coredns/man/coredns-rewrite.7

.\" Generated by Mmark Markdown Processer - mmark.miek.nl
.TH "COREDNS-REWRITE" 7 "March 2021" "CoreDNS" "CoreDNS Plugins"

.SH "NAME"
.PP
\fIrewrite\fP - performs internal message rewriting.

.SH "DESCRIPTION"
.PP
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.

.SH "SYNTAX"
.PP
A simplified/easy-to-digest syntax for \fIrewrite\fP is...

.PP
.RS

.nf
rewrite [continue|stop] FIELD [FROM TO|FROM TTL]

.fi
.RE

.IP \(bu 4
\fBFIELD\fP indicates what part of the request/response is being re-written.

.RS
.IP \(en 4
\fB\fCtype\fR - the type field of the request will be rewritten. FROM/TO must be a DNS record type (\fB\fCA\fR, \fB\fCMX\fR, etc.);
e.g., to rewrite ANY queries to HINFO, use \fB\fCrewrite type ANY HINFO\fR.
.IP \(en 4
\fB\fCclass\fR - the class of the message will be rewritten. FROM/TO must be a DNS class type (\fB\fCIN\fR, \fB\fCCH\fR, or \fB\fCHS\fR); e.g., to rewrite CH queries to IN use \fB\fCrewrite class CH IN\fR.
.IP \(en 4
\fB\fCname\fR - the query name in the \fIrequest\fP is rewritten; by default this is a full match of the
name, e.g., \fB\fCrewrite name example.net example.org\fR. Other match types are supported, see the \fBName Field Rewrites\fP section below.
.IP \(en 4
\fB\fCanswer name\fR - the query name in the \fIresponse\fP is rewritten.  This option has special restrictions and requirements, in particular it must always combined with a \fB\fCname\fR rewrite.  See below in the \fBResponse Rewrites\fP section.
.IP \(en 4
\fB\fCedns0\fR - an EDNS0 option can be appended to the request as described below in the \fBEDNS0 Options\fP section.
.IP \(en 4
\fB\fCttl\fR - the TTL value in the \fIresponse\fP is rewritten.

.RE
.IP \(bu 4
\fBFROM\fP is the name (exact, suffix, prefix, substring, or regex) or type to match
.IP \(bu 4
\fBTO\fP is the destination name or type to rewrite to
.IP \(bu 4
\fBTTL\fP is the number of seconds to set the TTL value to


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

.IP \(bu 4
\fB\fCcontinue\fR will continue applying the next rule in the rule list.
.IP \(bu 4
\fB\fCstop\fR will consider the current rule the last rule and will not continue.  The default behaviour is \fB\fCstop\fR


.SH "EXAMPLES"
.SS "NAME FIELD REWRITES"
.PP
The \fB\fCrewrite\fR 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.

.PP
The syntax for name rewriting is as follows:

.PP
.RS

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

.fi
.RE

.PP
The match type, e.g., \fB\fCexact\fR, \fB\fCsubstring\fR, etc., triggers rewrite:

.IP \(bu 4
\fBexact\fP (default): on an exact match of the name in the question section of a request
.IP \(bu 4
\fBsubstring\fP: on a partial match of the name in the question section of a request
.IP \(bu 4
\fBprefix\fP: when the name begins with the matching string
.IP \(bu 4
\fBsuffix\fP: when the name ends with the matching string
.IP \(bu 4
\fBregex\fP: when the name in the question section of a request matches a regular expression


.PP
If the match type is omitted, the \fB\fCexact\fR match type is assumed.

.PP
The following instruction allows rewriting names in the query that
contain the substring \fB\fCservice.us-west-1.example.org\fR:

.PP
.RS

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

.fi
.RE

.PP
Thus:

.IP \(bu 4
Incoming Request Name: \fB\fCftp.service.us-west-1.example.org\fR
.IP \(bu 4
Rewritten Request Name: \fB\fCftp.service.us-west-1.consul\fR


.PP
The following instruction uses regular expressions. Names in requests
matching the regular expression \fB\fC(.*)-(us-west-1)\.example\.org\fR are replaced with
\fB\fC{1}.service.{2}.consul\fR, where \fB\fC{1}\fR and \fB\fC{2}\fR are regular expression match groups.

.PP
.RS

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

.fi
.RE

.PP
Thus:

.IP \(bu 4
Incoming Request Name: \fB\fCftp-us-west-1.example.org\fR
.IP \(bu 4
Rewritten Request Name: \fB\fCftp.service.us-west-1.consul\fR


.PP
The following example rewrites the \fB\fCschmoogle.com\fR suffix to \fB\fCgoogle.com\fR.

.PP
.RS

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

.fi
.RE

.SS "RESPONSE REWRITES"
.PP
When rewriting incoming DNS requests' names, CoreDNS re-writes the \fB\fCQUESTION SECTION\fR
section of the requests. It may be necessary to rewrite the \fB\fCANSWER SECTION\fR of the
requests, because some DNS resolvers treat mismatches between the \fB\fCQUESTION SECTION\fR
and \fB\fCANSWER SECTION\fR as a man-in-the-middle attack (MITM).

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

.PP
.RS

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

.fi
.RE

.PP
CoreDNS rewrote the request from \fB\fCftp-us-west-1.coredns.rocks\fR to
\fB\fCftp.service.us-west-1.consul\fR and ultimately resolved it to 3 records.
The resolved records, in the \fB\fCANSWER SECTION\fR below, were not from \fB\fCcoredns.rocks\fR, but
rather from \fB\fCservice.us-west-1.consul\fR.

.PP
.RS

.nf
$ 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

.fi
.RE

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

.PP
The following configuration snippet allows for rewriting of the
\fB\fCANSWER SECTION\fR, provided that the \fB\fCQUESTION SECTION\fR was rewritten:

.PP
.RS

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

.fi
.RE

.PP
Now, the \fB\fCANSWER SECTION\fR matches the \fB\fCQUESTION SECTION\fR:

.PP
.RS

.nf
$ 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

.fi
.RE

.PP
It is also possible to rewrite other values returned in the DNS response records
(e.g. the server names returned in \fB\fCSRV\fR and \fB\fCMX\fR records). This can be enabled by adding
the \fB\fCanswer value\fR to a name regex rule as specified below. \fB\fCanswer value\fR takes a
regular expression and a rewrite name as parameters and works in the same way as the
\fB\fCanswer name\fR rule.

.PP
Note that names in the \fB\fCAUTHORITY SECTION\fR and \fB\fCADDITIONAL SECTION\fR will also be
rewritten following the specified rules. The names returned by the following
record types: \fB\fCCNAME\fR, \fB\fCDNAME\fR, \fB\fCSOA\fR, \fB\fCSRV\fR, \fB\fCMX\fR, \fB\fCNAPTR\fR, \fB\fCNS\fR will be rewritten
if the \fB\fCanswer value\fR rule is specified.

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

.PP
.RS

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

.fi
.RE

.PP
Note that the above syntax is strict.  For response rewrites, only \fB\fCname\fR
rules are allowed to match the question section, and only by match type
\fB\fCregex\fR. The answer rewrite must be after the name, as in the
syntax example.

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

.PP
.RS

.nf
rewrite [continue|stop] name regex STRING STRING answer name STRING STRING [answer value STRING STRING]

.fi
.RE

.PP
When using \fB\fCexact\fR name rewrite rules, the answer gets rewritten automatically,
and there is no need to define \fB\fCanswer name\fR. The rule below
rewrites the name in a request from \fB\fCRED\fR to \fB\fCBLUE\fR, and subsequently
rewrites the name in a corresponding response from \fB\fCBLUE\fR to \fB\fCRED\fR. The
client in the request would see only \fB\fCRED\fR and no \fB\fCBLUE\fR.

.PP
.RS

.nf
rewrite [continue|stop] name exact RED BLUE

.fi
.RE

.SS "TTL FIELD REWRITES"
.PP
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 (\fB\fC0\fR). An administrator
may want to increase the TTL to ensure it is cached, e.g., by increasing it to 15 seconds.

.PP
In the below example, the TTL in the answers for \fB\fCcoredns.rocks\fR domain are
being set to \fB\fC15\fR:

.PP
.RS

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

.fi
.RE

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

.PP
The syntax for the TTL rewrite rule is as follows. The meaning of
\fB\fCexact|prefix|suffix|substring|regex\fR is the same as with the name rewrite rules.

.PP
.RS

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

.fi
.RE

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

.IP \(bu 4
\fB\fCreplace\fR will modify any "matching" option with the specified option. The criteria for "matching" varies based on EDNS0 type.
.IP \(bu 4
\fB\fCappend\fR will add the option only if no matching option exists
.IP \(bu 4
\fB\fCset\fR will modify a matching option or add one if none is found


.PP
Currently supported are \fB\fCEDNS0_LOCAL\fR, \fB\fCEDNS0_NSID\fR and \fB\fCEDNS0_SUBNET\fR.

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

.IP \(bu 4
A string data is treated as hex if it starts with \fB\fC0x\fR. Example:


.PP
.RS

.nf
\&. {
    rewrite edns0 local set 0xffee 0x61626364
    whoami
}

.fi
.RE

.PP
rewrites the first local option with code 0xffee, setting the data to "abcd". This is equivalent to:

.PP
.RS

.nf
\&. {
    rewrite edns0 local set 0xffee abcd
}

.fi
.RE

.IP \(bu 4
A variable data is specified with a pair of curly brackets \fB\fC{}\fR. Following are the supported variables:
{qname}, {qtype}, {client\fIip}, {client\fPport}, {protocol}, {server\fIip}, {server\fPport}.
.IP \(bu 4
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.


.PP
Examples:

.PP
.RS

.nf
rewrite edns0 local set 0xffee {client\_ip}

.fi
.RE

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

.PP
.RS

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

.fi
.RE

.SS "EDNS0_NSID"
.PP
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 \fB\fCreplace\fR or \fB\fCset\fR, then the NSID in the option will be set to the empty string.

.SS "EDNS0_SUBNET"
.PP
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.

.PP
Example:

.PP
.RS

.nf
rewrite edns0 subnet set 24 56

.fi
.RE

.IP \(bu 4
If the query's source IP address is an IPv4 address, the first 24 bits in the IP will be the network subnet.
.IP \(bu 4
If the query's source IP address is an IPv6 address, the first 56 bits in the IP will be the network subnet.


.SH "FULL SYNTAX"
.PP
The full plugin usage syntax is harder to digest...

.PP
.RS

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

.fi
.RE

.PP
The syntax above doesn't cover the multi-line block option for specifying a name request+response rewrite rule described in the \fBResponse Rewrite\fP section.