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Dynamic configuration is cool, but we still live in a world of static configuration. nss-etcd provides a host resolution module for GNU libc Name Service Switch (NSS). Think of it as a distributed and consistent hosts file without the complication and delay of normal DNS. nss-etcd does not require any modification to your existing DNS setup.

$ curl -L http://127.0.0.1:2379/v2/keys/database/primary -XPUT -d value="10.0.0.20"
$ ping database.primary
PING database.primary (10.0.0.20) 56(84) bytes of data.
64 bytes from 10.0.0.20: icmp_seq=1 ttl=64 time=0.320 ms

#Getting started

Prerequisites

CMake, libyajl and a C compiler are required to build and install nss-etcd

apt-get install cmake libyajl-dev gcc

Currently etcd is also required to be running on localhost.

Building and installing nss-etcd

git clone https://github.com/ryandoyle/nss-etcd.git
cd nss-etcd
cmake -DCMAKE_INSTALL_PREFIX:PATH=/usr .
make install

Configuring

nss-etcd has to be enabled in /etc/nsswitch.conf to start to be used for host resolution. Add etcd in the hosts section of this file.

hosts:          files myhostname etcd dns

Resolution is ordered left to right. For most cases, it should be before dns.

Adding and resolving names

The namespace in etcd is mapped to dot-delimitered names. You have full control of the namespace you create in etcd.

$ curl -L http://127.0.0.1:2379/v2/keys/database/primary -XPUT -d value="10.0.0.20"
$ getent hosts database.primary
10.0.0.20       database.primary
  • mysql/info/address -> mysql.info.address
  • dns/services/db/us-east/primary -> dns.services.db.us-east.primary
  • You get the idea

Issues

Minimal tolerance for bugs

Bugs in nss-etcd have an impact on all processes that do name resolution. I would highly suggest that nss-etcd is not run in production environments.

Caching

nss-etcd does not cache (on purpose). It is possible that your application does cache resolution though.

Out-of-band resolution

Most applications will use glibc and gethostbyname but it's possible some directly query DNS by reading /etc/resolv.conf and performing their own resolution. This would bypass nss and therefore nss-etcd.

Other works

  • Consul provides a DNS interface to their service discovery tool.
  • SkyDNS is a DNS server built on-top of etcd