dkim-filter - DKIM filter for sendmail
dkim-filter -p socketspec [-a peerlist] [-A] [-b modes] [-c canon] [-C
config] [-d domains] [-D] [-f] [-F time] [-i ilist] [-I eilist] [-h]
[-k keyfile] [-K] [-l] [-L min] [-m mtas] [-M macro[=value][,...]] [-n]
[-o hdrlist] [-P pidfile] [-q] [-r] [-R] [-s selector] [-S signalg] [-t
testfile] [-T secs] [-u userid[:group]] [-U popdb] [-v] [-V] [-W] [-x
dkim-filter implements the DKIM standard for signing and verifying e-
mail messages on a per-domain basis.
dkim-filter uses the milter interface, originally distributed as part
of version 8.11 of sendmail(8), to provide DKIM signing and/or
verifying service for mail transiting a milter-aware MTA.
Most, if not all, of the command line options listed below can also be
set using a configuration file. See the -x option for details.
Identifies a file of "peers" which identifies clients whose
connections should be accepted without processing by this
filter. The 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.
-A Automatically re-start on failures. Use with caution; if the
filter fails instantly after it starts, this can cause a tight
fork(2) loop. This can be mitigated using some values in the
configuration file to limit restarting. See dkim-
Selects operating modes. modes is a concatenation of characters
that indicate which mode(s) of operation are desired. Valid
modes are s (signer) and v (verifier). The default is sv except
in test mode (see -t below) in which case the default is v.
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 relaxed and simple as defined by the DKIM specification.
The default is 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
Configuration control. See the CONFIGURATION section for
-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.
Matching is case-insensitive.
This parameter is not required if -K is in use; in that case,
the list of signed domains is implied by the lines in the key
-D Sign subdomains of those listed by the -d option as well as the
-f Normally 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.
Specifies a fixed time to use when generating signatures.
Ignored unless also used in conjunction with -t (see below).
The time must be expressed in the usual UNIX time_t (seconds
since epoch) format.
-h Causes 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.
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 -a option above. If not specified, the default
of "127.0.0.1" is applied. Naturally, providing a value here
overrides the default, so if mail from 127.0.0.1 should be
signed, the list provided here should include that address
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 eilist file should be of the same form as those
of the -a option above. The list is empty by default.
-K Requests multiple-key processing. See also -k below.
Without -K, gives the location of a PEM-formatted private key to
be used for signing all messages. With -K, gives the location
of a file listing rules for signing with multiple keys.
In the latter mode, the keyfile should contain a set of lines of
the form sender-pattern:signing-domain:keypath where sender-
pattern is a pattern to match against message senders (with the
special character "*" interpreted as "zero or more characters"),
signing-domain is the domain to announce as the signing domain
when generating signatures, and keypath is the path to the PEM-
formatted private key to be used for signing messages which
match the sender-pattern. The selector used in the signature
will be the filename portion of keypath.
If the file referenced by keypath cannot be opened, the filter
will try again by appending ".pem" and then ".private" before
-l Log via calls to syslog(3) any interesting activity.
Instructs the verification code to fail messages for which a
partial signature was received. There are three possible
formats: min indicating at least min bytes of the message must
be signed (or if the message is smaller than min then all of it
must be signed); min% requiring that at least min percent of the
received message must be signed; and min+ meaning there may be
no more than min bytes of unsigned data appended to the message
for it to be considered valid.
A comma-separated list of MTA names (a la the sendmail(8)
DaemonPortOptions Name parameter) whose mail should be signed by
this filter. If not set, the MTA name is not used when deciding
whether or not a message should be signed.
Defines a set of MTA-provided 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
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,
meaning macros are not used when deciding whether or not a
message should be signed.
The general format of the string is test1[,test2[,...]] where a
"test" is of the form 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
-n Parse the configuration file and command line arguments,
reporting any errors found, and then exit. The exit value will
be 0 if the filter would start up without complaint, or non-zero
Specifies a list of headers which should be omitted when
generating signatures. 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).
Specifies the socket that should be established by the filter to
receive connections from sendmail(8) in order to provide
service. socketspec is in one of two forms: local:path which
creates a UNIX domain socket at the specified path, or
inet:port[@host] which creates a TCP socket on the specified
port. If the 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, local is assumed, meaning the
parameter is interpreted as a path at which the socket should be
created. This parameter is mandatory.
Writes the process ID of the filter, once started, to the
-q Requests that messages which fail verification be quarantined by
the MTA. (Requires a sufficiently recent version of the milter
-r Checks all messages for compliance with RFC2822 header count
requirements. Non-compliant messages are rejected.
-R When a signature verification fails and the signing site
advertises a reporting address (i.e. r=user@host in its policy
record), send a structured report to that address containing
details needed to reproduce the problem.
Defines the name of the selector to be used when signing
messages. See the DKIM specification for details.
Selects the signing algorithm to use when generating signatures.
If the filter was compiled against version 0.9.8 or later of
OpenSSL then both rsa-sha1 and rsa-sha256 are available and the
latter is the default. Otherwise, only the former is available
and it is (obviously) the default.
Evaluates (verifies) an RFC2822-formatted message found in
testfile and exits. The value of testfile may be "-" if the
message should be read from standard input.
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
Attempts to be come the specified userid before starting
operations. The process will be assigned all of the groups and
primary group ID of the named userid unless an alternate group
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.
-v Increase verbose output during test mode (see -t above). May be
specified more than once to request increasing amounts of
-V Print the version number and supported canonicalization and
signature algorithms, and then exit without doing anything else.
-W If logging is enabled (see -l above), issues very detailed
logging about the logic behind the filter’s decision to either
sign a message or verify it. The "W" stands for "Why?!" since
the logic behind the decision is non-trivial and can be
confusing to administrators not familiar with its operation. A
description of how the decision is made can be found in the
OPERATION section of this document. This causes a large
increase in the amount of log data generated for each message,
so it should be limited to debugging use and not enabled for
Read the named configuration file. See the dkim-filter.conf(5)
man page for details. Values in the configuration file are
overridden when their equivalents are provided on the command
line until a configuration reload occurs. The OPERATION section
describes how reloads are triggered.
The value of the -C switch is a comma-separated list of settings of the
form 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.
badsignature (bad) the signature found in the message did not
verify successfully against the message; dnserror (dns) an error
was encountered attempting to retrieve a public key from the
nameserver; internal (int) an internal error occurred;
nosignature (no) no signature was present on the message;
security (sec) the message tripped internal security concerns
(e.g. unusually large header blocks). There is also a special
result called default (def) whose action is copied onto all of
the other results.
action accept (a) accept the message; discard (d) discard the message;
tempfail (t) temp-fail the message; reject (r) reject the
In the interests of minimal initial impact, the defaults for
badsignature and nosignature are accept, and the default for the others
Results and actions are processed in order, so use of the 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.
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 -d command line switch or the Domain
configuration file setting, and (2) (a) 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 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 -m command line switch, or (d)
the MTA must have set one or more macros matching the criteria set by
the -M command line switch.
When signing a message, a 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 sendmail(8) recent enough to be able
to do header prepend operations (8.13.0 or later).
When verifying a message, an 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.
Upon receiving SIGUSR1, if the filter was started with a configuration
file, it will be re-read and the new values used. Note that any
command line overrides provided at startup time will be lost when this
is done. Also, the following configuration file values (and their
corresponding command line items, if any) are not reloaded through this
process: AutoRestart (-A), AutoRestartCount, AutoRestartRate,
Background, MilterDebug, PidFile (-P), POPDBFile, Quarantine (-q),
QueryCache, Socket (-p), StrictTestMode, TestPublicKeys, UMask, UserID
(-u). The filter does not automatically check the configuration file
for changes and reload.
The following environment variable(s) can be used to adjust the
behaviour of this filter:
The directory to use when creating temporary files. The default
When using DNS timeouts (see the -T option above), be sure not to use a
timeout that is larger than the timeout being used for interaction
between 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.
The POP authentication database is expected to be a Sleepycat DB file
(formerly known as a Berkeley DB) in hash format with keys containing
the IP address in text form without a terminating NULL. The values of
these records are not checked; only the existence of such records is of
interest. The filter will attempt to establish a shared lock on the
database before reading from it, so any programs which write to the
database should keep their lock use to a minimum or else this filter
will appear to hang while waiting for the lock operation to complete.
Features that involve specification of IPv4 addresses or CIDR blocks
will use the inet_addr(3) function to parse that information. Users
should be familiar with the way that function handles the non-trivial
cases (for example, "1.2.3/24" and "22.214.171.124/24" are not the same
DKIM is an amalgam of Yahoo!’s DomainKeys proposal, and Cisco’s
Internet Identified Mail (IIM) proposal.
This man page covers version 2.8.0 of dkim-filter.
Copyright (c) 2005-2008, Sendmail, Inc. and its suppliers. All rights
Sendmail Operations Guide
RFC2821 - Simple Mail Transfer Protocol
RFC2822 - Internet Messages
RFC4871 - DomainKeys Identified Mail
RFC5451 - Message Header Field for Indicating Message Authentication
Sendmail, Inc. dkim-filter(8)