[DOCS] add DNS discovery setup guide (#20399)

This adds setup instructions for the DHT crawler and DNS node
list publishing tools.

docs/_developers/dns-discovery-setup.md

@@ -0,0 +1,113 @@
+---
+title: DNS Discovery Setup Guide
+sort_key: C
+---
+
+This document explains how to set up an [EIP 1459][dns-eip] node list using the devp2p
+developer tool. The focus of this guide is creating a public list for the Ethereum mainnet
+and public testnets, but you may also find this helpful if you want to set 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, we'll create node lists 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 Geth distribution. You can
+install this command using `go get`:
+
+```shell
+go get -u github.com/ethereum/go-ethereum/cmd/devp2p
+```
+
+To create a signing key, you might also need the `ethkey` utility.
+
+```shell
+go get -u github.com/ethereum/go-ethereum/cmd/ethkey
+```
+
+### 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 should schedule this command
+to run at a regular interval. To create a node list, run
+
+```shell
+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.
+
+```shell
+mkdir mainnet.nodes.example.org
+```
+
+Then, to create the output set containing Ethereum mainnet nodes only, run
+
+```shell
+devp2p nodeset filter all-nodes.json -eth-network mainnet > mainnet.nodes.example.org/nodes.json
+```
+
+The following filter flags are available:
+
+* `-eth-network ( mainnet | ropsten | rinkeby | goerli )` selects an Ethereum network.
+* `-les-server` selects LES server nodes.
+* `-ip <mask>` restricts nodes to the given IP range.
+* `-min-age <duration>` 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, you
+need a key pair. To create the key file in the correct format, you can use the cmd/ethkey
+utility. Please choose a good password to encrypt the key on disk.
+
+```shell
+ethkey generate dnskey.json
+```
+
+Now use `devp2p dns sign` to update the signature of the node list. If your list's
+directory name differs from the name you want to publish it at, please specify the DNS
+name the using the `-domain` flag. This command will prompt for the key file password and
+update the tree signature.
+
+```shell
+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. You can also export TXT records as a JSON file and
+publish them yourself.
+
+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:
+
+```shell
+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.
+
+[dns-eip]: https://eips.ethereum.org/EIPS/eip-1459