localdns

localdns

A simple, local, wildcarding DNS server

Features

• map wildcarded domains...
  • ...to local IP
  • ...to loopback
  • ...to a specific target
• recursively resolve domains...
  • ...via servers specified for that domain
  • ...via a specified resolv.conf-formatted file
    • so you can use this as your primary local DNS resolver

Compile

Local go

go get github.com/miekg/dns
go build

Docker

install=$HOME/bin
case "$(uname)" in
  Darwin) GOOS=darwin ;;
  *) GOOS=linux ;;
esac
docker run --rm \
  -v $(pwd):/localdns \
  -v ${install}:/out \
  -w /localdns \
  -e GOOS=$GOOS \
  -e GOARCH=amd64 \
  -e GOBIN=/out \
  golang:1.3.3-cross \
  go get -v ./...

Configuration

All configuration is via environment variables.

• CNAMES=src:target,src2:target2,...

  • *.src will return a CNAME to target
  • localdns will also look up targets A record(s)
  • no default

• SELF=name,name2,...

  • *.name will point to local, non-loopback IP address(es)
  • no default

• IFACE=name%iface1%iface2...

  • *.name will point to IP addresses assigned to interface(s) iface1...
  • no default

• LOOP=name,name2,...

  • *.name will point to 127.0.0.1
  • no default

• SERVERS=name/name2/server,server2,...

  • use server for upstream resolving
  • if name/ is present, only use server for names ending in name
  • otherwise server is used unconditionally
  • if any server matches (including unconditionally), RESOLV will not be used
  • no default (will fall through to RESOLV)

• RESOLV=filename

  • use filename as the resolv.conf-formatted source for upstream servers
  • default /etc/resolv.conf won't work if using as primary DNS server

• ADDR=ip:port

  • listen on port, binding ip
  • no default

• PORT=port

  • listen on port, not bound to a specific interface
  • default 9753

Examples

Server command

sudo env \
  CNAMES=nginx.nginx.dev.docker:example.dev \
  SELF=me \
  IFACE=ether%enp0s25 \
  LOOP=dev \
  SERVERS=docker/127.0.0.1:5553,int.vpc/10.10.1.51,int.vpc/10.10.1.50 \
  RESOLV=/etc/resolv.conf.upstream \
  PORT=53 \
  localdns

Example results

$ dig @localhost +noall +answer my.machine.dev
my.machine.dev.  60  IN  A  127.0.0.1

$ dig @localhost +noall +answer an.arbitrarily.long.name.me
an.arbitrarily.long.name.me.  60  IN  A  192.168.30.40
an.arbitrarily.long.name.me.  60  IN  A  172.17.42.1

$ dig @localhost +noall +answer bhaskell.example.dev
bhaskell.example.dev.    60  IN  CNAME  nginx.nginx.dev.docker.
nginx.nginx.dev.docker.  22  IN  A      172.17.0.26

$ dig @localhost +noall +answer google.com
google.com.  193  IN  A  74.125.228.7
google.com.  193  IN  A  74.125.228.2
google.com.  193  IN  A  74.125.228.8
...

$ dig @localhost +noall +answer reflect.ether
reflect.ether.  60  IN  A     10.50.50.54
reflect.ether.  60  IN  AAAA  fe80::d7db:9dbf:f031:761c

Permissions

sudo is only needed when listening on a "privileged" port (<= 1024), and can be replaced by capabilities on Linux:

setcap cap_net_bind_service=+ep `which localdns`

Why?

I've been a huge fan of dnsmasq for a long time, but I needed an extra feature: arbitrary CNAME mapping.

From the dnsmasq man page, under the --cname=<cname>,<target> option:

Return a CNAME record which indicates that <cname> is really <target>.  There
are significant limitations on the target; it must be a DNS name which is known
to dnsmasq from /etc/hosts (or additional hosts files), from DHCP, from
--interface-name or from another --cname. If the target does not satisfy this
criteria, the whole cname is ignored.