Provided by: mailavenger_0.8.2-1_amd64 bug

NAME

       avenger - Mail Avenger

DESCRIPTION

       Mail Avenger is a highly-configurable MTA-independent SMTP (Simple Mail Transport
       Protocol) server designed to let you filter and fight SPAM before accepting incoming mail
       from a client machine.  avenger is the script run on behalf of each user to decide whether
       to accept incoming mail.

       When a client attempts to send mail to a user on the system, the avenger SMTP daemon,
       asmtpd, runs avenger to process the file .avenger/rcpt in the user's home directory.  That
       file, a shell script with access to special functions, determines how the SMTP server
       should proceed.  The possible outcomes are:

       ·   Provisionally accept the mail, falling back to system-default rules

       ·   Accept the mail immediately with no further checks

       ·   Reject the mail immediately

       ·   Defer the mail, telling the client to re-send it later

       ·   Redirect the processing to another local name.  The name can be another email address
           belonging to the current user, or an email address belonging to the special
           AvengerUser user.  In the later case, avenger will be re-run with a different user ID,
           and hence can, for example, employ utilities that maintain state across multiple users
           (assuming they all redirect processing the same way).

       ·   Run a "bodytest" rule.  With this outcome, the the SMTP transaction continues on to
           receive the entire contents of the mail message, after which a program is run on the
           contents of the mail message.  That program can decide, based on the contents, whether
           to accept, reject, defer, or silently discard the message.

       Mail Avenger should typically be configured to have a Separator character, allowing each
       user to maintain multiple email addresses.  With sendmail, Separator is typically "+",
       with qmail it is typically "-".  If the separator is "+", then any email sent to
       user+ext@your-host will be processed by files in user's .avenger directory.

       Avenger first checks for a file named rcpt+ext in a user's .avenger directory, then for
       rcpt+default.  If ext itself contains the separator character, for example
       user+ext1+ext2@your-host, avenger will check first for rcpt+ext1+ext2, then for
       rcpt+ext1+default, then for rcpt+default.  The same algorithm is extended for arbitrarily
       many separator characters.  (If separator is "-", simply replace "+" with "-" throughout
       the above description, including in the names of files such as rcpt-default.)

       If mail is rejected by the recipient checks but the sender address of a message is local
       and UserMail is 1 in asmtpd.conf (which is not the default), then before rejecting mail,
       avenger will be run on behalf of the sending user.  In this case, the address will be
       parsed as above, but avenger will look for rules in files beginning mail instead of rcpt.
       This mechanism can be used by local users who want to relay mail through the server from
       an untrusted IP address.

       Using the mail configuration files, each user can, for instance, configure a mail+... file
       to accept mail from an IP address he or she trusts, even if that address is not trusted by
       all users.  (Alternatively, using tools such as macutil, a user might set up relaying of
       mail in which the envelope sender contains a cryptographic code, checked by the mail+...
       script.)

       Error output of an avenger script rcpt+ext or mail+ext is redirected to a file called
       log+ext in the same directory, for use in debugging.

AVENGER SYNTAX

       Avenger configuration files are simply shell scripts, using the syntax described in sh(1).
       Each line of the file contains a variable assignment, command, or function to run.
       Scripts can additionally make use of a number of avenger-specific functions and variables.
       This section describes avenger functions.  The next two sections describe variables.

       errcheck
           Certain error conditions result in Mail Avenger rejecting mail by default, unless the
           message is explicitly accepted through an accept or successful bodytest check.  These
           conditions are indicated by the MAIL_ERROR environment variable described below.  If
           your script either rejects mail or falls through to the default behavior, there is
           often no reason to run tests on a message that will end up being rejected either way.
           errcheck exits immediately with the default error if the default would be to reject or
           defer the mail.

       accept [message]
           Immediately accepts the message (without falling back to any default rules).  If
           message is supplied, it will be returned to the SMTP client.  The default message is
           "ok".

       reject [message]
           Reject the mail, with message.  (The default message is "command rejected for policy
           reasons").

       defer [message]
           Reject the mail with a temporary error code, so that a legitimate mail client will
           attempt to re-send it later.  The default for message is "temporary error in
           processing".

       bodytest command [arg ...]
           Accept the current SMTP "RCPT" command.  However, once the whole mail message has been
           received with the SMTP "DATA" command, run command with the message as its standard
           input.  Depending on the exit status of command return to the client's "DATA" command
           either success, temporary, or permanent failure.  Exit code 0 means accept the mail,
           100 means reject, 111 means reject with a temporary error code (i.e., defer the mail).
           See the description of bodytest in the asmtpd/avenger interface description for more
           information on bodytest (since this function directly invokes bodytest in asmtpd).

           Error output from command will be redirected to the same log file as output from the
           rcpt+... avenger script invoking the bodytest function.  Standard output of command
           will be included as a diagnostic the bounce message if the exit code defers or rejects
           the mail.

           Note that command and the arguments passed to bodytest will be run by the shell.
           Thus, it is important not to pass any arguments that might contain shell
           metacharacters such as ">" and "$".

       redirect local
           Finish processing, and re-run avenger as if mail were being sent to a different
           username local (possibly belonging to the special AvengerUser user).  See the
           description of redirect in the asmtpd/avenger interface description for more
           information on redirect (since this function directly invokes redirect in asmtpd).

       greylist [sender-key]
           This command defers mail the first time mail is received from a particular sender at a
           particular IP address.  However, after a certain interval, greylist_delay, if the
           client re-sends the mail, it will be accepted.  Furthermore, from that point on, all
           mail will be immediately accepted from that sender and IP address, unless the sender
           stops sending mail for a period of greylist_ttl2 or more.  If, however, after sending
           the initial, defered piece of mail, the client does not try again within a period of
           greylist_ttl1, then any record of the client will be erased, and the next time it
           tries to send mail it will be defered again.

           The parameters can be tuned by setting variables in the script.  The default values
           are:

               greylist_delay=30m  # Time to wait before allowing message
               greylist_ttl1=5h    # How long to remember first-time senders
               greylist_ttl2=36D   # How long to remember ok senders

           m means minutes, h hours, and D days.  For a complete list of allowed suffixes, see
           the documentation for dbutil(1) (in particular for the --expire option).

           sender-key, if supplied, is used to identify the sender.  The default value is
           "$CLIENT_IP $RECIPIENT $SENDER".  If, for example, you wanted to record only the first
           24-bits of IP address and didn't care about the recipient, you could use the command:

               greylist "${CLIENT_IP%.*} $SENDER"

       setvars
           All functions that set a variable by means of an external query to asmtpd are
           performed asynchronously.  setvars actually waits for results and sets the values of
           those variables.  In this way, a number of potentially slow requests (such as DNS
           lookups) can be initiated concurrently, and their latencies overlapped.  However, one
           must remember to call setvars, or else variables that should contain the results of
           operations will remain unset.

       dns var type domain-name
           Performs a DNS lookup of domain-name for records of type type, and assigns the result
           to variable var when you call setvars.  type must be one of a, mx, ptr, or txt (lower-
           case only).

       rbl [-ipf] var domain
           Looks up the current mail sender in a real-time blackhole list (RBL).  domain is the
           domain name of the RBL (e.g., "bl.spamcop.net").  If the sender is listed, set var to
           the result of the DNS lookup when you next call setvars.  -i looks up the sender's IP
           address (the default if no options are specified).  -p looks up the sender's domain
           name (verified DNS PTR record).  -f looks up the envelope sender domain name in the
           RBL.

       spf0 var [spf-mechanism ...]
       spf var [spf-mechanism ...]
           Tests the sender against an arbitrary query formulated in the SPF language.  This is a
           powerful way to whitelist or blacklist particular senders.  For example, suppose you
           want to accept any mail from machines in the list maintained by trusted-forwarder.org,
           accept mail from any machine name ending "yahoo.com" reject any mail from users in the
           spamcop RBL, and for other users fall back to the default system-wide rules.  You
           might use the following rcpt file:

               spf MYSPF +include:spf.trusted-forwarder.org \
                   +ptr:yahoo.com -exists:%{ir}.bl.spamcop.net ?all
               setvars
               case "$MYSPF" in
                   pass)
                       accept "I like you"
                       ;;
                   fail)
                       reject "I don't like you"
                       ;;
                   error)
                       # Note, could instead fall through to default here
                       defer "Temporary DNS error"
                       ;;
               esac

           Note that commands spf0 and spf are synonymous, but spf is deprecated, because in a
           later release of Mail Avenger spf will become synonymous with spf1.

       spf1 var [spf-mechanism ...]
           Performs the same tests as the spf directive, but returns the result strings None,
           Neutral, Pass, Fail, SoftFail, TempError, and PermError instead of none, neutral,
           pass, fail, softfail, error, and unknown.

AVENGER VARIABLES

       These variables are set by the avenger script.  In addition, asmtpd sets a number of
       environment variables before running avenger.  These are documented in the next section,
       ENVIRONMENT.

       FILEX
           The extension on the file currently being processed.  For example, if file rcpt+ext is
           being processed, will be set to "+ext".  Empty when processing just rcpt (or mail).
           May also contain default when a default rule file for some suffix is being run.

       PREFIX
       SUFFIX
           Assuming the separator is "+", when processing a file rcpt+base+default or
           mail+base+default, PREFIX is set to base, while SUFFIX is set to the portion of the
           name for which default was substituted.  When the file does not end with default,
           SUFFIX is empty.  When the file is just rcpt with no extension, both PREFIX and SUFFIX
           are empty.  When SUFFIX itself contains a "+" character, SUFFIX1 contains to the part
           of SUFFIX after the first "+" character, SUFFIX2 contains the part after the second
           "+", and so on for each "+" character in suffix.

ENVIRONMENT

       AUTH_USER
           If Mail Avenger was compiled with SASL support (which is not the default, unless you
           supplied the --enable-sasl argument to "configure"), and if the client successfully
           authenticates to the server using SASL, then AUTH_USER will be set to the name of the
           authenticated user.

       AVENGER_MODE
           Set to "rcpt" when testing whether a recipient should receive mail.  Set to "mail"
           (possibly after an "rcpt" check fails) when checking whether to relay mail (possibly
           on behalf of a local user).

       AVUSER
           The effective local username for which avenger is being run.  Ordinarily, this will be
           the same as:

           $USER${PREFIX+$SEPARATOR}$PREFIX\
           ${SUFFIX+$SEPARATOR}$SUFFIX

           However, for special avenger files like unknown and default, it can contain useful
           information, because unlike the RECIPIENT_LOCAL environment variable, AVUSER reflects
           substitutions from the Mail Avenger domains and aliases files.

       CLIENT
           This variable contains the name of the client machine, as typically reported in
           "Received:" headers.  Its value has the form:

               [user@]host

           user is the user name for the connection reported by the client, if the client
           supports the RFC 1413 identification protocol, otherwise it is omitted.  host is a
           verified DNS hostname for the IP, if asmtpd could find one.  Otherwise, it is simply
           the numeric IP address.

       CLIENT_COLONSPACE
           Set to 1 if the client included a space between the colon in the command "MAIL FROM:"
           or "RCPT TO:" and the subsequent "<" that begins an email address.

       CLIENT_DNSFAIL
           If AllowDNSFail is set to 1 in the asmtpd.conf file and resolving the client's IP to a
           hostname returns a temporary error, then this variable will be set to a description of
           the error.

       CLIENT_HELO
           Set to the argument the client supplied to the SMTP "HELO" or "EHLO" command.

       CLIENT_IP
           Set to the IP address of the client.

       CLIENT_NAME
           Set to the verified DNS name of the client, if asmtpd can find one.

       CLIENT_NETHOPS
           Set to the number of network hops between the server and the client, if asmtpd can get
           the client or its firewall to return an ICMP destination unreachable (type 3 packet)
           in response to a UDP probe.  Whether or not this is set will depend on firewall
           configurations.

       CLIENT_NETPATH
           Set to as many intermediary network hops as asmtpd can determine between the server
           and the client.  How close to the client asmtpd can probe will depend on firewalls.

       CLIENT_PIPELINING
           Set to 1 if the client wrote data after the SMTP HELO or EHLO command, before
           receiving its response.  A correct SMTP client should not "pipeline" commands until
           after receiving the result of the HELO command and verifying that the server accepts
           pipelined commands.

       CLIENT_PORT
           The TCP port number of the client.

       CLIENT_POST
           Set to 1 if the client sent a "POST" command at some point during the SMTP session.
           "POST" is not a valid SMTP command; it is an HTTP command.  However, one technique for
           sending spam involves exploiting an open web proxy to "post" an SMTP session to a mail
           server.  The initial HTTP headers (including the HTTP post command) simply cause SMTP
           syntax errors, while the body of the POST command contains SMTP commands.  By checking
           the CLIENT_POST environment variable, you to reject mail sent in this way.

       CLIENT_REVIP
           The value of CLIENT_IP with the order of the bytes reversed.  Suitable for prepending
           to ".in-addr.arpa" or an RBL domain to perform a DNS lookup based on IP address.

       CLIENT_SYNFP
           Contains a fingerprint, abstracting the contents of the initial TCP SYN packet the
           client sent to establish the TCP connection.  The exact contents of SYN packets
           depends on the operating system and version of the client, and can therefore reveal
           interesting information about the type of client connecting to your mail server.  The
           format of the fingerprint is:

               wwww:ttt:D:ss:OOO

           Where the fields are as follows:

           wwww
               the initial TCP window size

           ttt the IP ttl of the received packet

           D   the IP "don't fragment" bit

           ss  total size of the SYN packet (including IP header)

           OOO a comma-separated list of TCP options, as follows:

               N   NOP option

               Wnnn
                   window scaling option with value nnn

               Mnnn
                   maximum segment size value nnn

               S   Selective ACK OK

               T   timestamp option

               T0  timestamp option with value zero

       CLIENT_SYNOS
           If asmtpd can guess the client's operating system based on CLIENT_SYNFP, it will set
           CLIENT_SYNOS to the value of that guess.  For example, to greylist mail from Windows
           machines, you can run:

              match -q "*Windows*" "$CLIENT_SYNOS" && greylist

       DATA_BYTES
           This variable is not really an avenger variable, as it is only available in bodytest
           commands.  It specifies the number of bytes of message transfered in the SMTP DATA
           command, but after converting CR NL sequences to NL.  Roughly speaking this is how
           many bytes are in the message including all headers after the X-Avenger:, SPF-
           Received, or Received: header.

       ETCDIR
           The value of EtcDir from the asmtpd configuration file (or /etc/avenger by default).

       EXT When avenger runs on behalf of a user EXT is set to the part of the address that
           determines the suffix of the rcpt or mail file.  For example, suppose Separator is "-"
           and the recipient is list-subscribe@host, where host is not a virtual domain.  If the
           AliasFile contains:

               list: user-mylist

           Then avenger will be run on behalf of "user" (because alias expansion yields user-
           mylist-subscribe).  EXT will be set to mylist-subscribe.

           Note that EXT is empty when there is no suffix, and that it is equal to the name of
           the system file being processed when avenger is run on a system file.  Like RECIPIENT,
           this variable is not set for bodytest commands.

       HOST
           Set to the name of the local host, as specified by the HostName directive in
           avenger.conf.

       MAIL_ERROR
           This variable is set when the SPF disposition of the sender is fail, or when asmtpd is
           unable to send a bounce message to the sender address.  In either case, Mail Avenger
           will reject the mail if the script falls through to the default.

       MSGID
           A randomly generated string for this message, which can be useful to correlate calls
           to rcpt scripts with bodytest scripts.  Note this is unrelated to the Message-ID
           header in the message, but does show up in the Received header that Mail Avenger
           inserts.

       MYIP
           IP address of local end of SMTP TCP connection.

       MYPORT
           TCP port number of local end of SMTP TCP connection.  Ordinarily this will be 25.

       RECIPIENT
           The envelope recipient of the message.  Note that this environment variable is not
           present for bodytest programs, since such programs may be run on behalf of multiple
           users.

       RECIPIENT_HOST
           The domain part of RECIPIENT, folded to lower-case--i.e., host when RECIPIENT is
           local@host.  Not present for bodytest programs, as noted in the description of
           RECIPIENT.

       RECIPIENT_LOCAL
           The local part of RECIPIENT, folded to lower-case--i.e., local when RECIPIENT is
           local@host.  Not present for bodytest programs, as noted in the description of
           RECIPIENT.

       SENDER
           The envolope sender of this mail message (i.e., the argument supplied by the client to
           the "MAIL FROM:" SMTP command.)

       SENDER_HOST
           The hostname part of SENDER, converted to lower-case (i.e., host in user@host).

       SENDER_LOCAL
           The local part of SENDER, converted to lower-case (i.e., user in user@host).

       SENDER_MXES
           A list of DNS MX records for SENDER_HOST, if that hostname has any MX records.

       SENDER_BOUNCERES
           For non-empty envelope senders, asmtpd attempts to see if it is possible to deliver
           bounce messages for the sender.  If not, SENDER_BOUNCERES is set to a three-digit SMTP
           error code.  If the first digit is 4, the error was temporary.  If the first digit is
           5, the error was permanent.  Note that failure to accept bounce messages is considered
           a MAIL_ERROR as described above, and will cause mail to be rejected by default.

       SEPARATOR
           The value of Separator from the asmtpd configuration file.  There is no default
           (SEPARATOR will not be set if no Separator is specified in the configuration file).
           However, it should be configured for "+" with sendmail and "-" with qmail.

       SPF0
       SPF The result of performing an SPF check on the message.  Will be one of: none, neutral,
           pass, fail, softfail, error, or unknown.  Note that SPF0 and SPF are synonymous, but
           SPF is deprecated as a future release of Mail Avenger will make SPF synonymous with
           SPF1.

       SPF1
           Also the result of performing an SPF check on the message, but returns different names
           for the results, to be compatible with newer revisions of the SPF protocol
           specification.  The new names are None, Neutral, Pass, Fail, SoftFail, TempError, and
           PermError.

       SPF_EXPL
           The explanation string that goes along with a bad SPF status.

       SSL_CIPHER
           If the Mail Avenger has been compiled with support for the STARTTLS command (using the
           --enable-ssl option to "configure"), and the client is communicating over SSL/TLS,
           this variable will contain a textual description of the algorithm.

       SSL_CIPHER_BITS
       SSL_ALG_BITS
           SSL_CIPHER_BITS contains the number of secret key bits used by the SSL/TLS ciphers.
           SSL_ALG_BITS is the number of bits used by the algorithm.  For example, if you are
           using 128-bit RC4 with 88 bits sent in cleartext, SSL_CIPHER_BITS will only be 40,
           since that is the effective security, while SSL_ALG_BITS will be 128.

       SSL_ISSUER
       SSL_ISSUER_DN
           If the client has successfully authenticated itself using an SSL certificate,
           SSL_ISSUER will be set to the certificate signer's common name, while SSL_ISSUER_DN
           will be set to a compact representation of the signer's full distinguished name.  The
           full distinguished name is in the form output by the command:

                   openssl x509 -noout -issuer -in cert.pem

           Note that this variable is mostly useful if the SSLCAcert file you have given to Mail
           Avenger contains more than one certificate authority, or signs other CA certificates.
           Mail Avenger will not accept client certificates if it does not recognize the signer
           of the certificate.

       SSL_SUBJECT
       SSL_SUBJECT_DN
           If the client has successfully authenticated itself using an SSL certificate,
           SSL_SUBJECT will be set to the client's common name in the certificate, while
           SSL_SUBJECT_DN will be set to a compact representation of the client's full
           distinguished name.  The full distinguished name is in the form output by the command:

                   openssl x509 -noout -subject -in cert.pem

       SSL_VERSION
           The version of the SSL/TLS protocol in use.

       UFLINE
           An mbox "From " line suitable for prepending to the message before passing the message
           to a delivery program.  (This is mostly useful for bodytest rules.)

       USER
           The name of the user under which avenger is running.

AVENGER/ASMTPD INTERFACE

       avenger is just a simple shell script.  You can inspect the file to see what it is doing.
       Most of the interesting operations happen in either asmtpd, or in external programs
       spawned from avenger.  This section documents the interface between asmtpd and avenger.

       avenger inherits a unix-domain socket connected to asmtpd on its standard input and
       output.  It sends commands to asmtpd over this socket, and similarly reads replies from
       it.  In order to avoid mixing messages to and from asmtpd with the output of other
       programs you run, however, the avenger shell script reorganizes its file descriptors so
       that all communication to and from asmtpd happens over file descriptor number 3.

       Each command consists of a single line, followed by a newline (except the return command,
       which can optionally take multiple lines).  There may or may not be a reply, possibly
       depending on the outcome of the command.  Most replies consist of zero or more lines of
       the form

           VARIABLE=value

       VARIABLE is typically a variable name that was supplied as part of the command.  The
       avenger shell script records results by setting the environment variable VARIABLE to
       value, so that it can be accessed by subsequent lines of the script.

       Replies are sent in the order in which the corresponding commands were received.  However,
       asmtpd executes requests asynchronously.  Thus, one can perform several concurrent
       operations (such as DNS requests or SPF tests) by simply writing multiple commands to
       asmtpd before receiving any of the responses.

       The "." command is a no-op, but asmtpd echoes the "." back to avenger as the reply.  This
       allows one to synchronize the avenger process's state after issuing one or more commands.
       For example, one might issue several DNS lookups to check various RBLs (real-time
       blackhole lists), then issue a . command, then wait for replies.  When the . comes back,
       all previous commands will also have completed.  The avenger setvars command simply sends
       a ".", then loops until it reads back the ".", setting variables from any previous
       commands whose replies it reads in the process.

       The following commands are available:

       .   The . command is simply echoed back by asmtpd.

       bodytest command
           Ends the current avenger script.  Specifies that asmtpd should receive the entire body
           of the message, then run command (under the same user ID as the current avenger
           script) with the entire mail message as its standard input.  asmtpd then replies to
           the SMTP "DATA" command based on the exit status of command as follows:

           0   If command exits with status 0, asmtpd will reply to the "DATA" command with
               success (SMTP code 250), and will pass the message to sendmail (or whatever you
               have configured as Sendmail in asmtpd.conf) for delivery.

           99  If command exits with status 99, asmtpd will still reply to the "DATA" command
               with a successful 250 reply code, but will not spool the data.  Either command
               must have done something with the data, or the message will be lost.

           100 (also 64, 65, 70, 76, 77, 78, 112)
               If command exits with status 100 (or any of the above exit statuses), avenger will
               reject the mail with a hard SMTP error (code 554).  If command wrote output to its
               standard output, this output will be passed back to the mail client.  Otherwise,
               asmtpd will supply the text "message contents rejected."

           111 (or any other exit status)
               If command exits with status 111, the result is the same as exit status 100,
               except that asmtpd will use a temporary error code (451) instead of 554.

           signal
               If command exits abnormally because of a signal, asmtpd will also use 451, but in
               this case will not pass the program's output back to the client.  It will instead
               pass back a description of the problem.

           Note that asmtpd can only run one bodytest command per message.  If there are multiple
           recipients of a message, all must run the same bodytest under the same user ID.  If
           two users wish to run different bodytest commands, or even run the same command under
           different user IDs, asmtpd will defer the second SMTP "RCPT" command with the message:

               452 send a separate copy of the message to this user

           This will cause the mail client to re-send the message later to the second user.  To
           avoid forcing clients to send multiple copies of messages, you can place bodytest
           commands in system wide files (such as the default rule file), or use a redirect
           command to redirect to the AvengerUser, so that commands for multiple users can be run
           under the AvengerUser user ID.

           Note that file descriptor 0 inherited by command is opened for both reading and
           writing.  Thus, it is possible to modify the message before it is spooled by the local
           MTA.  The command edinplace(1) is useful for running messages through spam filters
           that annotate messages before spooling them.

       dns-a VARIABLE domain-name
           Requests that asmtpd perform a DNS lookup for A (IPv4 address) records on domain-name.
           If such an A record exists, the reply is a list of one or more IP addresses:

               VARIABLE=IP-address ...

           If no such A record exists, the reply is simply:

               VARIABLE=

           With the standard avenger script, this sets VARIABLE to the empty string.  If there is
           a temporary error in DNS name resolution, there is no reply, and hence with the
           default avenger script VARIABLE will remain unset.

           When checking such things as RBLs, it is advisable not to reject mail because of a
           temporary DNS error.  You can use the shell construct ${VARIABLE-default}$ to return
           $VARIABLE when VARIABLE is set, and default when VARIABLE is not set.  Similarly
           ${VARIABLE+set} returns set if VARIABLE is set, and the empty string otherwise.

           For example, if bad-senders.org contained an RBL of undesirable sender hosts:

               echo dns-a BADSENDER "$SENDER_HOST".bad-senders.org >&3
               setvars
               test -n "$BADSENDER" && reject "$SENDER_HOST is a bad sender"
               test -z "${BADSENDER+set}" \
                   && defer "$SENDER_HOST.bad-senders.org: DNS error"

           Note that when using the avenger script, there is already a function rbl to check
           RBLs.

       dns-mx VARIABLE domain-name
           Similar to dns-a, but looks up MX records.  A successful reply is of the form:

               VARIABLE=priority-1:host-1 [priority-2:host-2 ...]

           Where priority-1 is the MX priority of host-1.  As before, an empty string indicates
           no MX records exist, and no reply indicates an error.

       dns-ptr VARIABLE IP-address
           Returns a list of verified DNS hostnames for IP-address.  As before, an empty string
           for VARIABLE indicates no PTR records exist, and no reply indicates an error.

       dns-txt VARIABLE domain-name
           Similar to the other dns commands, but looks up a record of type TXT.  If multiple TXT
           records exist, returns only one.  Places some restrictions on the TXT records, for
           example will not return one that contains a newline character.

       netpath VARIABLE IP-address
           Maps out the network hops to IP-address (this is similar to the traceroute system
           utility, but more efficient).  The reply is of the form:

               VARIABLE=#hops hop1 hop2 ...

           #hops is the total number of network hops to IP-address if asmtpd can figure this out.
           (It won't always be able to if IP-address is behind a firewall.)  If asmtpd cannot
           figure this out, the value is -1.  hop1 and the remaining arguments are the addresses
           of routers along the way to IP-address.

       redirect local
           Terminates the current avenger process, and instead processes the mail as though it is
           being sent to local.  This command is only available in "rcpt" mode, as opposed to
           "mail" mode (in which asmtpd runs avenger to see if it should relay mail for a local
           user on a non-local client machine).

           local can be a local user name, or a local user name followed by the separator
           character and an extension.  The name is mapped using the aliases (specified by
           AliasFile in asmtpd.conf).

           Note that while the AvengerUser user can redirect to other users, ordinary users can
           only redirect to themselves or the AvengerUser.

       return code explanation
                   or
       return code-explanation
       code-explanation
       code explanation
           Specifies the SMTP reponse desired.  Also avoids further processing of the message
           with system-wide default rulesets (as typically happens when avenger simply exits with
           status 0).  code must be a three digit number beginning 2, 4, or 5.  (usually 250 for
           success, 451 to defer mail, and 554 to reject mail).

           The first form of this command (with a space between code and explanation) gives a
           single line explanation along with the result code.  In the second form, avenger
           specifies a multi-line response.  In this case all but the last line must contain a -
           between the code and explanation, while the last line must contain a space.  (Note
           that the return keyword only appears on the first line; after starting to issue a
           return command, no further commands can be issued.)

       spf VARIABLE SPF-mechanism ...
       spf0 VARIABLE SPF-mechanism ...
       spf1 VARIABLE SPF-mechanism ...
           Evaluates the mail client based on SPF mechanisms.  It will return:

               VARIABLE=disposition

           where, for spf0, disposition is one of:  none, neutral, pass, fail, softfail, error,
           or unknown (though the disposition none is actually impossible).  For spf1, the
           equivalent disposition names are None, Neutral, Pass, Fail, SoftFail, TempError,
           PermError.  (Currently spf is a synonym for spf0, but it is recommended that you avoid
           using spf as in a future release it may become an alias for spf1.)

           As an example, suppose that your username is "joe", Separator is "+", and you have
           subscribed to a number of yahoo mailing lists using email address "joe+yahoo".  If
           spammers started sending mail to "joe+yahoo", you would want to reject all mail to
           that address except that originating from yahoo's computers.  Yahoo's computers might
           correspond to anything ending ".yahoo.com" or sharing a 24-bit IP-address prefix with
           any of yahoo.com's MX records.  This can be accomplished with the following script in
           $HOME/.avenger/rcpt+yahoo:

               echo spf YAHOO ptr:yahoo.com mx:yahoo.com/24 -all >&3
               setvars
               case "$YAHOO" in
               fail)
                   reject "Sorry, this private alias for Yahoo lists only"
                   ;;
               error)
                   defer "Sorry, temporary DNS error"
                   ;;
               esac

EXAMPLES

       If you never use your email address as an envelope sender, you can reject all bounces to
       that address with these commands in your rcpt file:

           test -z "$SENDER" \
               && reject "<$RECIPIENT> not a valid sender;" \
               " should not receive bounces"

       The following script runs spamassassin (a popular spam filter, available from
       <http://www.spamassassin.org/>) on the body of a message, unless the sender of the message
       has an SPF disposition of pass or is already going to be rejected by default.

           # The next line immediately falls through to the default reject
           # disposition when mail has an SPF disposition of fail or the
           # sender does not accept bounce messages.
           errcheck

           test "$SPF" = pass \
               || bodytest edinplace -x 111 spamassassin -e 100

       The following script immediately accepts any mail from any machine at MIT or NYU (provided
       MAIL_ERROR is not set), "greylists" machines not in one of those domains, and if the
       greylist passes, falls through to the the default, system-wide rules:

           errcheck

           spf TRUSTED ptr:nyu.edu ptr:mit.edu ?all
           setvars
           test pass = "$TRUSTED" && accept Trusted sender OK

           greylist_delay=5m
           greylist

       The following script rejects mail from clients that have issued an SMTP "POST" command
       (which doesn't exist) or used aggressive, premature pipelining of commands.  If the client
       put a space after the colon in the MAIL FROM: or RCPT TO: SMTP commands, it greylists the
       message using a key that includes the SYN fingerprint and first 24-bits of the IP address.
       If the SPF disposition of the message is error, it defers the message.  If the SPF
       disposition of the message is softfail or none, it runs the body of the message through
       spamassassin.

           errcheck

           test -n "$CLIENT_POST" -o -n "$CLIENT_PIPELINING" \
               && reject "no spam please"

           test -n "$CLIENT_COLONSPACE" \
               && greylist "${CLIENT_IP%.*} $CLIENT_SYNFP $SENDER"

           case "$SPF" in
               error)
                   defer "Temporary error in SPF record processing"
                   ;;
               softfail|none)
                   bodytest edinplace -x 111 spamassassin -e 100
                   ;;
           esac

       If you set your MACUTIL_SENDER environment variable to be "user+bounce+*@your.host.com"
       and send mail with macutil --sendmail, you can create the following rcpt+bounce+default to
       accept mail only to valid bounce addresses.

           macutil --check "$SUFFIX" > /dev/null \
               || reject "<$RECIPIENT>.. user unknown"

       In conjunction with this script, you may want to reject bounce messages to your regular
       email addresss with your rcpt script, as described in the first example.

       This example is slightly more complicated, and shows how to use a bodytest to reject mail
       based on message contents.  The goal of this set-up is to check each message with the
       ClamAV anti-virus software (from <http://www.clamav.net/>) and the spamassassin mail
       filter.  If the message contains a virus or is flagged as spam, it should be rejected with
       an explanation of the problem.  We construct a shell script, $HOME/.avenger/body, to run
       these tests on message bodies.  The script can be invoked with the line

           bodytest $HOME/.avenger/body

       in your $HOME/.avenger/rcpt file.  Or, alternatively the script could be configured to run
       in the system-wide /etc/avenger/default file (in which case you want to make sure that the
       AvengerUser can write its own home directory, so as to store spamassassin files).  The
       script is as follows:

           #!/bin/sh
           out="`clamscan -i --no-summary --mbox -  2>&1`"
           if test "$?" = 1; then
               echo This message appears to be infected with a virus
               printf "%s\n" "$out" \
                   | sed -e '/Warning:/d' -e 's/^[^:]*: //' | sort -u
               exit 100
           fi

           out="`edinplace -x 111 spamassassin -e 100`"
           case "$?" in
               0)
                   exit 0
                   ;;
               100)
                   echo Sorry, spamassassin has flagged your message as spam
                   while read a b c; do
                       test "$a $b" = "Content analysis" && break
                   done
                   read a
                   read a
                   read a
                   while read a b c; do
                       case "$a" in
                       "")
                           break
                           ;;
                       -*)
                           ;;
                       [0-9]*)
                           printf "  %s\n" "$c"
                           ;;
                       *)
                           printf "    %s\n" "$a $b $c"
                           ;;
                       esac
                   done
                   exit 100
                   ;;
               *)
                   if test -n "$out"; then
                       echo spamassassin failure:
                       printf "%s\n" "$out"
                   else
                       echo system error in spamassassin
                   fi
                   exit 111
                   ;;
           esac

       The first half of this script runs the clamscan virus checker, storing the output in
       variable out.  clamscan exits with code 1 when a virus is found, exits 0 on success, and
       uses other error codes to indicate various system errors.  We only want to reject mail if
       clamscan exits with code 1.  When this happens, we take the output of clamscan, format it
       in a more pleasing way (stripping out warnings), and send it to standard output.  An
       example of an SMTP transaction using this bodytest and detecting a virus will look like
       this (tested with the special EICAR test string that flags a positive with most virus
       checkers):

           DATA
           354 enter mail, end with "." on a line by itself
           Subject: eicar test

           X5O!P%@AP[4\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*
           .
           554-This message appears to be infected with a virus
           554 Eicar-Test-Signature FOUND

       If the virus check fails, the script runs the message through spamassassin to check for
       spam.  Note that spamassassin modifies the mail message, so that we must run it with
       edinplace.  Note also that clamscan will read to the end of the input file, but this is
       okay since edinplace rewinds its standard input.  We use the -e flag to tell spamassassin
       to exit 100 on spam.  Then, if spamassassin exits 0, we accept the mail.  If it exits with
       anything but 100, something went wrong and we temporarily defer the mail.  Note that it
       might also be possible to accept the mail at this point, but since spamassassin edits the
       file in place, the message may be truncated if spamassassin exits unexpectedly.

       If spamassassin exits 100, we reject the mail.  We also report on why spamassassin has
       rejected the mail.  Here again we take advantage of the fact that edinplace rewinds its
       standard input both before and after processing a message.  Because the file descriptor
       has been rewound, we can start processing the message one line at a time with the shell
       script.  Spamassassin by default (if you have not configred it with "report_safe 0")
       contains a spam report like this:

        Content analysis details:   (11.7 points, 5.0 required)

         pts rule name        description
        ---- --------------- --------------------------------------------------
         1.0 RATWARE_RCVD_AT Bulk email fingerprint (Received @) found
         4.2 X_MESSAGE_INFO  Bulk email fingerprint (X-Message-Info) found
         0.0 MONEY_BACK      BODY: Money back guarantee
         0.5 BIZ_TLD         URI: Contains a URL in the BIZ top-level domain
         0.6 URIBL_SBL       Contains a URL listed in the SBL blocklist
                             [URIs: crocpeptide.biz]
         0.5 URIBL_WS_SURBL  Contains a URL listed in the WS SURBL blocklist
                             [URIs: crocpeptide.biz]
        ...

       We skip over the headers, and for each result, print it to the SMTP session.
       Negative/whitelist results (those starting -), we do not report, and comment lines (not
       starting with a number) we print indented.  A typical SMTP session looks like this (using
       the special GTUBE test line that triggers spam filters):

           DATA
           354 enter mail, end with "." on a line by itself
           Subject: gtube test

           XJS*C4JDBQADN1.NSBN3*2IDNEN*GTUBE-STANDARD-ANTI-UBE-TEST-EMAIL*C.34X
           .
           554-Sorry, spamassassin has flagged your message as spam
           554-  Missing Date: header
           554   BODY: Generic Test for Unsolicited Bulk Email

       Here's an example of how to use SSL client certificates for authentication.  If you have a
       private CA with common name "My CA" that signs the certificates of all your authorized
       mail clients, you can place the following in /etc/avenger/relay to permit those clients to
       relay:

           test "My CA" = "$SSL_ISSUER" \
               && accept "Relaying permitted for client $SSL_SUBJECT"
           reject "relaying denied"

FILES

       /usr/local/libexec/avenger, /etc/avenger/default, $HOME/.avenger/rcpt,
       $HOME/.avenger/rcpt* $HOME/.avenger/mail, $HOME/.avenger/mail*

SEE ALSO

       dbutil(1), deliver(1), edinplace(1), escape(1), macutil(1), match(1), synos(1),
       asmtpd.conf(5), asmtpd(8), avenger.local(8)

       The Mail Avenger home page: <http://www.mailavenger.org/>.

BUGS

       avenger (and the configuration files it reads) are shell scripts.  In a shell script, it
       is sometimes tempting to use "echo ..." where one should instead use the command "printf
       '%s\n' ...".  (The later just prints its argument to standard output, while the former
       interprets various "\" escape codes.)

       In shell scripts, one must be careful about variables containing shell metacharacters.
       For example, it is not safe to run something like:

               bodytest "echo $VAR > $PWD/log"

       if variable "VAR" has untrusted contents that might contain characters like ">" or ";".
       The reason is that $VAR will be expanded and sent back to the SMTP server, which will then
       pass the expansion to the shell to execute the bodytest.  ($VAR effectively gets expanded
       twice.)  The escape utility can be used to avoid these problems.  For example:

               bodytest echo `escape "$VAR"` ">" $PWD/log

       It is easy to forget to call setvars after a dns, rbl, or spf command.

AUTHOR

       David Mazieres