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Here is a FAQ posting that I have worked up. It is intended to answer most questions about how UUCP works, what the UUCP protocols are, etc., in nauseating detail. It does not answer such useful questions as how to configure UUCP, how to make an X.25 PAD not trap ^P, etc., and other questions for which I am not necessarily qualified to provide answers. Let this posting serve as a challenge to other people to produce more FAQ answers, which can either be folded into this posting, or, more likely, form a separate posting. This is the first trial of this posting. I'll consider any comments, make a relatively final version, and start posting it every month. Enjoy! Frequently Asked Questions about UUCP internals Last updated 1992-02-25 This article was written by Ian Lance Taylor <ian@airs.com> and I may even update it periodically. Send me mail about suggestions or inaccuracies. This article describes how the various UUCP protocols work. It does not describe how to configure UUCP, nor how to solve UUCP problems, nor how to deal with UUCP mail. There are currently no FAQ postings on any of these topics, and I do not plan to write any. If you haven't read the news.announce.newusers articles, read them. I took a lot of the information from Jamie E. Hanrahan's paper in the Fall 1990 DECUS Symposium, and from Managing UUCP and Usenet by Tim O'Reilly and Grace Todino (with contributions by several other people). The latter includes most of the former, and is published by O'Reilly & Associates, Inc. 632 Petaluma Avenua Sebastopol, CA 95472 It is currently in its tenth edition. The ISBN number is 0-937175-48-X. Some information is originally due to a Usenet article by Chuck Wegrzyn. The information on the 'g' protocol comes partially from a paper by G.L. Chesson of Bell Laboratories, partially from Jamie E. Hanrahan's paper, and partially from source code by John Gilmore. The information on the 'f' protocol comes from the source code by Piet Berteema. The information on the 't' protocol comes from the source code by Rick Adams. The information on the 'e' protocol comes from a Usenet article by Matthias Urlichs. This article answers the following questions. If one of these questions is posted to comp.mail.uucp, you are encouraged to send mail to the poster referring her or him to this FAQ. There is no reason to post a followup, as most of us know the answer already. *) What is the UUCP protocol? *) What is the 'g' protocol? *) What is the 'f' protocol? *) What is the 't' protocol? *) What is the 'e' protocol? *) What is the 'x' protocol? *) What is the UUCP protocol? The UUCP protocol is a conversation between two UUCP packages. A UUCP conversation consists of three parts: an initial handshake, a series of file transfer requests, and a final handshake. Before the initial handshake, the caller will usually have logged in the called machine and somehow started the UUCP package there. On Unix this is normally done by setting the shell of the login name used to /usr/lib/uucp/uucico. All messages in the initial handshake begin with a ^P (a byte with the octal value \020) and end with a null byte (\000). The initial handshake goes as follows. It is begun by the called machine. called: \020Shere=hostname\000 The hostname is the UUCP name of the called machine. Older UUCP packages do not output it, and simply send \020Shere\000. caller: \020Shostname options\000 The hostname is the UUCP name of the calling machine. The following options may appear (or there may be none): -QSEQ Report sequence number for this conversation. The sequence number is stored at both sites, and incremented after each call. If there is a sequence number mismatch, something has gone wrong (somebody may have broken security by pretending to be one of the machines) and the call is denied. -xLEVEL Requests the called system to set its debugging level to the specified value. This is not supported by all systems. -pGRADE -vgrade=GRADE Requests the called system to only transfer files of the specified grade or higher. This is not supported by all systems. Some systems support -p, some support -vgrade=. -R Indicates that the calling UUCP understands how to restart failed file transmissions. Supported only by System V Release 4 UUCP. -ULIMIT Reports the ulimit value of the calling UUCP. The limit is specified as a base 16 number in C notation (e.g. -U0x1000000). This number is the number of 512 byte blocks in the largest file which the calling UUCP can create. The called UUCP may not transfer a file larger than this. Supported only by System V Release 4 UUCP. -N Indicates that the calling UUCP understands the Taylor UUCP size limiting extensions. Supported only by Taylor UUCP. called: \020ROK\000 There are actually several possible responses. ROK The calling UUCP is acceptable, and the handshake proceeds to the protocol negotiation. Some options may also appear; see below. ROKN The calling UUCP is acceptable, it specified -N, and the called UUCP understands the Taylor UUCP size limiting extensions. Supported only by Taylor UUCP. RLCK The called UUCP already has a lock for the calling UUCP, which normally indicates the two machines are already communicating. RCB The called UUCP will call back. This may be used to avoid impostors (but only one machine out of each pair should call back, or no conversation will ever begin). RBADSEQ The call sequence number is wrong (see the -Q discussion above). RLOGIN The calling UUCP is using the wrong login name. RYou are unknown to me The calling UUCP is not known to the called UUCP, and the called UUCP does not permit connections from unknown systems. If the response is ROK, the following options are supported by System V Release 4 UUCP. -R The called UUCP knows how to restart failed file transmissions. -ULIMIT Reports the ulimit value of the called UUCP. The limit is specified as a base 16 number in C notation. This number is the number of 512 byte blocks in the largest file which the called UUCP can create. The calling UUCP may not send a file larger than this. -xLEVEL I'm not sure about this one. It may request the calling UUCP to set its debugging level to the specified value. If the response is not ROK (or ROKN) both sides hang up the phone, abandoning the call. called: \020Pprotocols\000 Note that the called UUCP outputs two strings in a row. The protocols string is a list of UUCP protocols supported by the caller. Each UUCP protocol has a single character name. These protocols are discussed in more detail later in this document. For example, the called UUCP might send \020Pgf\000. caller: \020Uprotocol\000 The calling UUCP selects which protocol to use out of the protocols offered by the called UUCP. If there are no mutually supported protocols, the calling UUCP sends \020UN\000 and both sides hang up the phone. Otherwise the calling UUCP sends something like \020Ug\000. At this point the initial handshake has been completed, and both sides turn on the selected protocol. For some protocols (notably 'g') a further handshake is done at this point. Each protocol supports a method for sending a command to the remote system. This method is used to transmit a series of commands between the two UUCP packages. At all times, one package is the master and the other is the slave. Initially, the calling UUCP is the master. If a protocol error occurs during the exchange of commands, both sides move immediately to the final handshake. The master will send one of four commands: S, R, X or H. Any file name referred to below is either an absolute pathname beginning with "/", a public directory pathname beginning with "~/", a pathname relative to a user's home directory beginning with "~USER/", or a spool directory file name. File names in the spool directory are not pathnames, but instead are converted to pathnames within the spool directory by UUCP. They always begin with "C." (for a command file created by uucp or uux), "D." (for a data file created by uucp, uux or by an execution, or received from another system for an execution), or "X." (for an execution file created by uux or received from another system). master: S FROM TO USER -OPTIONS TEMP MODE NOTIFY SIZE The S and the - are literal characters. This is a request by the master to send a file to the slave. FROM The name of the file to send. If the C option does not appear in OPTIONS, the master will actually open and send this file. Otherwise the file has been copied to the spool directory, where it is named TEMP. The slave ignores this field unless TO is a directory, in which case the basename of FROM will be used as the file name. If FROM is a spool directory filename, it must be a data file created for or by an execution, and must begin with "D.". TO The name to give the file on the slave. If this field names a directory (a name ending in '/' is taken to name a directory even if one does not already exist with that name) the file is placed within that directory with the basename of FROM. If TO begins with "X.", an execution file will be created on the slave. Otherwise, if TO begins with "D." it names a data file to be used by some execution file. Otherwise, TO may not be in the spool directory. USER The name of the user who requested the transfer. OPTIONS A list of options to control the transfer. The following options are defined (all options are single characters): C The file has been copied to the spool directory (the master should use TEMP rather than FROM). c The file has not been copied to the spool directory (this is the default). d The slave should create directories as necessary (this is the default). f The slave should not create directories if necessary, but should fail the transfer instead. m The master should send mail to USER when the transfer is complete. n The slave should send mail to NOTIFY when the transfer is complete. TEMP If the C option appears in OPTIONS, this names the file to be sent. Otherwise if FROM is in the spool directory, TEMP is the same as FROM. Otherwise TEMP is a dummy string, normally "D.0". After the transfer has been succesfully completed, the master will delete the file TEMP. MODE This is an octal number giving the mode of the file on MASTER. If the file is not in the spool directory, the slave will always create it with mode 0666, except that if (MODE & 0111) is not zero (the file is executable), the slave will create the file with mode 0777. If the file is in the spool directory, some UUCP packages will use the algorithm above and some will always create the file with mode 0600. NOTIFY This field is only present if the n option appears in OPTIONS. When the transfer is successfully completed, the slave will send mail to NOTIFY, which must be a legal mailing address on the slave. If a SIZE field will appear but the n option does not appear, NOTIFY will be the string "dummy" or simply a pair of double quotes. SIZE This field is only present when doing size negotiation, either with Taylor UUCP or SVR4 UUCP. It is the size of the file in bytes. SVR4 UUCP sends the size in base 16 as 0x.... while Taylor UUCP sends the size as a decimal integer. The slave then responds with an S command response. SY START The slave is willing to accept the file, and file transfer begins. The START field will only be present when using SVR4 file restart. It specifies the byte offset into the file at which to start sending. If this is a new file, START will be 0x0. SN2 The slave denies permission to transfer the file. This can mean that the destination directory may not be accessed, or that no requests are permitted. It implies that the file transfer will never succeed. SN4 The slave is unable to create the necessary temporary file. This implies that the file transfer may succeed later. SN6 This is only used by Taylor UUCP size negotiation. It means that the slave considers the file too large to transfer at the moment, but it may be possible to transfer it at some other time. SN7 This is only used by Taylor UUCP size negotiation. It means that the slave considers the file too large to ever transfer. If the slave responds with SY, a file transfer begins. When the file transfer is complete, the slave sends a C command response. CY The file transfer was successful. CN5 The temporary file could not be moved into the final location. This implies that the file transfer will never succeed. After the C command response has been received (in the SY case) or immediately (in an SN case) the master will send another command. master: R FROM TO USER -OPTIONS SIZE The R and the - are literal characters. This is a request by the master to receive a file from the slave. I do not know how SVR4 UUCP implements file transfer restart in this case. FROM This is the name of the file on the slave which the master wishes to receive. It must not be in the spool directory, and it may not contain any wildcards. TO This is the name of the file to create on the master. I do not believe that it can be a directory. It may only be in the spool directory if this file is being requested to support an execution either on the master or on some system other than the slave. USER The name of the user who requested the transfer. OPTIONS A list of options to control the transfer. The following options are defined (all options are single characters): d The master should create directories as necessary (this is the default). f The master should not create directories if necessary, but should fail the transfer instead. m The master should send mail to USER when the transfer is complete. SIZE This only appears if Taylor UUCP size negotiation is being used. It specifies the largest file which the master is prepared to accept (when using SVR4 UUCP, this was specified in the -U option during the initial handshake). The slave then responds with an R command response. RY MODE The slave is willing to send the file, and file transfer begins. MODE is the octal mode of the file on the slave. The master treats this just as the slave does the MODE argument in the send command, q.v. RN2 The slave is not willing to send the file, either because it is not permitted or because the file does not exist. This implies that the file request will never succeed. RN6 This is only used by Taylor UUCP size negotiation. It means that the file is too large to send, either because of the size limit specifies by the master or because the slave considers it too large. The file transfer may succeed later, or it may not (this will be cleared up in a later release of Taylor UUCP). If the slave responds with RY, a file transfer begins. When the file transfer is complete, the master sends a C command. The slave pretty much ignores this, although it may log it. CY The file transfer was successful. CN5 The temporary file could not be moved into the final location. After the C command response has been sent (in the RY case) or immediately (in an RN case) the master will send another command. master: X FROM TO USER -OPTIONS The X and the - are literal characters. This is a request by the master to, in essence, execute uucp on the slave. The slave should execute "uucp FROM TO". FROM This is the name of the file or files on the slave which the master wishes to transfer. It will often contain wildcard characters, since otherwise an R command will normally suffice (however, this command can also be used to request the transfer of a file on the slave to a third system). Any wildcards should be expanded on the slave. TO This is the name of the file or directory to which the files should be transferred. This will normally use a UUCP name. For example, if the master wishes to receive the files itself, it would use "master!path". USER The name of the user who requested the transfer. OPTIONS A list of options to control the transfer. It is not clear which, if any, options are supported by most UUCP packages. The slave then responds with an X command response. XY The request was accepted, and the appropriate file transfer commands have been queued up for later processing. XN The request was denied. No particular reason is given. In either case, the master will then send another command. master: H This is used by the master to hang up the connection. The slave will respond with an H command response. HY The slave agrees to hang up the connection. In this case the master sends another HY command. In some UUCP packages the slave will then send a third HY command. At this point the protocol is shut down, and the final handshake is begun. HN The slave does not agree to hang up. In this case the master and the slave exchange roles. The next command will be sent by the former slave, which is the new master. The roles may be reversed several times during a single connection. After the protocol has been shut down, the final handshake is performed. This handshake has no real purpose, and some UUCP packages simply drop the connection rather than do it (in fact, some will drop the connection immediately after both sides agree to hangup, without even closing down the protocl). caller: \020OOOOOO\000 called: \020OOOOOOO\000 That is, the calling UUCP sends six O's and the called UUCP replies with seven O's. Some UUCP packages always send six O's. *) What is the 'g' protocol? The 'g' protocol is a packet based flow controlled error correcting protocol that requires an eight bit clear connection. It is the original UUCP protocol, and is supported by all UUCP implementations. Many implementations of it are only able to support small window and packet sizes, specifically a window size of 3 and a packet size of 64 bytes, but the protocol itself can support up to a window size of 7 and a packet size of 4096 bytes. Complaints about the inefficiency of the 'g' protocol generally refer to specific implementations, rather than the correctly implemented protocol. The 'g' protocol was originally designed for general packet drivers, and thus contains some features that are not used by UUCP, including an alternate data channel and the ability to renegotiate packet and window sizes during the communication session. The 'g' protocol is spoofed by many Telebit modems. When spoofing is in effect, each Telebit modem uses the 'g' protocol to communicate with the attached computer, but the data between the modems is sent using a Telebit proprietary error correcting protocol. This allows for very high throughput over the Telebit connection, which, because it is half-duplex, would not normally be able to handle the 'g' protocol very well at all. This discussion of the 'g' protocol explains how it works, but does not discuss useful error handling techniques. Some discussion of this can be found in Jamie E. Hanrahan's paper, cited above. All 'g' protocol communication is done with packets. Each packet begins with a six byte header. Control packets consist only of the header. Data packets contain additional data. The header is as follows: \020 Every packet begins with a ^P. k (1 <= k <= 9) The k value is always 9 for a control packet. For a data packet, the k value indicates how must data follows the six byte header. The amount of data is 2 ** (k + 4), where ** indicates exponentiation. Thus a k value of 1 means 32 data bytes and a k value of 8 means 4096 data bytes. The k value for a data packet must be between 1 and 8 inclusive. checksum low byte checksum high byte The checksum value is described below. control byte The control packet indicates the type of packet, and is described below. xor byte This byte is the xor of k, the checksum low byte, the checksum high byte and the control byte (i.e. the second, third, fourth and fifth header bytes). It is used to ensure that the header data is valid. The control byte in the header is composed of three bit fields, referred to here as TT (two bits), XXX (three bits) and YYY (three bits). The control is TTXXXYYY, or (TT << 6) + (XXX << 3) + YYY. The TT field takes on the following values: 0 This is a control packet. In this case the k byte in the header must be 9. The XXX field indicates the type of control packet; these types are described below. 1 This is an alternate data channel packet. This is not used by UUCP. 2 This is a data packet, and the entire contents of the attached data field (whose length is given by the k byte in the header) are valid. The XXX and YYY fields are described below. 3 This is a short data packet. Let the length of the data field (as given by the k byte in the header) be L. Let the first byte in the data field be B1. If B1 is less than 128 (if the most significant bit of B1 is 0), then there are L - B1 valid bytes of data in the data field, beginning with the second byte. If B1 >= 128, let B2 be the second byte in the data field. Then there are L - ((B1 & 0x7f) + (B2 << 7)) valid bytes of data in the data field, beginning with the third byte. In all cases L bytes of data are sent (and all data bytes participate in the checksum calculation) but some of the trailing bytes may be dropped by the receiver. The XXX and YYY fields are described below. In a data packet (short or not) the XXX field gives the sequence number of the packet. Thus sequence numbers can range from 0 to 7, inclusive. The YYY field gives the sequence number of the last correctly received packet. Each communication direction uses a window which indicates how many unacknowledged packets may be transmitted before waiting for an acknowledgement (the window may be different in each direction). The window may range from 1 to 7. For example, if the window is 3 and the last packet acknowledged was packet number 6, packet numbers 7, 0 and 1 may be sent but the sender must wait for an acknowledgement before sending packet number 2. This acknowledgement could come as the YYY field of a data packet or as the YYY field of a RJ or RR control packet (described below). Each packet must be transmitted in order (the sender may not skip sequence numbers). Each packet must be acknowledged, and each packet must be acknowledged in order. In a control packet, the XXX field takes on the following values: 1 CLOSE The connection should be closed immediately. This is typically sent when one side has seen too many errors and wants to give up. It is also sent when shutting down the protocol. If an unexpected CLOSE packet is received, a CLOSE packet should be sent and the 'g' protocol should halt, causing UUCP to enter the final handshake. 2 RJ or NAK The last packet was not received correctly. The YYY field contains the sequence number of the last correctly received packet. 3 SRJ Selective reject. The YYY field contains the sequence number of a packet that was not received correctly, and should be retransmitted. This is not used by UUCP, and most implementations will not recognize it. 4 RR or ACK Packet acknowledgement. The YYY field contains the sequence number of the last correctly received packet. 5 INITC Third initialization packet. The YYY field contains the maximum window size to use. 6 INITB Second initialization packet. The YYY field contains the packet size to use. It requests a size of 2 ** (YYY + 5); note that this is not the same coding used for the k byte in the packet header (it is 1 less). Some UUCP implementations can handle any packet size up to that specified; some can only handled exactly the size specified. 7 INITA First initialization packet. The YYY field contains the maximum window size to use. The checksum of a control packet is simply 0xaaaa - the control byte. The checksum of a data packet is 0xaaaa - (CHECK ^ the control byte), where ^ denotes exclusive or, and CHECK is the result of the following routine as run on the contents of the data field (every byte in the data field participates in the checksum, even for a short data packet). This is the routine used by Taylor UUCP, and is a slightly modified version of a routine which John Gilmore patched from G.L. Chesson's original paper. The z argument points to the data and the c argument indicates how much data there is. int igchecksum (z, c) register const char *z; register int c; { register unsigned int ichk1, ichk2; ichk1 = 0xffff; ichk2 = 0; do { register unsigned int b; /* Rotate ichk1 left. */ if ((ichk1 & 0x8000) == 0) ichk1 <<= 1; else { ichk1 <<= 1; ++ichk1; } /* Add the next character to ichk1. */ b = *z++ & 0xff; ichk1 += b; /* Add ichk1 xor the character position in the buffer counting from the back to ichk2. */ ichk2 += ichk1 ^ c; /* If the character was zero, or adding it to ichk1 caused an overflow, xor ichk2 to ichk1. */ if (b == 0 || (ichk1 & 0xffff) < b) ichk1 ^= ichk2; } while (--c > 0); return ichk1 & 0xffff; } When the 'g' protocol is started, the calling UUCP sends an INITA control packet with the window size it wishes the called UUCP to use. The called UUCP responds with an INITA packet with the window size it wishes the calling UUCP to use. Pairs of INITB and INITC packets are then similarly exchanged. When these exchanges are completed, the protocol is considered to have been started. When a UUCP package transmits a command, it sends one or more data packets. All the data packets will normally be complete, although some UUCP packages may send a the last one as a short packet. The UUCP package receiving the command will know when the command has finished because it will be terminated by a null byte sent as data. Some UUCP packages require the last byte of the last packet sent to be null, even if the command ends earlier in the packet. Some packages may require all the trailing bytes in the last packet to be null, but I have not confirmed this. When a UUCP package sends a file, it will send a sequence of data packets. The end of the file is signalled by a short data packet containing zero valid bytes (it will normally be preceeded by a short data packet containing the last few bytes in the file). Note that the sequence numbers cover the entire communication session, including both command and file data. When the protocol is shut down, each UUCP package sends a CLOSE control packet. *) What is the 'f' protocol? The 'f' protocol is a seven bit protocol which checksums an entire file at a time. It only uses the characters between \040 and \176 (ASCII space and ~) inclusive as well as the carriage return character. It can be very efficient for transferring text only data, but it is very inefficient at transferring eight bit data (such as compressed news). It is not flow controlled, and the checksum is fairly insecure over large files, so using it over a serial connection requires handshaking (XON/XOFF can be used) and error correcting modems. Some people think it should not be used even under those circumstances. I believe the 'f' protocol originated in BSD versions of UUCP. It was originally intended for transmission over X.25 PAD links. The 'f' protocol has no startup or finish protocol. However, both sides should sleep for a couple of seconds before starting up, because typically they will switch the terminal into XON/XOFF mode and want to allow the changes to settle before beginning transmission. When a UUCP package transmits a command, it simply sends a string terminated by a carriage return. When a UUCP package transmits a file, each byte b of the file is translated according to the following table: 0 <= b <= 037: 0172, b + 0100 (0100 to 0137) 040 <= b <= 0171: b ( 040 to 0171) 0172 <= b <= 0177: 0173, b - 0100 ( 072 to 077) 0200 <= b <= 0237: 0174, b - 0100 (0100 to 0137) 0240 <= b <= 0371: 0175, b - 0200 ( 040 to 0171) 0372 <= b <= 0377: 0176, b - 0300 ( 072 to 077) That is, a byte between \040 and \171 inclusive is transmitted as is, and all other bytes are prefixed and modified as shown. When all the file data is sent, a seven byte sequence is sent: two bytes of \176 followed by four ASCII bytes of the checksum as printed in base 16 followed by a carriage return. For example, if the checksum was 0x1234, this would be sent: "\176\1761234\r". The checksum is initialized to 0xffff. For each byte that is sent it is modified as follows (where b is the byte before it has been transformed as described above): /* Rotate the checksum left. */ if ((ichk & 0x8000) == 0) ichk <<= 1; else { ichk <<= 1; ++ichk; } /* Add the next byte into the checksum. */ ichk += b; When the receiving UUCP sees the checksum, it compares it against its own calculated checksum and replies with a single character followed by a carriage return. G The file was received correctly. R The checksum did not match, and the file should be resent from the beginning. Q The checksum did not match, but too many retries have occurred and the communication session should be abandoned. The sending UUCP checks the returned character and acts accordingly. *) What is the 't' protocol? The 't' protocol is intended for TCP links. It does no error checking or flow control, and requires an eight bit clear channel. I believe the 't' protocol originated in BSD versions of UUCP. When a UUCP package transmits a command, it first gets the length of the command string, C. It then sends ((C / 512) + 1) * 512 bytes (the smallest multiple of 512 which can hold C bytes plus a null byte) consisting of the command string itself followed by trailing null bytes. When a UUCP package sends a file, it sends it in blocks. Each block contains at most 1024 bytes of data. Each block consists of four bytes containing the amount of data in binary (most significant byte first, the same format as used by the Unix function htonl) followed by that amount of data. The end of the file is signalled by a block containing zero bytes of data. *) What is the 'e' protocol? The 'e' protocol is similar to the 't' protocol. It does no flow control or error checking and is intended for use over TCP. The 'e' protocol originated in versions of HDB UUCP. When a UUCP package transmits a command, it simply sends the command as an ASCII string terminated by a null byte. When a UUCP package transmits a file, it sends the complete size of the file as an ASCII decimal number. The ASCII string is padded out to 20 bytes with null bytes (i.e. if the file is 1000 bytes long, it sends "1000\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"). It then sends the entire file. *) What is the 'x' protocol? I believe that the 'x' protocol was intended for use over X.25 virtual circuits. It relies on a write of zero bytes being read as zero bytes without stopping communication. I have heard that it does not work correctly. If someone would care to fill this in more, I would be grateful. -- Ian Taylor ian@airs.com uunet!airs!ian First person to identify this quote wins a free e-mail message: ``The will to be stupid is a very powerful force.''