Mail::SpamAssassin::PlUser:Contributed PeMail::SpamAssassin::Plugin::AskDNS(3)
NAME
AskDNS - form a DNS query using tag values, and look up the DNSxL lists
SYNOPSIS
loadplugin Mail::SpamAssassin::Plugin::AskDNS
askdns D_IN_DWL _DKIMDOMAIN_._vouch.dwl.spamhaus.org TXT /\b(transaction|list|all)\b/
DESCRIPTION
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.
USER SETTINGS
rbl_timeout t [t_min] [zone] (default: 15 3)
The rbl_timeout setting is common to all DNS querying rules (as
implemented by other plugins). It can specify a DNS query timeout
globally, or individually for each zone. When the zone parameter is
specified, the settings affects DNS queries when their query domain
equals the specified zone, or is its subdomain. See the
"Mail::SpamAssassin::Conf" POD for details on "rbl_timeout".
RULE DEFINITIONS
askdns NAME_OF_RULE query_template [rr_type [subqueryfilter]]
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. 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. Tag names may appear anywhere in
the template - each queried DNS zone prescribes how a query should
be formed.
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
_A_._B_.example._A_.com where tag A is a list (11,22) and B is
(xx,yy,zz), will result in queries: 11.xx.example.11.com,
22.xx.example.22.com, 11.yy.example.11.com, 22.yy.example.22.com,
11.zz.example.11.com, 22.zz.example.22.com .
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.
Currently recognized RR types in the rr_type parameter are: ANY, A,
AAAA, MX, TXT, PTR, NAPTR, NS, SOA, CERT, CNAME, DNAME, DHCID,
HINFO, MINFO, RP, HIP, IPSECKEY, KX, LOC, SRV, SSHFP, SPF.
https://www.iana.org/assignments/dns-parameters/dns-parameters.xml
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: "127.0.0.1", "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 127.0.0.0/8 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:
127.0.1.2, 127.0.1.20-127.0.1.39, 127.0.1.0/255.255.255.0,
0.0.0.16/0.0.0.16, 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
https://www.iana.org/assignments/dns-parameters 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] .
perl v5.26.3 2021-04Mail::SpamAssassin::Plugin::AskDNS(3)
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