Upon first joining the Lightning Network, a node must open a few connections to existing nodes in the network. However, it can only learn of nodes that are present in the network through its peers. This Seed helps bootstrapping new nodes by indexing nodes that are present in the network and returning a random sample when queried.
In addition the seed provides a way to query for specific nodes, e.g., allowing nodes to quickly find their peers even though the switched IP or port.
Generally this implementation supports both IPv4 and IPv6 queries, i.e., A and AAAA queries.
In addition it supports SRV queries that return a mix of IPv4 nodes and IPv6 nodes, and their associated A and AAAA answers.
The seed answers incoming A and AAAA queries with up to 25 known nodes in the network.
The nodes are filtered by their listening port, and only nodes that listen on the default Lightning port, 9735, are returned.
This is necessary since it is not possible to specify the port in A and AAAA answers.
Upon receiving an SRV query the seed will sample up to 25 nodes from the known nodes, regardless of their listening port, and return them, specifying an alias and the port.
The SRV query attempts to return a balanced set of IPv4 and IPv6 nodes.
In addition to the alias and port, the seed will also attach the matching A and AAAA records, such that a single query return both IP and port, and nodes may initiate connections without further queries.
Given the alias from the SRV queries, a client can also directly query for a specific node.
If the node's ID is 03edd9462482dbe1f1ea75db38a345c2b1d8a325c5c86f72aa9ea191a94be8b664 then the corresponding alias will be:
3edd9462482dbe1f1ea75db38a345c2b1d8a325c5c86f72aa9ea191a94be8b6.64.lseed.bitcoinstats.com
The leading 0 character is removed from the pubkey, and the pubkey is split into two chunks. The first chunk is 63 characters long, while the second chunk is 2 characters long.
The two chunks are then dot-separated and prefixed to the seed's domain, lseed.bitcoinstats.com in this case.
The answer contains the record matching the query, or the record of the other IP version type in the additional section if IP versions do not match.
Currently the seed will poll a local Lightning node periodically and update its local view accordingly. In future I'd like to introduce a number of different information sources and add further tests, such as testing for reachability before returning nodes.
Please refer to this issue