LOCATION

/etc/mail/dkim-filter.conf

DESCRIPTION

dkim-filter(8) implements the DKIM specification for signing and verifying e-mail messages on a per-domain basis. This file is its configuration file, read on startup only.

Blank lines are ignored. Lines containing a hash ("#") character are truncated at the hash character to allow for comments in the file.

Other content should be the name of a parameter, followed by white space, followed by the value of that parameter, each on a separate line.

For parameters which are Boolean in nature, only the first byte of the value is processed. For positive values, the following are accepted: "T", "t", "Y", "y", "1". For negative values, the following are accepted: "F", "f", "N", "n", "0".

Many, but not all, of these parameters are also available as command line options to dkim-filter(8). However, new parameters are generally not added as command line options so the complete set of options is available here, and thus use of the configuration file is encouraged. In some future release, the set of available command line options is likely to get trimmed.

See the dkim-filter(8) man page for details about how and when the configuration file contents are reloaded.

PARAMETERS

AllowSHA1Only (Boolean)

Permit verify mode when only SHA1 support is available. RFC4871 requires that verifiers implement both SHA1 and SHA256 support. Setting this feature changes the absence of SHA256 support from an error to a warning.

AlwaysAddARHeader (Boolean)

Add an "Authentication-Results:" header even to unsigned messages from domains with no "signs all" policy. The reported DKIM result will be "none" in such cases. Normally unsigned mail from non-strict domains does not cause the results header to be added.

AlwaysSignHeaders (string)

Specifies a list of headers which should be included in all signature header lists (the "h=" tag) even if they were not present at the time the signature was generated. The string should be a comma-separated list of header names. The list is empty by default. The purpose of listing an absent header is to prevent its addition between the signer and the verifier, since the verifier would include that header if it were added when performing verification, which would mean the signed message and the verified message were different and the verification would fail.

ADSPDiscard (Boolean)

If "true", requests rejection of messages which are determined to be suspicious according to the author domain's published signing practises (ADSP) record if that record also recommends discard of such messages.

ADSPNoSuchDomain (Boolean)

If "true", requests rejection of messages which are determined to be from nonexistent domains according to the author domain signing practises (ADSP) test.

AuthservID (string)

Sets the "authserv-id" to use when generating the Authentication-Results: header after verifying a message. The default is to use the local machine's hostname.

AuthservIDWithJobID (Boolean)

If "true", requests that the authserv-id portion of the added Authentication-Results: headers contain the job ID of the message being evaluated.

AutoRestart (Boolean)

Automatically re-start on failures. Use with caution; if the filter fails instantly after it starts, this can cause a tight fork(2) loop.

AutoRestartCount (integer)

Sets the maximum automatic restart count. After this number of automatic restarts, the filter will give up and terminate. A value of 0 implies no limit; this is the default.

AutoRestartRate (string)

Sets the maximum automatic restart rate. If the filter begins restarting faster than the rate defined here, it will give up and terminate. This is a string of the form n/t[u] where n is an integer limiting the count of restarts in the given interval and t[u] defines the time interval through which the rate is calculated; t is an integer and u defines the units thus represented ("s" or "S" for seconds, the default; "m" or "M" for minutes; "h" or "H" for hours; "d" or "D" for days). For example, a value of "10/1h" limits the restarts to 10 in one hour. There is no default, meaning restart rate is not limited.

Background (Boolean)

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.

BaseDirectory (string)

If set, instructs the filter to change to the specified directory using chdir(2) before doing anything else. This means any files referenced elsewhere in the configuration file can be specified relative to this directory. It's also useful for arranging that any crash dumps will be saved to a specific location.

BodyLengths (Boolean)

Requests that dkim-filter include the "l=" body length tag when generating signatures. This indicates to the verifier that only a certain amount of the original message was signed, allowing tolerance of things like mailing list managers which append list-specific text to the end of mailings it processes. However, this also enables an abuse attack. See the DKIM specification for more information.

BogusKey (string)

(Only available if the filter was compiled with libunbound to enable DNSSEC support.) Instructs the filter to treat a passing signature associated with a bogus (forged) key in a special way. Possible values are neutral (return a "neutral" result), none (take no special action) and fail (return a "fail" result; this is the default).

BogusPolicy (string)

(Only available if the filter was compiled with libunbound to enable DNSSEC support.) Instructs the filter to treat an ADSP policy found in an bogus (forged) DNS record in a special way. Possible values are apply (apply the policy) and ignore (ignore the policy; this is the default).

Canonicalization (string)

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.

ClockDrift (integer)

Sets the tolerance in seconds to be applied when determining whether a signature was either expired or generated in the future. The default is 300.

Diagnostics (Boolean)

Requests the inclusion of "z=" tags in signatures, which encode the original header set for use by verifiers when diagnosing verification failures. Not recommended for normal operation.

DNSTimeout (integer)

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.

DontSignMailTo (string)

A comma-separated list of e-mail addresses (with "*" allowed as a wildcard character), mail to which should never be signed by the filter. Note that this is an "any" feature; if any one of the recipients of the message matches a member of this list, the message will not be signed.

Domain (string)

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.

This parameter is not required if a KeyList is in use; in that case, the list of signed domains is implied by the lines in that file.

EnableCoredumps (boolean)

On systems which have such support, make an explicit request to the kernel to dump cores when the filter crashes for some reason. Some modern UNIX systems suppress core dumps during crashes for security reasons if the user ID has changed during the lifetime of the process. Currently only supported on Linux.

ExternalIgnoreList (string)

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 file should be of the same form as those of the PeerList option below. The list is empty by default.

FixCRLF (Boolean)

Requests that the DKIM library convert bare CRs and LFs to CRLFs during body canonicalization, anticipating that an MTA somewhere before delivery will do that conversion anyway. The default is to leave them as-is.

Include (string)

Names a file to be opened and read as an additional configuration file. Nesting is allowed to a maximum of five levels.

InsecureKey (string)

(Only available if the filter was compiled with libunbound to enable DNSSEC support.) Instructs the filter to treat a passing signature associated with an insecure key in a special way. Possible values are neutral (return a "neutral" result), none (take no special action; this is the default) and fail (return a "fail" result).

InsecurePolicy (string)

(Only available if the filter was compiled with libunbound to enable DNSSEC support.) Instructs the filter to treat an ADSP policy found in an insecure DNS record in a special way. Possible values are apply (apply the policy; this is the default) and ignore (ignore the policy).

InternalHosts (string)

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 PeerList option below. 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 explicitly.

KeepTemporaryFiles (boolean)

Instructs the filter to create temporary files containing the header and body canonicalizations of messages which are signed or verified. The location of these files can be set using the TemporaryDirectory parameter. Intended only for debugging verification problems.

KeyFile (string)

Gives the location of a PEM-formatted private key to be used for signing all messages. Ignored if KeyList is defined.

KeyList (string)

Gives the location of a file listing rules for signing with multiple keys. If present, overrides any KeyFile setting in the conifguration file. The file named here 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 giving up.

LocalADSP (string)

Allows specification of local ADSP overrides for domains. This is expected to be a file containing entries, one per line, with comments and blank lines allowed. An entry is of the form domain:policy where domain is either a fully-qualified domain name (e.g. "foo.example.com") or a subdomain name preceded by a period (e.g. ".example.com"), and policy is either unknown, all, or discardable, as per the current ADSP draft specification. This allows local overrides of policies to enforce for domains which either don't publish ADSP or publish weaker policies than the verifier would like to enforce.

LogWhy (boolean)

If logging is enabled (see Syslog below), issues very detailed logging about the logic behind the filter's decision to either sign a message or verify it. 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 OPERATIONS section of the dkim-filter(8) man page. 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.

MacroList (string)

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 a value specified (matching is case-sensitive), otherwise the macro must be defined but may contain any value. The set is empty by default, meaning macros are not considered when making the sign-verify decision. 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 any value.

MaximumHeaders (integer)

Defines the maximum number of bytes the header block of a message may consume before the filter will reject the message. This mitigates a denial-of-service attack in which a client connects to the MTA and begins feeding an unbounded number of header fields of arbitrary size; since the filter keeps a cache of these, the attacker could cause the filter to allocate an unspecified amount of memory. The default is 65536; a value of 0 removes the limit.

MaximumSignedBytes (integer)

Specifies the maximum number of bytes of message body to be signed. Messages shorter than this limit will be signed in their entirety. Setting this value forces BodyLengths to be "True".

MilterDebug (integer)

Sets the debug level to be requested from the milter library. The default is 0.

Minimum (string)

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.

Mode (string)

Selects operating modes. The string is a concatenation of characters which 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 the dkim-filter(8) man page) in which case the default is v.

MTA (string)

A comma-separated list of MTA names (a la the sendmail(8) DaemonPortOptions Name parameter) whose mail should be signed by this filter. There is no default, meaning MTA name is not considered when making the sign-verify decision.

MustBeSigned (string)

Specifies a list of headers which, if present, must be covered by the DKIM signature when verifying a message. The string should be a comma-separated list of header names. If a header in this list is present in the message and is not signed, the filter will treat even an otherwise valid signature as invalid. The default is an empty list.

OmitHeaders (string)

Specifies a list of headers which should be omitted when generating signatures. The string 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). Note that specifying a list with this parameter replaces the default entirely.

On-BadSignature (string)

Selects the action to be taken when a signature fails to validate. Possible values (with abbreviated forms in parentheses): accept (a) accept the message; discard (d) discard the message; tempfail (t) temp-fail the message; reject (r) reject the message. The default is accept.

On-Default (string)

Selects the action to be taken when any verification or internal error of any kind is encountered. This is processed before the other "On-" values so it can be used as a blanket setting followed by specific overrides.

On-DNSError (string)

Selects the action to be taken when a transient DNS error is encountered. Possible values are the same as those for On-BadSignature. The default is tempfail.

On-InternalError (string)

Selects the action to be taken when an internal error of some kind is encountered. Possible values are the same as those for On-BadSignature. The default is tempfail.

On-NoSignature (string)

Selects the action to be taken when a message arrives unsigned. Possible values are the same as those for On-BadSignature. The default is accept.

On-Security (string)

Selects the action to be taken when a message arrives containing properties that may be a security concern. Possible values are the same as those for On-BadSignature. The default is tempfail.

PeerList (string)

Identifies a file of "peers" which identifies clients whose connections should be accepted without processing by this filter. The file 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.

PidFile (string)

Specifies the path to a file which should be created at process start containing the process ID.

POPDBFile (string)

Requests that the filter consult a POP authentication database named in the string for IP addresses that should be allowed for signing. The filter must be compiled with the POPAUTH flag to enable this feature, since it adds a library dependency.

Quarantine (Boolean)

Requests that messages which fail verification be quarantined by the MTA. (Requires a sufficiently recent version of the milter library.)

QueryCache (Boolean)

Instructs the DKIM library to maintain its own local cache of keys and policies retrieved from DNS, rather than relying on the nameserver for caching service. Useful if the nameserver being used by the filter is not local. The filter must be compiled with the QUERY_CACHE flag to enable this feature, since it adds a library dependency.

RemoveARAll (Boolean)

Removes all Authentication-Results: header fields which also satisfy the requirements of RemoveARFrom below. By default, only those containing a DKIM result are removed.

RemoveARFrom (string)

Lists patterns of hostnames whose Authentication-Results: header fields should be removed before the message is passed for delivery. By default only those headers matching the local host's canonical name will be removed. If more than one pattern is desired, the list should be comma-separated. Matching is only done on full hostnames (e.g. "host.example.com") or on domain names (e.g. ".example.com").

RemoveOldSignatures (Boolean)

Removes all existing signatures when operating in signing mode.

ReportAddress (string)

Specifies the string to use in the From: header field for outgoing reports (see SendReports and SendADSPReports below). If not specified, the executing user and local hostname will be used to construct the address.

RequiredHeaders (boolean)

Checks all messages for compliance with RFC2822 header count requirements. Non-compliant messages are rejected.

Selector (string)

Defines the name of the selector to be used when signing messages. See the DKIM specification for details. Used only when signing with a single key; see the KeyList parameter above for more information.

SendADSPReports (Boolean)

If true, when a policy evaluation fails and the signing site advertises a reporting address (i.e. r=user in its policy record) and a request for reports of such failures, the filter will send a structured report to that address containing details of the incident.

SendReports (Boolean)

If true, when a signature verification fails and the signing site advertises a reporting address (i.e. r=user in its policy record) and a request for reports of such failures, the filter will send a structured report to that address containing details needed to reproduce the problem.

SignatureAlgorithm (string)

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.

SignatureTTL (integer)

Sets the time-to-live, in seconds, of signatures generated by the filter. If not set, no expiration time is added to signatures.

SignHeaders (string)

Specifies the list of headers which should be included when generating signatures. The string should be a comma-separated list of header names. If the list omits any header which is mandated by the DKIM specification, those headers are implicitly added. By default, those headers listed in the DKIM specification as "SHOULD" be signed will be signed by the filter. Specifying a list here replaces that list entirely. See the OmitHeaders configuration option for more information.

Socket (string)

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. This option is mandatory either in the configuration file or on the command line.

StrictTestMode (Boolean)

Selects strict CRLF mode during testing (see the -t command line flag in the dkim-filter(8) man page); messages for which all header fields and body lines are not CRLF-terminated are considered malformed and will produce an error.

SubDomains (Boolean)

Sign subdomains of those listed by the Domain parameter as well as the actual domains.

Syslog (Boolean)

Log via calls to syslog(3) any interesting activity.

SyslogFacility (string)

Log via calls to syslog(3) using the named facility. The facility names are the same as the ones allowed in syslog.conf(5). The default is mail.

SyslogSuccess (Boolean)

Log via calls to syslog(3) additional entries indicating successful signing or verification of messages.

TemporaryDirectory (string)

Specifies the directory in which temporary canonicalization files should be written. The default is to use the libdkim default location, currently /var/tmp.

TestPublicKeys (string)

Names a file from which public keys should be read. Intended for use only during automated testing.

TrustAnchorFile (string)

Specifies a file from which trust anchor data should be read when doing DNS queries and applying the DNSSEC protocol. Requires that the filter be compiled with USE_UNBOUND set. See the Unbound documentation at http://unbound.net for the expected format of this file.

TrustSignaturesFrom (string)

Like Domain, this value consists of either a comma-separated list of domain names or a file containing a list of domains. In either case, the list of domains is used to decide which domains are considered trustworthy in terms of third-party signatures. That is, if a message arrives with a signature from a domain that doesn't match the domain in the From: header, this setting determines whether or not that signature will be trusted. If this value is undefined, all signatures are trusted.

UMask (integer)

Requests a specific permissions mask to be used for file creation. This only really applies to creation of the socket when Socket specifies a UNIX domain socket, and to the PidFile (if any); temporary files are created by the mkstemp(3) function which enforces a specific file mode on creation regardless of the process umask. See umask(2) for more information.

UserID (string)

Attempts to become the specified userid before starting operations. The value is of the form userid[:group]. The process will be assigned all of the groups and primary group ID of the named userid unless an alternate group is specified.

X-Header (Boolean)

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.

NOTES

When using DNS timeouts (see the DNSTimeout 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.

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 "1.2.3.0/24" are not the same thing).

VERSION

This man page covers version 2.8.0 of dkim-filter.

COPYRIGHT

Copyright (c) 2007, 2008, Sendmail, Inc. and its suppliers. All rights reserved.