COREDNS-REWRITE(7)

NAME
rewrite - performs internal message rewriting.

DESCRIPTION
Rewrites are invisible to the client. There are simple rewrites (fast) and complex rewrites (slower), but they're powerful enough to accommodate most dynamic back-end applications.

SYNTAX
rewrite [continue|stop] FIELD FROM TO

•   FIELD is (type, class, name, ...)

•   FROM is the exact name of type to match

•   TO is the destination name or type to rewrite to

When the FIELD is type and FROM is (A, MX, etc.), the type of the message will be rewritten; e.g., to rewrite ANY queries to HINFO, use rewrite type ANY HINFO.

When the FIELD is class and FROM is (IN, CH, or HS) the class of the message will be rewritten; e.g., to rewrite CH queries to IN use rewrite class CH IN.

When the FIELD is name the query name in the message is rewritten; this needs to be a full match of the name, e.g., rewrite name miek.nl example.org.

When the FIELD is edns0 an EDNS0 option can be appended to the request as described below.

If you specify multiple rules and an incoming query matches on multiple rules, the rewrite will behave as following * continue will continue apply the next rule in the rule list. * stop will consider the current rule is the last rule and will not continue. Default behaviour for not specifying this rule processing mode is stop

EDNS0 OPTIONS
Using FIELD edns0, you can set, append, or replace specific EDNS0 options on the request.

•   replace will modify any matching (what that means may vary based on EDNS0 type) option with the specified option

•   append will add the option regardless of what options already exist

•   set will modify a matching option or add one if none is found

Currently supported are EDNS0_LOCAL, EDNS0_NSID and EDNS0_SUBNET.

  EDNS0_LOCAL
This has two fields, code and data. A match is defined as having the same code. Data may be a string or a variable.

A string data can be treated as hex if it starts with 0x. Example:

    . {
        rewrite edns0 local set 0xffee 0x61626364
        whoami
    }

rewrites the first local option with code 0xffee, setting the data to "abcd". Equivalent:

    . {
        rewrite edns0 local set 0xffee abcd
    }

A variable data is specified with a pair of curly brackets {}. Following are the supported variables
    {qname}, {qtype}, {client_ip}, {client_port}, {protocol}, {server_ip}, {server_port}.

Example:

    rewrite edns0 local set 0xffee {client_ip}

  EDNS0_NSID
This has no fields; it will add an NSID option with an empty string for the NSID. If the option already exists and the action is replace or set, then the NSID in the option will be set to the empty string.

  EDNS0_SUBNET
This has two fields, IPv4 bitmask length and IPv6 bitmask length. The bitmask length is used to extract the client subnet from the source IP address in the query.

Example:

    rewrite edns0 subnet set 24 56

•   If the query has source IP as IPv4, the first 24 bits in the IP will be the network subnet.

•   If the query has source IP as IPv6, the first 56 bits in the IP will be the network subnet.

  NAME FIELD REWRITES
The rewrite plugin offers the ability to match on the name in the question section of a DNS request. The match could be exact, substring, or based on a prefix, suffix, or regular expression.

The syntax for the name re-writing is as follows:

    rewrite [continue|stop] name [exact|prefix|suffix|substring|regex] STRING STRING

The match type, i.e. exact, substring, etc., triggers re-write:

•   exact (default): on exact match of the name in the question section of a request

•   substring: on a partial match of the name in the question section of a request

•   prefix: when the name begins with the matching string

•   suffix: when the name ends with the matching string

•   regex: when the name in the question section of a request matches a regular expression

If the match type is omitted, the exact match type is being assumed.

The following instruction allows re-writing the name in the query that contains service.us-west-1.example.org substring.

    rewrite name substring service.us-west-1.example.org service.us-west-1.consul

Thus:

•   Incoming Request Name: ftp.service.us-west-1.example.org

•   Re-written Request Name: ftp.service.us-west-1.consul

The following instruction uses regular expressions. The name in a request matching (.*)-(us-west-1)\.example\.org regular expression is being replaces with {1}.service.{2}.consul, where {1} and {2} are regular expression match groups.

    rewrite name regex (.*)-(us-west-1)\.example\.org {1}.service.{2}.consul

Thus:

•   Incoming Request Name: ftp-us-west-1.example.org

•   Re-written Request Name: ftp.service.us-west-1.consul