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.
The Initial 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'). A few systems end these messages with a line feed character (`\012') instead of a null byte; the examples below assume a null byte is being used.
Some options below are supported by QFT, which stands for Queued File Transfer, and is (or was) an internal Bell Labs version of UUCP.
Taylor UUCP size negotiation was introduced by Taylor UUCP, and is also supported by DOS based UUPlus and Amiga based wUUCP and UUCP-1.17.
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. If the sequence number changes on one of the machines, perhaps because of an attempted breakin or because a disk backup was restored, the sequence numbers on the two machines must be reconciled manually.
`-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='. UUPlus allows either `-p' or `-v' to be specified on a per-system basis in the `SYSTEMS' file (`gradechar' option).
`-R' Indicates that the calling UUCP understands how to restart failed file transmissions. Supported only by System V Release 4 UUCP, QFT, and Taylor 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, QFT and UUPlus. UUPlus reports the lesser of the available disk space on the spool directory drive and the ulimit variable in `UUPLUS.CFG'. Taylor UUCP understands this option, but does not generate it.
`-N[NUMBER]' Indicates that the calling UUCP understands the Taylor UUCP size negotiation extension. Not supported by traditional UUCP packages. Supported by UUPlus. The optional number is a bitmask of features supported by the calling UUCP, and is described below.
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[NUMBER]' The calling UUCP is acceptable, it specified `-N', and the called UUCP also understands the Taylor UUCP size limiting extensions. The optional number is a bitmask of features supported by the called UUCP, and is described below.
`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. Some versions of UUCP just drop the line rather than sending this message.
If the response is `ROK', the following options are supported by System V Release 4 UUCP and QFT. `-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. Also supported by UUPlus. Taylor UUCP understands this option, but does not generate it.
`-xLEVEL' I'm not sure just what this means. 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'.
Most UUCP packages will consider each locally supported protocol in turn and select the first one supported by the called UUCP. With some versions of HDB UUCP, this can be modified by giving a list of protocols after the device name in the `Devices' file or the `Systems' file. For example, to select the `e' protocol in `Systems', airs Any ACU,e ... or in Devices, ACU,e ttyXX ... Taylor UUCP provides the `protocol' command which may be used either for a system or a port. UUPlus allows specification of the protocol string on a per-system basis in the `SYSTEMS' file.
The optional number following a `-N' sent by the calling system, or an `ROKN' sent by the called system, is a bitmask of features supported by the UUCP package. The optional number was introduced in Taylor UUCP version 1.04. The number is sent as an octal number with a leading zero. The following bits are currently defined. A missing number should be taken as `011'.
`01' UUCP supports size negotiation.
`02' UUCP supports file restart.
`04' UUCP supports the `E' command.
`010' UUCP requires the file size in the `S' and `R' commands to be in base 10. This bit is used by default if no number appears, but should not be explicitly sent.
`020' UUCP expects a dummy string between the notify field and the size field in an `S' command. This is true of SVR4 UUCP. This bit should not be used.
After the protocol has been selected and the initial handshake has been completed, both sides turn on the selected protocol. For some protocols (notably `g') a further handshake is done at this point.
UUCP Protocol Commands
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 five commands: `S', `R', `X', `E', or `H'.
Any file name referred to below is either an absolute file name beginning with `/', a public directory file name beginning with ` /', a file name relative to a user's home directory beginning with ` USER/', or a spool directory file name. File names in the spool directory are not absolute, but instead are converted to file names 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).
The S Command .............
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 the file is placed within that directory with the basename of FROM. A name ending in `/' is taken to be a directory even if one does not already exist with that name. 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 should 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 may be a dummy string, such as `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 the 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. This field is ignored by UUPlus, since it is meaningless on DOS; UUPlus uses 0666 for outgoing files.
NOTIFY This field may not be present, and in any case is only meaningful if the `n' option appears in OPTIONS. If the `n' option appears, then, 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 always be present, typically as the string `dummy' or simply a pair of double quotes.
SIZE This field is only present when doing Taylor UUCP or SVR4 UUCP size negotiation. It is the size of the file in bytes. Taylor UUCP version 1.03 sends the size as a decimal integer, while versions 1.04 and up, and all other UUCP packages that support size negotiation, send the size in base 16 with a leading 0x.
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 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 might 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.
`SN8' This is only used by Taylor UUCP. It means that the file was already received in a previous conversation. This can happen if the receive acknowledgement was lost after it was sent by the receiver but before it was received by the sender.
`SN9' This is only used by Taylor UUCP (versions 1.05 and up) and UUPlus (versions 2.0 and up). It means that the remote system was unable to open another channel (see the discussion of the `i' protocol for more information about channels). This implies that the file transfer might succeed later.
`SN10' This is reportedly used by SVR4 UUCP to mean that the file size is too large.
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.
`CYM' The file transfer was successful, and the slave wishes to become the master; the master should send an `H' command, described below.
`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.
The R 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 or QFT implement 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 or QFT, this was specified in the `-U' option during the initial handshake).
The slave then responds with an `R' command response. UUPlus does not support `R' requests, and always responds with `RN2'.
`RY MODE [SIZE]' The slave is willing to send the file, and file transfer begins. The MODE argument 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. I am told that SVR4 UUCP sends a trailing SIZE argument. For some versions of BSD UUCP, the MODE argument may have a trailing `M' character (e.g., `RY 0666M'). This means that the slave wishes to become the master.
`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 might succeed later, or it might not (this may be cleared up in a later release of Taylor UUCP).
`RN9' This is only used by Taylor UUCP (versions 1.05 and up) and FSUUCP (versions 1.5 and up). It means that the remote system was unable to open another channel (see the discussion of the `i' protocol for more information about channels). This implies that the file transfer might succeed later.
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.
The X 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. Any wildcards are expanded on the slave. If the master is requesting that the files be transferred to itself, the request would normally contain wildcard characters, since otherwise an `R' command would suffice. The master can also use this command to request that the slave transfer files to a third system.
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. FSUUCP does not support `X' requests, and always responds with `XN'.
`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.
The E Command .............
master: `E FROM TO USER -OPTIONS TEMP MODE NOTIFY SIZE COMMAND' The `E' command is only supported by Taylor UUCP 1.04 and up. It is used to make an execution request without requiring a separate `X.*' file. It is only used when the command to be executed requires a single input file which is passed to it as standard input.
All the fields have the same meaning as they do for an `S' command, except for OPTIONS and COMMAND.
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).
`N' No mail message should be sent, even if the command fails. This is the equivalent of the `N' command in an `X.*' file.
`Z' A mail message should be sent if the command fails (this is generally the default in any case). This is the equivalent of the `Z' command in an `X.*' file.
`R' Mail messages about the execution should be sent to the address in the NOTIFY field. This is the equivalent of the `R' command in an `X.*' file.
`e' The execution should be done with `/bin/sh'. This is the equivalent of the `e' command in an `X.*' file.
COMMAND The command which should be executed. This is the equivalent of the `C' command in an `X.*' file.
The slave then responds with an `E' command response. These are the same as the `S' command responses, but the initial character is `E' rather than `S'.
If the slave responds with `EY', the file transfer begins. When the file transfer is complete, the slave sends a `C' command response, just as for the `S' command. After a successful file transfer, the slave is responsible for arranging for the command to be executed. The transferred file is passed as standard input, as though it were named in the `I' and `F' commands of an `X.*' file.
After the `C' command response has been received (in the `EY' case) or immediately (in an `EN' case) the master will send another command.
The H 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.
The Final Handshake
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 protocol).
caller: `\020OOOOOO\000' called: `\020OOOOOOO\000' That is, the calling UUCP sends six `O' characters and the called UUCP replies with seven `O' characters. Some UUCP packages always send six `O' characters.