weavedns

README

Weave DNS server

The Weave DNS server answers name queries in a Weave network. It is run per-host, to be supplied as the nameserver for containers on that host. It is then told about hostnames for the local containers. For other names it will ask the other weave hosts, or fall back to using the host's configured name server.

Using weaveDNS

The weave script command launch-dns starts the DNS container. Subsquently, giving any container a hostname in the domain .weave.local will register it in DNS. For example:

$ weave launch
$ weave launch-dns 10.1.0.2/16
$ weave run 10.1.1.25/24 -ti -h pingme.weave.local ubuntu
$ shell1=$(weave run 10.1.1.26/24 -ti -h ubuntu.weave.local ubuntu)
$ docker attach $shell1

# ping pingme
...

The IP address supplied to weave launch-dns must not be used by any other container, and the supplied network must contain all application networks.

Domain search paths

If you don't supply a domain search path (with --dns-search=), weave run ... will tell a container to look for "bare" hostnames, like pingme, in its own domain. That's why you can just say ping pingme above -- since the hostname is ubuntu.weave.local, it will look for pingme.weave.local.

If you want to supply other entries for the domain search path, you will need also to supply the weave.local domain or subdomain:

weave run 10.1.1.4/24 -ti --dns-search=weave.local --dns-search=local ubuntu

Doing things more manually

If a weaveDNS container is running, weave run will automatically supply it as the DNS server to the new container. Similarly, both weave run and weave attach will register the hostname of the given container against the given weave network IP address.

In some circumstances, you may not want to use the weave command. You can still take advantage of a running weaveDNS, using the HTTP API.

Supplying the DNS server

If you want to use docker run to start a container, rather than weave run, you can supply the weaveDNS IP as the --dns option to make it use weaveDNS:

$ dns_ip=$(docker inspect --format='{{ .NetworkSettings.IPAddress }}' weavedns)
$ shell2=$(docker run --dns=$dns_ip -ti ubuntu)
$ weave attach 10.1.1.27/24 $shell2

This isn't very useful unless the container is also attached to the weave network (as in the last line above).

Supplying the domain search path

Again, when using docker run, you will usually want to supply a domain search path so that you can use unqualified hostnames. Use --dns-search=. to make the resolver use the container's domain, or e.g., --dns-search=weave.local to make it look in weave.local.

Adding containers to DNS

If DNS is started after you've attached a container to the weave network, or you want to give the container a name in DNS other than its hostname, you can register it using the HTTP API:

$ docker start $shell2
$ shell2_ip=$(docker inspect --format='{{ .NetworkSettings.IPAddress }}' $shell2)
$ curl -X PUT "http://$dns_ip:6785/name/$shell2/10.1.1.27" -d local_ip=$shell2_ip -d fqdn=shell2.weave.local

Not watching docker events

By default, the server will watch docker events and remove entries for any containers that die. You can tell it not to, by adding --watch=false to the container args:

$ weave launch-dns 10.1.0.2/16 --watch=false

You can manually delete entries for a host, by poking weaveDNS's HTTP API with e.g., curl:

$ docker stop $shell2
$ dns_ip=$(docker inspect --format='{{ .NetworkSettings.IPAddress }}' weavedns)
$ curl -X DELETE "http://$dns_ip:6785/name/$shell2/10.1.1.27"

Supplying --dns-search=

By default, Docker will provide containers with a /etc/resolv.conf that matches that for the host. In some circumstances, this may include a DNS search path, which will break the nice "bare names resolve" property above.

Present limitations

• The server will not know about restarted containers, but if you re-attach a restarted container to the weave network, it will be re-registered with weaveDNS.
• The server will currently forget names if it is itself restarted, and otherwise not know about containers running when it starts. In the future it will look at existing container hostnames upon starting up.
• The server may give unreachable IPs as answers, since it doesn't try to filter by reachability. If you use subnets, align your hostnames with the subnets.
• We use UDP multicast to find out about remote names (from Weave DNS servers on other hosts); this likely won't scale well beyond a certain point T.B.D., so we'll have to come up with another scheme.