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opendkim(8) System Manager's Manual opendkim(8)

NAME
opendkim - DKIM signing and verifying filter for MTAs

SYNOPSIS
opendkim [-A] [-b modes] [-c canon] [-d domain[,...]] [-D] [-e name] [-f] [-F time] [-k
keyfile] [-l] [-L min] [-n] [-o hdrlist] [-p socketspec] [-P pidfile] [-Q] [-r] [-s selec‐
tor] [-S signalg] [-t testfiles] [-T secs] [-u userid[:group]] [-v] [-V] [-W] [-x config‐
file]

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 mil‐
ter-aware MTA.

Most, if not all, of the command line options listed below can also be set using a config‐
uration 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. Note that not all
of these are necessarily supported for all installations; most of them depend on the
availability of a particular third-party library at compile time.

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 ("#") char‐
acter 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 associ‐
ated values. The pattern is taken as the start of the line to the first white‐
space, 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 con‐
tains multiple entries, the entries should be separated by colons.

c) If the string begins with "db:" and the program was compiled with Sleepycat DB sup‐
port, 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 mem‐
bership 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 describ‐
ing the type, location parameters and access credentials for an ODBC or SQL data‐
base. 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.

No value within the DSN may contain any of the six punctuation characters (":",
"/", "@", "+", "?" and "=") used to separate portions of the DSN from each other.

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 tem‐
plate and optional scope, filter and attribute names to use in queries. When con‐
structing a DN template or filter, the special tokens "$d" and "$D" are replaced
with the key being queried and the 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 begins with "memcache:", it is presumed to refer to a memory cache
database provided by memcached. The remainder of the string is a comma-separated
list of hosts to which query attempts should be made, each optionally followed by
":" and a port number; that list must be followed by a slash ("/") character and a
string that will be used to prefix queries send to the cache. For example:

memcache:localhost,otherhost/keyname

This would use either "localhost" or "otherhost" to conduct queries, and all
strings sent to the dataset will be prefixed with "keyname:".

h) 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).

i) If the string contains none of these prefixes but starts with a slash ("/") charac‐
ter, it is presumed to be a flat file as described above.

j) If the string begins with "csl:", the string is treated as a comma-separated list
as described in m) below.

k) If the string begins with "erlang:", it is presumed to refer to a function called
to be made to the specified distributed Erlang node(s). The specification is of the
form

erlang:node@host[,...]:cookie:module:function

where node[,...] is a list of comma-separated erlang nodes, cookie is the cookie
for the known nodes of the distributed Erlang setup, module is the name of the
Erlang module where the function to be called resides, function is the name of the
Erlang function to be called. For example, (all in one line):

erlang:mynode@myhost,myothernode@myotherhost:
chocolate:dkim:lookup

will join the distributed Erlang setup connecting to either "mynode@myhost" or
"myothernode@myotherhost" (connections to nodes are tried in order) using "choco‐
late" as the cookie, and use the function "dkim:lookup/1" for lookups.

l) If the string begins with "mdb:", it refers to a directory that contains a memory
database, as provided by libmdb from OpenLDAP.

m) 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 miti‐
gated using some values in the configuration file to limit restarting. See opend‐
kim.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 (veri‐
fier). 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 ver‐
ifying, 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 sepa‐
rated 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.

-e name
Extracts the value of name from the configuration file (if any).

-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 mes‐
sages. 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 with‐
out 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 connec‐
tions 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] or inet6:port[@host] which creates a TCP socket on the specified
port using the requested protocol family. If the host is not given as either a
hostname or an IP address, the socket will be listening on all interfaces. A lit‐
eral IP address must be enclosed in square brackets. 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 either here or in
the configuration file.

-P pidfile
Specifies a file into which the filter should write its process ID at startup.

-Q Query test mode. The filter will read two lines from standard input, one contain‐
ing 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 'opendkim -V'
to see the list of supported algorithms. The default is rsa-sha256 if it is avail‐
able, otherwise it will be rsa-sha1.

-t testfiles
Evaluates (verifies) one or more RFC5322-formatted message found in testfiles and
exits. The value of testfiles should be a comma-separated list of one or more
filenames, one of which 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 an 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. See the FILE PERMISSIONS section for more infor‐
mation.

-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 confus‐
ing 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. The default is to read a configuration file
from /etc/opendkim.conf if one exists, or otherwise to apply defaults to all val‐
ues.

OPERATION
A message will be verified unless it conforms to the signing criteria, which are: (1) the
domain on the From: 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 InternalHosts configuration file setting (or be in the default list for
that option), or (c) the client must be connected to a daemon port named by the MTAs con‐
figuration file setting, or (d) the MTA must have set one or more macros matching the cri‐
teria set by the MacroList configuration file setting.

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
before or during the end-of-header (EOH) phase in order for its value to be tested. Check
your MTA's configuration documentation for details.

For (1) above, other header fields may be selected using the SenderHeaders configuration
file setting. See opendkim.conf(5) for more information.

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.

MTA MACROS
opendkim makes use of three MTA-provided macros, plus any demanded by configuration. The
basic three are: "i" (the envelope ID, also known as the job ID or the queue ID), which is
used for logging; "daemon_name" (the symbolic name given to the MTA instance that accepted
the connection), which is used when performing tests against any "MTAs" setting used; and
"auth_type", which is used to determine whether or not the SMTP client authenticated to
the MTA. If the MTA does not provide them to opendkim then it is not able to apply their
corresponding tests or do useful logging. Consult your MTA documentation to determine how
to adjust your its configuration if some or all of these are not available.

FILE PERMISSIONS
When the filter is started as the superuser and the UserID (-u) setting is used, the fil‐
ter gives up its root privileges by changing to the specified user after the following
steps are taken: (1) the configuration file (if any) is loaded; (2) if the KeyFile (-k)
setting is used, that key is loaded into memory; (3) all data sets in the configuration
file are opened, and those that are based on flat files are also read into memory; and (4)
if ChangeRootDirectory is set, the process root is changed to that directory. This means
on configuration reload, the filter will not be accessing these files or the configuration
file as the superuser (and possibly from a different root), and any key files referenced
by the KeyTable will also be accessed by the new user.

Thus, keys referenced by the KeyTable must always be accessible for read by the unprivi‐
leged user. Also, run-time reloads are not possible if any of the other files will not be
readable by the unprivileged user.

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 /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. Oth‐
erwise, 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 data‐
base 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).

EXIT STATUS
Filter exit status codes are selected according to sysexits(3).

HISTORY
DKIM is an amalgam of Yahoo!'s DomainKeys proposal, and Cisco's Internet Identified Mail
(IIM) proposal.

VERSION
This man page covers version 2.9.2 of opendkim.