Mail::SpamAssassin::Plugin::AskDNS(3) User Contributed Perl Documentation Mail::SpamAssassin::Plugin::AskDNS(3)

AskDNS - form a DNS query using tag values, and look up the DNSxL lists

loadplugin  Mail::SpamAssassin::Plugin::AskDNS
askdns D_IN_DWL TXT /\b(transaction|list|all)\b/

Using a DNS query template as specified in a parameter of a askdns rule, the plugin replaces tag names as found in the template with their values and launches DNS queries as soon as tag values become available. When DNS responses trickle in, filters them according to the requested DNS resource record type and optional subrule filtering expression, yielding a rule hit if a response meets filtering conditions.

A query template is a string which will be expanded to produce a domain name to be used in a DNS query. The template may include SpamAssassin tag names, which will be replaced by their values to form a final query domain.

The final query domain must adhere to rules governing DNS domains, i.e. must consist of fields each up to 63 characters long, delimited by dots, not exceeding 255 characters. International domain names (in UTF-8) are allowed and will be encoded to ASCII-compatible encoding (ACE) according to IDN rules. Syntactically invalid resulting queries will be discarded by the DNS resolver code (with some info warnings).

There may be a trailing dot at the end, but it is redundant / carries no semantics, because SpamAssassin uses a Net::DSN::Resolver::send method for querying DNS, which ignores any 'search' or 'domain' DNS resolver options. Domain names in DNS queries are case-insensitive.

A tag name is a string of capital letters, preceded and followed by an underscore character. This syntax mirrors the add_header setting, except that tags cannot have parameters in parenthesis when used in askdns templates (exceptions found below). Tag names may appear anywhere in the template - each queried DNS zone prescribes how a query should be formed.

Special supported tag HEADER() can be used to query any header content, using same header names/modifiers that as header rules support. For example _HEADER(Reply-To:addr:domain)_ can be used to query the trimmed domain part of Reply-To address. See Mail::SpamAssassin::Conf documentation about header rules.

A query template may contain any number of tag names including none, although in the most common anticipated scenario exactly one tag name would appear in each askdns rule. Specified tag names are considered dependencies. Askdns rules with dependencies on the same set of tags are grouped, and all queries in a group are launched as soon as all their dependencies are met, i.e. when the last of the awaited tag values becomes available by a call to set_tag() from some other plugin or elsewhere in the SpamAssassin code.

Launched queries from all askdns rules are grouped too according to a pair of: query type and an expanded query domain name. Even if there are multiple rules producing the same type/domain pair, only one DNS query is launched, and a reply to such query contributes to all the constituent rules.

A tag may produce none, one or multiple values. Askdns rules awaiting for a tag which never receives its value never result in a DNS query. Tags which produce multiple values will result in multiple queries launched, each with an expanded template using one of the tag values. An example is a DKIMDOMAIN tag which yields a list of signing domains, one for each valid signature in a signed message.

When more than one distinct tag name appears in a template, each potentially resulting in multiple values, a Cartesian product is formed, and each tuple results in a launch of one DNS query (duplicates excluded). For example, a query template where tag A is a list (11,22) and B is (xx,yy,zz), will result in queries:,,,,, .

A parameter rr_type following the query template is a comma-separated list of expected DNS resource record (RR) types. Missing rr_type parameter implies an 'A'. A DNS result may bring resource records of multiple types, but only resource records of a type found in the rr_type parameter list are considered, other resource records found in the answer section of a DNS reply are ignored for this rule. A value ANY in the rr_type parameter list matches any resource record type. An empty DNS answer section does not match ANY.

The rr_type parameter not only provides a filter for RR types found in the DNS answer, but also determines the DNS query type. If only a single RR type is specified in the parameter (e.g. TXT), than this is also the RR type of a query. When more than one RR type is specified (e.g. A, AAAA, TXT) or if ANY is specified, then the DNS query type will be ANY and the rr_type parameter will only act as a filter on a result.


The last optional parameter of a rule is a filtering expression, a.k.a. a subrule. Its function is much like the subrule in URIDNSBL plugin rules, or in the check_rbl eval rules. The main difference is that with askdns rules there is no need to manually group rules according to their queried zone, as the grouping is automatic and duplicate queries are implicitly eliminated.

The subrule filtering parameter can be: a plain string, a regular expression, a single numerical value or a pair of numerical values, or a list of rcodes (DNS status codes of a response). Absence of the filtering parameter implies no filtering, i.e. any positive DNS response (rcode=NOERROR) of the requested RR type will result in a rule hit, regardless of the RR value returned with the response.

When a plain string is used as a filter, it must be enclosed in single or double quotes. For the rule to hit, the response must match the filtering string exactly, and a RR type of a response must match the query type. Typical use is an exact text string for TXT queries, or an exact quad-dotted IPv4 address. In case of a TXT or SPF resource record which can return multiple character-strings (as defined in Section 3.3 of [RFC1035]), these strings are concatenated with no delimiters before comparing the result to the filtering string. This follows requirements of several documents, such as RFC 5518, RFC 7208, RFC 4871, RFC 5617. Examples of a plain text filtering parameter: "", "transaction", 'list' .

A regular expression follows a familiar perl syntax like /.../ or m{...} optionally followed by regexp flags (such as 'i' for case-insensitivity). If a DNS response matches the requested RR type and the regular expression, the rule hits. Examples: /^127\.0\.0\.\d+$/, m{\bdial up\b}i .

A single numerical value can be a decimal number, or a hexadecimal number prefixed by 0x. Such numeric filtering expression is typically used with RR type-A DNS queries. The returned value (an IPv4 address) is masked with a specified filtering value and tested to fall within a network range - the rule hits if the result is nonzero: ((r & n) != 0) && ((r & 0xff000000) == 0x7f000000). An example: 0x10 .

A pair of numerical values (each a decimal, hexadecimal or quad-dotted) delimited by a '-' specifies an IPv4 address range, and a pair of values delimited by a '/' specifies an IPv4 address followed by a bitmask. Again, this type of filtering expression is primarily intended with RR type-A DNS queries. The rule hits if the RR type matches, and the returned IP address falls within the specified range: (r >= n1 && r <= n2), or masked with a bitmask matches the specified value: (r & m) == (n & m) .

As a shorthand notation, a single quad-dotted value is equivalent to a n-n form, i.e. it must match the returned value exactly with all its bits.

Some typical examples of a numeric filtering parameter are:,,,, 0x10/0x10, 16, 0x10 .

Lastly, the filtering parameter can be a comma-separated list of DNS status codes (rcode), enclosed in square brackets. Rcodes can be represented either by their numeric decimal values (0=NOERROR, 3=NXDOMAIN, ...), or their names. See for the list of names. When testing for a rcode where rcode is nonzero, a RR type parameter is ignored as a filter, as there is typically no answer section in a DNS reply when rcode indicates an error. Example: [NXDOMAIN], or [FormErr,ServFail,4,5] .

DNS timeout can be set with "rbl_timeout" option. See the "Mail::SpamAssassin::Conf" POD for details on "rbl_timeout".

2024-06-30 perl v5.38.2