batv-filter(8) BATV (Bounce Address Tag Validation) filter for sendmail


batv-filter -p socketspec -k key [-a allowlist] [-A] [-b] [-d domainlist] [-D debuglevel] [-f] [-i ilist] [-l] [-n] [-P pidfile] [-s] [-S] [-u user] [-V] [-x]


batv-filter implements the experimental Bounce Address Tag Validation specification as a means to filter mail which claims to come from the local domain which in fact does not.

For details regarding the proposal, consult the draft text.


-a allowlist
Identifies a file of envelope addresses which should be immediately allowed (not processed) by the filter. Matching is done on both sender (MAIL FROM) and recipient (RCPT TO) addresses. The hash character ("#") denotes comments in the usual shell script style; data after a hash character on any given line is ignored. Blank lines are also ignored. The entries in this file may include wildcard ("*") characters.
Automatically re-start on failures. Use with caution; if the filter fails instantly after it starts, this can cause a tight fork(2) loop.
By default, arriving messages addressed to users in domains listed by the -d option must be signed if they are bounces (i.e. empty envelope sender) but not otherwise; bounces to unsigned addresses will be rejected. This option applies the verification to all envelope senders, not just the empty sender.
-d domainlist
Specifies a comma-separated list of domain names whose mail should be signed and verified by this filter. Specify all possible subdomains by prefixing a dot (".") to the appropriate entries. If no domains are specified, mail for all domains is signed. See also the -i option below.
-D debuglevel
Requests milter debugging at the specified debug level.
Suppresses the call to fork(), so that the filter runs in the foreground.
Causes batv-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.
-i ilist
Specifies a file which contains a list of hostnames, IP addresses or CIDR-style IP specifications (e.g. ""). Mail coming from these sources should be signed rather than being verified. See also the -d option above. If unspecified, a single-entry list referring to "" is assumed.
-k key
Specifies the file in which to find the key to be used when signing outgoing mail and verifying arriving mail. The key can be any arbitrary text string and should appear one the first line of this file.
Log via calls to syslog(3) any interesting activity.
-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. Multiple tests may be specified, separated by commas. The set is empty by default, meaning macros are not used when deciding whether or not a message should be signed.

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.

Don't reject things which fail validation.
-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.
Causes mail from clients authenticated to the MTA to be ignored (passed) by the filter. The default is to process even authenticated messages.
Requests that the MTA provide meaningful SMTP reply codes when rejecting messages rather than their un-descriptive defaults.
-u userid
Attempts to be come the specified userid before starting operations.
Print the version number and exit without doing anything else.
Activates an experimental Sendmail variant of BATV. See the SENDMAIL MODIFICATIONS section below for details.


A message will be verified unless it conforms to the signing criteria, which are: (1) the domain on the envelope sender address 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.


Adding -x to the command line will cause the filter to sign and verify using a Sendmail, Inc. variant of BATV. This method uses the basic idea of BATV but appends the tag rather than prepending it, and uses the "+" character instead of the "=" character to separate the BATV tag from the actual address. This is more compatible with configurations and filters that accomodate the existing MTA-supported address tagging system which has been around since at least the mid 1990s.


This man page covers version 0.4.0 of batv-filter.


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