persist recent peers. implement dns seed bootstrapping.

dns seeds are currently disabled though, as they always seem to return mainnet nodes.

electrum/util.py

@@ -46,6 +46,7 @@ import aiohttp
 from aiohttp_socks import SocksConnector, SocksVer
 from aiorpcx import TaskGroup
 import certifi
+import dns.resolver
 
 from .i18n import _
 from .logging import get_logger, Logger
@@ -1174,3 +1175,17 @@ def list_enabled_bits(x: int) -> Sequence[int]:
     binary = bin(x)[2:]
     rev_bin = reversed(binary)
     return tuple(i for i, b in enumerate(rev_bin) if b == '1')
+
+
+def resolve_dns_srv(host: str):
+    srv_records = dns.resolver.query(host, 'SRV')
+    # priority: prefer lower
+    # weight: tie breaker; prefer higher
+    srv_records = sorted(srv_records, key=lambda x: (x.priority, -x.weight))
+
+    def dict_from_srv_record(srv):
+        return {
+            'host': str(srv.target),
+            'port': srv.port,
+        }
+    return [dict_from_srv_record(srv) for srv in srv_records]