Pipe Backend

The PipeBackend allows for easy dynamic resolution based on a 'Coprocess' which can be written in any programming language that can read a question on standard input and answer on standard output.

The PipeBackend is primarily meant for allowing rapid development of new backends without tight integration with PowerDNS. It allows end-users to write PowerDNS backends in any language, a perl sample is provided. The PipeBackend is also very well suited for dynamic resolution of queries. Example applications include DNS based load balancing, geo-direction, DNS-based failover with low TTLs.

Note: The Remote Backend offers a superset of the functionality of the PipeBackend.

Note: Please do read the Backend Writer' guide carefully. The PipeBackend, like all other backends, must not do any DNS thinking, but answer all questions (INCLUDING THE ANY QUESTION) faithfully. Specifically, the queries that the PipeBackend receives will not correspond to the queries that arrived over DNS. So, a query for an AAAA record may turn into a backend query for an ANY record. There is nothing that can or should be done about this.

Configuration Parameters

pipe-abi-version

This is the version of the question format that is sent to the co-process (pipe-command) for the pipe backend.

If not set the default pipe-abi-version is 1. When set to 2, the local-ip-address field is added after the remote-ip-address, the local-ip-address refers to the IP address the question was received on. When set to 3, the real remote IP/subnet is added based on edns-subnet support (this also requires enabling edns-subnet-processing). When set to 4 it sends zone name in AXFR request. See also PipeBackend Protocol below.

pipe-command

Command to launch as backend or the path to a unix domain socket file. The socket should already be open and listening before PowerDNS starts.

pipe-timeout

Number of milliseconds to wait for an answer from the backend. If this time is ever exceeded, the backend is declared dead and a new process is spawned.

pipe-regex

If set, only questions matching this regular expression are even sent to the backend. This makes sure that most of PowerDNS does not slow down if you deploy a slow backend. A query for 'www.powerdns.com' would be presented to the regex as 'www.powerdns.com', a matching regex would be ^www\.powerdns\.com$. Note: to match the root domain, use a dot, e.g. ^\.$

PipeBackend protocol

Questions come in over a file descriptor, by default standard input. Answers are sent out over another file descriptor, standard output by default. Questions and answers are terminated by single newline (\n) characters. Fields in lines must be separated by tab ('\t') characters.

Handshake

PowerDNS sends out HELO\t1, indicating that it wants to speak the protocol as defined in this document, version 1. For abi-version 2 or 3, PowerDNS sends HELO\t2 or HELO\t3. A PowerDNS Coprocess must then send out a banner, prefixed by OK\t, indicating it launched successfully. If it does not support the indicated version, it should respond with FAIL, but not exit. Suggested behaviour is to try and read a further line, and wait to be terminated.

Note: fields are separated by a tab ('\t') character, even though they are displayed with spaces in this document.

Q: Regular queries for data

The question format, for type Q questions.

pipe-abi-version = 1 [default]

Q qname       qclass  qtype   id  remote-ip-address

pipe-abi-version = 2

Q   qname       qclass  qtype   id  remote-ip-address   local-ip-address

pipe-abi-version = 3

Q   qname       qclass  qtype   id  remote-ip-address   local-ip-address    edns-subnet-address

Fields are tab separated, and terminated with a single \n. The remote-ip-address is the IP address of the nameserver asking the question, the local-ip-address is the IP address on which the question was received.

Type is the tag above, qname is the domain the question is about. qclass is always 'IN' currently, denoting an INternet question. qtype is the kind of information desired, the record type, like A, CNAME or AAAA. id can be specified to help your backend find an answer if the id is already known from an earlier query. You can ignore it unless you want to support AXFR.

edns-subnet-address is the actual client subnet as provided via edns-subnet support. Note that for the SOA query that precedes an AXFR, edns-subnet is always set to 0.0.0.0/0.

Note: Queries for wildcard names should be answered literally, without expansion. So, if a backend gets a question for "*.powerdns.com", it should only answer with data if there is an actual "*.powerdns.com" name.

Note: In some (broken) network setups, the remote-ip-address and/or local-ip-address, when it is an IPv6 address, may be suffixed with a % and the name of the network interface (e.g. %eth1). Keep this in mind when checking the IP addresses.

AXFR: List an entire zone

AXFR-queries look like this:

AXFR    id  zone-name

The id is gathered from the answer to a SOA query. zone-name is given in ABI version 4.

Answers

Each answer starts with a tag, possibly followed by a TAB and more data.

• DATA: Indicating a successful line of DATA.
• END: Indicating the end of an answer - no further data.
• FAIL: Indicating a lookup failure. Also serves as 'END'. No further data.
• LOG: For specifying things that should be logged. Can only be sent after a query and before an END line. After the tab, the message to be logged.

ABI version 1 and 2

So, letting it be known that there is no data consists of sending 'END' without anything else. The answer format (for abi-version 1 and 2):

DATA    qname       qclass  qtype   ttl id  content

Again, all fields are tab-separated.

content is as specified in Types. For MX and SRV, content consists of the priority, followed by a tab, followed by the actual content.

A sample dialogue may look like this (note that in reality, almost all queries will actually be for the ANY qtype):

Q   www.example.org IN  CNAME   -1  203.0.113.210
DATA    www.example.org IN  CNAME   3600    1 ws1.example.org
END
Q   ws1.example.org IN  CNAME   -1  203.0.113.210
END
Q   wd1.example.org IN  A   -1  203.0.113.210
DATA    ws1.example.org IN  A   3600    1   192.0.2.4
DATA    ws1.example.org IN  A   3600    1   192.0.2.5
DATA    ws1.example.org IN  A   3600    1   192.0.2.6
END

This would correspond to a remote webserver 203.0.113.210 wanting to resolve the IP address of www.example.org, and PowerDNS traversing the CNAMEs to find the IP addresses of ws1.example.org. Another dialogue might be:

Q   example.org     IN  SOA -1  203.0.113.210
DATA    example.org     IN  SOA 86400   1 ahu.example.org ...
END
AXFR    1
DATA    example.org     IN  SOA 86400   1 ahu.example.org ...
DATA    example.org     IN  NS  86400   1 ns1.example.org
DATA    example.org     IN  NS  86400   1 ns2.example.org
DATA    ns1.example.org IN  A   86400   1 203.0.113.210
DATA    ns2.example.org IN  A   86400   1 63.123.33.135
.
.
END

This is a typical zone transfer.

ABI version 3 and higher

For abi-version 3, DATA-responses get two extra fields:

DATA    scopebits   auth    qname       qclass  qtype   ttl id  content

scopebits indicates how many bits from the subnet provided in the question (originally from edns-subnet) were used in determining this answer. This can aid caching (although PowerDNS does not currently use this value).

The auth field indicates whether this response is authoritative, this is for DNSSEC. The auth field should be set to '1' for data for which the zone itself is authoritative, which includes the SOA record and its own NS records. The auth field should be 0 for NS records which are used for delegation, and also for any glue (A, AAAA) records present for this purpose. Do note that the DS record for a secure delegation should be authoritative!

For abi-versions 1 and 2, the two new fields fall back to default values. The default value for scopebits is 0. The default for auth is 1 (meaning authoritative).

Direct backend commands

With abi-version 5 you can use backend-cmd for executing commands on your backend. PowerDNS will use the following query/answer format:

CMD     Whatever you wrote
Answer goes here
And can be multiple lines
until we see
END

Sample backends

• ABI version 1
• ABI version 3
• ABI version 5