When a full node first connects to the network it must first discover peers using the DNS seed endpoints. Bithereum mainnet and testnet both have their own endpoints which can be found hardcoded into core.

vSeeds.emplace_back("us-dnsseed.bithereum.network", true);
vSeeds.emplace_back("eu-dnsseed.bithereum.network", true);
vSeeds.emplace_back("sg-dnsseed.bithereum.network", true);

vSeeds.emplace_back("us-testnet-dnsseed.bithereum.network", true);
vSeeds.emplace_back("eu-testnet-dnsseed.bithereum.network", true);
vSeeds.emplace_back("sg-testnet-dnsseed.bithereum.network", true);

When setting up your DNS configuration, make sure that you have created an NS record within the domain provider that points to this server. For example if we want this server to server IP addresses of nodes on the network from a query on us-testnet-dnsseed.bithereum.network, then an NS record for us-testnet-dnsseed subdomain should be handled by this server. In most cases, you'll need to create a seperate subdomain that points to this server's IP address, as NS record's typically need to be fully qualified domain names.

A Record for dns --> IP of this seed server
NS Record for us-testnet-dnsseed --> dns.bithereum.network
NS Record for eu-testnet-dnsseed --> dns.bithereum.network
NS Record for sg-testnet-dnsseed --> dns.bithereum.network

To install a DNS seed server, you'll need to have NPM and Node JS v9+ installed prior to running the following commands.

sudo apt-get update -y
sudo apt-get install npm -y
sudo npm install n -g -y
sudo n v9
git clone https://github.com/BTHPOS/dns-seed-handler.git dns-seed-handler
cd dns-seed-handler
npm install

Within server.js, configure the hostname and ip addresses that you would like returned based on the DNS query.

      if (hostname.indexOf("x9.us-dnsseed") === 0) {
          res.answer.push({name:hostname, type:'A', data:"54.162.119.37", ttl:ttl});
          res.answer.push({name:hostname, type:'A', data:"18.212.4.44", ttl:ttl});
      }

To start the DNS server, run sudo npm start or sudo node server.js. If you would like to run it in the background append > /dev/null 2>&1 &, for example sudo npm start > /dev/null 2>&1 &.

Within the terminal or cmd of your requesting machine, type in the following to test a route.

dig x9.us-dnsseed.bithereum.network A

The corresponding A records should appear. Replace A with the record you intend on requesting.

In some cases, port 53 will be in use by a process acting as a DNS.

DNS Server Running
events.js:165
      throw er; // Unhandled 'error' event
      ^

Error: bind EACCES 0.0.0.0:53
    at _handle.lookup (dgram.js:266:18)
    at process._tickCallback (internal/process/next_tick.js:178:19)
    at Function.Module.runMain (internal/modules/cjs/loader.js:697:11)
    at startup (internal/bootstrap/node.js:201:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:516:3)
Emitted 'error' event at:
    at Socket.<anonymous> (/home/ubuntu/dns-seed-handler/node_modules/dnsd/server.js:41:44)
    at Socket.emit (events.js:180:13)
    at _handle.lookup (dgram.js:267:14)
    at process._tickCallback (internal/process/next_tick.js:178:19)
    [... lines matching original stack trace ...]
    at bootstrapNodeJSCore (internal/bootstrap/node.js:516:3)

If you are running this on a unix based system like Ubuntu, it often times can be systemd-resolve. Run sudo service systemd-resolved stop and then try running node server.js once more. In a nutshell, you'll have to figure out what process is using port 53 so that your DNS seed server will be able to handle incoming requests.