custom_dns_server

A DNS server written in python primarily using scapy. This server is intended to replace your computer's main DNS server, giving you the ability to decide which DNS server to use for certain domains.

This server was created with VPNs in mind. Let's say your computer is configured to use primary DNS server located behind a VPN at 192.168.255.1. You connect to that VPN using a domain name, say myvpn.me. When your computer tries to lookup myvpn.me at 192.168.255.1, it cannot reach that IP address while your VPN is disconnected. To circumvent that, this server allows you to specify an alternate DNS server for myvpn.me. Now your computer can perform a DNS lookup for myvpn.me, while the rest of your DNS traffic is redirected through your VPN.

Alternatively, let's say that certain local domains are only accessible within the local network. You might want to resolve local domains using the local network's DNS server instead of your VPN server. Again, this server will provide you with the capability to redirect requests for the local domain to the local DNS server.

Instructions to Run

After installing scapy and pyyaml, simply run python main.py

Configuration

All configuration options for this server is provided in the config.yml file. The sections below explain each configuration option.

Resolvers

The resolvers configuration section contains a series of entries describing which servers should be used for the corresponding domain

resolvers:
    - google.com: 8.8.8.8
    - facebook.com: [1.1.1.1, 2.2.2.2]
    
fallbacks: 192.168.255.1 # or you can provide multiple servers here too

The above configuration would lookup google.com sub-domains at 8.8.8.8 By specfiying two servers for facebook.com subdomains, the server will first query1.1.1.1 and then 2.2.2.2 if the former is unable to successfully fulfil the request

For domains with no configured resolver, the server will query servers specified by the fallbacks option. This option can be automatically configured using DHCP (see the next section)

- google.com: [8.8.8.8, fallback, 1.1.1.1]

Use fallback in the list of domains to specify that the server should use the fallback servers to query for that domain.

DHCP Configuration

If a DHCP server is available on the network, you can configure the server to request DNS server information from it by sending a DHCPINFORM message. To activate this functionality, set the use-dhcp option to be true.

dhcp-options:
  use-dhcp: true
  use-network-broadcast: true
  send-decline: true
  # local-iface: eth0 or "Ethernet"
  # local-addr: 192.168.10.5
  # dhcp-server: 192.168.10.1

use-network-broadcast

Set this to true to use the subnet's broadcast address when sending DHCPDISCOVER message instead of the universal broadcast address. For example, if the DHCP server is on the 192.168.10.0/24 network, setting this option to true will cause it to broadcast the DHCPINFORM message using 192.168.10.255 instead of 255.255.255.255. Turning this option will contain the server's DHCP packets to the local subnet, but may not work with networks that rely on DHCP relays

local-iface and local-addr

local-iface informs this server that the DHCP server should be reached using the given network interface, while local-addr informs this server that the DHCP server can be reached using the network interface with that specified local IP address. If neither of these options are given, then the interface returned by scapy's conf.iface variable is used

If both options are specified, then local-iface is ignored in favour of the value given by local-addr

dhcp-server and force-broadcast

If this option is specified, this server will send DHCPINFORM messages straight to the IP address provided instead of sending a broadcast message. force-broadcast overrides this option, forcing this server to always use broadcast messages when sending DHCPINFORM.

DNS Server Configuration

Configuration options pertaining to the DNS server

dns-server:
  ip: 127.0.0.1

ip

The IP address which the DNS server is supposed to bind to and listen for requests on.

Full Sample config.yml

fallbacks: 192.168.255.1

resolvers:
    - google.com: 8.8.8.8
    - facebook.com: 1.1.1.1

dhcp-options:
  use-dhcp: true
  use-network-broadcast: true
  send-decline: true

dns-server:
  ip: 127.0.0.1