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 socket-
spec] [-P pidfile] [-Q] [-r] [-s selector] [-S signalg] [-t testfiles]
[-T secs] [-u userid[:group]] [-v] [-V] [-W] [-x configfile] [-X]
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.
Note that not all of these are necessarily supported for all installa-
tions; 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 ele-
ments of the data set, one per line. If a line contains white-
space-separated values, then the line is presumed 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 pat-
terns, 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 ("*") char-
acter 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 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 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 mecha-
nism (e.g. "mysql"), user and password are optional login cre-
dentials for the database, port and host describe the destina-
tion 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 con-
sider as the key, and the name(s) of the column(s) to be consid-
ered 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 "db-
host"; 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.
The key may also include a "filter" value which will be included
in all generated SQL as an AND clause after the WHERE clause
that declares the search criteria. For example, given the above
DSN specification with an additional "filter" value of "ID >
1000", the generated SQL for a query for "foo" would look like
so:
SELECT v1,v2 FROM macros WHERE host = 'foo' AND ID > 1000
No value within the DSN may contain any of the six punctuation
characters (":", "/", "@", "+", "?" and "=") used to delimit
portions of the DSN. To include such characters within a value,
encode them in quoted-printable style (e.g., "=20" will be
translated into a single space character). Encoding of spaces
is also advised.
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
the key broken into components, separated at "." characters,
each component preceded by "dc=" and followed by "," (so "exam-
ple.com" would become "dc=example,dc=com"). If a data set re-
quires multiple values to be returned, the appropriate attribute
names should be given in the correct order to satisfy such re-
quests.
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 vari-
able 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 ("/") charac-
ter 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 ("/") character, it is presumed to be a flat file as de-
scribed 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 Er-
lang setup, module is the name of the Erlang module where the
function to be called resides, function is the name of the Er-
lang 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 "chocolate" 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-sepa-
rated list. Elements in the list are either simple data ele-
ments 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.
-e name
Extracts the value of name from the configuration file (if any).
-f Normally opendkim forks and exits immediately, leaving the ser-
vice running in the background. This flag suppresses that be-
haviour so that it runs in the foreground.
-F time
Specifies a fixed time to use when generating signatures. Ig-
nored 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 refer-
enced 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 for-
mats: 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, report-
ing any errors found, and then exit. The exit value will be 0
if the filter would start up without complaint, or non-zero oth-
erwise.
-o dataset
Specifies a list of headers that should be omitted when generat-
ing 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 ser-
vice. socketspec is in one of two forms: local:path which cre-
ates 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 literal IP ad-
dress 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 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 re-
quirements. Non-compliant messages are rejected.
-s selector
Defines the name of the selector to be used when signing mes-
sages. 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 available, 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 infi-
nite wait. The default is 5. Ignored if not using an asynchro-
nous resolver package. See also the NOTES section below.
-u userid[:group]
Attempts to be come the specified userid before starting opera-
tions. The process will be assigned all of the groups and pri-
mary group ID of the named userid unless an alternate group is
specified. See the FILE PERMISSIONS section for more informa-
tion.
-v Increase verbose output during test mode (see -t above). May be
specified more than once to request increasing amounts of out-
put.
-V Print the version number and supported canonicalization and sig-
nature algorithms, and then exit without doing anything else.
-W If logging is enabled (see -l above), issues very detailed log-
ging 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 sec-
tion 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 overrid-
den when their equivalents are provided on the command line un-
til a configuration reload occurs. The OPERATION section de-
scribes 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 values.
-X Tolerates configuration file items that have been internally
marked as "deprecated". Normally when a configuration file item
is removed from the package, it is flagged in this way for at
least one full release cycle. The presence of a deprecated con-
figuration file item typically causes the filter to return an
error and refuse to start. Setting this flag will allow the
filter to start and a warning is logged. In some future release
when the item is removed completely, a different error results,
and it will not be possible to start the filter. Use of this
flag is NOT RECOMMENDED; it could effectively hide a major con-
figuration change with serious security implications.
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 authen-
ticated, 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 con-
nected to a daemon port named by the MTAs configuration file setting,
or (d) the MTA must have set one or more macros matching the criteria
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 Sender-
Headers configuration file setting. See opendkim.conf(5) for more in-
formation.
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 com-
mand line overrides provided at startup time will be lost when this is
done. Also, the following configuration file values (and their corre-
sponding command line items, if any) are not reloaded through this
process: AutoRestart (-A), AutoRestartCount, AutoRestartRate, Back-
ground, 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; "dae-
mon_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 pro-
vide them to opendkim then it is not able to apply their corresponding
tests or do useful logging. Consult your MTA documentation to deter-
mine 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 filter gives up its root privileges by changing to the
specified user after the following steps are taken: (1) the configura-
tion 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 unprivileged 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 behav-
iour 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 be-
tween sendmail and the filter. Otherwise, the MTA could abort a mes-
sage 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).
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 Inter-
net Identified Mail (IIM) proposal.
VERSION
This man page covers version 2.11.0 of opendkim.
COPYRIGHT
Copyright (c) 2005-2008, Sendmail, Inc. and its suppliers. All rights
reserved.
Copyright (c) 2009-2013, 2015, The Trusted Domain Project. All rights
reserved.
SEE ALSO
opendkim.conf(5), sendmail(8)
Sendmail Operations Guide
RFC5321 - Simple Mail Transfer Protocol
RFC5322 - Internet Messages
RFC5451 - Message Header Field for Indicating Message Authentication
Status
RFC6008 - Authentication-Results Registration for Differentiating among
Cryptographic Results
RFC6376 - DomainKeys Identified Mail
The Trusted Domain Project opendkim(8)
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