NAME

dk-filter - DomainKeys filter for sendmail

SYNOPSIS

dk-filter -p socketspec [-a peerlist] [-A] [-b modes] [-c canon] [-C config] [-d domains] [-D] [-f] [-i ilist] [-I eilist] [-h] [-H] [-k] [-l] [-m mtas] [-M macro[=value][,...]] [-o hdrlist] [-P pidfile] [-q] [-R] [-s keyfile] [-S selector] [-T secs] [-u userid] [-U popdb] [-V]

DESCRIPTION

dk-filter implements Yahoo!, Inc.'s DomainKeys draft standard for signing and verifying e-mail messages on a per-domain basis.

Details regarding the protocol and other issues related to the draft standard can be found at http://antispam.yahoo.com/domainkeys.

OPTIONS

-a peerlist

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

-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.

-b modes

Selects operating modes. modes 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.

-c canon

Selects the canonicalization method to be used when signing messages. When verifying, the message's DomainKey-Signature: header specifies the canonicalization method. The recognized values are nofws and simple as defined by the DomainKeys draft. The default is simple.

-C config

Configuration control. See the CONFIGURATION section for details.

-d domain [,...]

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.

If the value of this parameter starts with a "/" character, it is assumed to be a filename from which the domain list will be read, one per line, with "#" characters indicating the beginning of a comment.

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.

-D

Sign subdomains of those listed by the -d option as well as the actual domains.

-f

Normally dk-filter forks and exits immediately, leaving the service running in the background. This flag suppresses that behaviour so that it runs in the foreground.

-h

Causes dk-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.

-H

Includes on DomainKey signatures the list of headers that were included in the signature. This makes the signature header larger by explicitly listing the included headers, but this also allows verifying agents to ignore headers that were added in transit.

-i ilist

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 -a option above. If not specified, the default of "127.0.0.1" is applied.

-I eilist

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 this file follow the same form as those of the -a option above.

-k

Causes -s to be interpreted as the location of a key list, which is a file listing rules for signing with multiple keys. The key list should contain a set of lines of the form sender-pattern:keypath where sender-pattern is a pattern to match against message senders (with the special character "*" interpreted as "zero or more characters"), 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.

-l

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

-m mta [,...]

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.

-M macro[=value][,...]

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 the value specified (matching is case-insensitive), otherwise the macro must be defined but may contain any value. The list is empty by default.

-o header [,...]

A comma-separated list of headers which should not be signed. Ignored when verifying.

-p socketspec

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.

-P pidfile

Writes the process ID of the filter, once started, to the filename given.

-q

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

-R

When a signature verification fails and the signing site advertises a reporting address (i.e. r=user@host in its policy record), send a structured report to that address containing details needed to reproduce the problem.

-s keyfile

Gives the location of a PEM-formatted private key to be used for message signing.

-S selector

Defines the name of the selector to be used when signing messages. See the DomainKeys specification for details.

-T secs

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.

-u userid

Attempts to be come the specified userid before starting operations.

-U popdb

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

-V

Print the version number and exit without doing anything else.

CONFIGURATION

The value of the -C switch is a comma-separated list of settings of the form result=action which defines what the filter should do with messages that produce certain results. Each result and each action has a full name and an abbreviated name. Either is accepted. Below, the abbreviated name appears in parentheses.

results: badsignature (bad) the signature found in the message did not verify successfully against the message; dnserror (dns) an error was encountered attempting to retrieve a public key from the nameserver; internal (int) an internal error occurred; nosignature (no) no signature was present on the message; signaturemissing (miss) no signature was present on the message which claims to sign all messages.
action: accept (a) accept the message; discard (d) discard the message; tempfail (t) temp-fail the message; reject (r) reject the message.

In the interests of minimal initial impact, the defaults for badsignature, nosignature and signaturemissing are all accept, and the default for the others is tempfail.

OPERATION

A message will be verified unless it conforms to the signing criteria, which are: (1) the domain on the From: address or Sender: address (if present) must be listed by the -d command line switch, and (2) the client connecting to the MTA must (a) have authenticated, or (b) be listed in the file referenced by the -i command line switch (or be in the default list for that option), or (c) be connected to daemon port named by the -m command line switch.

When signing a message, a DomainKey-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.

ENVIRONMENT

The following environment variable(s) can be used to adjust the behaviour of this filter:

DK_TMPDIR: The directory to use when creating temporary files. The default is /var/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. 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.

VERSION

This man page covers version 1.0.1 of dk-filter.