rewrite

Source

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

A simplified/easy-to-digest syntax for rewrite is…

rewrite [continue|stop] FIELD [FROM TO|FROM TTL]
  • FIELD indicates what part of the request/response is being re-written.

    • type - the type field of the request will be rewritten. FROM/TO must be a DNS record type (A, MX, etc.); e.g., to rewrite ANY queries to HINFO, use rewrite type ANY HINFO.
    • class - the class of the message will be rewritten. FROM/TO must be a DNS class type (IN, CH, or HS); e.g., to rewrite CH queries to IN use rewrite class CH IN.
    • name - the query name in the request is rewritten; by default this is a full match of the name, e.g., rewrite name example.net example.org. Other match types are supported, see the Name Field Rewrites section below.
    • answer name - the query name in the response is rewritten. This option has special restrictions and requirements, in particular it must always combined with a name rewrite. See below in the Response Rewrites section.
    • edns0 - an EDNS0 option can be appended to the request as described below in the EDNS0 Options section.
    • ttl - the TTL value in the response is rewritten.
  • FROM is the name (exact, suffix, prefix, substring, or regex) or type to match

  • TO is the destination name or type to rewrite to

  • TTL is the number of seconds to set the TTL value to

If you specify multiple rules and an incoming query matches multiple rules, the rewrite will behave as follows:

  • continue will continue applying the next rule in the rule list.
  • stop will consider the current rule the last rule and will not continue. The default behaviour is stop

Examples

Name Field Rewrites

The rewrite plugin offers the ability to match the name in the question section of a DNS request. The match could be exact, a substring match, or based on a prefix, suffix, or regular expression. If the newly used name is not a legal domain name, the plugin returns an error to the client.

The syntax for name rewriting is as follows:

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

The match type, e.g., exact, substring, etc., triggers rewrite:

  • exact (default): on an 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 assumed.

The following instruction allows rewriting names in the query that contain the substring service.us-west-1.example.org:

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

The following instruction uses regular expressions. Names in requests matching the regular expression (.*)-(us-west-1)\.example\.org are replaced 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
  • Rewritten Request Name: ftp.service.us-west-1.consul

The following example rewrites the schmoogle.com suffix to google.com.

rewrite name suffix .schmoogle.com. .google.com.

Response Rewrites

When rewriting incoming DNS requests’ names, CoreDNS re-writes the QUESTION SECTION section of the requests. It may be necessary to rewrite the ANSWER SECTION of the requests, because some DNS resolvers treat mismatches between the QUESTION SECTION and ANSWER SECTION as a man-in-the-middle attack (MITM).

For example, a user tries to resolve ftp-us-west-1.coredns.rocks. The CoreDNS configuration file has the following rule:

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

CoreDNS rewrote the request from ftp-us-west-1.coredns.rocks to ftp.service.us-west-1.consul and ultimately resolved it to 3 records. The resolved records, in the ANSWER SECTION below, were not from coredns.rocks, but rather from service.us-west-1.consul.

$ dig @10.1.1.1 ftp-us-west-1.coredns.rocks

;; QUESTION SECTION:
;ftp-us-west-1.coredns.rocks. IN A

;; ANSWER SECTION:
ftp.service.us-west-1.consul. 0    IN A    10.10.10.10
ftp.service.us-west-1.consul. 0    IN A    10.20.20.20
ftp.service.us-west-1.consul. 0    IN A    10.30.30.30

The above is a mismatch between the question asked and the answer provided.

The following configuration snippet allows for rewriting of the ANSWER SECTION, provided that the QUESTION SECTION was rewritten:

    rewrite stop {
        name regex (.*)-(us-west-1)\.coredns\.rocks {1}.service.{2}.consul
        answer name (.*)\.service\.(us-west-1)\.consul {1}-{2}.coredns.rocks
    }

Now, the ANSWER SECTION matches the QUESTION SECTION:

$ dig @10.1.1.1 ftp-us-west-1.coredns.rocks

;; QUESTION SECTION:
;ftp-us-west-1.coredns.rocks. IN A

;; ANSWER SECTION:
ftp-us-west-1.coredns.rocks. 0    IN A    10.10.10.10
ftp-us-west-1.coredns.rocks. 0    IN A    10.20.20.20
ftp-us-west-1.coredns.rocks. 0    IN A    10.30.30.30

The syntax for the rewrite of DNS request and response is as follows:

rewrite [continue|stop] {
    name regex STRING STRING
    answer name STRING STRING
}

Note that the above syntax is strict. For response rewrites, only name rules are allowed to match the question section, and only by match type regex. The answer rewrite must be after the name, as in the syntax example. There must only be two lines (a name followed by an answer) in the brackets; additional rules are not supported.

An alternate syntax for rewriting a DNS request and response is as follows:

rewrite [continue|stop] name regex STRING STRING answer name STRING STRING

When using exact name rewrite rules, the answer gets rewritten automatically, and there is no need to define answer name. The rule below rewrites the name in a request from RED to BLUE, and subsequently rewrites the name in a corresponding response from BLUE to RED. The client in the request would see only RED and no BLUE.

rewrite [continue|stop] name exact RED BLUE

TTL Field Rewrites

At times, the need to rewrite a TTL value could arise. For example, a DNS server may not cache records with a TTL of zero (0). An administrator may want to increase the TTL to ensure it is cached, e.g., by increasing it to 15 seconds.

In the below example, the TTL in the answers for coredns.rocks domain are being set to 15:

    rewrite continue {
        ttl regex (.*)\.coredns\.rocks 15
    }

By the same token, an administrator may use this feature to prevent or limit caching by setting the TTL value really low.

The syntax for the TTL rewrite rule is as follows. The meaning of exact|prefix|suffix|substring|regex is the same as with the name rewrite rules.

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

EDNS0 Options

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

  • replace will modify any “matching” option with the specified option. The criteria for “matching” varies based on EDNS0 type.
  • append will add the option only if no matching option exists
  • 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 is 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”. This is equivalent to:

. {
    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}.

  • If the metadata plugin is enabled, then labels are supported as variables if they are presented within curly brackets. The variable data will be replaced with the value associated with that label. If that label is not provided, the variable will be silently substituted with an empty string.

Examples:

rewrite edns0 local set 0xffee {client_ip}

The following example uses metadata and an imaginary “some-plugin” that would provide “some-label” as metadata information.

metadata
some-plugin
rewrite edns0 local set 0xffee {some-plugin/some-label}

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’s source IP address is an IPv4 address, the first 24 bits in the IP will be the network subnet.
  • If the query’s source IP address is an IPv6 address, the first 56 bits in the IP will be the network subnet.

Full Syntax

The full plugin usage syntax is harder to digest…

rewrite [continue|stop] {type|class|edns0|name [exact|prefix|suffix|substring|regex [FROM TO answer name]]} FROM TO

The syntax above doesn’t cover the multi-line block option for specifying a name request+response rewrite rule described in the Response Rewrite section.