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CoreDNS

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CoreDNS is a DNS server that started as a fork of Caddy. It has the same model: it chains middleware. In fact it's so similar that CoreDNS is now a server type plugin for Caddy.

CoreDNS is also a Cloud Native Computing Foundation inception level project.

CoreDNS is the successor to SkyDNS. SkyDNS is a thin layer that exposes services in etcd in the DNS. CoreDNS builds on this idea and is a generic DNS server that can talk to multiple backends (etcd, kubernetes, etc.).

CoreDNS aims to be a fast and flexible DNS server. The keyword here is flexible: with CoreDNS you are able to do what you want with your DNS data. And if not: write some middleware!

CoreDNS can listen for DNS request coming in over UDP/TCP (go'old DNS), TLS (RFC 7858) 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).
  • 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).
  • Automatically load zone files from disk (auto).
  • Caching (cache).
  • Health checking endpoint (health).
  • Use etcd as a backend, i.e., a 101.5% replacement for SkyDNS (etcd).
  • Use k8s (kubernetes) as a backend (kubernetes).
  • Serve as a proxy to forward queries to some other (recursive) nameserver (proxy).
  • Provide metrics (by using Prometheus) (metrics).
  • Provide query (log) and error (error) logging.
  • Support the CH class: version.bind and friends (chaos).
  • Profiling support (pprof).
  • Rewrite queries (qtype, qclass and qname) (rewrite).
  • Echo back the IP address, transport and port number used (whoami).

Each of the middlewares has a README.md of its own.

Status

CoreDNS can be used as an authoritative nameserver for your domains, and should be stable enough to provide you with good DNS(SEC) service.

There are still a few known issues, and work is ongoing on making things fast and to reduce the memory usage.

All in all, CoreDNS should be able to provide you with enough functionality to replace parts of BIND 9, Knot, NSD or PowerDNS and SkyDNS. Most documentation is in the source and some blog articles can be found here. If you do want to use CoreDNS in production, please let us know and how we can help.

https://caddyserver.com/ is also full of examples on how to structure a Corefile (renamed from Caddyfile when forked).

Compilation

CoreDNS (as a servertype plugin for Caddy) has a dependency on Caddy, but this is not different than any other Go dependency. If you have the source of CoreDNS, get all dependencies:

go get ./...

And then go build as you would normally do:

go build

This should yield a coredns binary.

Examples

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

.:53
2016/09/18 09:20:50 [INFO] CoreDNS-001
CoreDNS-001

Any query send to port 53 should return some information; your sending address, port and protocol used.

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.

Start a simple proxy, you'll need to be root to start listening on port 53.

Corefile contains:

.:53 {
    proxy . 8.8.8.8:53
    log stdout
}

Just start CoreDNS: ./coredns. And then just 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.

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

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

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.

.:1053 {
    rewrite ANY HINFO
    proxy . 8.8.8.8:53

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

Zone Specification

The following Corefile fragment is legal, but does not explicitly define a zone to listen on:

{
   # ...
}

This defaults to .:53 (or whatever -dns.port is).

The next one only defines a port:

:123 {
    # ...
}

This defaults to the root zone ., but can't be overruled with the -dns.port flag.

Just specifying a zone, default to listening on port 53 (can still be overridden with -dns.port):

example.org {
    # ...
}

Listening on TLS and for gRPC? Use:

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

Specifying ports works in the same way:

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

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

Blog and Contact

Systemd Service File

Use this as a systemd service file. It defaults to a coredns with a homedir of /home/coredns and the binary lives in /opt/bin and the config in /etc/coredns/Corefile:

[Unit]
Description=CoreDNS DNS server
Documentation=https://coredns.io
After=network.target

[Service]
PermissionsStartOnly=true
LimitNOFILE=8192
User=coredns
WorkingDirectory=/home/coredns
ExecStartPre=/sbin/setcap cap_net_bind_service=+ep /opt/bin/coredns
ExecStart=/opt/bin/coredns -conf=/etc/coredns/Corefile
ExecReload=/bin/kill -SIGUSR1 $MAINPID
Restart=on-failure

[Install]
WantedBy=multi-user.target