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