1Mail::SpamAssassin::PluUgsienr::CAosnktDrNiSb(u3t)ed PerMlaiDlo:c:uSmpeanmtAastsiaosnsin::Plugin::AskDNS(3)
2
6 AskDNS - form a DNS query using tag values, and look up the DNSxL lists
7
9 loadplugin Mail::SpamAssassin::Plugin::AskDNS
10 askdns D_IN_DWL _DKIMDOMAIN_._vouch.dwl.spamhaus.org TXT /\b(transaction|list|all)\b/
11
13 Using a DNS query template as specified in a parameter of a askdns
14 rule, the plugin replaces tag names as found in the template with their
15 values and launches DNS queries as soon as tag values become available.
16 When DNS responses trickle in, filters them according to the requested
17 DNS resource record type and optional subrule filtering expression,
18 yielding a rule hit if a response meets filtering conditions.
19
21 rbl_timeout t [t_min] [zone] (default: 15 3)
22 The rbl_timeout setting is common to all DNS querying rules (as
23 implemented by other plugins). It can specify a DNS query timeout
24 globally, or individually for each zone. When the zone parameter is
25 specified, the settings affects DNS queries when their query domain
26 equals the specified zone, or is its subdomain. See the
27 "Mail::SpamAssassin::Conf" POD for details on "rbl_timeout".
28
30 askdns NAME_OF_RULE query_template [rr_type [subqueryfilter]]
31 A query template is a string which will be expanded to produce a
32 domain name to be used in a DNS query. The template may include
33 SpamAssassin tag names, which will be replaced by their values to
34 form a final query domain.
35 The final query domain must adhere to rules governing DNS domains,
37 i.e. must consist of fields each up to 63 characters long,
38 delimited by dots, not exceeding 255 characters. International
39 domain names (in UTF-8) are allowed and will be encoded to ASCII-
40 compatible encoding (ACE) according to IDN rules. Syntactically
41 invalid resulting queries will be discarded by the DNS resolver
42 code (with some info warnings).
43 There may be a trailing dot at the end, but it is redundant /
45 carries no semantics, because SpamAssassin uses a
46 Net::DSN::Resolver::send method for querying DNS, which ignores any
47 'search' or 'domain' DNS resolver options. Domain names in DNS
48 queries are case-insensitive.
49 A tag name is a string of capital letters, preceded and followed by
51 an underscore character. This syntax mirrors the add_header
52 setting, except that tags cannot have parameters in parenthesis
53 when used in askdns templates (exceptions found below). Tag names
54 may appear anywhere in the template - each queried DNS zone
55 prescribes how a query should be formed.
56 Special supported tag HEADER() can be used to query any header
58 content, using same header names/modifiers that as header rules
59 support. For example _HEADER(Reply-To:addr:domain)_ can be used to
60 query the trimmed domain part of Reply-To address. See
61 Mail::SpamAssassin::Conf documentation about header rules.
62 A query template may contain any number of tag names including
64 none, although in the most common anticipated scenario exactly one
65 tag name would appear in each askdns rule. Specified tag names are
66 considered dependencies. Askdns rules with dependencies on the
67 same set of tags are grouped, and all queries in a group are
68 launched as soon as all their dependencies are met, i.e. when the
69 last of the awaited tag values becomes available by a call to
70 set_tag() from some other plugin or elsewhere in the SpamAssassin
71 code.
72 Launched queries from all askdns rules are grouped too according to
74 a pair of: query type and an expanded query domain name. Even if
75 there are multiple rules producing the same type/domain pair, only
76 one DNS query is launched, and a reply to such query contributes to
77 all the constituent rules.
78 A tag may produce none, one or multiple values. Askdns rules
80 awaiting for a tag which never receives its value never result in a
81 DNS query. Tags which produce multiple values will result in
82 multiple queries launched, each with an expanded template using one
83 of the tag values. An example is a DKIMDOMAIN tag which yields a
84 list of signing domains, one for each valid signature in a signed
85 message.
86 When more than one distinct tag name appears in a template, each
88 potentially resulting in multiple values, a Cartesian product is
89 formed, and each tuple results in a launch of one DNS query
90 (duplicates excluded). For example, a query template
91 _A_._B_.example._A_.com where tag A is a list (11,22) and B is
92 (xx,yy,zz), will result in queries: 11.xx.example.11.com,
93 22.xx.example.22.com, 11.yy.example.11.com, 22.yy.example.22.com,
94 11.zz.example.11.com, 22.zz.example.22.com .
95 A parameter rr_type following the query template is a comma-
97 separated list of expected DNS resource record (RR) types. Missing
98 rr_type parameter implies an 'A'. A DNS result may bring resource
99 records of multiple types, but only resource records of a type
100 found in the rr_type parameter list are considered, other resource
101 records found in the answer section of a DNS reply are ignored for
102 this rule. A value ANY in the rr_type parameter list matches any
103 resource record type. An empty DNS answer section does not match
104 ANY.
105 The rr_type parameter not only provides a filter for RR types found
107 in the DNS answer, but also determines the DNS query type. If only
108 a single RR type is specified in the parameter (e.g. TXT), than
109 this is also the RR type of a query. When more than one RR type is
110 specified (e.g. A, AAAA, TXT) or if ANY is specified, then the DNS
111 query type will be ANY and the rr_type parameter will only act as a
112 filter on a result.
113 Currently recognized RR types in the rr_type parameter are: ANY, A,
115 AAAA, MX, TXT, PTR, NAPTR, NS, SOA, CERT, CNAME, DNAME, DHCID,
116 HINFO, MINFO, RP, HIP, IPSECKEY, KX, LOC, GPOS, SRV, OPENPGPKEY,
117 SSHFP, SPF, TLSA, URI, CAA, CSYNC.
118 https://www.iana.org/assignments/dns-parameters/dns-parameters.xml
120 The last optional parameter of a rule is a filtering expression,
122 a.k.a. a subrule. Its function is much like the subrule in URIDNSBL
123 plugin rules, or in the check_rbl eval rules. The main difference
124 is that with askdns rules there is no need to manually group rules
125 according to their queried zone, as the grouping is automatic and
126 duplicate queries are implicitly eliminated.
127 The subrule filtering parameter can be: a plain string, a regular
129 expression, a single numerical value or a pair of numerical values,
130 or a list of rcodes (DNS status codes of a response). Absence of
131 the filtering parameter implies no filtering, i.e. any positive DNS
132 response (rcode=NOERROR) of the requested RR type will result in a
133 rule hit, regardless of the RR value returned with the response.
134 When a plain string is used as a filter, it must be enclosed in
136 single or double quotes. For the rule to hit, the response must
137 match the filtering string exactly, and a RR type of a response
138 must match the query type. Typical use is an exact text string for
139 TXT queries, or an exact quad-dotted IPv4 address. In case of a TXT
140 or SPF resource record which can return multiple character-strings
141 (as defined in Section 3.3 of [RFC1035]), these strings are
142 concatenated with no delimiters before comparing the result to the
143 filtering string. This follows requirements of several documents,
144 such as RFC 5518, RFC 7208, RFC 4871, RFC 5617. Examples of a
145 plain text filtering parameter: "127.0.0.1", "transaction", 'list'
146 .
147 A regular expression follows a familiar perl syntax like /.../ or
149 m{...} optionally followed by regexp flags (such as 'i' for case-
150 insensitivity). If a DNS response matches the requested RR type
151 and the regular expression, the rule hits. Examples:
152 /^127\.0\.0\.\d+$/, m{\bdial up\b}i .
153 A single numerical value can be a decimal number, or a hexadecimal
155 number prefixed by 0x. Such numeric filtering expression is
156 typically used with RR type-A DNS queries. The returned value (an
157 IPv4 address) is masked with a specified filtering value and tested
158 to fall within a 127.0.0.0/8 network range - the rule hits if the
159 result is nonzero: ((r & n) != 0) && ((r & 0xff000000) ==
160 0x7f000000). An example: 0x10 .
161 A pair of numerical values (each a decimal, hexadecimal or quad-
163 dotted) delimited by a '-' specifies an IPv4 address range, and a
164 pair of values delimited by a '/' specifies an IPv4 address
165 followed by a bitmask. Again, this type of filtering expression is
166 primarily intended with RR type-A DNS queries. The rule hits if the
167 RR type matches, and the returned IP address falls within the
168 specified range: (r >= n1 && r <= n2), or masked with a bitmask
169 matches the specified value: (r & m) == (n & m) .
170 As a shorthand notation, a single quad-dotted value is equivalent
172 to a n-n form, i.e. it must match the returned value exactly with
173 all its bits.
174 Some typical examples of a numeric filtering parameter are:
176 127.0.1.2, 127.0.1.20-127.0.1.39, 127.0.1.0/255.255.255.0,
177 0.0.0.16/0.0.0.16, 0x10/0x10, 16, 0x10 .
178 Lastly, the filtering parameter can be a comma-separated list of
180 DNS status codes (rcode), enclosed in square brackets. Rcodes can
181 be represented either by their numeric decimal values (0=NOERROR,
182 3=NXDOMAIN, ...), or their names. See
183 https://www.iana.org/assignments/dns-parameters for the list of
184 names. When testing for a rcode where rcode is nonzero, a RR type
185 parameter is ignored as a filter, as there is typically no answer
186 section in a DNS reply when rcode indicates an error. Example:
187 [NXDOMAIN], or [FormErr,ServFail,4,5] .
188perl v5.36.0 2023-01-M2a1il::SpamAssassin::Plugin::AskDNS(3)