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20070708 - Breaking Firewalls

+
+ + +If the world ran only on static IPs with no firewalls, doing P2P connections would be easy. +Now when was anything worthwhile easy? Connecting the rest of the world requires breaking through firewalls and dealing with dynamic IP addresses. +
+
+
  1. End point = 6 bytes.
    2. +
  2. Public end point is what server sees as ip:port for client.
    3. +
  3. Private end point is what client sees as ip:port for self.
    4. +
  4. All clients connect to server and server responds with public end point.
    5. +
  5. So all clients know their public end point.
    6. +
  6. Clients also give private end point to server.
    7. +
  7. NATs might mangle private end point in UDP packet so encript it.
+
+Here is how S (server or peer) can connect A (peer) to B (peer). This works both on local LAN and across the wide open internet with NAT and firewalls. +

+
  1. A is connected with S.
    2. +
  2. B is connected with S.
    3. +
  3. S sends private and public end points of A to B.
    4. +
  4. S sends private and public end points of B to A.
    5. +
  5. A sends packets to private and public end points of B.
    6. +
  6. B sends packets to private and public end points of A.
    7. +
  7. A locks in ip:port it sees from B.
    8. +
  8. B locks in ip:port it sees from A.
+
+Keep in mind that NAT (network address translation) might result in a different public end point on each connection attempt, +so during steps 5 and 6, they might have to IP/port scan by attempting multiple public end points seeded from the public end point sent in steps 3 and 4. + + +
+ + +