NAME
opendkim - DKIM signing and verifying filter for MTAs
SYNOPSIS
opendkim -p socketspec [-A] [-b modes] [-c canon] [-d domain[,...]]
[-D] [-f] [-F time] [-k keyfile] [-l] [-L min] [-n] [-o hdrlist] [-P
pidfile] [-q] [-Q] [-r] [-s selector] [-S signalg] [-t testfile] [-T
secs] [-u userid[:group]] [-v] [-V] [-W] [-x configfile]
DESCRIPTION
opendkim implements the DKIM standard for signing and verifying e-mail
messages on a per-domain basis.
opendkim 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.
DATA SETS
Many of the command line and configuration file parameters will refer
to a "dataset" as their values. This refers to a string that either
contains the list of desirable values, or to a file that contains them,
or (if enabled at compile time) a database containing the data.
Some data sets require that the value contain more than one entry. How
this is done depends on which data set type is used.
Which type is used depends on the format of the specification string.
In particular:
a) If the string begins with "file:", then the remainder of the
string is presumed to refer to a flat file that contains
elements of the data set, one per line. If a line contains
whitespace-separated values, then the line is preusmed to define
a key and its corresponding value. Blank lines are ignored, and
the hash ("#") character denotes the start of a comment. If a
value contains multiple entries, the entries should be separated
by colons.
b) If the string begins with "refile:", then the remainder of the
string is presumed to specify a file that contains a set of
patterns, one per line, and their associated values. The
pattern is taken as the start of the line to the first
whitespace, and the portion after that whitespace is taken as
the value to be used when that pattern is matched. Patterns are
simple wildcard patterns, matching all text except that the
asterisk ("*") character is considered a wildcard. If a value
contains multiple entries, the entries should be separated by
colons.
c) If the string begins with "db:" and the program was compiled
with Sleepycat DB support, then the remainder of the string is
presumed to identify a Sleepycat database containing keys and
corresponding values. These may be used only to test for
membership in the data set, or for storing keys and
corresponding values. If a value contains multiple entries, the
entries should be separated by colons.
d) If the string begins with "dsn:" and the OpenDKIM library was
compiled to support that database type, then the remainder of
the string is a Data Store Name describing the type, location
parameters and access credentials for an ODBC or SQL database.
The DSN is of the form:
backend://[user[:pwd]@][port+]host/dbase[/key=value[?...]]
where backend is the name of a supported backend database
mechanism (e.g. "mysql"), user and password are optional login
credentials for the database, port and host describe the
destination of a TCP connection to connect to that database,
dbase is the name of the database to be accessed, and the
key=value pairs must specify at least "table", "keycol" and
"datacol" values specifying the name of the table, the name of
the column to consider as the key, and the name(s) of the
column(s) to be considered as the values (separated by commas).
For example (all in one line):
mysql:://dbuser:dbpass@3306+dbhost/odkim/table=macros
?keycol=host?datacol=v1,v2
defines a MySQL database listening at port 3306 on host
"dbhost"; the userid "dbuser" and password "dbpass" should be
used to access the database; the database name is "odkim", and
the data are in columns "host" (the keys) and "v1" and "v2" (the
values) inside table "macros". This example would thus return
two values when a match is found.
e) If the string begins with "ldap:", "ldaps:" or "ldapi:", it is
presumed to be a space-separated set of one or more LDAP URLs
that identify a set of servers to be queried. The first one
should be a full RFC4516 LDAP URL indicating a base DN template
and optional scope, filter and attribute names to use in
queries. When constructing a DN template or filter, the special
tokens "$d" and "$D" are replaced with the key being queried and
they key broken into components, separated at "." characters,
each component preceded by "dc=" and followed by "," (so
"example.com" would become "dc=example,dc=com"). If a data set
requires multiple values to be returned, the appropriate
attribute names should be given in the correct order to satisfy
such requests.
f) If the string begins with "lua:", it is presumed to refer to a
file that contains a Lua script to be executed whenever a query
is performed. The key for the query is placed in a global
variable called "query", which the called script can then
access. The script may return any number of values as required
for the type of query being performed.
g) If the string contains none of these prefixes but ends with
".db", it is presumed to be a Sleepycat DB as described above
(if support for same is compiled in).
h) If the string contains none of these prefixes but starts with a
slash ("/") character, it is presumed to be a flat file as
described above.
i) If the string begins with "csl:", the string is treated as a
comma-separated list as described in j) below.
j) In any other case, the string is presumed to be a comma-
separated list. Elements in the list are either simple data
elements that are part of the set or, in the case of an entry of
the form "x=y", are stored as key-value pairs as described
above.
OPTIONS
-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 opendkim.conf(5).
-b modes
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.
-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 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
the body.
-d dataset
A set of domains whose mail should be signed by this filter.
Mail from other domains will be verified rather than being
signed.
-D Sign subdomains of those listed by the -d option as well as the
actual domains.
-f Normally opendkim forks and exits immediately, leaving the
service running in the background. This flag suppresses that
behaviour so that it runs in the foreground.
-F time
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.
-k keyfile
Gives the location of a PEM-formatted private key to be used for
signing all messages. Ignored if a configuration file is
referenced that defines a KeyTable.
-l Log via calls to syslog(3) any interesting activity.
-L min[%+]
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.
-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
otherwise.
-o dataset
Specifies a list of headers that should be omitted when
generating signatures. 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 that will
match no headers).
-p socketspec
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.
-P pidfile
Specifies a file into which the filter should write its process
ID at startup.
-q Requests that messages which fail verification be quarantined by
the MTA. (Requires a sufficiently recent version of the milter
library.)
-Q Query test mode. The filter will read two lines from standard
input, one containing a database description to be opened and
one containing a string of the form "q/n" where "q" is the query
to be performed and "n" is the number of fields to be retrieved.
-r Checks all messages for compliance with RFC5322 header count
requirements. Non-compliant messages are rejected.
-s selector
Defines the name of the selector to be used when signing
messages. See the DKIM specification for details.
-S signalg
Selects the signing algorithm to use when generating signatures.
Use 'dkim-filter -V' to see the list of supported algorithms.
The default is rsa-sha256 if it is available, otherwise it will
be rsa-sha1.
-t testfile
Evaluates (verifies) an RFC5322-formatted message found in
testfile and exits. The value of testfile may be "-" if the
message should be read from standard input.
-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.
-u userid[:group]
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
is specified.
-v Increase verbose output during test mode (see -t above). May be
specified more than once to request increasing amounts of
output.
-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
general operation.
-x configfile
Read the named configuration file. See the opendkim.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.
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 -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.
For (a) above, the test is whether or not the MTA macro "{auth_type}"
is set and contains any non-empty value. This means the MTA must pass
the value of that macro to the filter in order for its value to be
tested. Check your MTA's configuration documentation for details.
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.
ENVIRONMENT
The following environment variable(s) can be used to adjust the
behaviour of this filter:
DKIM_TMPDIR
The directory to use when creating temporary files. The default
is /var/tmp.
NOTES
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, "192.0.2/24" and "192.0.2.0/24" are not the same
thing).
HISTORY
DKIM is an amalgam of Yahoo!'s DomainKeys proposal, and Cisco's
Internet Identified Mail (IIM) proposal.
VERSION
This man page covers version 2.1.3 of opendkim.
COPYRIGHT
Copyright (c) 2005-2008, Sendmail, Inc. and its suppliers. All rights
reserved.
Copyright (c) 2009, 2010, The OpenDKIM Project. All rights reserved.
SEE ALSO
opendkim.conf(5), sendmail(8)
Sendmail Operations Guide
RFC4871 - DomainKeys Identified Mail
RFC5321 - Simple Mail Transfer Protocol
RFC5322 - Internet Messages
RFC5451 - Message Header Field for Indicating Message Authentication
Status
The OpenDKIM Project opendkim(8)