etcd

Source

etcd enables SkyDNS service discovery from etcd.

Description

The etcd plugin implements the (older) SkyDNS service discovery service. It is not suitable as a generic DNS zone data plugin. Only a subset of DNS record types are implemented, and subdomains and delegations are not handled at all. The plugin will also recursively descend the tree and return all records found, see “Special Behavior” below for details.

The data in the etcd instance has to be encoded as a message like SkyDNS. It works just like SkyDNS.

The etcd plugin makes extensive use of the forward plugin to forward and query other servers in the network - if that plugin has been enabled as well.

Syntax

etcd [ZONES...]

ZONES zones etcd should be authoritative for.

The path will default to /skydns the local etcd3 proxy (http://localhost:2379). If no zones are specified the block’s zone will be used as the zone.

etcd [ZONES...] {
    fallthrough [ZONES...]
    path PATH
    endpoint ENDPOINT...
    credentials USERNAME PASSWORD
    tls CERT KEY CACERT
}

fallthrough If zone matches but no record can be generated, pass request to the next plugin. If [ZONES…] is omitted, then fallthrough happens for all zones for which the plugin is authoritative. If specific zones are listed (for example in-addr.arpa and ip6.arpa), then only queries for those zones will be subject to fallthrough.
PATH the path inside etcd. Defaults to “/skydns”.
ENDPOINT the etcd endpoints. Defaults to “http://localhost:2379”.
credentials is used to set the USERNAME and PASSWORD for accessing the etcd cluster.
tls followed by:
- no arguments, if the server certificate is signed by a system-installed CA and no client cert is needed
- a single argument that is the CA PEM file, if the server cert is not signed by a system CA and no client cert is needed
- two arguments - path to cert PEM file, the path to private key PEM file - if the server certificate is signed by a system-installed CA and a client certificate is needed
- three arguments - path to cert PEM file, path to client private key PEM file, path to CA PEM file - if the server certificate is not signed by a system-installed CA and client certificate is needed.

Special Behaviour

The etcd plugin leverages directory structure to look for related entries. For example an entry /skydns/test/skydns/mx would have entries like /skydns/test/skydns/mx/a, /skydns/test/skydns/mx/b and so on. Similarly a directory /skydns/test/skydns/mx1 will have all mx1 entries. Note this plugin will search through the entire (sub)tree for records. In case of the first example, a query for mx.skydns.test will return both the contents of the a and b records. If the directory extends deeper those records are returned as well.

With etcd3, support for hierarchical keys are dropped. This means there are no directories but only flat keys with prefixes in etcd3. To accommodate lookups, the etcd plugin now does a lookup on prefix /skydns/test/skydns/mx/ to search for entries like /skydns/test/skydns/mx/a etc, and if there is nothing found on /skydns/test/skydns/mx/, it looks for /skydns/test/skydns/mx to find entries like /skydns/test/skydns/mx1.

This causes two lookups from CoreDNS to etcd in certain cases.

Examples

This is the default SkyDNS setup, with everything specified in full:

skydns.local {
    etcd {
        path /skydns
        endpoint http://localhost:2379
    }
    prometheus
    cache
    loadbalance
}

. {
    forward . 8.8.8.8:53 8.8.4.4:53
    cache
}

Or a setup where we use /etc/resolv.conf as the basis for the proxy and the upstream when resolving external pointing CNAMEs.

skydns.local {
    etcd {
        path /skydns
    }
    cache
}

. {
    forward . /etc/resolv.conf
    cache
}

Multiple endpoints are supported as well.

etcd skydns.local {
    endpoint http://localhost:2379 http://localhost:4001
...

Before getting started with these examples, please setup etcdctl (with etcdv3 API) as explained here. This will help you to put sample keys in your etcd server.

If you prefer, you can use curl to populate the etcd server, but with curl the endpoint URL depends on the version of etcd. For instance, etcd v3.2 or before uses only [CLIENT-URL]/v3alpha/* while etcd v3.5 or later uses [CLIENT-URL]/v3/* . Also, Key and Value must be base64 encoded in the JSON payload. With etcdctl these details are automatically taken care of. You can check this document for details.

Reverse zones

Reverse zones are supported. You need to make CoreDNS aware of the fact that you are also authoritative for the reverse. For instance if you want to add the reverse for 10.0.0.0/24, you’ll need to add the zone 0.0.10.in-addr.arpa to the list of zones. Showing a snippet of a Corefile:

etcd skydns.local 10.0.0.0/24 {
...

Next you’ll need to populate the zone with reverse records, here we add a reverse for 10.0.0.127 pointing to reverse.skydns.local.

% etcdctl put /skydns/arpa/in-addr/10/0/0/127 '{"host":"reverse.skydns.local."}'

Querying with dig:

% dig @localhost -x 10.0.0.127 +short
reverse.skydns.local.

Zone name as A record

The zone name itself can be used as an A record. This behavior can be achieved by writing special entries to the ETCD path of your zone. If your zone is named skydns.local for example, you can create an A record for this zone as follows:

% etcdctl put /skydns/local/skydns/ '{"host":"1.1.1.1","ttl":60}'

If you query the zone name itself, you will receive the created A record:

% dig +short skydns.local @localhost
1.1.1.1

If you would like to use DNS RR for the zone name, you can set the following:

% etcdctl put /skydns/local/skydns/x1 '{"host":"1.1.1.1","ttl":60}'
% etcdctl put /skydns/local/skydns/x2 '{"host":"1.1.1.2","ttl":60}'

If you query the zone name now, you will get the following response:

% dig +short skydns.local @localhost
1.1.1.1
1.1.1.2

Zone name as AAAA record

If you would like to use AAAA records for the zone name too, you can set the following:

% etcdctl put /skydns/local/skydns/x3 '{"host":"2003::8:1","ttl":60}'
% etcdctl put /skydns/local/skydns/x4 '{"host":"2003::8:2","ttl":60}'

If you query the zone name for AAAA now, you will get the following response:

% dig +short skydns.local AAAA @localhost
2003::8:1
2003::8:2

SRV record

If you would like to use SRV records, you can set the following:

% etcdctl put /skydns/local/skydns/x5 '{"host":"skydns-local.server","ttl":60,"priority":10,"port":8080}'

Please notice that the key host is the target in SRV, so it should be a domain name.

If you query the zone name for SRV now, you will get the following response:

% dig +short skydns.local SRV @localhost
10 100 8080 skydns-local.server.

TXT record

If you would like to use TXT records, you can set the following:

% etcdctl put /skydns/local/skydns/x6 '{"ttl":60,"text":"this is a random text message."}'
% etcdctl put /skydns/local/skydns/x7 '{"ttl":60,"text":"this is a another random text message."}'

If you query the zone name for TXT now, you will get the following response:

% dig +short skydns.local TXT @localhost
"this is a random text message."
"this is a another random text message."