How To Bypass Router Firewall

How To Bypass Router Firewall – We covered a lot of space in our article on how it works. However, we took a look at how to bypass NATs (Network Address Translators) and connect your devices directly to each other, whatever in between. Let’s talk about it now!

Let’s start with a simple task: establishing a peer-to-peer connection between two machines. For example, we want to build a WireGuard® tunnel, but that is not important. The methods we use are widely used and have been the work of many people for decades. For example, WebRTC uses these tricks to send peer-to-peer audio, video and data between web browsers. VoIP phones and some video games use similar techniques, though not always successful.

How To Bypass Router Firewall

How To Bypass Router Firewall

We will discuss these techniques generally, using others as examples where appropriate. Let’s say you are configuring your own protocol and you want to go through NAT. You need two things.

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NAT traversal with TCP however adds another layer of complexity to an already complex problem and may even require kernel configuration depending on how deep you go. We’ll focus on UDP for the remainder of this article.

How To Bypass Router Firewall

If you want to reach TCP, consider using QUIC instead if you want a streaming connection after NAT traversal is complete. It is built on top of UDP so we can focus on UDP for NAT traversal and eventually become a good streaming protocol.

Second, you need to directly monitor the network socket sending and receiving network packets. As a general rule, you cannot take an existing network library and pass it through NAT because you have to send and receive additional packets that are not included in the “base” protocol you are trying to talk to. Some protocols tightly integrate NAT traversal with others (eg WebRTC). However, if you’re building your own, it’s helpful to think of NAT traversal as a separate entity that shares the same socket as your underlying protocol. Both run in parallel, one activates the other.

How To Bypass Router Firewall

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Accessing a live socket can be difficult depending on your situation. One solution is to run a local proxy. Your protocol talks to this proxy and the proxy both traverses NAT and distributes your packets to peers. This indirect layer allows you to use NAT traversals without changing your original implementation.

With the prerequisites gone, let’s move on from first principles to NAT traversal. Our goal is to have UDP packets flow bi-directionally between two devices so that our other protocol (WireGuard, QUIC, WebRTC,…) can do something great. There are two barriers to doing this Just Work: firewalls and NAT devices.

How To Bypass Router Firewall

Government firewalls are the simpler of the two problems. In fact, most NAT devices include a stateful firewall, so you should resolve this section before resolving NATs.

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There are many characters to consider. Some you might recognize are Windows Defender firewall, Ubuntu’s ufw (which uses iptables/nftables), BSD’s pf (also used by macOS), and AWS Security Groups. All are highly configurable, but the most common configuration allows all “outgoing” connections and blocks all “incoming” connections. There may be a few selected exceptions, such as allowing SSH access.

How To Bypass Router Firewall

But links and “paths” are just part of the protocol designer’s imagination. In a cable, every link is bidirectional; they all fly back and forth. How does the firewall know what goes in and what goes out?

Stateful firewalls remember which packets they have seen in the past and can use this information to decide what to do with new packets.

How To Bypass Router Firewall

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For UDP, this rule is very simple: The firewall will allow an incoming UDP packet if it has seen a packet before it. For example, if our laptop firewall sees a UDP packet from the laptop

For UDP traffic, this rule is only a minor issue for us as long as all firewalls in the path are “looking” in the same direction. This usually happens when communicating with a server on the Internet. Our only limitation is that device

How To Bypass Router Firewall

The firewall should initiate all connections. Nothing can speak to him unless he speaks first.

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Good, but not very interesting: We reopened client/server communication where the server is easily accessible to clients. In the VPN world, this results in a centralized and scalable topology: there are no firewalls blocking the concentrator from accessing it, and the firewalls are connected to the hub.

How To Bypass Router Firewall

The problems start when two of our “customers” want to speak directly. The defensive walls are now facing each other. According to our rule above, this means that both parties must go first, but neither party can be first, because the other must go first!

How to solve this problem? One way would be to require users to “open a port” and reconfigure one or both firewalls to allow other device traffic. This is not very user friendly. It also doesn’t scale to create networks that expect peers to move consistently across the Internet. Of course, in most cases you can’t control firewalls: you can’t change your router at your favorite cafe or airport. (I hope at least!)

How To Bypass Router Firewall

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The trick is to carefully read the rules we’ve created for our state firewalls. The convention of UDP is that packets must go out before packets can return.

So, to get past these many firewalls, we need to share some information to continue: peers need to know ahead of time.

How To Bypass Router Firewall

Uses a colleague. One approach is to manually configure each peer statically, but this approach is not that scalable. To go even further, we created a sync server for storage

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Peers then start sending UDP packets to each other. They must wait for some of these packets to be lost, so they cannot receive valuable information unless you are ready to resend them. This is usually true in UDP, but particularly true here. We

How To Bypass Router Firewall

Bypasses the Windows Defender firewall and exits to the Internet. A corporate firewall blocks the packet on the other side as there is no registration.

Through the corporate firewall and the internet. When it comes to the laptop, Windows Defender thinks “oh, that’s the answer to that request I saw” and skips the pack! Additionally, the corporate firewall now remembers to wait for responses

How To Bypass Router Firewall

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Alerted by receiving a packet from the workstation, the laptop sends back another packet. Windows Defender goes through the firewall, the corporate firewall (because it’s a “reply” to the already sent packet) and reaches the endpoint.

Success! We established two-way communication through a double wall that could block the view.

How To Bypass Router Firewall

It’s not always this easy. We rely on certain indirect links to third-party systems that should be handled with care. What should we pay attention to when managing connections through a firewall?

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Both nodes should try to communicate at the same time so both peers are still out and all firewalls will open. One approach is to constantly try peers, but to no avail. Wouldn’t it be great if two peers started hanging out at the same time?

How To Bypass Router Firewall

This may seem a bit repetitive: in order to communicate, you must first be able to communicate. However, the pre-existing “side channel” shouldn’t be too fancy: it can have a delay of a few seconds and transmit a few thousand bytes in total, so a small VM can easily fit thousands. machines.

In the past, I’ve used XMPP chat messages as a side channel and had great results. As another example, WebRTC requires you to find your own “notification channel” (a name that represents WebRTC’s IP address) and connect it to the WebRTC APIs. In this, our coordination server and DERP (Detour Encrypted Routing Protocol) server group serve as our side channel.

How To Bypass Router Firewall

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Stateful firewalls have limited memory, which means they have to communicate occasionally to maintain connections. If no packets are seen for a period of time (common value for UDP is 30 seconds), the firewall forgets the session and we have to start over. To avoid this we use timers and we need to send regular packets to reset the timers otherwise there is no way to reopen the connection on demand.

What we should be worried about is how many firewalls there are between our two peers. A synchronous transmission method will pass through any layer as long as it allows both stateful and outgoing connections. This is really good because it means we only have to apply the logic once and it will work anywhere.

How To Bypass Router Firewall

We can think of NAT (Network Address Translator) devices as state firewalls with a really annoying feature: in addition to all firewall protection devices, they also manipulate packets.

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A NAT device is anything that translates or modifies any address.

How To Bypass Router Firewall

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