Provided by: mailavenger_0.8.1-4ubuntu1_i386 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:

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

       o   Accept the mail immediately with no further checks

       o   Reject the mail immediately

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

       o   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).

       o   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