/coredns

CoreDNS is a DNS server that chains plugins

Primary LanguageGoApache License 2.0Apache-2.0

CoreDNS

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CoreDNS is a DNS server/forwarder, written in Go, that chains plugins. Each plugin performs a (DNS) function.

CoreDNS is a Cloud Native Computing Foundation graduated project.

CoreDNS is a fast and flexible DNS server. The key word here is flexible: with CoreDNS you are able to do what you want with your DNS data by utilizing plugins. If some functionality is not provided out of the box you can add it by writing a plugin.

CoreDNS can listen for DNS requests coming in over UDP/TCP (go'old DNS), TLS (RFC 7858), also called DoT, DNS over HTTP/2 - DoH - (RFC 8484) and gRPC (not a standard).

Currently CoreDNS is able to:

  • Serve zone data from a file; both DNSSEC (NSEC only) and DNS are supported (file and auto).
  • Retrieve zone data from primaries, i.e., act as a secondary server (AXFR only) (secondary).
  • Sign zone data on-the-fly (dnssec).
  • Load balancing of responses (loadbalance).
  • Allow for zone transfers, i.e., act as a primary server (file + transfer).
  • Automatically load zone files from disk (auto).
  • Caching of DNS responses (cache).
  • Use etcd as a backend (replacing SkyDNS) (etcd).
  • Use k8s (kubernetes) as a backend (kubernetes).
  • Serve as a proxy to forward queries to some other (recursive) nameserver (forward).
  • Provide metrics (by using Prometheus) (prometheus).
  • Provide query (log) and error (errors) logging.
  • Integrate with cloud providers (route53).
  • Support the CH class: version.bind and friends (chaos).
  • Support the RFC 5001 DNS name server identifier (NSID) option (nsid).
  • Profiling support (pprof).
  • Rewrite queries (qtype, qclass and qname) (rewrite and template).
  • Block ANY queries (any).
  • Provide DNS64 IPv6 Translation (dns64).

And more. Each of the plugins is documented. See coredns.io/plugins for all in-tree plugins, and coredns.io/explugins for all out-of-tree plugins.

Compilation from Source

To compile CoreDNS, we assume you have a working Go setup. See various tutorials if you don’t have that already configured.

First, make sure your golang version is 1.17 or higher as go mod support and other api is needed. See here for go mod details. Then, check out the project and run make to compile the binary:

$ git clone https://github.com/coredns/coredns
$ cd coredns
$ make

This should yield a coredns binary.

Compilation with Docker

CoreDNS requires Go to compile. However, if you already have docker installed and prefer not to setup a Go environment, you could build CoreDNS easily:

$ docker run --rm -i -t -v $PWD:/v -w /v golang:1.17 make

The above command alone will have coredns binary generated.

Examples

When starting CoreDNS without any configuration, it loads the whoami and log plugins and starts listening on port 53 (override with -dns.port), it should show the following:

.:53
CoreDNS-1.6.6
linux/amd64, go1.16.10, aa8c32

The following could be used to query the CoreDNS server that is running now:

dig @127.0.0.1 -p 53 www.example.com

Any query sent to port 53 should return some information; your sending address, port and protocol used. The query should also be logged to standard output.

The configuration of CoreDNS is done through a file named Corefile. When CoreDNS starts, it will look for the Corefile from the current working directory. A Corefile for CoreDNS server that listens on port 53 and enables whoami plugin is:

.:53 {
    whoami
}

Sometimes port number 53 is occupied by system processes. In that case you can start the CoreDNS server while modifying the Corefile as given below so that the CoreDNS server starts on port 1053.

.:1053 {
    whoami
}

If you have a Corefile without a port number specified it will, by default, use port 53, but you can override the port with the -dns.port flag: coredns -dns.port 1053, runs the server on port 1053.

You may import other text files into the Corefile using the import directive. You can use globs to match multiple files with a single import directive.

.:53 {
    import example1.txt
}
import example2.txt

You can use environment variables in the Corefile with {$VARIABLE}. Note that each environment variable is inserted into the Corefile as a single token. For example, an environment variable with a space in it will be treated as a single token, not as two separate tokens.

.:53 {
    {$ENV_VAR}
}

A Corefile for a CoreDNS server that forward any queries to an upstream DNS (e.g., 8.8.8.8) is as follows:

.:53 {
    forward . 8.8.8.8:53
    log
}

Start CoreDNS and then query on that port (53). The query should be forwarded to 8.8.8.8 and the response will be returned. Each query should also show up in the log which is printed on standard output.

To serve the (NSEC) DNSSEC-signed example.org on port 1053, with errors and logging sent to standard output. Allow zone transfers to everybody, but specifically mention 1 IP address so that CoreDNS can send notifies to it.

example.org:1053 {
    file /var/lib/coredns/example.org.signed
    transfer {
        to * 2001:500:8f::53
    }
    errors
    log
}

Serve example.org on port 1053, but forward everything that does not match example.org to a recursive nameserver and rewrite ANY queries to HINFO.

example.org:1053 {
    file /var/lib/coredns/example.org.signed
    transfer {
        to * 2001:500:8f::53
    }
    errors
    log
}

. {
    any
    forward . 8.8.8.8:53
    errors
    log
}

IP addresses are also allowed. They are automatically converted to reverse zones:

10.0.0.0/24 {
    whoami
}

Means you are authoritative for 0.0.10.in-addr.arpa..

This also works for IPv6 addresses. If for some reason you want to serve a zone named 10.0.0.0/24 add the closing dot: 10.0.0.0/24. as this also stops the conversion.

This even works for CIDR (See RFC 1518 and 1519) addressing, i.e. 10.0.0.0/25, CoreDNS will then check if the in-addr request falls in the correct range.

Listening on TLS (DoT) and for gRPC? Use:

tls://example.org grpc://example.org {
    whoami
}

And for DNS over HTTP/2 (DoH) use:

https://example.org {
    whoami
    tls mycert mykey
}

in this setup, the CoreDNS will be responsible for TLS termination

you can also start DNS server serving DoH without TLS termination (plain HTTP), but beware that in such scenario there has to be some kind of TLS termination proxy before CoreDNS instance, which forwards DNS requests otherwise clients will not be able to communicate via DoH with the server

https://example.org {
    whoami
}

Specifying ports works in the same way:

grpc://example.org:1443 https://example.org:1444 {
    # ...
}

When no transport protocol is specified the default dns:// is assumed.

Community

We're most active on Github (and Slack):

More resources can be found:

Contribution guidelines

If you want to contribute to CoreDNS, be sure to review the contribution guidelines.

Deployment

Examples for deployment via systemd and other use cases can be found in the deployment repository.

Deprecation Policy

When there is a backwards incompatible change in CoreDNS the following process is followed:

  • Release x.y.z: Announce that in the next release we will make backward incompatible changes.
  • Release x.y+1.0: Increase the minor version and set the patch version to 0. Make the changes, but allow the old configuration to be parsed. I.e. CoreDNS will start from an unchanged Corefile.
  • Release x.y+1.1: Increase the patch version to 1. Remove the lenient parsing, so CoreDNS will not start if those features are still used.

E.g. 1.3.1 announce a change. 1.4.0 a new release with the change but backward compatible config. And finally 1.4.1 that removes the config workarounds.

Security

Security Audit

A third party security audit was performed by Cure53, you can see the full report here.

Reporting security vulnerabilities

If you find a security vulnerability or any security related issues, please DO NOT file a public issue, instead send your report privately to security@coredns.io. Security reports are greatly appreciated and we will publicly thank you for it.

Please consult security vulnerability disclosures and security fix and release process document