fixes #384

also adds MultiError to libp2p/exceptions.py

and an additional fixme I have noticed

libp2p/exceptions.py

@@ -8,3 +8,9 @@ class ValidationError(BaseLibp2pError):
 
 class ParseError(BaseLibp2pError):
     pass
+
+
+class MultiError(BaseLibp2pError):
+    """Raised with multiple exceptions."""
+
+    # todo: find some way for this to fancy-print all encapsulated errors

libp2p/network/swarm.py

@@ -19,6 +19,7 @@ from libp2p.transport.transport_interface import ITransport
 from libp2p.transport.upgrader import TransportUpgrader
 from libp2p.typing import StreamHandlerFn
 
+from ..exceptions import MultiError
 from .connection.raw_connection import RawConnection
 from .connection.swarm_connection import SwarmConn
 from .exceptions import SwarmException
@@ -90,14 +91,49 @@ class Swarm(INetwork):
         except PeerStoreError:
             raise SwarmException(f"No known addresses to peer {peer_id}")
 
-        if not addrs:
+        if len(addrs) == 0:
             raise SwarmException(f"No known addresses to peer {peer_id}")
 
-        multiaddr = addrs[0]
+        exceptions: List[SwarmException] = []
+
+        # Try all known addresses
+        for multiaddr in addrs:
+            try:
+                return await self.dial_addr(multiaddr, peer_id)
+            except SwarmException as e:
+                exceptions.append(e)
+                logger.debug(
+                    "encountered swarm exception when trying to connect to %s, "
+                    "trying next address...",
+                    multiaddr,
+                    exc_info=e,
+                )
+
+        # Tried all addresses, raising exception.
+        if len(exceptions) > 0:
+            raise SwarmException(
+                "unable to connect to %s, all addresses failed to dial (with exceptions)",
+                peer_id,
+            ) from MultiError(exceptions)
+        else:
+            raise SwarmException(
+                "unable to connect to %s, all addresses failed to dial", peer_id
+            )
+
+    async def dial_addr(self, addr: Multiaddr, peer_id: ID) -> INetConn:
+        """
+        dial_addr try to create a connection to peer_id with addr.
+
+        :param addr: the address we want to connect with
+        :param peer_id: the peer we want to connect to
+        :raises SwarmException: raised when an error occurs
+        :return: muxed connection
+        """
+
         # Dial peer (connection to peer does not yet exist)
         # Transport dials peer (gets back a raw conn)
         try:
-            raw_conn = await self.transport.dial(multiaddr)
+            raw_conn = await self.transport.dial(addr)
         except OpenConnectionError as error:
             logger.debug("fail to dial peer %s over base transport", peer_id)
             raise SwarmException(
@@ -137,7 +173,7 @@ class Swarm(INetwork):
     async def new_stream(self, peer_id: ID) -> INetStream:
         """
         :param peer_id: peer_id of destination
-        :param protocol_id: protocol id
+        :param protocol_id: protocol id fixme: protocol_id not in parameters
         :raises SwarmException: raised when an error occurs
         :return: net stream instance
         """