOAD 1 if users can upload into this room, else 0.
251 QR_DOWNLOAD 1 if users can download from this room, else 0.
252 QR_VISDIR 1 if users are allowed to read the directory, else 0.
253 QR_ANONONLY 1 if all messages are to recieve the "****" anon header.
254 QR_ANON2 1 if the user will be asked if he/she wants an anon message.
255 QR_NETWORK 1 if this room is shared on a network, else 0.
256 QR_PREFONLY 1 if the room is only accessible to preferred users, else 0.
258 QRname is just an ASCII string (null-terminated, like all strings)
259 giving the name of the room.
261 QRdirname is meaningful only in QR_DIRECTORY rooms, in which case
262 it gives the directory name to window.
264 QRpasswd is the room's password, if it's a QR_PASSWORDED room. Note that
265 if QR_PASSWORDED or QR_GUESSNAME are set, you MUST also set QR_PRIVATE.
266 QR_PRIVATE by itself designates invitation-only. Do not EVER set all three
267 flags at the same time.
269 QRroomaide is the user number of the room's room-aide (or zero if the room
270 doesn't have a room aide). Note that if a user is deleted, his/her user number
271 is never used again, so you don't have to worry about a new user getting the
272 same user number and accidentally becoming a room-aide of one or more rooms.
274 The only field new to us in quickroom is QRhighest, recording the
275 most recent message in the room. When we are searching for rooms with
276 messages a given caller hasn't seen, we can check this number
277 and avoid a whole lot of extra disk accesses.
279 There used to also be a structure called "fullroom" which resided in one
280 file for each room on the system. This has been abandoned in favour of
281 "message lists" which are variable sized and simply contain zero or more
282 message numbers. The message numbers, in turn, point to messages on disk.
284 User records (usersupp)
286 This is the fun one. Get some fresh air and plug in your thinking cap
287 first. (Time, space and complexity are the eternal software rivals.
288 We've got lots of log entries times lots of messages spread over up to nnn
289 rooms to worry about, and with multitasking, disk access time is important...
290 so perforce, we opt for complexity to keep time and space in bounds.)
292 To understand what is happening in the log code takes a little persistence.
293 You also have to disentangle the different activities going on and
294 tackle them one by one.
296 o We want to remember some random things such as terminal screen
297 size, and automatically set them up for each caller at login.
299 o We want to be able to locate all new messages, and only new
300 messages, efficiently. Messages should stay new even if it
301 takes a caller a couple of calls to get around to them.
303 o We want to remember which private rooms a given caller knows
304 about, and treat them as normal rooms. This means mostly
305 automatically seeking out those with new messages. (Obviously,
306 we >don't< want to do this for unknown private rooms!) This
307 has to be secure against the periodic recycling of rooms
310 o We want to support private mail to a caller.
312 o We want to provide some protection of this information (via
313 passwords at login) and some assurance that messages are from
314 who they purport to be from (within the system -- one shouldn't
315 be able to forge messages from established users).
317 Lifting another page from citadel.h gives us:
319 struct usersupp { /* User record */
320 int USuid; /* uid account is logged in under */
321 char password[20]; /* password */
322 long lastseen[MAXROOMS]; /* Last message seen in each room */
323 char generation[MAXROOMS]; /* Generation # (for private rooms) */
324 char forget[MAXROOMS]; /* Forgotten generation number */
325 unsigned flags; /* See US_ flags below */
326 int screenwidth; /* For formatting messages */
327 int timescalled; /* Total number of logins */
328 int posted; /* Number of messages posted (ever) */
329 char fullname[26]; /* Bulletin Board name for messages */
330 char axlevel; /* Access level */
331 long usernum; /* Eternal user number */
332 long lastcall; /* Last time the user called */
335 #define US_PERM 1 /* Permanent user; don't scroll off */
336 #define US_LASTOLD 16 /* Print last old message with new */
337 #define US_EXPERT 32 /* Experienced user */
338 #define US_UNLISTED 64 /* Unlisted userlog entry */
339 #define US_NOPROMPT 128 /* Don't prompt after each message */
340 #define US_PREF 1024 /* Preferred user */
342 Looks simple enough, doesn't it? One topic at a time:
344 Random configuration parameters:
345 -screenwidth is the caller's screen width. We format all messages to this
346 width, as best we can. flags is another bit-bag, recording whether we want
347 prompts, people who want to suppress the little automatic hints all through
350 Attachments, names & numbers:
351 -USuid is the uid the account was established under. For most users it will
352 be the same as BBSUID, but it won't be for users that logged in from the shell.
353 -fullname is the user's full login name.
354 -usernum is the user's ID number. It is unique to the entire system:
355 once someone has a user number, it is never used again after the user is
356 deleted. This allows an easy way to numerically represent people.
357 -password is the user's password.
358 -axlevel is the user's access level, so we know who's an Aide, who's a problem
359 user, etc. These are defined and listed in the system.
362 -timescalled is the number of times the user has called.
363 -posted is the number of messages the user has posted, public or private.
366 -lastcall holds the date and time (standard Unix format) the user called, so
367 we can purge people who haven't called in a given amount of time.
369 Finding new messages:
370 This is the most important. Thus, it winds up being the most
371 elaborate. Conceptually, what we would like to do is mark each
372 message with a bit after our caller has read it, so we can avoid
373 printing it out again next call. Unfortunately, with lots of user
374 entries this would require adding lots of bits to each message... and
375 we'd wind up reading off disk lots of messages which would never
376 get printed. So we resort to approximation and a small table.
378 The approximation comes in doing things at the granularity of
379 rooms rather than messages. Messages in a given room are "new"
380 until we visit it, and "old" after we leave the room... whether
381 we read any of them or not. This can actually be defended: anyone
382 who passes through a room without reading the contents probably just
383 isn't interested in the topic, and would just as soon not be dragged
384 back every visit and forced to read them. Given that messages are
385 numbered sequentially, we can simply record the most recent message ID#
386 of each room as of the last time we visited it. Very simple.
388 Putting it all together, we can now compute whether a given room
389 has new messages for our current caller without going to the message base
390 index (fullroom) at all:
392 > We get the usersupp.lastseen[] for the room in question
393 > We compare this with the room's quickroom.QRhighest, which tells us
394 what the most recent message in the room is currently.
397 REMEMBERING WHICH PRIVATE ROOMS TO VISIT
399 This looks trivial at first glance -- just record one bit per room per
400 caller in the log records. The problem is that rooms get recycled
401 periodically, and we'd rather not run through all the log entries each
402 time we do it. So we adopt a kludge which should work 99% of the time.
404 As previously noted, each room has a generation number, which is bumped
405 by one each time it is recycled. As not noted, this generation number
406 runs from 0 -> 127 (and then wraps around and starts over).
407 When someone visits a room, we set usersupp.generation for the room
408 equal to that of the room. This flags the room as being available.
409 If the room gets recycled, on our next visit the two generation numbers
410 will no longer match, and the room will no longer be available -- just
411 the result we're looking for. (Naturally, if a room is public,
412 all this stuff is irrelevant.)
414 This leaves only the problem of an accidental matchup between the two
415 numbers giving someone access to a Forbidden Room. We can't eliminate
416 this danger completely, but it can be reduced to insignificance for
417 most purposes. (Just don't bet megabucks on the security of this system!)
418 Each time someone logs in, we set all "wrong" generation numbers to -1.
419 So the room must be recycled 127 times before an accidental matchup
420 can be achieved. (We do this for all rooms, INUSE or dead, public
421 or private, since any of them may be reincarnated as a Forbidden Room.)
423 Thus, for someone to accidentally be led to a Forbidden Room, they
424 must establish an account on the system, then not call until some room
425 has been recycled 127 to 128 times, which room must be
426 reincarnated as a Forbidden Room, which someone must now call back
427 (having not scrolled off the userlog in the mean time) and read new
428 messages. The last clause is about the only probable one in the sequence.
429 The danger of this is much less than the danger that someone will
430 simply guess the name of the room outright (if it's a guess-name room)
431 or some other human loophole.
435 This is exactly the opposite of private rooms. When a user chooses to
436 forget a room, we put the room's generation number in usersupp.forget for
437 that room. When doing a <K>nown rooms list or a <G>oto, any matchups cause
438 the room to be skipped. Very simple.
440 SUPPORTING PRIVATE MAIL
442 Can one have an elegant kludge? This must come pretty close.
444 Private mail is sent and recieved in the Mail> room, which otherwise
445 behaves pretty much as any other room. To make this work, we have a
446 separate Mail> room for each user behind the scenes. The actual room name
447 in the database looks like "0000001234.Mail" (where '1234' is the user
448 number) and it's flagged with the QR_MAILBOX flag. The user number is
449 stripped off by the server before the name is presented to the client.
451 This requires a little fiddling to get things just right. For example,
452 make_message() has to be kludged to ask for the name of the recipient
453 of the message whenever a message is entered in Mail>. But basically
454 it works pretty well, keeping the code and user interface simple and
458 PASSWORDS AND NAME VALIDATION
460 This has changed a couple of times over the course of Citadel's history. At
461 this point it's very simple, again due to the fact that record managers are
462 used for everything. The user file (usersupp) is indexed using the user's
463 name, converted to all lower-case. Searching for a user, then, is easy. We
464 just lowercase the name we're looking for and query the database. If no
465 match is found, it is assumed that the user does not exist.
467 This makes it difficult to forge messages from an existing user. (Fine
468 point: nonprinting characters are converted to printing characters, and
469 leading, trailing, and double blanks are deleted.)