NAME

mailsurr − surrogate commands for routing and transport of mail

DESCRIPTION

The mailsurr file contains routing and transport surrogate commands used by the mail command.  Each entry in mailsurr has three whitespace-separated, single quote delimited fields:

’sender’   ’recipient’   ’command’

or a line that begins

Defaults:

Entries and fields may span multiple lines, but leading whitespace on field continuation lines is ignored.  Fields must be less than 1024 characters long after expansion (see below). 

The sender and recipient fields are regular expressions.  If the sender and recipient fields match those of the message currently being processed, the associated command is invoked. 

The command field may have one of the following five forms:

A[ccept]
D[eny]
T[ranslate] R=[│]string
< S=. . .;C=. . .;F=. . .; command
> command

Regular Expressions

The sender and recipient fields are composed of regular expressions (REs) which are digested by the regexp(5) compile and advance procedures in the C library.  The regular expressions matched are those from ed(1), with simple parentheses () playing the role of \(\) and the addition of the + and ? operators from egrep(1).  Any single quotes embedded within the REs must be escaped by prepending them with a backslash or the RE is not interpreted properly. 

The mail command prepends a circumflex (^) to the start and appends a dollar sign ($) to the end of each RE so that it matches the entire string.  Therefore it would be an error to use ^RE$ in the sender and recipient fields.  To provide case insensitivity, all REs are converted to lower case before compilation, and all sender and recipient information is converted to lower case before comparison.  This conversion is done only for the purposes of RE pattern matching; the information contained within the message’s header is not modified. 

The sub-expression pattern matching capabilities of regexp may be used in the command field, that is, (. . .), where 1 ≤ n ≤ 9.  Any occurrences of \\n in the replacement string are themselves replaced by the corresponding (. . .) substring in the matched pattern.  The sub-expression fields from both the sender and recipient fields are accessible, with the fields numbered 1 to 9 from left to right. 

Accept and Deny Commands

Accept instructs rmail to continue its processing with the mailsurr file, but to ignore any subsequent matching Deny.  That is, unconditionally accept this message for delivery processing.  Deny instructs rmail to stop processing the mailsurr file and to send a negative delivery notification to the originator of the message.  Whichever is encountered first takes precedence. 

Translate Command

Translate allows optional on-the-fly translation of recipient address information.  The recipient replacement string is specified as R=string. 

For example, given a command line of the form

’.+’ ’([^!]+)@(.+)\.EUO\.ATT\.com’ ’Translate R=attmail!\\2!\\1’

and a recipient address of rob@sysa.EUO.ATT.COM the resulting recipient address would be attmail!sysa!rob. 

Should the first character after the equal sign be a ‘│’, the remainder of the string is taken as a command line to be directly executed by rmail.  If any sh(1) syntax is required (metacharacters, redirection, and so on), then the surrogate command must be of the form:

sh −c "shell command line. . ."

Special care must be taken to escape properly any embedded back-slashes and single or double quotes, since rmail uses double quoting to group whitespace delimited fields that are meant to be considered as a single argument to execl(2).  It is assumed that the executed command will write one or more replacement strings on stdout, one per line.  If more than one line is returned, each is assumed to be a different recipient for the message.  This mechanism is useful for mailing list expansions.  As stated above, any occurrences of \\n are replaced by the appropriate substring before the command is executed.  If the invoked command does not return at least one replacement string (no output or just a newline), the original string is not modified.  For example, the command line

’.+’’(.+)’ ’Translate R=│/usr/bin/findpath \\1’

allows local routing decisions to be made. 

If the recipient address string is modified, mailsurr is rescanned from the beginning with the new address(es), and any prior determination of Accept (see above) is discarded. 

< command

The intent of a < command is that it is invoked as part of the transport and delivery mechanism, with the ready-for-delivery message available to the command at its standard input.  As such, there are three conditions possible when the command exits:

Success The command successfully delivered the message.  What actually constitutes successful delivery may be different within the context of different surrogates.  The rmail process assumes that no more processing is required for the message for the current recipient. 

Continue The command performed some function (logging remote message traffic, for example) but did not do what would be considered message delivery.  The rmail process continues to scan the mailsurr file looking for some other delivery mechanism. 

Failure The command encountered some catastrophic failure.  The rmail process stops processing the message and sends to the originator of the message a non-delivery notification that includes any stdout and stderr output generated by the command. 

The semantics of the < command field in the mailsurr file allow the specification of exit codes that constitute success, continue, and failure for each surrogate command individually.  The syntax of the exit state specification is:

< WS [exit_state_id=ec[,ec[,. . .]];][exit_state_id=ec[,ec[,. . .]];
        [. . .]]] WS surrogate_cmd_line

WS is whitespace.  exit_state_id can have the value S, C, or F.  exit_state_ids can be specified in any order.  ec can be:

any integer 0 ≤ n ≤ 255 [Negative exit values are not possible.  See exit(2) and wait(2).]

a range of integers of the form lower_limit−upper_limit where the limits are ≥ 0 and ≤ 255, and

∗, which implies anything

For example, a command field of the form:

’< S=1-5,99;C=0,12;F=∗;   command %R’

indicates that exit values of 1 through 5, and 99, are to be considered success, values of 0 (zero) and 12 indicate continue, and that anything else implies failure.  If not explicitly supplied, default settings are S=0;C=∗;. 

It may be possible for ambiguous entries to exist if two exit states have the same value, for example, S=12,23;C=∗;F=23,52; or S=∗;C=9;F=∗;.  To account for this, rmail looks for explicit exit values (that is, not “∗”) in order of success, continue, failure.  Not finding an explicit match, rmail then scans for “∗” in the same order. 

It is possible to eliminate an exit state completely by setting that state’s value to an impossible number.  Since exit values must be between 0 and 255 (inclusive), a value of 256 is a good one to use.  For example, if you had a surrogate command that was to log all message traffic, a mailsurr entry of

’(.+)’ ’(.+)’ ’<S=256;C=∗; /usr/lib/mail/surrcmd/logger \\1 \\2’

would always indicate continue. 

Surrogate commands are executed by rmail directly.  If any shell syntax is required (metacharacters, redirection, and so on), then the surrogate command must be of the form:

sh −c "shell command line. . ."

Special care must be taken to properly escape any embedded back-slashes and other characters special to the shell as stated in the “Translate” section above. 

If there are no matching < commands, or all matching < commands exit with a continue indication, rmail attempts to deliver the message itself by assuming that the recipient is local and delivering the message to /var/mail/recipient. 

> command

The intent of a > command is that it is invoked after a successful delivery to do any post-delivery processing that may be required.  Matching > commands are executed only if some < command indicates a successful delivery (see the previous section) or local delivery processing is successful.  The mailsurr file is re­scanned and all matching > commands, not just those following the successful < command, are executed in order.  The exit status of an > command is ignored. 

Defaults: Line

The default settings may be redefined by creating a separate line in the mailsurr file of the form

Defaults: [S=. . .;][C=. . .;][F=. . .;]

Defaults: lines are honored and the indicated default values redefined when the line is encountered during the normal processing of the mailsurr file.  Therefore, to redefine the defaults globally, the Defaults: line should be the first line in the file.  It is possible to have multiple Defaults: lines in the mailsurr file, where each subsequent line overrides the previous one. 

Surrogate Command Keyword Replacement.

Certain special sequences are textually-substituted in surrogate commands before they are invoked:

%n the recipient’s full name. 

%R the full return path to the originator (useful for sending replies, delivery failure notifications, and so on)

%c value of the Content-Type: header line if present. 

%C “text” or “binary”, depending on an actual scan of the content.  This is independent of the value of any Content-Type header line encountered (useful when calling ckbinarsys.) 

%S the value of the Subject: header line, if present. 

%l value of the Content-Length: header line. 

%L the local system name.  This will be either CLUSTER from mailcnfg or the value returned by uname. 

%U the local system name, as returned by uname. 

%X the value of SMARTERHOST in mailcnfg. 

%D the local domain name.  This will be either DOMAIN from mailcnfg, or the value returned by getdomainame. 

\\n as described above, the corresponding (. . .)  substring in the matched patterns.  This implies that the regexp limitation of 9 substrings is applied to the sender and recipient REs collectively. 

%keywords Other keywords as specified in /etc/mail/mailcnfg.  See mailcnfg(4). 

The sequences %L, %U, %D, and %keywords are permitted within the sender and recipient fields as well as in the command fields. 

An example of the mailsurr entry that replaces the uux “built-in” of previous versions of rmail is:

’.+’’([^@!]+)!(.+)’’< /usr/bin/uux − \\1!rmail (\\2)’

Mail Surrogate Examples

Some examples of mail surrogates include the distribution of message-waiting notifications to LAN-based recipients and lighting Message-Waiting Lamps, the ability to mail output to printers, and the logging of all rmail requests between remote systems (messages passing through the local system).  The following is a sample mailsurr file:

#
# Some common remote mail surrogates follow. To activate any
# or all of them, remove the ‘#’ (comment indicators) from
# the beginning of the appropriate lines. Remember that they
# will be tried in the order they are encountered in the file,
# so put preferred surrogates first.
 #Prevent all shell meta-characters
’.+’’.∗[‘;&|^<>()].∗’       ’Deny’
 #Map all names of the form local-machine!user -> user
’.+’’%L!(.+)’               ’Translate R=\1’
 #Map all names of the form uname!user -> user
#Must be turned on when using mail in a cluster environment.
#’.+’’%U!(.+)’               ’Translate R=\1’
 #Map all names of the form user@host -> host!user
’.+’’([^!@]+)@(.+)’         ’Translate R=\2!\1’
 #Map all names of the form host.uucp!user -> host!user
’.+’’([^!@]+)\.uucp!(.+)’  ’Translate R=\1!\2’
 #Map all names of the form host.local-domain!user -> host!user
#DOMAIN= within /etc/mail/mailcnfg will override getdomainame(3).
’.+’’([^!@]+)%D!(.+)’       ’Translate R=\1!\2’
 #Allow access to ‘attmail’ from remote system ‘sysa’
’sysa!.∗’’attmail!.+’    ’Accept’
 #Deny access to ‘attmail’ from all other remotes
’.+!.+’’attmail!.+’    ’Deny’
 #Send mail for ‘laser’ to attached laser printer
#Make certain that failures are reported via return mail.
’.+’’laser’’< S=0;F=∗; lp −dlaser’
 #Run all local names through the mail alias processor
#
’.+’’[^!@]+’         ’Translate R=|/usr/bin/mailalias %n’
 #For remote mail via nusend
#’.+’’([^!]+)!(.+)’   ’< /usr/bin/nusend −d \\1 −s −e −!"rmail \\2" −’
 #For remote mail via usend
’.+’’([^!]+)!(.+)’
         ’< /usr/bin/usend −s −d\\1 −uNoLogin −!"rmail \\2" − ’
 #For remote mail via uucp
’.+’’([^!@]+)!.+’   ’<S=256;C=0;
  /usr/lib/mail/surrcmd/ckbinarsys −t %C −s \\1’
’.+’’([^!@]+)!(.+)’   ’< /usr/bin/uux − \\1!rmail (\\2)’
 #For remote mail via smtp
#’.+’’([^!@]+)!(.+)’         ’< /usr/lib/mail/surrcmd/smtpqer %R %n’
 #If none of the above work, then let a router change the address.
#’.+’’.∗[!@].∗’      ’Translate R=| /usr/lib/mail/surrcmd/smail -A %n’
 #If none of the above work, then ship remote mail off to a smarter host.
#Make certain that SMARTERHOST= is defined within /etc/mail/mailcnfg.
#’.+’’.∗[!@].∗’              ’Translate R=%X!%n’
 #Log successful message deliveries
’(.+)’ ’(.+)’ ’>/usr/lib/mail/surrcmd/logger \1 \2’

Note that invoking mail to read mail does not involve the mailsurr file or any surrogate processing. 

Security

Surrogate commands execute with the permissions of rmail (user ID of the invoker, group ID of mail).  This allows surrogate commands to validate themselves, checking that their effective group ID was mail at invocation time.  This requires that all additions to mailsurr be scrutinized before insertion to prevent any unauthorized access to users’ mail files.  All surrogate commands are executed with the path /usr/lib/mail/surrcmd:/usr/bin. 

Debugging New mailsurr Entries

To debug mailsurr files, use the −T option of the mail command.  The −T option requires an argument that is taken as the pathname of a test mailsurr file.  If null (as in −T ""), the system mailsurr file is used.  Enter

mail −T test_file recipient

and some trivial message (like “testing”), followed by a line with either just a dot (“.”) or a cntl-D.  The result of using the −T option is displayed on standard output and shows the inputs and resulting transformations as mailsurr is processed by the mail command for the indicated recipient.  Mail messages will never be sent or delivered when using the −T option. 

FILES

/etc/mail/mailsurr

/usr/lib/mail/surrcmd/∗ surrogate commands

/etc/mail/mailcnfg initialization information for mail

SEE ALSO

ckbinarsys(1M), ed(1), egrep(1), mail(1), sh(1), uux(1), exec(2), exit(2), wait(2), getdomainname(3) popen(3), mailcnfg(4), regexp(5). 

NOTES

It would be unwise to install new entries into the system mailsurr file without verifying at least their syntactical correctness via ‘mail −T . . .’ as described above. 

  —  Essential Utilities