Today we take a quick look at running dnsmasq as a docker container.

dnsmasq is a simple lightweight DNS (amongst other features) that can be used to easily set up various DNS records within your infrastructure. Our particular usecase is to set the TXT records Eureka requires for DNS based bootstrapping.

Getting started

I've built and release an alpine based dnsmasq image which configures itself from a directory of .conf files. I chose this approach so that different services can include themselves in dnsmasq during provisioning.

You can see the Dockerfile here - https://github.com/storytel/dnsmasq

Starting the container

docker run --name dnsmasq --cap-add=NET_ADMIN --net=host -v /etc/dnsmasq:/etc/dnsmasq storytel/dnsmasq

There are three options here to note :-

1. --cap-add=NET_ADMIN is required for dnsmasq to interact with the network stack
2. --net=host runs the container with the host network stack, so that port 53 on the host becomes the DNS and is accessible from the rest of the network.
3. -v /etc/dnsmasq:/etc/dnsmasq maps our folder of .conf files inside the container.

Configuration

dnsmasq will read all the .conf files added to /etc/dnsmasq (a container restart is required to load changes). So we can add each service as it's own file, or put them all in one. Whichever suits your configuration needs.

# 0.base.conf
domain-needed
bogus-priv
no-hosts
keep-in-foreground
no-resolv
expand-hosts
server=8.8.8.8
server=8.8.4.4

# 1.eureka.conf
address=/001.eureka.storytel/10.10.10.21
address=/002.eureka.storytel/10.10.10.22
address=/003.eureka.storytel/10.10.10.23
txt-record=txt.global.eureka.storytel,sweden.eureka.storytel
txt-record=txt.sweden.eureka.storytel,10.10.10.21,10.10.10.22,10.10.10.23

# 2.etcd.conf
address=/001.etcd.storytel/10.10.10.21
address=/002.etcd.storytel/10.10.10.22
address=/003.etcd.storytel/10.10.10.23

Access from another server

Once we have our container up and running we can then use the DNS from other servers on the network.

$ dig @10.10.10.1 TXT txt.global.eureka.storytel

; <<>> DiG 9.10.2-P4 <<>> @10.10.10.1 TXT txt.global.eureka.storytel
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 13897
;; flags: qr aa rd ra ad; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;txt.global.eureka.storytel.        IN      TXT

;; ANSWER SECTION:
txt.global.eureka.storytel. 0       IN      TXT     "sweden.eureka.storytel"

;; Query time: 1 msec
;; SERVER: 10.10.10.1#53(10.10.10.1)
;; WHEN: Sat Sep 03 09:04:25 UTC 2016
;; MSG SIZE  rcvd: 85