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dkim-filter.conf(5) dkim- filter.conf(5) NAME dkim-filter.conf - Configuration file for dkim-filter LOCATION /etc/mail/dkim-filter.conf DESCRIPTION dkim-filter(8) implements the DKIM specification for signing and veri- fying e-mail messages on a per-domain basis. This file is its configu- ration 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, fol- lowed 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 avail- able 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 con- figuration file contents are reloaded. PARAMETERS AllowSHA1Only (Boolean) Permit verify mode when only SHA1 support is avail- able. 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 mes- sages from domains with no "signs all" policy. The reported DKIM result will be "none" in such cases. Nor- mally 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 includ- ed in all sig- nature 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 ver- ifier would include that header if it were added when performing verification, which would mean the signed message and the veri- fied 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 do- main's published signing practises (ADSP) record if that record al- so 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. AuthservIDWithJobID (Boolean) If "true", requests that the authserv-id portion of the added Authentication-Results: headers contain the job ID of the mes- sage 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 repre- sented ("s" or "S" for seconds, the default; "m" or "M" for min- utes; "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. BodyLengths (Boolean) Requests that dkim-filter include the "l=" body length tag when generating signatures. This indicates to the veri- fier 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 pro- cesses. How- ever, this also enables an abuse attack. See the DKIM specifi- cation for more information. Canonicalization (string) Selects the canonicalization method(s) to be used when signing messages. When verifying, the message's DKIM-Sig- nature: header specifies the canonicalization method. The rec- ognized 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 ("/") char- acter, 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 gen- erated 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 infi- nite wait. The default is 5. Ignored if not us- ing the asyn- chronous resolver package. See also the NOTES sec- tion 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 file- name 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 char- acter "*" 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. ExternalIgnoreList (string) Identifies a file of "external" hosts which may send mail through the server as one of the signing domains without creden- tials as such. Basically suppresses the "external host (host- name) tried to send mail as (domain)" log mes- sages. 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 addition- al configura- tion file. Nesting is allowed to a maximum of five levels. 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, pro- viding 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. KeyFile (string) Gives the location of a PEM-formatted private key to be used for signing all messages. Ignored if KeyList is de- fined. KeyList (string) Gives the location of a file listing rules for signing with mul- tiple 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 an- nounce 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 se- lector 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" be- fore 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 deci- sion 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 deci- sion 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, other- wise the macro must be set but can contain any val- ue. MaximumHeaders (integer) Defines the maximum number of bytes the header block of a mes- sage may consume before the filter will reject the message. This mitigates a denial-of-service attack in which a client con- nects to the MTA and begins feeding an unbound- ed 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 for- mats: min indicating at least min bytes of the mes- sage 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 concate- nation of char- acters 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) Dae- monPortOptions Name parameter) whose mail should be signed by this filter. There is no default, meaning MTA name is not con- sidered 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 mes- sage. 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 omit- ted when gener- ating 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 specifica- tion as "SHOULD NOT" be signed; the default list for this param- eter contains those headers (Return-Path, Received, Comments, Keywords, Bcc, Resent-Bcc and DKIM-Signature). To omit no head- ers, 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 vali- date. 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) re- ject the mes- sage. The default is accept. On-Default (string) Selects the action to be taken when any verifica- tion or internal error of any kind is encountered. This is pro- cessed before the other "On-" values so it can be used as a blanket setting fol- lowed 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 ar- rives 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 ar- rives containing properties that may be a security concern. Possi- ble values are the same as those for On-BadSignature. The default is tempfail. PeerList (string) Identifies a file of "peers" which identifies clients whose con- nections should be accepted without processing by this filter. The file should contain on each line a host- name, domain name (e.g. ".example.com"), IP address, an IPv6 address (including an IPv4 mapped address), or a CIDR-style IP speci- fication (e.g. "192.168.1.0/24"). An entry beginning with a bang ("!") charac- ter means "not", allowing exclusions of specific hosts that are otherwise members of larger sets. The order of en- tries in this file is therefore significant. PidFile (string) Specifies the path to a file which should be cre- ated at process start containing the process ID. POPDBFile (string) Requests that the filter consult a POP authentica- tion 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 depen- dency. 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 deliv- ery. 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 outgo- ing reports (see SendReports and SendADSPReports below). If not specified, the executing user and local hostname will be used to construct the address. Selector (string) Defines the name of the selector to be used when signing mes- sages. See the DKIM specification for details. Used only when signing with a single key; see the KeyList parame- ter 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 fail- ures, the filter will send a structured report to that address con- taining 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 fail- ures, the filter will send a structured report to that address con- taining details needed to reproduce the problem. SignatureAlgorithm (string) Selects the signing algorithm to use when generat- ing 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 signa- tures. SignHeaders (string) Specifies the list of headers which should be in- cluded when gen- erating 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 im- plicitly 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 con- figuration 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 ser- vice. socketspec is in one of two forms: local:path which cre- ates a UNIX domain socket at the speci- fied 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 in- terfaces. 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-terminat- ed are consid- ered malformed and will produce an error. SubDomains (Boolean) Sign subdomains of those listed by the Domain pa- rameter as well as the actual domains. Syslog (Boolean) Log via calls to syslog(3) any interesting activi- ty. SyslogFacility (string) Log via calls to syslog(3) using the named facili- ty. The facil- ity 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 in- dicating suc- cessful signing or verification of messages. TestPublicKeys (string) Names a file from which public keys should be read. Intended for use only during automated testing. 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-par- ty 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 trust- ed. UMask (integer) Requests a specific permissions mask to be used for file cre- ation. 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 informa- tion. UserID (string) Attempts to become the specified userid before starting opera- tions. 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 speci- fied. X-Header (Boolean) Causes dkim-filter to add a header indicating the presence of this filter in the path of the message from injec- tion to deliv- ery. 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 inter- action 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 infor- mation. 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 2.7.0 of dkim-filter. COPYRIGHT Copyright (c) 2007, 2008, Sendmail, Inc. and its suppli- ers. All rights reserved. SEE ALSO dkim-filter(8), sendmail(8) RFC4871 - DomainKeys Identified Mail Authentication-Results Internet draft (draft-kucher- awy-sender-auth- header) Author Domain Signing Practises draft (draft-ietf-dkim- ssp-04) Sendmail, Inc. dkim- filter.conf(5)