rewrite
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
A simplified/easy-to-digest syntax for rewrite is...
rewrite [continue|stop] FIELD [TYPE] [(FROM TO)|TTL] [OPTIONS]
-
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
.
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.
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
.
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.
cname
- the CNAME target if the response has a CNAME record
-
TYPE this optional element can be specified for a name
or ttl
field.
If not given type exact
will be assumed. If options should be specified the
type must be given.
-
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 (only for field ttl
)
-
OPTIONS
for field name
further options are possible controlling the response rewrites.
All name matching types support the following options
answer auto
- the names in the response is rewritten in a best effort manner.
answer name FROM TO
- the query name in the response is rewritten matching the from regex pattern.
answer value FROM TO
- the names in the response is rewritten matching the from regex pattern.
See below in the Response Rewrites section for further details.
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 [OPTIONS]
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. If OPTIONS are
given, the type must be specified.
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 (field name
), 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.
There are three possibilities to specify an answer rewrite:
- A rewrite can request a best effort answer rewrite by adding the option
answer auto
.
- A rewrite may specify a dedicated regex based response name rewrite with the
answer name FROM TO
option.
- A regex based rewrite of record values like
CNAME
, SRV
, etc, can be requested by
an answer value FROM TO
option.
Hereby FROM/TO follow the rules for the regex
name rewrite syntax.
Auto Response Name Rewrite
The following configuration snippet allows for rewriting of the
ANSWER SECTION
according to the rewrite of the QUESTION SECTION
:
rewrite stop {
name suffix .coredns.rocks .service.consul answer auto
}
Any occurrence of the rewritten question in the answer is mapped
back to the original value before the rewrite.
Please note that answers for rewrites of type exact
are always rewritten.
For a suffix
name rule auto
leads to a reverse suffix response rewrite,
exchanging FROM and TO from the rewrite request.
Explicit Response Name Rewrite
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
Rewriting other Response Values
It is also possible to rewrite other values returned in the DNS response records
(e.g. the server names returned in SRV
and MX
records). This can be enabled by adding
the answer value FROM TO
option to a name rule as specified below. answer value
takes a
regular expression and a rewrite name as parameters and works in the same way as the
answer name
rule.
Note that names in the AUTHORITY SECTION
and ADDITIONAL SECTION
will also be
rewritten following the specified rules. The names returned by the following
record types: CNAME
, DNAME
, SOA
, SRV
, MX
, NAPTR
, NS
, PTR
will be rewritten
if the answer value
rule is specified.
The syntax for the rewrite of DNS request and response is as follows:
rewrite [continue|stop] {
name regex STRING STRING
answer name STRING STRING
[answer value STRING STRING]
}
Note that the above syntax is strict. For response rewrites, only name
rules are allowed to match the question section. The answer rewrite must be
after the name, as in the syntax example.
Example: PTR Response Value Rewrite
The original response contains the domain service.consul.
in the VALUE
part
of the ANSWER SECTION
$ dig @10.1.1.1 30.30.30.10.in-addr.arpa PTR
;; QUESTION SECTION:
;30.30.30.10.in-addr.arpa. IN PTR
;; ANSWER SECTION:
30.30.30.10.in-addr.arpa. 60 IN PTR ftp-us-west-1.service.consul.
The following configuration snippet allows for rewriting of the value
in the ANSWER SECTION
:
rewrite stop {
name suffix .arpa .arpa
answer name auto
answer value (.*)\.service\.consul\. {1}.coredns.rocks.
}
Now, the VALUE
in the ANSWER SECTION
has been overwritten in the domain part:
$ dig @10.1.1.1 30.30.30.10.in-addr.arpa PTR
;; QUESTION SECTION:
;30.30.30.10.in-addr.arpa. IN PTR
;; ANSWER SECTION:
30.30.30.10.in-addr.arpa. 60 IN PTR ftp-us-west-1.coredns.rocks.
Multiple Response Rewrites
name
and value
rewrites can be chained by appending multiple answer rewrite
options. For all occurrences but the first one the keyword answer
might be
omitted.
answer (auto | (name|value FROM TO)) { [answer] (auto | (name|value FROM TO)) }
For example:
rewrite [continue|stop] name regex FROM TO answer name FROM TO [answer] value FROM TO
When using exact
name rewrite rules, the answer gets rewritten automatically,
and there is no need to define answer name auto
. But it is still possible to define
additional answer value
and answer value
options.
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.
An omitted type is defaulted to exact
.
rewrite [continue|stop] ttl [exact|prefix|suffix|substring|regex] STRING [SECONDS|MIN-MAX]
It is possible to supply a range of TTL values in the SECONDS
parameters instead of a single value.
If a range is supplied, the TTL value is set to MIN
if it is below, or set to MAX
if it is above.
The TTL value is left unchanged if it is already inside the provided range.
The ranges can be unbounded on either side.
TTL examples with ranges:
# rewrite TTL to be between 30s and 300s
rewrite ttl example.com. 30-300
# cap TTL at 30s
rewrite ttl example.com. -30 # equivalent to rewrite ttl example.com. 0-30
# increase TTL to a minimum of 30s
rewrite ttl example.com. 30-
# set TTL to 30s
rewrite ttl example.com. 30 # equivalent to rewrite ttl example.com. 30-30
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.
CNAME Field Rewrites
There might be a scenario where you want the CNAME
target of the response to be rewritten. You can do this by using the CNAME
field rewrite. This will generate new answer records according to the new CNAME
target.
The syntax for the CNAME rewrite rule is as follows. The meaning of
exact|prefix|suffix|substring|regex
is the same as with the name rewrite rules.
An omitted type is defaulted to exact
.
rewrite [continue|stop] cname [exact|prefix|suffix|substring|regex] FROM TO
Consider the following CNAME
rewrite rule with regex type.
rewrite cname regex (.*).cdn.example.net. {1}.other.cdn.com.
If you were to send the following DNS request without the above rule, an example response would be:
$ dig @10.1.1.1 my-app.com
;; QUESTION SECTION:
;my-app.com. IN A
;; ANSWER SECTION:
my-app.com. 200 IN CNAME my-app.com.cdn.example.net.
my-app.com.cdn.example.net. 300 IN A 20.2.0.1
my-app.com.cdn.example.net. 300 IN A 20.2.0.2
If you were to send the same DNS request with the above rule set up, an example response would be:
$ dig @10.1.1.1 my-app.com
;; QUESTION SECTION:
;my-app.com. IN A
;; ANSWER SECTION:
my-app.com. 200 IN CNAME my-app.com.other.cdn.com.
my-app.com.other.cdn.com. 100 IN A 30.3.1.2
Note that the answer will contain a completely different set of answer records after rewriting the CNAME
target.