.TH dkim-filter 8 "Sendmail, Inc."
.SH NAME
.B dkim-filter
- DKIM filter for sendmail
.SH SYNOPSIS
.B dkim-filter
-p socketspec [-a peerlist] [-A] [-b modes] [-c canon] [-C config] [-d domains] [-D] [-f] [-F time] [-i ilist] [-I eilist] [-h] [-H hdrlist] [-k keyfile] [-K] [-l] [-L min] [-m mtas] [-M macro[=value][,...]] [-o hdrlist] [-P pidfile] [-q] [-R] [-s selector] [-S signalg] [-t testfile] [-T secs] [-u userid[:group]] [-U popdb] [-v] [-V] [-x configfile]
.SH DESCRIPTION
.B dkim-filter
implements the
.B DKIM
standard for signing and verifying e-mail messages on a per-domain basis.

.B dkim-filter
use the milter interface to
.B sendmail(8)
to provide DKIM signing and/or verifying service for mail transiting a
sendmail MTA.

Most, if not all, of the command line options listed below can also be set
using a configuration file.  See the
.I -x
option for details.
.SH OPTIONS
.TP
.I -a peerlist
Identifies a file of "peers" which identifies clients whose connections
should be accepted without processing by this filter.  The
.I peerlist
should contain on each line a hostname, domain name (e.g. ".example.com"),
IP address, an IPv6 address (including an IPv4 mapped address), or a
CIDR-style IP specification (e.g. "192.168.1.0/24").  An entry beginning
with a bang ("!") character means "not", allowing exclusions of specific
hosts that are otherwise members of larger sets.  The order of entries
in this file is therefore significant.
.TP
.I -A
Automatically re-start on failures.  Use with caution; if the filter
fails instantly after it starts, this can cause a tight
.I fork(2)
loop.
.TP
.I -b modes
Selects operating modes.
.I modes
is a concatenation of characters which indicate which mode(s) of operation
are desired.  Valid modes are
.I s
(signer) and
.I v
(verifier).  The default is
.I sv
except in test mode (see
.I -t
below) in which case the default is
.I v.
.TP
.I -c canon
Selects the canonicalization method(s) to be used when signing messages.
When verifying, the message's DKIM-Signature: header specifies
the canonicalization method.  The recognized values are
.I relaxed
and
.I simple
as defined by the DKIM specification.  The default is
.I simple.
The value may include two different canonicalizations separated by a
slash ("/") character, in which case the first will be applied to the
headers and the second to the body.
.TP
.I -C config
Configuration control.  See the CONFIGURATION section for details.
.TP
.I -d domain [,...]
A comma-separated list of domains whose mail should be signed by this
filter.  Mail from other domains will be verified rather than being signed.

The value of this parameter may also be a filename from which domain names
will be read.  The "#" character in such a file is assumed to indicate a
comment.  An absolute path must be used (i.e. the first character must be
a "/").

In either case, the domain name(s) may contain the special character "*"
which is treated as a wildcard character matching zero or more characters
in a domain name.
.TP
.I -D
Sign subdomains of those listed by the
.I -d
option as well as the actual domains.
.TP
.I -f
Normally
.I dkim-filter
forks and exits immediately, leaving the service running in the background.
This flag suppresses that behaviour so that it runs in the foreground.
.TP
.I -F time
Specifies a fixed time to use when generating signatures.  Ignored unless
also used in conjunction with
.I -t
(see below).  The time must be expressed in the usual UNIX
.I time_t
(seconds since epoch) format.
.TP
.I -h
Causes
.I dkim-filter
to add a header indicating the presence of this filter in the path of
the message from injection to delivery.  The product's name, version, and
the job ID are included in the header's contents.
.TP
.I -H hdrlist
Specifies the list of headers which should be included when generating
signatures.
.I hdrlist
should be a comma-separated list of header names.  If the list omits any
header which is mandated by the DKIM specification, those headers are
implicitly added.  By default, all headers present at the time of signing
except those which the DKIM specification says "SHOULD NOT" be signed will
be included when generating the signature.  See the
.I -o
option for more information.
.TP
.I -i ilist
Identifies a file of internal hosts whose mail should be signed rather
than verified.  Entries in this file follow the same form as those of
the
.I -a
option above.  If not specified, the default of "127.0.0.1" is applied.
.TP
.I -I eilist
Identifies a file of "external" hosts which may send mail through the server
as one of the signing domains without credentials as such.  Basically
suppresses the "external host (hostname) tried to send mail as (domain)"
log messages.  Entries in the
.I eilist
file should be of the same form as those of the
.I -a
option above.  The list is empty by default.
.TP
.I -K
Requests multiple-key processing.  See also
.I -k
below.
.TP
.I -k keyfile
Without
.I -K,
gives the location of a PEM-formatted private key to be used for signing
all messages.  With
.I -K,
gives the location of a file listing rules for signing with multiple keys.
In the latter mode, the
.I keyfile
should contain a set of lines of the form
.I sender-pattern:signing-domain:keypath
where
.I sender-pattern
is a pattern to match against message senders (with the special character
"*" interpreted as "zero or more characters"),
.I signing-domain
is the domain to announce as the signing domain when generating signatures, and
.I keypath
is the path to the PEM-formatted private key to be used for signing messages
which match the
.I sender-pattern.
The selector used in the signature will be the filename portion of
.I keypath.
If the file referenced by
.I keypath
cannot be opened, the filter will try again by appending ".pem"
and then ".private" before giving up.
.TP
.I -l
Log via calls to
.I syslog(3)
any interesting activity.
.TP
.I -L min[%+]
Instructs the verification code to fail messages for which a partial
signature was received.  There are three possible formats:
.I min
indicating at least
.I min
bytes of the message must be signed (or if the message is smaller than
.I min
then all of it must be signed);
.I min%
requiring that at least
.I min
percent of the received message must be signed; and
.I min+
meaning there may be no more than
.I min
bytes of unsigned data appended to the message for it to be considered
valid.
.TP
.I -m mta[,...]
A comma-separated list of MTA names (a la the
.I sendmail(8)
DaemonPortOptions Name parameter) whose mail should be signed by this
filter.  There is no default.
.TP
.I -M macro[=value][,...]
Defines a set of MTA-provided
.I macros
which should be checked to see if the sender has been determined to be a
local user and therefore whether or not the message should be signed.  If a
.I value
is specified, the value of the macro must match the value specified
(matching is case-insensitive), otherwise the macro must be defined
but may contain any value.  Multiple tests may be specified, separated
by commas.  The set is empty by default.  The general
format of the string is
.I test1[,test2[,...]]
where a "test" is of the form
.I macro[=value1[|value2[|...]]];
if one or more value is defined then the macro must be set to one of the
listed values, otherwise the macro must be set but can contain any
value.
.TP
.I -o hdrlist
Specifies a list of headers which should be omitted when generating
signatures.
.I hdrlist
should be a comma-separated list of header names.  If an entry in the list
names any header which is mandated by the DKIM specification, the entry
is ignored.  A set of headers is listed in the DKIM specification as
"SHOULD NOT" be signed; the default list for this parameter contains those
headers (Return-Path, Received, Comments, Keywords, Bcc, Resent-Bcc and
DKIM-Signature).  To omit no headers, simply use the string "-" (or any
string which will match no headers).
.TP
.I -p socketspec
Specifies the socket that should be established by the filter to receive
connections from
.I sendmail(8)
in order to provide service.
.I socketspec
is in one of two forms:
.I local:path
which creates a UNIX domain socket at the specified
.I path,
or
.I inet:port[@host]
which creates a TCP socket on the specified
.I port.
If the
.I host
is not given as either a hostname or an IP address, the socket will be
listening on all interfaces.  If neither socket type is specified,
.I local
is assumed, meaning the parameter is interpreted as a path at which
the socket should be created.  This option is mandatory.
.TP
.I -P pidfile
Writes the process ID of the filter, once started, to the filename given.
.TP
.I -q
Requests that messages which fail verification be quarantined by the
MTA.  (Requires a sufficiently recent version of the milter library.)
.TP
.I -R
When a signature verification fails and the signing site advertises a
reporting address (i.e.
.I r=user@host
in its policy record), send a structured report to that address containing
details needed to reproduce the problem.
.TP
.I -s selector
Defines the name of the selector to be used when signing messages.
See the
.B DKIM
specification for details.
.TP
.I -S signalg
Selects the signing algorithm to use when generating signatures.
If the filter was compiled against version 0.9.8 or later of
.B OpenSSL
then both
.I rsa-sha1
and 
.I rsa-sha256
are available and the latter is the default.  Otherwise, only the former
is available and it is (obviously) the default.
.TP
.I -t testfile
Evaluates (verifies) an RFC2822-formatted message found in
.I testfile
and exits.  The value of
.I testfile
may be "-" if the message should be read from standard input.
.TP
.I -T secs
Sets the DNS timeout in seconds.  A value of 0 causes an infinite wait.
The default is 5.  Ignored if not using the asynchronous resolver package.
See also the NOTES section below.
.TP
.I -u userid[:group]
Attempts to be come the specified
.I userid
before starting operations.  The process will be assigned all of the groups
and primary group ID of the named
.I userid
unless an alternate
.I group
is specified.
.TP
.I -U popdb
Requests that the filter consult a POP authentication database for IP
addresses that should be allowed for signing.  The filter must be specially
compiled to enable this feature, since it adds a library dependency.
.TP
.I -v
Increase verbose output during test mode (see
.I -t
above).  May be specified more than once to request increasing amounts of
output.
.TP
.I -V
Print the version number and supported canonicalization and signature
algorithms, and then exit without doing anything else.
.TP
.I -x configfile
Read the named configuration file.  See the
.I dkim-filter.conf(5)
man page for details.
.SH ACTION CONFIGURATION
The value of the
.I -C
switch is a comma-separated list of settings of the form
.I result=action
which defines what the filter should do with messages that produce
certain results.  Each result and each action has a full name and an
abbreviated name.  Either is accepted.  Below, the abbreviated name appears
in parentheses.
.TP
.I results
.I badsignature
(bad) the signature found in the message did not verify successfully
against the message;
.I dnserror
(dns) an error was encountered attempting to retrieve a public key from
the nameserver;
.I internal
(int) an internal error occurred;
.I nosignature
(no) no signature was present on the message;
.I signaturemissing
(miss) no signature was present on the message which came from a domain that
claims to sign all messages;
.I security
(sec) the message tripped internal security concerns (e.g. unusually large
header blocks).  There is also a special result called
.I default
(def) whose action is copied onto all of the other results.
.TP
.I action
.I accept
(a) accept the message;
.I discard
(d) discard the message;
.I tempfail
(t) temp-fail the message;
.I reject
(r) reject the message.
.PP
In the interests of minimal initial impact, the defaults for
.I badsignature,
.I nosignature
and
.I signaturemissing
are all
.I accept,
and the default for the others is
.I tempfail.
.PP
Results and actions are processed in order, so use of the
.I default
action can be overridden by later specifications.  For example, using
"def=a,int=t" sets all result actions to "accept" except for internal
errors which will generate a temporary failure.
.SH OPERATION
A message will be verified unless it conforms to the signing criteria,
which are: (1) the domain on the From: address or Sender: address (if present)
must be listed by the
.I -d
command line switch or the
.I Domain
configuration file setting, and (2) the client connecting to the MTA must
have authenticated, or (b) the client connecting to the MTA must be listed in
the file referenced by the
.I -i
command line switch (or be in the default list for that option), or (c)
the client must be connected to a daemon port named by the
.I -m
command line switch, or (d) the MTA must have set one or more macros
matching the criteria set by the
.I -M
command line switch.

When signing a message, a
.I DKIM-Signature:
header will be prepended to the message.  The signature is computed using
the private key provided.  You must be running a version of
.I sendmail(8)
recent enough to be able to do header prepend operations (8.13.0 or later).

When verifying a message, an
.I Authentication-Results:
header will be prepended to indicate the presence of a signature and whether
or not it could be validated against the body of the message using the
public key advertised by the sender's nameserver.  The value of this header
can be used by mail user agents to sort or discard messages that were not
signed or could not be verified.
.SH ENVIRONMENT
The following environment variable(s) can be used to adjust the behaviour
of this filter:
.TP
.I DKIM_TMPDIR
The directory to use when creating temporary files.  The default is
.I /var/tmp.
.SH NOTES
When using DNS timeouts (see the
.I -T
option above), be sure not to use a timeout that is larger than the timeout
being used for interaction between
.I sendmail
and the filter.  Otherwise, the MTA could abort a message while waiting for
a reply from the filter, which in turn is still waiting for a DNS reply.
.SH HISTORY
DKIM is an amalgam of Yahoo!'s
.B DomainKeys
proposal, and Cisco's
.B Internet Identified Mail
(IIM) proposal.
.SH VERSION
This man page covers version 2.4.1 of
.I dkim-filter.
.SH COPYRIGHT
Copyright (c) 2005-2007, Sendmail, Inc. and its suppliers.  All rights
reserved.
.SH SEE ALSO
.I dkim-filter.conf(5), sendmail(8)
.P
Sendmail Operations Guide
.P
RFC2821 - Simple Mail Transfer Protocol
.P
RFC2822 - Internet Messages
.P
RFC4871 - DomainKeys Identified Mail
.P
Authentication-Results Internet Draft