dnsrouter

A simplistic dns daemon that you can use as your local DNS server and have it route DNS requests to upstream servers based on the requested domain.

I've created this so that I can effectively set up split DNS (sometimes called split horizon DNS) such that my setup will properly forward DNS requests through a VPN connection only when the domain being queried is told to do so.

Why? Split DNS is apparently supported by common DNS server software but my experience is that it's not implemented in the way I expect. For example dnsmasq will send DNS requests to all servers regardless of the rules and when an upstream DNS server is dropping connections it will hang the whole server.

This service can also answer with internal DNS entries, avoiding the need for host file modifications.

Installation

Only the following methods are available for installation. For all other systems, see Building below.

MacOS Homebrew

Homebrew won't accept this package until it has higher reputation (stars, forks) so until then you'll have to use the development Formula and rebuild it to install it:

brew update
cd $(brew --repo homebrew/core)
git remote add jc21 https://github.com/jc21/homebrew-core.git
git fetch --all
git checkout jc21/dnsrouter
brew install --build-from-source dnsrouter

# Edit config which is located at:
# /opt/homebrew/etc/dnsrouter/config.json

sudo brew services start dnsrouter

# Switch back to homebrew-core master
git checkout master

Centos 8/Stream

RPM's are built here and hosted here.

sudo yum localinstall https://yum.jc21.com/jc21-yum.rpm
sudo yum install dnsrouter

# Edit config which is located at:
# /etc/dnsrouter/config.json

sudo systemctl enable dnsrouter --now

Configuration

The command is able to write it's default configuration and exit:

./dnsrouter -w
# optionally specify the file to write
./dnsrouter -w -c /path/to/config.json

Then it's up to you to edit this file to your liking. The default location is /etc/dnsrouter/config.json

Refer to the config.json.example file for upstream routing examples.

Examples

Given the following configuration:

{
  "servers": [
    {
      "host": "127.0.0.1",
      "port": 53,
      "default_upstream": "1.1.1.1",
      "internal": [
        {
          "regex": "mail.example.com",
          "A": "192.168.0.10",
          "AAAA": "2001:db8::1234:5678",
          "TXT": "omgyesitworked",
          "MX": "10 mailserver1.example.com.\n20 mailserver2.example.com."
        }
      ],
      "upstreams": [
        {
          "regex": "local",
          "nxdomain": true
        },
        {
          "regex": ".*\\.example.com",
          "upstream": "8.8.8.8"
        },
        {
          "regex": ".*\\.localdomain",
          "upstream": "10.0.0.1"
        },
        {
          "regex": ".*\\.(office\\.lan|myoffice\\.com)",
          "upstream": "10.0.0.1"
        }
      ]
    }
  ]
}

Requesting DNS for test.example.com

1. DNS client connects to dnsrouter and asks for test.example.com
2. dnsrouter matches with the 2nd upstream rule
3. dnsrouter forwards the DNS question to upstream DNS server 8.8.8.8
4. dnsrouter returns the answer to the DNS client

Requesting DNS for google.com

1. DNS client connects to dnsrouter and asks for google.com
2. dnsrouter does not match with any defined rules
3. dnsrouter forwards the DNS question to default upstream DNS server 1.1.1.1
4. dnsrouter returns the answer to the DNS client

Requesting DNS for local

1. DNS client connects to dnsrouter and asks for local
2. dnsrouter matches with the 1st upstream rule
3. dnsrouter returns an error to the client with NXDOMAIN

Requesting DNS for myoffice.com

1. DNS client connects to dnsrouter and asks for myoffice.com
2. dnsrouter does not match with any defined rules
3. dnsrouter forwards the DNS question to default upstream DNS server 1.1.1.1
4. dnsrouter returns the answer to the DNS client

Note: This is a trick example. The domain matching regex will match *.myoffice.com but not myoffice.com

Requesting DNS for mail.example.com

1. DNS client connects to dnsrouter and asks for mail.example.com
2. dnsrouter matches with the 1st internal rule
3. dnsrouter returns the answer value to the DNS client with the A/AAAA/MX/TXT record as requested

Building

git clone https://github.com/jc21/dnsrouter.git
cd dnsrouter
./scripts/build.sh

Binary will output to bin/dnsrouter

Running

./dnsrouter
# optionally specify the file to read
./dnsrouter -c /path/to/config.json

Be aware that running on port 53 will require root permissions on Linux systems.

After the service is running you just have to use it. Modify your network interface's DNS servers (or /etc/resolv.conf) to use the IP running dnsrouter ie 127.0.0.1 if it's the same machine.

You may choose to run in verbose mode by specifying -v this will output each incoming DNS request and the determined forwarding DNS server.

Multiple Servers

The configuration allows you to run multiple DNS servers on different interfaces and ports and have different rules for them.

Additional Notes

1. DNS Answers are cached for 30 seconds in memory, regardless of upstream TTL
2. Regex's are prefixed with ^ and appended with $ so there is no need to add them
3. Performance on a desktop used heavily appears to be great. Has not been tested for an entire office.