Background for non-technical users: An "exit" is a program supplied by the customer to modify the behavior of a product (such as LISTSERV) in ways that the supplier of the product could not anticipate, or could not afford to support via standard commands or options. The product checks for the presence of the "exit" program and calls it on a number of occasions, called "exit points". In some cases, the "exit" program supplies an answer ("return code") to the main program, which adjusts its behavior accordingly. For instance, LISTSERV may ask an exit program "Is it ok to add JOE@XYZ.EDU to the ABC-L list?", and the program will answer yes or no, and possibly send a message to the user explaining why his subscription was accepted or rejected. In other cases, the "exit point" call is purely informative: the exit program gets a chance to do something, such as sending an informational message to a user, but does not return any answer. Because the exit is a computer program, it must be prepared by a technical person and installed by the LISTSERV maintainer.
List "exits" are available to control the major events associated with list maintenance. While the implementation of list exits is necessarily system dependent, the list exits themselves (i.e. the tasks that they may carry out, as opposed to how such tasks would be carried out on a particular operating system) are system independent.
2.
Modify the LIST_EXITS configuration option in the LISTSERV site configuration file (create one if none was present in your configuration). This variable lists the names of all the "known" exits. For security reasons, LISTSERV will not call an exit which is not listed there. To enable XYZ and ABC as valid list exits, you would set LIST_EXITS to "XYZ ABC" (with or without the quotes, depending on your operating system). You must reboot LISTSERV after making this change.
Note: You do not code the extension of the exit program (if any) into the LIST_EXITS configuration variable. For instance if you are running under Windows NT and your exit is called XYZ.CMD, you set LIST_EXITS to "XYZ". Under OpenVMS, if your exit is called ABC.COM, you set LIST_EXITS to "ABC".
SECURITY NOTE: Once an exit has been listed in the LIST_EXITS option, ANY list owner may activate it for his own lists. In other words, step 2 merely tells LISTSERV that the program is a "bona fide" exit. There is no mechanism to restrict the use of an exit to a particular list, because it is very easy to implement this in the exit itself, by checking that the name of the list is what you expect or allow.
LISTSERV exits receive one or more parameters as input, and may provide a numeric and (in a few cases) supplemental string result as output. Each operating system has its own set of numeric return codes for various kinds of failures, but LISTSERV always uses the same internal return code system for its exits - anything else would quickly become unmanageable. The value 0 always means "success" or "normal/default action". Positive values indicate various non-default actions, depending on the particular exit point. Negative values indicate system errors. Exit programs are responsible for doing their own error reporting, since LISTSERV has no way to know which errors they may or may not run into.
Important: Even though the exit program is usually called from LISTSERV's root directory, it should not make any assumption about its current directory. For optimal portability, the program should always use absolute pathnames to access the various files it might need. For instance, list files should be accessed (if at all) as $A/ or A: or %A%\, and so forth.
It is particularly important to note that an intermediary script file (for instance a CMD file under Windows) must normally make use of absolute paths, including an absolute path to the interpreter that will run the program called by your exit. If your CMD file calls a perl file, your CMD file will need to look something like this (depending on the actual locations of the files in question, of course):
c:\bin\perl -w e:\listserv\main\MYEXIT.pl
Similarly, all files referenced in MYEXIT.pl (including, but not limited to, exit.input and exit.output) must be referenced with absolute paths.
Not using absolute paths in your programming is guaranteed to produce unexpected results, if indeed LISTSERV is even able to find your exit when the exit point is called.
The exit may receive one or more string parameters as input. Most operating systems provide a mechanism to pass one parameter to a script or program, with some restrictions. However, LISTSERV may need to pass several parameters, and the restrictions may not be acceptable. Thus, a system independent parameter passing convention had to be designed. This convention is used by all systems except VM, where multiple parameters of arbitrary length and contents may be passed to a REXX program. On VM, the system independent convention is never used, because it is unnecessary and less efficient than the native method. VM exits use the standard PARSE ARG directive to retrieve their parameters.
The system independent convention uses a disk file called 'exit.input' in the same directory as the exit program. This is a standard text file, where each record is one input string parameter. This file is always created if there are 2 or more string parameters for the exit, or if the EXIT_INPUT configuration parameter is set to the value 1. In addition, it is always created on Windows™ operating systems. Under OpenVMS™ and unix®, the file is not created when there is only one parameter and EXIT_INPUT defaults to 0. Since most exits only have a single parameter, this improves performance in most cases. Note that LISTSERV will take care of deleting the 'exit.input' when appropriate.
Regardless of whether or not the 'exit.input' file is created, the first parameter is always passed to the exit using system-specific methods under OpenVMS™ and unix®. Under Windows™ systems, the first parameter is only passed through the 'exit.input' file. Again, this may simplify programming for simple exits.
SECURITY NOTE: LISTSERV will always quote/escape the first parameter to prevent the recognition of special characters by the underlying operating system. However, your program should be very careful in its use of the parameters in any subsequent system call. For instance, if the parameter to your unix® exit is the string "a|b", LISTSERV will quote the vertical bar so that it does not result in the execution of the program 'b', and so that the value "a|b" is present in your argument vector. However, you must still be careful in the use of the arguments within your program, especially if you plan to launch a compiled program from a shell and pass it the same arguments. In that case L-Soft recommends the use of EXIT_INPUT = 1, which allows the second program to read its parameters safely from the 'exit.input' file.
where 'epname' is the name of the entry point being called (SUB_FILTER, SUB_FAIL, etc.), 'listname' is the name of the list, and 'more' depends on the particular exit point. Under VM, 'more' may contain '15'x characters delimiting additional parameters. Again, while 'epname' and 'listname' are unlikely to contain "tricky" characters, the same cannot be assumed about the remainder of the parameter string.
In most cases, your program will only handle a limited set of exit points. When it does not recognize the 'epname', it should exit with the numeric result 0, which tells LISTSERV to take the default action. To exit with the result 0, you can take a normal operating system dependent exit. In particular, success status codes are translated to 0 under OpenVMS™. To return any other numeric code, or to return a string code, you must use the system independent parameter return convention return below, except on VM where the operating system provides a suitable native convention for the return of one parameter of arbitrary length and contents. So, REXX programs return their results with a standard RETURN or EXIT statement. The first blank-delimited word is interpreted as the numeric result, and the rest, if any, is the string result. In other words, the return string is broken up into number and string in exactly the same manner as with a PARSE VAR RESULT NUMBER STRING instruction. On VM, the system independent return convention is not used, because it is unnecessary and less efficient than the native method.
The system independent convention is based on a file called 'exit.output', in the same directory as the exit program. LISTSERV erases this file before calling your program, and reads it when it regains control. This file is a standard text file, which contains a number of directives. To set the numeric return code, you use the EXIT directive:
Note: You must ALWAYS set the numeric code if you want the string result to have any effect. The default numeric code is 0, which means "default behavior", and the default behavior never uses the string you supply. By definition, the default behavior is whatever LISTSERV would do if the exit were not present.
In addition to EXIT and EXIT-STRING, a number of other directives are available. For instance, you can tell LISTSERV to send a message to the originating user, to explain why the exit rejected his subscription request. These directives are processed sequentially until the end of the file. Note that the EXIT directives merely set the final exit codes. They do not interrupt the processing of the 'exit.output' file, which is always read to the end of the file. This means that, if you change your mind about the exit code, you can write a new EXIT instructions and LISTSERV will use the last value that you supplied.
Each directive has 0 or more mandatory parameters, and may support a number of optional parameters, which are always listed after the mandatory ones. Some parameters may be simple blank-delimited keywords or options, while others may contain arbitrary data. The exit should not need to provide placeholders for optional parameters, and above all it should be possible to add new optional parameters without requiring all exits to be rewritten. To solve this problem, each directive was given two forms: a simple form, where the entire directive fits in a single line, and an explicit form, where you indicate the number of parameters that you intend to provide, and each parameter follows on a line by its own. In the simple form, the mandatory parameters are filled from the data supplied on the single directive line, and all the optional parameters are set to their default value. Each blank delimited word supplies one parameter, until the first N-1 parameters have been set. The remainder is used for the last parameter. Here is an example of the simple form:
If, on the other hand, you want to send the message to more than one person, you need to use the explicit form. In the explicit form, you specify the parameters on separate lines and modify the directive to add the number of lines which you are passing to the processor. By way of example, here is the explicit form of the TELL directive shown above, where the number 2 is added to the directive, telling LISTSERV that there will be two lines of parameters following:
TELL2jack@XYZ.COM cc: honcho@FOO.COM
The FOO-L list is only open to FOO Inc. employees.
TELL3jack@XYZ.COM cc: honcho@FOO.COM
The FOO-L list is only open to FOO Inc. employees.
ECHO
It is always safer to use the explicit form if you are not sure how many words you will have in the various parameters. The simple form is provided mostly for directives such as EXIT or EXIT-STRING which only take one parameter, and for hardcoded parameters.
Currently, the supported directives are as follows. The "VM API" indicates the corresponding REXX API for VM users (it is not possible to provide an API for non-VM systems because the exits run in a different virtual address space and may not call back into LISTSERV entry points).
- ECHO: echoes message to log
- RAGGED: flows but does not justify message
- ECHO: echoes message to log
Description: This exit point gives you a chance to reject a subscription request by returning a nonzero value. This adds to rather than replaces the "Filter=" keyword.
Description: This exit point is called whenever a subscription is rejected (in particular, it is called if you return 1 from SUB_FILTER). You may send additional messages to the user, log the event, and so on.
Description: The user's request for subscription is being forwarded to the list owners. You may send additional messages to the user, log the event, and so on. To implement custom subscription/rejection, use the SUB_FILTER exit with "Subscription= Open". When SUB_REQ is called, it is too late to let the user through.
Description: This exit point notifies you that the user has been successfully subscribed to the list (SUB_NEW is for the SUBSCRIBE command, ADD_NEW corresponds to the ADD command). You can send additional messages, log the event, and so on.
Description: This exit point is called for all SIGNOFF commands, and for DELETE commands issued by a registered Node Administrator. It is NOT called for DELETE commands originating from the list owner. If you return the value 1, LISTSERV does not delete the target e-mail address but continues to look for more addresses matching the pattern provided, and the exit will be called again as appropriate. If you return the value 2, LISTSERV simply interrupts the processing of the SIGNOFF command and terminates the command with no further message (i.e., you have to send your own explanation back to the command originator). If the request is rejected, you should check for the presence of the second parameter and send an explanatory message to that address, or to the target address if only one parameter was specified.
Description: This exit point is called for the SIGNOFF command only, when a user has been successfully removed from the list. The farewell message, if any, has already been sent. You may issue additional messages, log the event, and so on. A return value of 1 directs LISTSERV not to mail the "SIGNOFF1" form to the list owners.
Description: This exit point is called for the DELETE command only, after a user has been removed from the list. You may issue additional messages, log the event, and so on. A return value of 1 directs LISTSERV not to mail the "DELETE1" form to the list owners.
Description: This exit point is called for all SET commands that do not originate from the list owner. The first parameter (originator's address) is the address you should use to send replies or informational messages to the command originator. The second parameter (list of options to be altered) is a blank-separated list of command options, in their canonical spelling. If topics have been specified, they are listed last, after the word 'TOPICS:', with the spelling provided by the user. Bear in mind that topic change requests are not necessarily a list of the new topics to be enabled, and may contain complex '+' or '-' directives. Finally, the third parameter (target e-mail address) is the address as it appears in the list, and is provided for the sake of completeness; in most cases you will not need to examine it. If you return the value 1, the command is rejected and no option is modified. A return value of 2 indicates that the list of options and/or topics should be altered before the changes are performed. The exit string must contain a replacement for the second input parameter, in the same format. LISTSERV will assume that any options or topics specified in this fashion are syntactically correct; while incorrect values will not cause any problem and will be properly rejected as invalid options, the error message(s) returned to the user may not be as helpful as they ordinarily would.
Description: This exit point is called for messages posted to a list. It can be used to compare the e-mail address of the poster to a list of users who should (or should not) be allowed to post. It can further be used to scan the body of the e-mail message, i.e., to accept or reject the message based on its content. The exit places the message to be checked in the file $D/listserv.cmsut1 (where $D is the value assigned with .SD D in system.cfg or CORPORAT SYSVARS, D= in go.sys, or D\ in system_config.dat, depending on your OS platform; in any case, by default this is LISTSERV's TMP directory or minidisk).
Note: Your exit program MAY NOT edit or alter the listserv.cmsut1 file as LISTSERV has already loaded it preparatory to processing it. The exit point ONLY allows you to accept or reject the message.
Important: List exits may not make any change to the LIST file, because the command processor may have changes of its own to make to the file, or may have kept the file open for further processing. Under VM only, If you really have to change the file, use CP MSG * CMS EXEC to schedule a new call to the exit after command processing completes.
Note: The change in calling sequence from LSVIMAIL. LSVIMAIL was removed in version 1.8b and is no longer available.
It is possible to define "local" LISTSERV commands on non-VM systems. A "local" command is a locally developed extension to the LISTSERV command set, which can be installed without making any modification to LISTSERV itself. To install a local command, you must perform the following steps:
where 'origin' is the address of the command originator, 'command' is the name of the command ('/ABC' in the present example), and 'arguments' are the command arguments, if any, provided by the user. The second parameter is the optional DEF parameter from 'localcmd.file'.
Typically, your program will parse the arguments, decide on a course of action, and issue a number of messages to the user. The exit code is immaterial; there is no particular course of action to select for command processing.
This “exit point” allows you to use a third-party spam filter to scan messages processed by LISTSERV. Although this hook can in principle be used with any spam scanning product, all the examples and step-by-step instructions in this section will relate to SpamAssassin, a popular freeware spam filter that can be downloaded from
http://spamassassin.apache.org.
Note: L-Soft did not author SpamAssassin and is unable to correct problems with the SpamAssassin product itself. L-Soft does not make any legal representations or warranties about SpamAssassin. Although L Soft’s support department will be glad to answer questions about the integration of SpamAssassin and LISTSERV, we cannot answer questions about SpamAssassin itself.
SPAM_EXIT is a standard LISTSERV exit, with all that this entails. The same OS-specific naming requirements used for regular LISTSERV exit points are enforced for the SPAM_EXIT exit point. However, SPAM_EXIT is defined separately in the site configuration file as it is a server-level exit rather than a list-level exit.
The rationale for doing things in this order is that viruses are far more dangerous than spam, so LISTSERV wants to identify them as quickly as possible, and in particular before any whitelist rule has had the opportunity to accept them. Besides, virus scans are much faster than spam scans.
Its primary input is the file spam.tmp in the D directory (typically, unix, ~listserv/tmp ; Windows, \LISTSERV\TMP ; OpenVMS, LISTSERV_ROOT:[TMP]). This contains a copy of the whole message, header and body.
When using this exit, it is very important to test things carefully, since a mistake could mean that every message is rejected. If for instance the script is not found by the operating system due to a misspelling, and the operating system happens to return 2 in that case, then the message will be automatically rejected even though the message was never scanned. (Because Windows returns 1 for a misspelled exit name, we chose 2=reject instead of the usual 1=reject.)
The minimum implementation for the REPORT call is to do the same as SCAN does. In addition, it is desirable to create an output file called spam.report in the same directory where the input file spam.tmp is located. There is no special format for this output file, but it is a good idea to start with a line saying whether or not the message was identified as spam, and give the score if the spam filter uses a score system. LISTSERV does not process the report, it just ends up being shown to a human. If no spam.report file is created by the exit, LISTSERV will use the exit string as a one-line report. If there is no exit string, LISTSERV will generate a hard-coded message.
To enable spam filtering in LISTSERV, you must install the third-party spam filter, provide a script that will scan messages using the third-party filter (if using SpamAssassin, you can use one of the L-Soft supplied scripts), and activate this script by making changes to the LISTSERV configuration. LISTSERV will then scan every message it processes, with a few exceptions, and reject messages identified as spam.
Optionally, you can also activate LISTSERV 14.3’s built-in blacklist/whitelist functionality (see the release notes for version 14.3 for more information). This feature provides an additional level of spam filtering and can also improve performance significantly, because spam filters like SpamAssassin can take up to 5-20 seconds to scan a message.
This section contains step-by-step instructions for configuring LISTSERV to use SpamAssassin using one of the L-Soft supplied scripts. Throughout this section, we will make the following assumptions:
Unix: Compile spamc on the LISTSERV host, then copy it to a directory in LISTSERV’s path. To test it, use a command similar to the following:
The flags you need to use may vary depending on your version of SpamAssassin and configuration. The response must be two numbers as shown above, but the numbers can be different than in the example (they are the SpamAssassin score of the test message). Any other response indicates an error. Refer to the spamc and spamd man pages for more information.
Windows: Download and install the spamc.exe executable from L-Soft, and place it in a directory in LISTSERV’s path – for instance, the LISTSERV\MAIN directory.
The response must be two numbers as shown above, but the numbers can be different than in the example (they are the SpamAssassin score of the test message). Any other response indicates an error. In that case, make sure that spamd is configured to allow connections from the LISTSERV host.
Unix: Install perl on the LISTSERV host, if not already installed.
Windows: Install a REXX interpreter, such as Regina REXX (
http://regina-rexx.sourceforge.net/, Windows kit download available at
http://prdownloads.sourceforge.net/regina-rexx/regina33.exe?download). When prompted to register .REXX as a path extension, you should do so. Alternatively, you can simply download
REXX.EXE from L Soft and place it in the same directory where you saved spamc.exe.
•
Windows: if you have not registered .REXX as a path extension when installing it in step 2 or if you downloaded it from the L Soft ftp site instead of installing the full kit, you will need to create a script called
saexit.cmd in the C:\LISTSERV\MAIN directory containing the following three lines:
@echo off
rexx saexit.rexx %*
exit %ERRORLEVEL%
•
Unix: SPAM_EXIT="saexit"•
The spam exit is implemented within LISTSERV’s message scanner, which is informally known as the “virus scanner,” because this was its original purpose. If the message scanner is disabled, for instance by setting the ANTI_VIRUS configuration parameter to 0, or by failing to install the maintenance LAK, both virus scanning and spam scanning are disabled. If ANTI_VIRUS is unset, LISTSERV will enable the message scanner if either virus scanning or spam scanning is configured and available.
The spam exit is also available on VM and VMS, but there is no version of spamc for these systems. Since VM and VMS hosts normally use the AVS for message scanning, spam protection can be provided by simply installing the spam exit on the AVS server.
The configuration parameter SPAM_MAXSIZE can be used to automatically accept messages larger than a certain size without wasting cycles scanning them. The rationale is that spam filters can take minutes to process very large messages, whereas spam messages are almost always very small. The following example sets the threshold to 512k:
The default value is 256k. If set to 0, all messages will be scanned, which again could take considerable time. Messages that are not scanned do not count as a spam scan in the LISTSERV statistics.
In late 1998/early 1999 a worm called HAPPY99 surfaced on the Internet. In an attempt to keep the worm off of lists, an L-Soft engineer wrote the following exit in Regina REXX to run under the Windows NT version of LISTSERV 1.8d.
Warning: USE AT YOUR OWN RISK: THIS SOFTWARE IS PROVIDED ON AN 'AS IS' BASIS. L-SOFT DOES NOT MAKE ANY EXPRESS OR IMPLIED WARRANTY OF ANY KIND WHATSOEVER WITH RESPECT TO THE SOFTWARE, INCLUDING, WITHOUT LIMITATION, ANY WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Neither L-Soft nor any of its employees, officers or agents will be liable for any direct, indirect or consequential damages, even if L-Soft had been advised of the possibility of such damage. No support is available from L-Soft or from the author for this program. If you give a copy of this program to someone else for their use, you must provide it with both the copyright notice and this paragraph intact. You are not authorized to make this program available for public access via anonymous FTP or by any other means of mass distribution.
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Then, we wrote the following HAPPY99.CMD file, and placed it in the \LISTSERV\MAIN directory. This CMD file, when executed by LISTSERV, calls the Regina interpreter and in turn tells it to run the HAPPY99.REXX file we wrote above.
Notes: The program makes no attempt to determine if the message actually contains a working copy of the virus. Its only function is to attempt to identify messages that MAY contain a working copy of the virus. Thus it may engender some false positives (but it's doubtful that anyone sending legitimate mail will send the specific strings searched for by the exit program).
The LISTSERV.CMSUT1 file that you search for evidence of the virus may not be edited "on the fly" to remove parts of the message--it is a read-only file for this purpose. You have only the option of accepting or rejecting the message in its entirety.
Given that the "classic" version of HAPPY99 is not sent as a MIME attachment, it is not possible to block it with the Attachments= list header keyword introduced in the 2000a level set release of LISTSERV 1.8d, and the exit is the only way to guard against it.
LISTSERV 1.8e and above, with its integrated anti-virus scanning and ability to filter encoded inline attachments removes the need for this particular exit, but the above remains a useful example of how to program an exit.
