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Today's Outage Post Mortem


5 min read

Today's Outage Post Mortem

This morning at 09:47 UTC CloudFlare effectively dropped off the Internet. The outage affected all of CloudFlare's services including DNS and any services that rely on our web proxy. During the outage, anyone accessing or any site on CloudFlare's network would have received a DNS error. Pings and Traceroutes to CloudFlare's network resulted in a "No Route to Host" error.

The cause of the outage was a system-wide failure of our edge routers. CloudFlare currently runs 23 data centers worldwide. These data centers are connected to the rest of the Internet using routers. These routers announce the path that, from any point on the Internet, packets should use to reach our network. When a router goes down, the routes to the network that sits behind the router are withdrawn from the rest of the Internet.

We regularly will shut down one or a small handful of routers when we are upgrading a facility. Because we use Anycast, traffic naturally fails to the next closest data center. However, this morning we encountered a bug that caused all of our routers to fail network wide.


We are largely a Juniper shop at CloudFlare and all the edge routers that were affected were from Juniper. One of the reasons we like Juniper is their support of a protocol called Flowspec. Flowspec allows you to propagate router rules to a large number of routers efficiently. At CloudFlare, we constantly make updates to the rules on our routers. We do this to fight attacks as well as to shift traffic so it can be served as fast as possible.

This morning, we saw a DDoS attack being launched against one of our customers. The attack specifically targeted the customer's DNS servers. We have an internal tool that profiles attacks and outputs signatures that our automated systems as well as our ops team can use to stop attacks. Often, we use these signatures in order to create router rules to either rate limit or drop known-bad requests.

In this case, our attack profiler output the fact that the attack packets were between 99,971 and 99,985 bytes long. That's odd to begin with because the largest packets sent across the Internet are typically in the 1,500-byte range and average around 500 – 600 bytes. We have the maximum packet size set to 4,470 on our network, which is on the large size, but well under what the attack profiler was telling us was the size of these attack packets.

Bad Rule

Someone from our operations team is monitoring our network 24/7. As is normal practice for us, one of our ops team members took the output from the profiler and added a rule based on its output to drop packets that were between 99,971 and 99,985 bytes long. Here's what the rule (somewhat simplified and with the IPs obscured) looked like in Junos, the Juniper operating system:

+    route 173.X.X.X/32-DNS-DROP {
+        match {
+            destination 173.X.X.X/32;
+            port 53;
+            packet-length [ 99971 99985 ];
+        }
+        then discard;
+    }

Flowspec accepted the rule and relayed it to our edge network. What should have happened is that no packet should have matched that rule because no packet was actually that large. What happened instead is that the routers encountered the rule and then proceeded to consume all their RAM until they crashed.

In all cases, we run a monitoring process that reboots the routers automatically when they crash. That worked in a few cases. Unfortunately, many of the routers crashed in such a way that they did not reboot automatically and we were not able to access the routers' management ports. Even though some data centers came back online initially, they fell back over again because all the traffic across our entire network hit them and overloaded their resources.

Sam Bowne, a computer science professor at City College of San Francisco, used BGPlay to capture the following video of BGP sessions being withdrawn as our routers crashed:

Incident Response

CloudFlare's ops and network teams were aware of the incident immediately because of both internal and external monitors we run on our network. While it wasn't initially clear the reason the routers had crashed, it was clear that it was an issue caused by an inability for packets to find a route to our network. We were able to access some routers and see that they were crashing when they encountered this bad rule. We removed the rule and then called the network operations teams in the data centers where our routers were unresponsive to ask them to physically access the routers and perform a hard reboot.

CloudFlare's 23 data centers span 14 countries so the response took some time but within about 30 minutes we began to restore CloudFlare's network and services. By 10:49 UTC, all of CloudFlare's services were restored. We continue to investigate some edge cases where people are seeing outages. In nearly all of these cases, the problem is that a bad DNS response has been cached. Typically clearing the DNS cache will resolve the issue.

We have already reached out to Juniper to see if this is a known bug or something unique to our setup and the kind of traffic we were seeing at the time. We will be doing more extensive testing of Flowspec provisioned filters and evaluating whether there are ways we can isolate the application of the rules to only those data centers that need to be updated, rather than applying the rules network wide. Finally, we plan to proactively issue service credits to accounts covered by SLAs. Any amount of downtime is completely unacceptable to us and the whole CloudFlare team is sorry we let our customers down this morning.

Parallels to Syria

In writing this up, I was reminded of the parallels to the Syrian Internet outage we reported on earlier this year. In that case, we were able to detect as the Syrian government shut down their board routers and effectively cut the country off from the rest of the Internet. In CloudFlare's case the cause was not intentional or malicious, but the net effect was the same: a router change caused a network to go offline.

At CloudFlare, we spend a significant amount of our time immersed in the dark arts of Internet routing. This incident, like the incident in Syria, illustrates the power and importance of the these network protocols. We let our customer down this morning, but we will learn from the incident and put more controls in place to eliminate problems like this in the future.

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Post MortemBGPOutageDDoSAttacks

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Matthew Prince|@eastdakota

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