Cases3 cases

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1. IL-0268 AdGuard DNS resolvers in Amsterdam, Frankfurt and London overloaded and answered with timeouts and SERVFAIL

   AdGuard DNS2025

   Symptoms

   Between about 15:30 and 18:00 UTC on 25 November 2025 some AdGuard DNS users saw slow or failed DNS resolution, with timeouts and SERVFAIL responses. Alerts fired for increased DNS error rates and server load, and a significant part of the traffic in several European locations, primarily Amsterdam, Frankfurt and London, had intermittent DNS failures for up to 2.5 hours.

   DNS resolution failuresIntermittent timeoutsHigh latencyMemory pressure

   Trigger

   A new version of AdGuard DNS, deployed on 25 November to improve how resolvers cache user data, had a bug that saved only the recently changed subset of user data into the local cache file. The first deployment failed partway on a network issue and left about half the servers on the new version; the second attempt made many resolvers start full synchronizations at once.

   Root cause

   Faulty deployment

   Component

   Recursive DNS resolver serviceLanguage runtime settings

   Contributing factor

   Dependence on a warm cacheRetry stormHarmful manual actionGap in automated tests

   Detection

   Alerts for increased DNS error rates and server load fired during the second deployment attempt, between 15:30 and 16:00 UTC.

   Remediation

   • Rolled back the buggy version on all servers
   • Partially rerouted Amsterdam's traffic to other European locations
   • Reduced connection concurrency and request pipeline depth in Frankfurt and London, from 5 to 1 in Frankfurt
   • Adjusted limits to the actual hardware capacity
   • Repaired corrupted cache files and user data snapshots to stop further full synchronizations
   • Normalised the traffic distribution once the rollback was complete

   Action items

   • Add automated tests that the full user dataset is correctly written to and restored from the local cache file, including detection of empty or truncated caches
   • Add integration tests that simulate resolver startup with an empty or corrupted cache under realistic load
   • Standardize server hardware profiles per location, or keep separate configurations per hardware class
   • Review and adjust connlimit, cache sizes and connection limits for each standard node type
   • Reevaluate and lower pipelining depth where needed
   • Reevaluate how connections and goroutines are limited so resource use stays bounded with decent throughput after a connection break
   • Monitor Go heap size, GC time, GC cycles per second and per-cluster request latency distributions
   • Alert on sustained increases in SERVFAIL rate and request duration while upstreams are healthy
   • Refine DNS resolver deployment and rollback procedures to avoid simultaneous cache invalidation across a large part of the fleet
   • Document and rehearse playbooks for adjusting pipeline depth, connection limits and connlimit during live incidents

   Lessons

   How nodes behaved depended on the interaction between available RAM, Go's garbage collector and the connlimit setting. On 64-GiB nodes a high limit let the heap grow large under heavy concurrency and pipelining, and the garbage collector then spent more and more CPU, a known worst case for high-memory applications in which memory is not exhausted but latency rises and throughput falls. On 32-GiB nodes the same configuration reached practical memory limits sooner because it had not been scaled down. Together with high pipelining and client retries, GC pressure, connection saturation and retries formed a feedback loop.

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2. IL-0340 Cloudflare's 1.1.1.1 resolver returned SERVFAIL to some valid DNS queries for four hours

   Cloudflare2023

   Symptoms

   From 07:00 to 11:00 UTC on 4 October 2023, some users of Cloudflare's 1.1.1.1 DNS resolver received SERVFAIL responses to valid queries. This reached users of WARP and Zero Trust and of third-party DNS resolvers that use 1.1.1.1, and the first external reports of unexpected SERVFAILs arrived at 7:57 UTC.

   DNS resolution failuresHigh error rate

   Trigger

   On 21 September 2023, as a planned change in root zone management, a ZONEMD resource record, in effect a checksum of the zone's contents, was added to the root zone for the first time. On 4 October at 07:00 UTC the DNSSEC signatures in the last root zone copy that 1.1.1.1 had loaded expired.

   Root cause

   Software defect

   Component

   Recursive DNS resolver service

   Contributing factor

   Cache-masked failureUnsafe failure behaviourMonitoring blind spotGap in automated tests

   Detection

   External reports of unexpected SERVFAILs started coming in at 7:57 UTC, and Cloudflare declared an internal incident at 8:03 UTC. Nothing had alerted on static_zone serving a stale root zone during the two weeks before.

   Remediation

   • Deployed an override rule meant to stop 1.1.1.1 from answering with the stale root zone, which did not cover forwarded queries
   • Stopped 1.1.1.1 from preloading the root zone file entirely

   Action items

   • Add alerts for when static_zone serves a stale root zone file
   • Re-evaluate how the root zone is ingested and distributed internally so new record types are handled
   • Improve test coverage for what happens when the root zone fails to parse
   • Limit how long stale copies of the root zone can be used, following RFC 8806

   Lessons

   Cloudflare's main lesson is never to assume an input will not change. Modern systems pull in long chains of libraries that may not be ready when their input changes, so they need tests that catch regressions and components that fail gracefully on new input, and format changes in DNS and BGP should be expected to have an impact. Cloudflare also writes that with better monitoring of the stale root zone, the existing caching would have prevented any impact.

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3. IL-0250 slack.com stopped resolving on some DNSSEC-validating resolvers for about 24 hours after DNSSEC signing was turned on

   Slack2021

   Symptoms

   Soon after Slack published the DS record for slack.com at its DNS registrar on 30 September 2021, the Customer Experience team started getting user reports of "ERR_NAME_NOT_RESOLVED" errors in Slack clients. Fewer than 1% of online users were affected, and most reports came from users of one private corporate recursive resolver and of Google Public DNS (8.8.8.8).

   DNS resolution failures

   Trigger

   On 30 September 2021 the Traffic Engineering team enabled DNSSEC signing on the slack.com zone in Amazon Route 53 and on its sub-delegated zones in NS1, and after a three-hour soak published the slack.com DS record at the DNS registrar.

   Root cause

   Software defect

   Component

   Authoritative DNS servers and recordsRecursive DNS resolver serviceTerraform

   Contributing factor

   Unrepresentative stagingUnverified assumptionHarmful manual actionIneffective rollbackLimited debug access

   Detection

   Customer Experience received user reports of "ERR_NAME_NOT_RESOLVED" errors in Slack clients, and customers sent NetLog captures through support cases. After signing was rolled back, Slack's external DNS and DNSSEC resolution probes alerted the Traffic team.

   Remediation

   • Removed the slack.com DS record from the DNS registrar
   • Rolled back DNSSEC signing on the slack.com authoritative and delegated zones, which made resolution worse
   • Asked major ISPs and public DNS resolver operators to flush their cached slack.com records
   • Flushed the slack.com records on Google's 8.8.8.8 and Cloudflare's 1.1.1.1 through their public flush sites

   Lessons

   The DNSSEC rollout had been tested only on domains without a wildcard record, so the Route 53 wildcard NSEC bug first appeared on slack.com; Slack names not testing a wildcard domain first as an oversight. Removing the DS record at the registrar does not stop validation at once, because resolvers cache the DS record from the .com zone for up to 24 hours, and removing the DNSKEY while that record is still cached makes every validating resolver fail. Managing DNS through Terraform hid the console warning about disabling DNSSEC. Re-enabling signing would neither have helped nor hurt, because Route 53 generates a new ZSK each time signing is enabled.

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