# DNS discovery setup guide
This document explains how to set up an [EIP 1459](https://eips.ethereum.org/EIPS/eip-1459) node list using the devp2p developer tool. The focus of this guide is creating a public list for the Electroneum mainnet and public testnets, but it may also be helpful for setting up DNS-based discovery for a private network.
DNS-based node lists can serve as a fallback option when connectivity to the discovery DHT is unavailable. In this guide, node lists will be created by crawling the discovery DHT, then publishing the resulting node sets under chosen DNS names.
## Installing the devp2p command
`cmd/devp2p` is a developer utility and is not included in the Etn-sc distribution. You can install this command using go get:
```sh
go get github.com/electroneum/electroneum-sc/cmd/devp2p
```
To create a signing key, the `etnkey` utility is needed.
```sh
go get github.com/electroneum/electroneum-sc/cmd/etnkey
```
## Crawling the v4 DHT
Our first step is to compile a list of all reachable nodes. The DHT crawler in cmd/devp2p is a batch process which runs for a set amount of time. You should schedule this command to run at a regular interval. To create a node list, run
```sh
devp2p discv4 crawl -timeout 30m all-nodes.json
```
This walks the DHT and stores the set of all found nodes in the `all-nodes.json` file. Subsequent runs of the same command will revalidate previously discovered node records, add newly-found nodes to the set, and remove nodes which are no longer alive. The quality of the node set improves with each run because the number of revalidations is tracked alongside each node in the set.
## Creating sub-lists through filtering
Once `all-nodes.json` has been created and the set contains a sizeable number of nodes, useful sub-sets of nodes can be extracted using the `devp2p nodeset filter` command. This command takes a node set file as argument and applies filters given as command-line flags.
To create a filtered node set, first create a new directory to hold the output set. You can use any directory name, though it's good practice to use the DNS domain name as the name of this directory.
```sh
mkdir mainnet.nodes.example.org
```
Then, to create the output set containing Electroneum mainnet nodes only, run
```sh
devp2p nodeset filter all-nodes.json -eth-network mainnet > mainnet.nodes.example.org/nodes.json
```
The following filter flags are available:
* `-eth-network` ( `mainnet` | `testnet` ) selects an Electroneum Smart Chain network.
* `-les-server` selects LES server nodes.
* `-ip ` restricts nodes to the given IP range.
* `-min-age ` restricts the result to nodes which have been live for the given duration.
## Creating DNS trees
To turn a node list into a DNS node tree, the list needs to be signed. To do this, a key pair is required. To create the key file in the correct format, the `cmd/etnkey` utility should be used. Choose a strong password to encrypt the key on disk!
```sh
etnkey generate dnskey.json
```
Now use `devp2p dns sign` to update the signature of the node list. If the list's directory name differs from the name it will be published at, specify the DNS name using the `-domain` flag. This command will prompt for the key file password and update the tree signature.
```sh
devp2p dns sign mainnet.nodes.example.org dnskey.json
```
The resulting DNS tree metadata is stored in the `mainnet.nodes.example.org/enrtree-info.json` file.
## Publishing DNS trees
Now that the tree is signed, it can be published to a DNS provider. `cmd/devp2p` currently supports publishing to CloudFlare DNS and Amazon Route53.TXT records can also be exported as a JSON file and published independently.
To publish to CloudFlare, first create an API token in the management console. `cmd/devp2p` expects the API token in the `CLOUDFLARE_API_TOKEN` environment variable. Now use the following command to upload DNS TXT records via the CloudFlare API:
```sh
devp2p dns to-cloudflare mainnet.nodes.example.org
```
Note that this command uses the domain name specified during signing. Any existing records below this name will be erased by `cmd/devp2p`.
## Using DNS trees with ETN-SC
Once a tree is available through a DNS name, Etn-sc can use it with the `--discovery.dns` command line flag. Node trees are referenced using the `enrtree://` URL scheme. The URL of the tree can be found in the `enrtree-info.json` file created by `devp2p dns sign`. Pass the URL as an argument to the flag in order to make use of the published tree.
```sh
etn-sc --discovery.dns "enrtree://AMBMWDM3J6UY3M32TMMROUNLX6Y3YTLVC3DC6HN2AVG5NHNSAXDW6@mainnet.nodes.example.org"
```
---
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