1dkim-filter(8) System Manager's Manual dkim-filter(8)
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6 dkim-filter - DKIM filter for sendmail
7
9 dkim-filter -p socketspec [-a peerlist] [-A] [-b modes] [-c canon] [-C
10 config] [-d domains] [-D] [-f] [-F time] [-i ilist] [-I eilist] [-h]
11 [-k keyfile] [-K] [-l] [-L min] [-m mtas] [-M macro[=value][,...]] [-n]
12 [-o hdrlist] [-P pidfile] [-q] [-r] [-R] [-s selector] [-S signalg] [-t
13 testfile] [-T secs] [-u userid[:group]] [-U popdb] [-v] [-V] [-W] [-x
14 configfile]
15
17 dkim-filter implements the DKIM standard for signing and verifying e-
18 mail messages on a per-domain basis.
19 dkim-filter uses the milter interface, originally distributed as part
21 of version 8.11 of sendmail(8), to provide DKIM signing and/or verify‐
22 ing service for mail transiting a milter-aware MTA.
23 Most, if not all, of the command line options listed below can also be
25 set using a configuration file. See the -x option for details.
26
28 -a peerlist
29 Identifies a file of "peers" which identifies clients whose con‐
30 nections should be accepted without processing by this filter.
31 The peerlist should contain on each line a hostname, domain name
32 (e.g. ".example.com"), IP address, an IPv6 address (including an
33 IPv4 mapped address), or a CIDR-style IP specification (e.g.
34 "192.168.1.0/24"). An entry beginning with a bang ("!") charac‐
35 ter means "not", allowing exclusions of specific hosts that are
36 otherwise members of larger sets. The order of entries in this
37 file is therefore significant.
38 -A Automatically re-start on failures. Use with caution; if the
40 filter fails instantly after it starts, this can cause a tight
41 fork(2) loop. This can be mitigated using some values in the
42 configuration file to limit restarting. See dkim-fil‐
43 ter.conf(5).
44 -b modes
46 Selects operating modes. modes is a concatenation of characters
47 that indicate which mode(s) of operation are desired. Valid
48 modes are s (signer) and v (verifier). The default is sv except
49 in test mode (see -t below) in which case the default is v.
50 -c canon
52 Selects the canonicalization method(s) to be used when signing
53 messages. When verifying, the message's DKIM-Signature: header
54 specifies the canonicalization method. The recognized values
55 are relaxed and simple as defined by the DKIM specification.
56 The default is simple. The value may include two different
57 canonicalizations separated by a slash ("/") character, in which
58 case the first will be applied to the headers and the second to
59 the body.
60 -C config
62 Configuration control. See the CONFIGURATION section for
63 details.
64 -d domain [,...]
66 A comma-separated list of domains whose mail should be signed by
67 this filter. Mail from other domains will be verified rather
68 than being signed.
69 The value of this parameter may also be a filename from which
71 domain names will be read. The "#" character in such a file is
72 assumed to indicate a comment. An absolute path must be used
73 (i.e. the first character must be a "/").
74 In either case, the domain name(s) may contain the special char‐
76 acter "*" which is treated as a wildcard character matching zero
77 or more characters in a domain name.
78 Matching is case-insensitive.
80 This parameter is not required if -K is in use; in that case,
82 the list of signed domains is implied by the lines in the key
83 file.
84 -D Sign subdomains of those listed by the -d option as well as the
86 actual domains.
87 -f Normally dkim-filter forks and exits immediately, leaving the
89 service running in the background. This flag suppresses that
90 behaviour so that it runs in the foreground.
91 -F time
93 Specifies a fixed time to use when generating signatures.
94 Ignored unless also used in conjunction with -t (see below).
95 The time must be expressed in the usual UNIX time_t (seconds
96 since epoch) format.
97 -h Causes dkim-filter to add a header indicating the presence of
99 this filter in the path of the message from injection to deliv‐
100 ery. The product's name, version, and the job ID are included
101 in the header's contents.
102 -i ilist
104 Identifies a file of internal hosts whose mail should be signed
105 rather than verified. Entries in this file follow the same form
106 as those of the -a option above. If not specified, the default
107 of "127.0.0.1" is applied. Naturally, providing a value here
108 overrides the default, so if mail from 127.0.0.1 should be
109 signed, the list provided here should include that address
110 explicitly.
111 -I eilist
113 Identifies a file of "external" hosts which may send mail
114 through the server as one of the signing domains without creden‐
115 tials as such. Basically suppresses the "external host (host‐
116 name) tried to send mail as (domain)" log messages. Entries in
117 the eilist file should be of the same form as those of the -a
118 option above. The list is empty by default.
119 -K Requests multiple-key processing. See also -k below.
121 -k keyfile
123 Without -K, gives the location of a PEM-formatted private key to
124 be used for signing all messages. With -K, gives the location
125 of a file listing rules for signing with multiple keys.
126 In the latter mode, the keyfile should contain a set of lines of
128 the form sender-pattern:signing-domain:keypath where sender-pat‐
129 tern is a pattern to match against message senders (with the
130 special character "*" interpreted as "zero or more characters"),
131 signing-domain is the domain to announce as the signing domain
132 when generating signatures, and keypath is the path to the PEM-
133 formatted private key to be used for signing messages which
134 match the sender-pattern. The selector used in the signature
135 will be the filename portion of keypath.
136 If the file referenced by keypath cannot be opened, the filter
138 will try again by appending ".pem" and then ".private" before
139 giving up.
140 -l Log via calls to syslog(3) any interesting activity.
142 -L min[%+]
144 Instructs the verification code to fail messages for which a
145 partial signature was received. There are three possible for‐
146 mats: min indicating at least min bytes of the message must be
147 signed (or if the message is smaller than min then all of it
148 must be signed); min% requiring that at least min percent of the
149 received message must be signed; and min+ meaning there may be
150 no more than min bytes of unsigned data appended to the message
151 for it to be considered valid.
152 -m mta[,...]
154 A comma-separated list of MTA names (a la the sendmail(8) Dae‐
155 monPortOptions Name parameter) whose mail should be signed by
156 this filter. If not set, the MTA name is not used when deciding
157 whether or not a message should be signed.
158 -M macro[=value][,...]
160 Defines a set of MTA-provided macros which should be checked to
161 see if the sender has been determined to be a local user and
162 therefore whether or not the message should be signed. If a
163 value is specified, the value of the macro must match the value
164 specified (matching is case-insensitive), otherwise the macro
165 must be defined but may contain any value. Multiple tests may
166 be specified, separated by commas. The set is empty by default,
167 meaning macros are not used when deciding whether or not a mes‐
168 sage should be signed.
169 The general format of the string is test1[,test2[,...]] where a
171 "test" is of the form macro[=value1[|value2[|...]]]; if one or
172 more value is defined then the macro must be set to one of the
173 listed values, otherwise the macro must be set but can contain
174 any value.
175 -n Parse the configuration file and command line arguments, report‐
177 ing any errors found, and then exit. The exit value will be 0
178 if the filter would start up without complaint, or non-zero oth‐
179 erwise.
180 -o hdrlist
182 Specifies a list of headers which should be omitted when gener‐
183 ating signatures. hdrlist should be a comma-separated list of
184 header names. If an entry in the list names any header which is
185 mandated by the DKIM specification, the entry is ignored. A set
186 of headers is listed in the DKIM specification as "SHOULD NOT"
187 be signed; the default list for this parameter contains those
188 headers (Return-Path, Received, Comments, Keywords, Bcc, Resent-
189 Bcc and DKIM-Signature). To omit no headers, simply use the
190 string "-" (or any string which will match no headers).
191 -p socketspec
193 Specifies the socket that should be established by the filter to
194 receive connections from sendmail(8) in order to provide ser‐
195 vice. socketspec is in one of two forms: local:path which cre‐
196 ates a UNIX domain socket at the specified path, or
197 inet:port[@host] which creates a TCP socket on the specified
198 port. If the host is not given as either a hostname or an IP
199 address, the socket will be listening on all interfaces. If
200 neither socket type is specified, local is assumed, meaning the
201 parameter is interpreted as a path at which the socket should be
202 created. This parameter is mandatory.
203 -P pidfile
205 Writes the process ID of the filter, once started, to the file‐
206 name given.
207 -q Requests that messages which fail verification be quarantined by
209 the MTA. (Requires a sufficiently recent version of the milter
210 library.)
211 -r Checks all messages for compliance with RFC2822 header count
213 requirements. Non-compliant messages are rejected.
214 -R When a signature verification fails and the signing site adver‐
216 tises a reporting address (i.e. r=user@host in its policy
217 record), send a structured report to that address containing
218 details needed to reproduce the problem.
219 -s selector
221 Defines the name of the selector to be used when signing mes‐
222 sages. See the DKIM specification for details.
223 -S signalg
225 Selects the signing algorithm to use when generating signatures.
226 If the filter was compiled against version 0.9.8 or later of
227 OpenSSL then both rsa-sha1 and rsa-sha256 are available and the
228 latter is the default. Otherwise, only the former is available
229 and it is (obviously) the default.
230 -t testfile
232 Evaluates (verifies) an RFC2822-formatted message found in test‐
233 file and exits. The value of testfile may be "-" if the message
234 should be read from standard input.
235 -T secs
237 Sets the DNS timeout in seconds. A value of 0 causes an infi‐
238 nite wait. The default is 5. Ignored if not using the asyn‐
239 chronous resolver package. See also the NOTES section below.
240 -u userid[:group]
242 Attempts to be come the specified userid before starting opera‐
243 tions. The process will be assigned all of the groups and pri‐
244 mary group ID of the named userid unless an alternate group is
245 specified.
246 -U popdb
248 Requests that the filter consult a POP authentication database
249 for IP addresses that should be allowed for signing. The filter
250 must be specially compiled to enable this feature, since it adds
251 a library dependency.
252 -v Increase verbose output during test mode (see -t above). May be
254 specified more than once to request increasing amounts of out‐
255 put.
256 -V Print the version number and supported canonicalization and sig‐
258 nature algorithms, and then exit without doing anything else.
259 -W If logging is enabled (see -l above), issues very detailed log‐
261 ging about the logic behind the filter's decision to either sign
262 a message or verify it. The "W" stands for "Why?!" since the
263 logic behind the decision is non-trivial and can be confusing to
264 administrators not familiar with its operation. A description
265 of how the decision is made can be found in the OPERATION sec‐
266 tion of this document. This causes a large increase in the
267 amount of log data generated for each message, so it should be
268 limited to debugging use and not enabled for general operation.
269 -x configfile
271 Read the named configuration file. See the dkim-filter.conf(5)
272 man page for details. Values in the configuration file are
273 overridden when their equivalents are provided on the command
274 line until a configuration reload occurs. The OPERATION section
275 describes how reloads are triggered.
276
278 The value of the -C switch is a comma-separated list of settings of the
279 form result=action which defines what the filter should do with mes‐
280 sages that produce certain results. Each result and each action has a
281 full name and an abbreviated name. Either is accepted. Below, the
282 abbreviated name appears in parentheses.
283 results
285 badsignature (bad) the signature found in the message did not
286 verify successfully against the message; dnserror (dns) an error
287 was encountered attempting to retrieve a public key from the
288 nameserver; internal (int) an internal error occurred; nosigna‐
289 ture (no) no signature was present on the message; security
290 (sec) the message tripped internal security concerns (e.g.
291 unusually large header blocks). There is also a special result
292 called default (def) whose action is copied onto all of the
293 other results.
294 action accept (a) accept the message; discard (d) discard the message;
296 tempfail (t) temp-fail the message; reject (r) reject the mes‐
297 sage.
298 In the interests of minimal initial impact, the defaults for badsigna‐
300 ture and nosignature are accept, and the default for the others is
301 tempfail.
302 Results and actions are processed in order, so use of the default
304 action can be overridden by later specifications. For example, using
305 "def=a,int=t" sets all result actions to "accept" except for internal
306 errors which will generate a temporary failure.
307
309 A message will be verified unless it conforms to the signing criteria,
310 which are: (1) the domain on the From: address or Sender: address (if
311 present) must be listed by the -d command line switch or the Domain
312 configuration file setting, and (2) (a) the client connecting to the
313 MTA must have authenticated, or (b) the client connecting to the MTA
314 must be listed in the file referenced by the -i command line switch (or
315 be in the default list for that option), or (c) the client must be con‐
316 nected to a daemon port named by the -m command line switch, or (d) the
317 MTA must have set one or more macros matching the criteria set by the
318 -M command line switch.
319 When signing a message, a DKIM-Signature: header will be prepended to
321 the message. The signature is computed using the private key provided.
322 You must be running a version of sendmail(8) recent enough to be able
323 to do header prepend operations (8.13.0 or later).
324 When verifying a message, an Authentication-Results: header will be
326 prepended to indicate the presence of a signature and whether or not it
327 could be validated against the body of the message using the public key
328 advertised by the sender's nameserver. The value of this header can be
329 used by mail user agents to sort or discard messages that were not
330 signed or could not be verified.
331 Upon receiving SIGUSR1, if the filter was started with a configuration
333 file, it will be re-read and the new values used. Note that any com‐
334 mand line overrides provided at startup time will be lost when this is
335 done. Also, the following configuration file values (and their corre‐
336 sponding command line items, if any) are not reloaded through this
337 process: AutoRestart (-A), AutoRestartCount, AutoRestartRate, Back‐
338 ground, MilterDebug, PidFile (-P), POPDBFile, Quarantine (-q),
339 QueryCache, Socket (-p), StrictTestMode, TestPublicKeys, UMask, UserID
340 (-u). The filter does not automatically check the configuration file
341 for changes and reload.
342
344 The following environment variable(s) can be used to adjust the behav‐
345 iour of this filter:
346 DKIM_TMPDIR
348 The directory to use when creating temporary files. The default
349 is /var/tmp.
350
352 When using DNS timeouts (see the -T option above), be sure not to use a
353 timeout that is larger than the timeout being used for interaction
354 between sendmail and the filter. Otherwise, the MTA could abort a mes‐
355 sage while waiting for a reply from the filter, which in turn is still
356 waiting for a DNS reply.
357 The POP authentication database is expected to be a Sleepycat DB file
359 (formerly known as a Berkeley DB) in hash format with keys containing
360 the IP address in text form without a terminating NULL. The values of
361 these records are not checked; only the existence of such records is of
362 interest. The filter will attempt to establish a shared lock on the
363 database before reading from it, so any programs which write to the
364 database should keep their lock use to a minimum or else this filter
365 will appear to hang while waiting for the lock operation to complete.
366 Features that involve specification of IPv4 addresses or CIDR blocks
368 will use the inet_addr(3) function to parse that information. Users
369 should be familiar with the way that function handles the non-trivial
370 cases (for example, "1.2.3/24" and "1.2.3.0/24" are not the same
371 thing).
372
374 DKIM is an amalgam of Yahoo!'s DomainKeys proposal, and Cisco's Inter‐
375 net Identified Mail (IIM) proposal.
376
378 This man page covers version 2.8.0 of dkim-filter.
379
381 Copyright (c) 2005-2008, Sendmail, Inc. and its suppliers. All rights
382 reserved.
383
385 dkim-filter.conf(5), sendmail(8)
386 Sendmail Operations Guide
388 RFC2821 - Simple Mail Transfer Protocol
390 RFC2822 - Internet Messages
392 RFC4871 - DomainKeys Identified Mail
394 RFC5451 - Message Header Field for Indicating Message Authentication
396 Status
397 Sendmail, Inc. dkim-filter(8)