Bitcoin uses a DNS-based bootstrapping mechanism to help new nodes find peers. As far as I'm aware, Lightning doesn't have such mechanism, and new users are supposed to just search for node addresses on the web (or use nodes maintained / recommended by their wallet developers, or whatever).
What is the rationale behind this design decision?
It does! There is even a BOLT for it (check out: https://github.com/lightningnetwork/lightning-rfc/blob/master/10-dns-bootstrap.md )
from there you have the following example:
dig lseed.bitcoinstats.com AAAA lseed.bitcoinstats.com. 60 IN AAAA 2a02:aa16:1105:4a80:1234:1234:37c1:9c9 Christian Decker is maintaining the dns seed list. I even used this in my autopilot (the c-lightning wrapper). check out:
def __get_seed_keys(self): """ retrieve the nodeids of the ln seed nodes from lseed.bitcoinstats.com """ domain = "lseed.bitcoinstats.com" srv_records = dns.resolver.query(domain,"SRV") res = [] for srv in srv_records: bech32 = str(srv.target).rstrip(".").split(".")[0] data = bech32_decode(bech32)[1] decoded = convertbits(data, 5, 4) res.append("".join( ['{:1x}'.format(integer) for integer in decoded])[:-1]) return res def __connect_to_seeds(self): """ sets up peering connection to seed nodes of the lightning network This is necessary in case the node operating the autopilot has never been connected to the lightning network. """ try: for nodeid in random.shuffle(self.__get_seed_keys()): self.__clogger.info("peering with node: " + nodeid) self.__rpc_interface.connect(nodeid) # FIXME: better strategy than sleep(2) for building up time.sleep(2) except: pass 
r is "used to specify what realm the returned nodes must support" with the default value of "0 (Bitcoin)", but it doesn't mention the values for other networks...