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SkyDNS Build Status

Version 2.5.3a

SkyDNS is a distributed service for announcement and discovery of services built on top of etcd. It utilizes DNS queries to discover available services. This is done by leveraging SRV records in DNS, with special meaning given to subdomains, priorities and weights.

This is the original announcement blog post for version 1. Since then, SkyDNS has seen some changes, most notably the ability to use etcd as a backend. Here you can find the SkyDNS2 announcement.

Changes since version 1


  • Does away with Raft and uses etcd (which uses raft).
  • Makes it possible to query arbitrary domain names.
  • Is a thin layer above etcd, that translates etcd keys and values to the DNS.
  • Does DNSSEC with NSEC3 instead of NSEC.

Note that bugs in SkyDNS1 will still be fixed, but the main development effort will be focussed on version 2. Version 1 of SkyDNS can be found here.

Setup / Install

Download/compile and run etcd. See the documentation for etcd at

Then get and compile SkyDNS:

go get
cd $GOPATH/src/
go build -v

SkyDNS' configuration is stored in etcd: but there are also flags and environment variables you can set. To start SkyDNS, set the etcd machines with the environment variable ETCD_MACHINES:

export ETCD_MACHINES=','

If ETCD_MACHINES is not set, SkyDNS will default to using to connect to etcd. Or you can use the flag -machines. Auto-discovering new machines added to the network can be enabled by enabling the flag -discover.

Optionally (but recommended) give it a nameserver:

curl -XPUT \
    -d value='{"host":""}'

Also see the section "NS Records".


SkyDNS' configuration is stored in etcd as a JSON object under the key /skydns/config. The following parameters may be set:

  • dns_addr: IP:port on which SkyDNS should listen, defaults to
  • domain: domain for which SkyDNS is authoritative, defaults to skydns.local..
  • dnssec: enable DNSSEC
  • hostmaster: hostmaster email address to use.
  • local: optional unique value for this skydns instance, default is none. This is returned when queried for local.dns.skydns.local.
  • round_robin: enable round-robin sorting for A and AAAA responses, defaults to true. Note that packets containing more than one CNAME are exempt from this (see issue #128 on Github).
  • nameservers: forward DNS requests to these (recursive) nameservers (array of IP:port combination), when not authoritative for a domain. This defaults to the servers listed in /etc/resolv.conf. Also see no_rec.
  • no_rec: never (ever) provide a recursive service (i.e. forward to the servers provided in -nameservers).
  • read_timeout: network read timeout, for DNS and talking with etcd.
  • ttl: default TTL in seconds to use on replies when none is set in etcd, defaults to 3600.
  • min_ttl: minimum TTL in seconds to use on NXDOMAIN, defaults to 30.
  • scache: the capacity of the DNSSEC signature cache, defaults to 10000 signatures if not set.
  • rcache: the capacity of the response cache, defaults to 0 messages if not set.
  • rcache_ttl: the TTL of the response cache, defaults to 60 if not set.
  • ndots: how many labels a name should have before we allow forwarding. Default to 2.
  • systemd: bind to socket(s) activated by systemd (ignores -addr).
  • path-prefix: backend(etcd) path prefix, defaults to skydns (i.e. if it is set to mydns, the SkyDNS's configuration object should be stored under the key /mydns/config).
  • etcd3: flag that toggles the etcd version 3 support by skydns during runtime. Defaults to false.

To set the configuration, use something like:

curl -XPUT \
    -d value='{"dns_addr":"","ttl":3600, "nameservers": ["",""]}'

SkyDNS needs to be restarted for configuration changes to take effect. This might change, so that SkyDNS can re-read the config from etcd after a HUP signal.

You can also use the command line options, however the settings in etcd take precedence.

Commandline flags

  • -addr: used to specify the address to listen on (note: this will be changed into -dns_addr to match the json.

  • -local: used to specify a unique service for this SkyDNS instance. This should point to a (unique) domain into etcd, when SkyDNS receives a query for the name local.dns.skydns.local it will fetch this service and return it. For instance: -local e2016c14-fbba-11e3-ae08-10604b7efbe2.dockerhosts.skydns.local and then

      curl -XPUT \
          -d value='{"host":""}'

    To register the local IP address. Now when SkyDNS receives a query for local.dns.skydns.local it will fetch the above key and returns that one service. In other words skydns will substitute e2016c14-fbba-11e3-ae08-10604b7efbe2.dockerhosts.skydns.local for local.dns.skydns.local. This follows the same rules as the other services, so it can also be an external names, which will be resolved.

    Also see the section Host Local Values.

Environment Variables

SkyDNS uses these environment variables:

  • ETCD_MACHINES - list of etcd machines, "http://localhost:4001,". Overwrite with -machines string flag.
  • ETCD_TLSKEY - path of TLS client certificate - private key. Overwrite with -tls-key string flag.
  • ETCD_TLSPEM - path of TLS client certificate - public key. Overwrite with -tls-pem string flag.
  • ETCD_CACERT - path of TLS certificate authority public key. Overwrite with -ca-cert string flag.
  • ETCD_USERNAME - username used for basic auth. Overwrite with -username string flag.
  • ETCD_PASSWORD - password used for basic auth. Overwrite with -password string flag.
  • SKYDNS_ADDR - specify address to bind to. Overwrite with -addr string flag.
  • SKYDNS_DOMAIN - set a default domain if not specified by etcd config. Overwrite with -domain string flag.
  • SKYDNS_NAMESERVERS - set a list of nameservers to forward DNS requests to when not authoritative for a domain, ",". Overwrite with -nameservers string flag.
  • SKYDNS_PATH_PREFIX - backend(etcd) path prefix, defaults to skydns (i.e. if it is set to mydns, the SkyDNS's configuration object should be stored under the key /mydns/config). Overwrite with -path-prefix string flag.
  • SKYDNS_SYSTEMD: set to true to bind to socket(s) activated by systemd (ignores SKYDNS_ADDR). Overwrite with -systemd bool flag.
  • SKYDNS_NDOTS: how many labels a name should have before we allow forwarding. Default to 2.

For Prometheus the following environment variables are available:

  • PROMETHEUS_PORT: port where the HTTP server for prometheus will run.
  • PROMETHEUS_PATH: path for the metrics, defaults to /metrics.
  • PROMETHEUS_NAMESPACE: namespace used in the metrics, no default.
  • PROMETHEUS_SUBSYSTEM: subsystem used in the metric, defaults to skydns.

If PROMETHEUS_PORT is set to an integer larger than 0, Prometheus support will be enabled.

Current counters are:

  • dns_request_count_total, total count of request made against SkyDNS.
  • dns_request_duration_seconds, duration of the request handling in seconds.
  • dns_response_size_bytes, size of the repsonses in bytes.
  • dns_error_count_total, total count of responses containing errors.
  • dns_cachemiss_count_total, total count of cache misses.

SSL Usage and Authentication with Client Certificates

In order to connect to an SSL-secured etcd, you will at least need to set ETCD_CACERT to be the public key of the Certificate Authority which signed the server certificate.

If the SSL-secured etcd expects client certificates to authorize connections, you also need to set ETCD_TLSKEY to the private key of the client, and ETCD_TLSPEM to the public key of the client.

Service Announcements

Announce your service by submitting JSON over HTTP to etcd with information about your service. This information will then be available for queries via DNS. We use the directory /skydns to anchor all names.

When providing information you will need to fill out (some of) the following values.

  • Path - The path of the key in etcd, e.g. if the domain you want to register is "rails.production.east.skydns.local", you need to reverse it and replace the dots with slashes. So the name here becomes: local/skydns/east/production/rails. Then prefix the /skydns/ string too, so the final path becomes /v2/keys/skydns/local/skydns/east/production/rails
  • Host - The name of your service, e.g., or an IP address (either v4 or v6);
  • Port - the port where the service can be reached;
  • Priority - the priority of the service, the lower the value, the more preferred;
  • Weight - a weight factor that will be used for services with the same Priority;
  • Text - text you want to add (this returned when doing a TXT query);
  • TTL - the time-to-live of the service, overriding the default TTL. If the etcd key also has a TTL, the minimum of this value and the etcd TTL is used.
  • TargetStrip - when synthesising a name for an IP only SRV record, take the path name and strip TargetStrip labels from the ride hand side.
  • Group - limit recursion and only return services that share the Group's value.

Path is the only mandatory field. The lookups into Etcd will be done with a lower cased path name.

Adding the service can thus be done with:

curl -XPUT \
    -d value='{"host":"","priority":20}'

Or with etcdctl:

etcdctl set /skydns/local/skydns/east/production/rails \

When doing a SRV query for these keys an SRV record is returned with the priority and a certain weight. The weight of a service is calculated as follows. We treat weight as a percentage, so if there are 3 services, the weight is set to 33 for each:

Service Weight SRV.Weight
a 100 33
b 100 33
c 100 33

If we add other weights to the equation some services will get a different Weight:

Service Weight SRV.Weight
a 120 34
b 100 28
c 130 37

Note, all calculations are rounded down, so the sum total might be lower than 100.

When querying the DNS for services you can use wildcards or query for subdomains. See the section named "Wildcards" below for more information.

Service Discovery via the DNS

You can find services by querying SkyDNS via any DNS client or utility. It uses a known domain syntax with subdomains to find matching services.

For the purpose of this document, let's suppose we have added the following services to etcd:

  • 1.rails.production.east.skydns.local, mapping to
  • 2.rails.production.west.skydns.local, mapping to
  • 4.rails.staging.east.skydns.local, mapping to
  • 6.rails.staging.east.skydns.local, mapping to 2003::8:1

These names can be added with:

curl -XPUT \
    -d value='{"host":"","port":8080}'
curl -XPUT \
    -d value='{"host":"","port":8080}'
curl -XPUT \
    -d value='{"host":"","port":8080}'
curl -XPUT \
    -d value='{"host":"2003::8:1","port":8080}'

Testing one of the names with dig:

% dig @localhost SRV 1.rails.production.east.skydns.local

1.rails.production.east.skydns.local. 3600 IN SRV 10 0 8080


Of course using the full names isn't that useful, so SkyDNS lets you query for subdomains, and returns responses based upon the amount of services matched by the subdomain or from the wildcard query.

If we are interested in all the servers in the east region, we simply omit the rightmost labels from our query:

% dig @localhost SRV east.skydns.local

east.skydns.local.      3600    IN      SRV     10 20 8080
east.skydns.local.      3600    IN      SRV     10 20 8080 4.rails.staging.east.skydns.local.
east.skydns.local.      3600    IN      SRV     10 20 8080 6.rails.staging.east.skydns.local.

4.rails.staging.east.skydns.local. 3600 IN A
6.rails.staging.east.skydns.local. 3600 IN AAAA 2003::8:1

Here all three entries of the east are returned.

There is one other feature at play here. The second and third names, {4,6}.rails.staging.east.skydns.local, only had an IP record configured. Here SkyDNS used the etcd path (also see TargetStrip) to construct a target name and then puts the actual IP address in the additional section. Directly querying for the A records of 4.rails.staging.east.skydns.local. of course also works:

% dig @localhost -p 5354 +noall +answer A 4.rails.staging.east.skydns.local.

4.rails.staging.east.skydns.local. 3600 IN A

Another way to leads to the same result it to query for *.east.skydns.local, you even put the wildcard (the * or any) in the middle of a name staging.*.skydns.local or staging.any.skydns.local is a valid query, which returns all name in staging, regardless of the region. Multiple wildcards per name are also permitted.

Note that any is synonymous for a *, as shown above.


Now we can try some of our example DNS lookups:

SRV Records

Get all Services in staging.east:

% dig @localhost staging.east.skydns.local. SRV

staging.east.skydns.local. 3600 IN  SRV 10 50 8080 4.rails.staging.east.skydns.local.
staging.east.skydns.local. 3600 IN  SRV 10 50 8080 6.rails.staging.east.skydns.local.

4.rails.staging.east.skydns.local. 3600 IN A
6.rails.staging.east.skydns.local. 3600 IN AAAA 2003::8:1

If you ask for a service who's Host value is an IP address you would (in theory) get back a SRV record such as:

% dig @localhost 4.rails.staging.east.skydns.local SRV

4.rails.staging.east.skydns.local 3600 IN SRV 10 100 8080

Where the target of the SRV is an IP address. This is not how SRV records work. SkyDNS will in this case synthesize a domain name and add the actual IP address to the additional section of the response:

% dig @localhost 4.rails.staging.east.skydns.local SRV

4.rails.staging.east.skydns.local 3600 IN SRV 10 100 4.rails.staging.east.skydns.local.

4.rails.staging.east.skydns.local. 3600 IN A

Which conveys the same information and is legal in the DNS. To have some control on how the target names look you can register a service with TargetStrip set to a non-zero value. Setting TargetStrip to "2" strips 2 labels from the generated target name:

4.rails.staging.east.skydns.local 3600 IN SRV 10 100 staging.east.skydns.local.

staging.east.skydns.local. 3600 IN A

Which removed the 4.rails from the target name.

A/AAAA Records

To return A records, simply run a normal DNS query for a service matching the above patterns.

Now do a normal DNS query:

% dig @localhost staging.east.skydns.local. A

staging.east.skydns.local. 3600 IN  A

Now you have a list of all known IP Addresses registered running in staging in the east area.

Because we're returning A records and not SRV records, there are no ports listed, so this is only useful when you're querying for services running on ports known to you in advance.

MX Records

If a service is added with "mail": true it is also an MX record, the Priority doubles as the MX's Preference.

CNAME Records

If for an A or AAAA query the IP address can not be parsed, SkyDNS will try to see if there is a chain of names that will lead to an IP address. The chain can not be longer than 8. So for instance if the following services have been registered:

curl -XPUT \
    -d value='{"host":"service1.skydns.local","port":8080}'


curl -XPUT \
    -d value='{"host":"","port":8080}'

We have created the following CNAME chain: 1.rails.production.east.skydns.local -> service1.skydns.local -> If you then query for an A or AAAA for 1.rails.production.east.skydns.local SkyDNS returns:

1.rails.production.east.skydns.local. 3600  IN  CNAME   service1.skydns.local.
service1.skydns.local.                 3600  IN  A
External Names

If the CNAME chains leads to a name that falls outside of the domain (i.e. does not end with skydns.local.), a.k.a. an external name, SkyDNS will attempt to resolve that name using the supplied nameservers. If this succeeds the reply is concatenated to the current one and send to the client. So if we register this service:

curl -XPUT \
    -d value='{"host":"","port":8080}'

Doing an A/AAAA query for this will lead to the following response:

1.rails.production.east.skydns.local. 3600 IN CNAME            3600    IN      CNAME              3600    IN      A

The first CNAME is generated from within SkyDNS, the other CNAME is returned from the remote name server.

TXT Records

SkyDNS also allows you to query for TXT records. Just register a json with the 'text' field set.

NS Records

For DNS to work properly SkyDNS needs to tell its parents its nameservers. This information is stored inside etcd, under the key local/skydns/dns/ns. There multiple services maybe stored. Note these services MUST use IP address, using names will not work. For instance:

curl -XPUT \
    -d value='{"host":""}'

Registers ns1.ns.dns.skydns.local as a nameserver with IP address

% dig @localhost NS skydns.local

skydns.local.       3600    IN  NS  ns1.ns.dns.skydns.local.

n1.ns.dns.skydns.local.    3600    IN  A

Having the nameserver(s) in etcd make sense because usually it is hard for SkyDNS to figure this out by itself, especially when running behind NAT or running on and being forwarded packets IPv6 packets, etc. etc.

PTR Records: Reverse Addresses

When registering a service with an IP address only, you might also want to register the reverse (adding a hostname the address points to). In the DNS these records are called PTR records.

So looking back at some of the services in the section "Service Discovery via the DNS", we register these IP only ones:


To add the reverse of this address you need to add the DNS name that will be used when doing a reverse lookup. With dig -x <IP address> you can easiliy find what the reverse name should be:

% dig -x +noall +question
;   IN  PTR

So the name must be which should point to 4.rails.staging.east.skydns.local.

These can be added with the following command. Note that the IP address is reversed again and is actually back in its original form.

curl -XPUT \
    -d value='{"host":"4.rails.staging.east.skydns.local"}'

If SkyDNS receives a PTR query it will check these paths and will return the contents. Note that these replies are sent with the AA (Authoritative Answer) bit off. If nothing is found locally the query is forwarded to the local recursor (if so configured), otherwise SERVFAIL is returned.

This also works for IPv6 addresses, except that the reverse path is quite long.

DNS Forwarding

By specifying nameservers in SkyDNS's config, for instance,, you create a DNS forwarding proxy. In this case it round-robins between the two nameserver IPs mentioned.

Requests for which SkyDNS isn't authoritative will be forwarded and proxied back to the client. This means that you can set SkyDNS as the primary DNS server in /etc/resolv.conf and use it for both service discovery and normal DNS operations.


SkyDNS supports signing DNS answers, also known as DNSSEC. To use it, you need to create a DNSSEC keypair and use that in SkyDNS. For instance, if the domain for SkyDNS is skydns.local:

% dnssec-keygen skydns.local
Generating key pair............++++++ ...................................++++++

This creates two files with the basename Kskydns.local.+005.49860, one with the extension .key (this holds the public key) and one with the extension .private which holds the private key. The basename of these files should be given to SkyDNS's DNSSEC configuration option like so (together with some other options):

curl -XPUT -d \

If you then query with dig +dnssec you will get signatures, keys and NSEC3 records returned. Authenticated denial of existence is implemented using NSEC3 white lies, see RFC7129, Appendix B.

Host Local Values

SkyDNS supports storing values which are specific for that instance of SkyDNS.

This can be useful when you have SkyDNS running on multiple hosts, but want to store values that are specific for a single host. For example the public IP-address of the host or the IP-address on the tenant network.

To do that you need to specify a unique value for that host with -local. A good unique value for that would be an UUID which you can generate with uuidgen for instance.

That unique value is used as a path in etcd to store the values separately from the normal values. It is still stored in the etcd backend so a restart of SkyDNS with the same unique value will give it access to the old data.

In the example here, we don't use an UUID, we use public.addresses:

% skydns -local public.addresses.skydns.local &

% curl -XPUT \
    -d value='{"host":""}'

% dig @ local.dns.skydns.local. A

local.dns.skydns.local. 3600 IN  A

The name local.dns.skydns.local. is fixed, i.e. you can retrieve the Host Local Value by querying for local.dns.<your domain>.


Groups can be used to group set of services together. The main use of this is to limit recursion, i.e. don't give back all records, but only a subset. Say that I have configuration like this:

/skydns/local/domain/a - {"host": "", "group": "g1"}
/skydns/local/domain/b - {"host": "", "group": "g1"}
/skydns/local/domain/subdom/c - {"host": "", "group": "g2"}
/skydns/local/domain/subdom/d - {"host": "", "group": "g2"}

And you want domain.local to return ( and and subdom.domain.local to return ( and For this the two domains, need to be in different groups. What those groups are does not matter, as long as a and b belong to the same group which is different from the group c and d belong to. If a service is found without a group it is always included.

Implementing a custom DNS backend

The SkyDNS server package may be used as a library, which allows a custom record retrieval implementation (referred to as a Backend) to be provided. The default Etcd implementation resides under backends/etcd/etcd.go. To provide your own backend implementation, you must implement the server.Backend interface.

If you want to preserve the ability to answer arbitrary queries from etcd, but use your custom implementation for certain subsets of the namespace, the server.FirstBackend helper type will allow you to chain multiple Backends in order. The first backend that answers a Records or ReverseRecord call with a record and with no error will be served.

Stub Zones

Stub Zones are pointers that point to another set of servers which should provide an answer for the current query. This is similar to the (recursive) forwarding SkyDNS does, but different in that you need to specify a domain name and a set of authoritative servers. Also this can be dynamically controlled by writing values into Etcd. Note, that when enabled SkyDNS will first consult the stub configuration, potentially bypassing any configured local records.

The stub zone configuration lives under stub.dns.skydns.local.. The following example shows on how to set this up. Suppose we want to create a stub zone for and point to the nameservers reachable by following address and (optional) ports:

  •, port 54
  •, port 53 (53 is the default that will be used if there isn't one specified)

We should then register 2 services under the name

% curl -XPUT \
    -d value='{"host":"", "port":54}'
% curl -XPUT \
    -d value='{"host":""}'

So the leaves should have the nameserver information.

xxx.<domain name>.stub.dns.skydns.local
|           |
v           |
nameservers |
      stub domain name

When SkyDNS receives a query for it will not forward it to the recursors, but instead will query on port 54 and if that fails will query (on 53) to get an answer. That answer will then be given back to the original client.

When forwarding to a stub, SkyDNS adds a EDNS0 meta data RR to the packet telling the remote server (if its a SkyDNS instance) that this is a stub request. SkyDNS will not (stub)forward packets with this EDNS0 meta data, instead the request will be dropped and logged.

Remember this will only work when SkyDNS is started with -stubzones.

How Do I Create an Address Pool and Round Robin Between Them

You have 3 machines with 3 different IP addresses and you want to have 1 name pointing to all 3 possible addresses. The name we want to use is: db.skydns.local and the 3 addresses are 127.0.0.{1,2,3}. For this to work we create the hosts named x{1,2,3}.db.skydns.local in etcd:

curl -XPUT -d \
curl -XPUT -d \
    value='{"host": ""'}
curl -XPUT -d \
    value='{"host": ""'}

Now the name db.skydns.local is the "load balanced" name for the database, SkyDNS will round-robin by default in this case unless -round-robin=false is enabled.

How I Do Create Multiple SRV Records For the Same Name

You want this response from SkyDNS, which says there are 2 open ports on bar.skydns.local and this name has IP addres

bar.skydns.local.   3600    IN  SRV 10 50 80 bar.skydns.local.
bar.skydns.local.   3600    IN  SRV 10 50 443 bar.skydns.local.

bar.skydns.local. 3600    IN  A

So you register a "dummy" host named x1:

curl -XPUT -d \
curl -XPUT -d \
    value='{"host": "bar.skydns.local","port":443}'

And try it out:

bar.skydns.local.   3600    IN  SRV 10 50 80
bar.skydns.local.   3600    IN  SRV 10 50 443 bar.skydns.local.


Which has x1 in the name, which is not the name you wanted to see there, and worse does not match the name in the other SRV record. To makes this work you'll need TargetStrip which allows you to tell SkyDNS to strip labels from the name it makes up:

curl -XPUT -d \

% dig @ bar.skydns.local. SRV

bar.skydns.local.   3600    IN  SRV 10 50 80 bar.skydns.local.
bar.skydns.local.   3600    IN  SRV 10 50 443 bar.skydns.local.

bar.skydns.local. 3600    IN  A

How do you limit recursion?

By default SkyDNS will returns all records under a name. Suppose you want we have bar.skydns.local... TODO.


Official Docker images are at the Docker Hub:

  • master -> skynetservices/skydns:latest
  • latest tag -> skynetservices/skydns:latest-tagged

The supplied Dockerfile can be used to build an image as well. Note that the image is based of Alpine Linux which used musl libc instead of glibc, so when building SkyDNS you must make sure if does not need glibc when run:

Build SkyDNS with:

% GOOS=linux go build -a -tags netgo -installsuffix netgo

And then build the docker image:

% docker build -t $USER/skydns .

If you run it, SkyDNS needs to access Etcd (or whatever backend), which usually runs on the host server (i.e. when using CoreOS), to make that work, just run:

docker run --net host <image>


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