4 ms·

They can, it's called firewall punching... Skype does that routinely: ever wondered how it can setup a point to point connection without port forwarding? So t

• dec0dedab0de 9y ago
  I believe that's called ICE. and still needs both clients aware that theyre about to connect, and making outbound connections to each other with the goal of opening up sourceports. This does not negate the fact that NAT(PAT) provides protection against directly connecting to a device.
  • mshook 9y ago
    If you know a bug in the DNS resolving stack of the client, you can make it send a query to your DNS server and exploit it to establish a connection. So no, it doesn't have to know it's about to connect. A query is easily triggered by sending an email with a an external picture embedded or something like that. Nothing NAT/PAT can protect you against.
    • slau 9y ago
      But why would the client use _your_ DNS server? It will just use its local resolver, which will most likely use the router, or ISPs DNS server, and so forth.
    • dec0dedab0de 9y ago
      thats also something a firewall cant protect you against.
      • rocqua 9y ago
        The difference is that NAT doesn't track the counter party, so after you reached out to the DNS, any other service can use the opened port to connect to your PC. With a stateful firewall, it tracks that the port was opened only used for the DNS server. If a connection to that port from a different IP address than the DNS server is made, the firewall will block it.
• rocqua 9y ago
  That is inside-out. It requires a client inside your network to initiate a connection to the outside. Obviously, NAT does nothing for preventing outbound connections. The whole point-to-point connection between 2 NATed PCs isn't so much about security either. If an attacker wants to connect to your PC behind a NAT, all the attacker needs is to be routable.
  • pdkl95 9y ago
    If your router is only NATing packets and routing them, your PC is routable. The router will simply translate the addresses as usual. There is a good chance most people have never seen such a setup, as the router almost always includes other features like a stateful firewall. The firewall may even take advantage of the NAT information when it decides if a packet should be routed or dropped. All NAT does is rewrite the Source Address and/or Destination Address fields in the IP header, and possibly the Source/Destination Port fields in the UDP or TCP header. There are many rewriting methods, including some that are designed to route packets from the public network ("port forwarding", etc).
    • rocqua 9y ago
      Ok, so below is how I understand NAT to make a PC effectively (with exceptions) unroutable. As I figured it, if pc Bob is behind a NAT, there is not a public IP address that will route to Bob. The NAT box (lets call it a router) does have a public IP address. However, when a packet arrives at the router, and the destination port isn't mapped (by mapped I don't just mean manual port forwarding but also the actual NAT process) to some port on Bob, the packet will never reach Bob. In order to figure out a destination port that will even reach Bob at all, you either need to somehow get a recognized request from Bob, and look at the 'return address'. If you already have some control over Bob (or another PC in the NAT) that seems feasible, otherwise it takes a rather large dragnet. My point being, unless you have info on the state of the router, anything behind it is effectively unroutable. I'd be very interested to hear where I am wrong, it's been a while since I covered this material.
      • pdkl95 9y ago