Magic Transit: Network functions at Cloudflare scale

Today we announced Cloudflare Magic Transit, which makes Cloudflare’s network available to any IP traffic on the Internet. Up until now, Cloudflare has primarily operated proxy services: our servers terminate HTTP, TCP, and UDP sessions with Internet users and pass that data through new sessions they create with origin servers. With Magic Transit, we are now also operating at the IP layer: in addition to terminating sessions, our servers are applying a suite of network functions (DoS mitigation, firewalling, routing, and so on) on a packet-by-packet basis.

Over the past nine years, we’ve built a robust, scalable global network that currently spans 193 cities in over 90 countries and is ever growing. All Cloudflare customers benefit from this scale thanks to two important techniques. The first is anycast networking. Cloudflare was an early adopter of anycast, using this routing technique to distribute Internet traffic across our data centers. It means that any data center can handle any customer’s traffic, and we can spin up new data centers without needing to acquire and provision new IP addresses. The second technique is homogeneous server architecture. Every server in each of our edge data centers is capable of running every task. We build our servers on commodity hardware, making it easy to quickly increase our processing capacity by adding new servers to existing data centers. Having no specialty hardware to depend on has also led us to develop an expertise in pushing the limits of what’s possible in networking using modern Linux kernel techniques.

Magic Transit is built on the same network using the same techniques, meaning our customers can now run their network functions at Cloudflare scale. Our fast, secure, reliable global edge becomes our customers’ edge. To explore how this works, let’s follow the journey of a packet from a user on the Internet to a Magic Transit customer’s network.

In the announcement blog post we describe an example deployment for Acme Corp. Let’s continue with this example here. When Acme brings their IP prefix 203.0.113.0/24 to Cloudflare, we start announcing that prefix to our transit providers, peers, and to Internet exchanges in each of our data centers around the globe. Additionally, Acme stops announcing the prefix to their own ISPs. This means that any IP packet on the Internet with a destination address within Acme’s prefix is delivered to a nearby Cloudflare data center, not to Acme’s router.

Let’s say I want to access Acme’s FTP server on 203.0.113.100 from my computer in Cloudflare’s office in Champaign, IL. My computer generates a TCP SYN packet with destination address 203.0.113.100 and sends it out to the Internet. Thanks to anycast, that packet ends up at Cloudflare’s data center in Chicago, which is the closest data center (in terms of Internet routing distance) to Champaign. The packet arrives on the data center’s router, which uses ECMP (Equal Cost Multi-Path) routing to select which server should handle the packet and dispatches the packet to the selected server.

Once at the server, the packet flows through our XDP- and iptables-based DoS detection and mitigation functions. If this TCP SYN packet were determined to be part of an attack, it would be dropped and that would be the end of it. Fortunately for me, the packet is permitted to pass.

So far, this looks exactly like any other traffic on Cloudflare’s network. Because of our expertise in running a global anycast network we’re able to attract Magic Transit customer traffic to every data center and apply the same DoS mitigation solution that has been protecting Cloudflare for years. Our DoS solution has handled some of the largest attacks ever recorded, including a 942Gbps SYN flood in 2018. Below is a screenshot of a recent SYN flood of 300M packets per second. Our architecture lets us scale to stop the largest attacks.

The above looked identical to how all other Cloudflare traffic is processed, but this is where the similarities end. For our other services, the TCP SYN packet would now be dispatched to a local proxy process (e.g. our nginx-based HTTP/S stack). For Magic Transit, we instead want to dynamically provision and apply customer-defined network functions like firewalls and routing. We needed a way to quickly spin up and configure these network functions while also providing inter-network isolation. For that, we turned to network namespaces.

Namespaces are a collection of Linux kernel features for creating lightweight virtual instances of system resources that can be shared among a group of processes. Namespaces are a fundamental building block for containerization in Linux. Notably, Docker is built on Linux namespaces. A network namespace is an isolated instance of the Linux network stack, including its own network interfaces (with their own eBPF hooks), routing tables, netfilter configuration, and so on. Network namespaces give us a low-cost mechanism to rapidly apply customer-defined network configurations in isolation, all with built-in Linux kernel features so there’s no performance hit from userspace packet forwarding or proxying.

When a new customer starts using Magic Transit, we create a brand new network namespace for that customer on every server across our edge network (did I mention that every server can run every task?). We built a daemon that runs on our servers and is responsible for managing these network namespaces and their configurations. This daemon is constantly reading configuration updates from Quicksilver, our globally distributed key-value store, and applying customer-defined configurations for firewalls, routing, etc, inside the customer’s namespace. For example, if Acme wants to provision a firewall rule to allow FTP traffic (TCP ports 20 and 21) to 203.0.113.100, that configuration is propagated globally through Quicksilver and the Magic Transit daemon applies the firewall rule by adding an nftables rule to the Acme customer namespace:

# Apply nftables rule inside Acme’s namespace
$ sudo ip netns exec acme_namespace nft add rule inet filter prerouting ip daddr 203.0.113.100 tcp dport 20-21 accept

Getting the customer’s traffic to their network namespace requires a little routing configuration in the default network namespace. When a network namespace is created, a pair of virtual ethernet (veth) interfaces is also created: one in the default namespace and one in the newly created namespace. This interface pair creates a “virtual wire” for delivering network traffic into and out of the new network namespace. In the default network namespace, we maintain a routing table that forwards Magic Transit customer IP prefixes to the veths corresponding to those customers’ namespaces. We use iptables to mark the packets that are destined for Magic Transit customer prefixes, and we have a routing rule that specifies that these specially marked packets should use the Magic Transit routing table.

(Why go to the trouble of marking packets in iptables and maintaining a separate routing table? Isolation. By keeping Magic Transit routing configurations separate we reduce the risk of accidentally modifying the default routing table in a way that affects how non-Magic Transit traffic flows through our edge.)

Network namespaces provide a lightweight environment where a Magic Transit customer can run and manage network functions in isolation, letting us put full control in the customer’s hands.

After passing through the edge network functions, the TCP SYN packet is finally ready to be delivered back to the customer’s network infrastructure. Because Acme Corp. does not have a network footprint in a colocation facility with Cloudflare, we need to deliver their network traffic over the public Internet.

This poses a problem. The destination address of the TCP SYN packet is 203.0.113.100, but the only network announcing the IP prefix 203.0.113.0/24 on the Internet is Cloudflare. This means that we can’t simply forward this packet out to the Internet—it will boomerang right back to us! In order to deliver this packet to Acme we need to use a technique called tunneling.

Tunneling is a method of carrying traffic from one network over another network. In our case, it involves encapsulating Acme’s IP packets inside of IP packets that can be delivered to Acme’s router over the Internet. There are a number of common tunneling protocols, but Generic Routing Encapsulation (GRE) is often used for its simplicity and widespread vendor support.

GRE tunnel endpoints are configured both on Cloudflare’s servers (inside of Acme’s network namespace) and on Acme’s router. Cloudflare servers then encapsulate IP packets destined for 203.0.113.0/24 inside of IP packets destined for a publicly-routable IP address for Acme’s router, which decapsulates the packets and emits them into Acme’s internal network.

Now, I’ve omitted an important detail in the diagram above: the IP address of Cloudflare’s side of the GRE tunnel. Configuring a GRE tunnel requires specifying an IP address for each side, and the outer IP header for packets sent over the tunnel must use these specific addresses. But Cloudflare has thousands of servers, each of which may need to deliver packets to the customer through a tunnel. So how many Cloudflare IP addresses (and GRE tunnels) does the customer need to talk to? The answer: just one, thanks to the magic of anycast.

Cloudflare uses anycast IP addresses for our GRE tunnel endpoints, meaning that any server in any data center is capable of encapsulating and decapsulating packets for the same GRE tunnel. How is this possible? Isn’t a tunnel a point-to-point link? The GRE protocol itself is stateless—each packet is processed independently and without requiring any negotiation or coordination between tunnel endpoints. While the tunnel is technically bound to an IP address it need not be bound to a specific device. Any device that can strip off the outer headers and then route the inner packet can handle any GRE packet sent over the tunnel. Actually, in the context of anycast the term “tunnel” is misleading since it implies a link between two fixed points. With Cloudflare’s Anycast GRE, a single “tunnel” gives you a conduit to every server in every data center on Cloudflare’s global edge.

One very powerful consequence of Anycast GRE is that it eliminates single points of failure. Traditionally, GRE-over-Internet can be problematic because an Internet outage between the two GRE endpoints fully breaks the “tunnel”. This means reliable data delivery requires going through the headache of setting up and maintaining redundant GRE tunnels terminating at different physical sites and rerouting traffic when one of the tunnels breaks. But because Cloudflare is encapsulating and delivering customer traffic from every server in every data center, there is no single “tunnel” to break. This means Magic Transit customers can enjoy the redundancy and reliability of terminating tunnels at multiple physical sites while only setting up and maintaining a single GRE endpoint, making their jobs simpler.

Magic Transit is a powerful new way to deploy network functions at scale. We’re not just giving you a virtual instance, we’re giving you a global virtual edge. Magic Transit takes the hardware appliances you would typically rack in your on-prem network and distributes them across every server in every data center in Cloudflare’s network. This gives you access to our global anycast network, our fleet of servers capable of running your tasks, and our engineering expertise building fast, reliable, secure networks. Our scale is now your scale.