Cases2 cases

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1. IL-0354 Fly.io private networking (6PN) broke for some machines after edges received gateway /80 routes

   Fly.io2026

   Symptoms

   On 23 September 2026, private networking (6PN) stopped working for a subset of Fly.io machines. Fly.io mitigated it in about 10 minutes, and the problem came back briefly the following day.

   Network unreachableMisrouted requests

   Trigger

   Fly.io changed its 6PN routing so that edges act as gateways for non-static peers. Every edge was then treated as a gateway and got a /80 route for its prefix in the Linux routing table, which overrode the per-machine routing for machines that had kept a reused host ID.

   Root cause

   Faulty deployment

   Component

   Host routes and routing rulesNetwork control plane (SDN)

   Contributing factor

   Unverified assumptionGap in automated tests

   Remediation

   • Deleted the bad /80 routes from the routing table on all affected hosts
   • Rolled back the changes to s6pnforwarderd, the daemon that manages 6PN routes
   • Shipped a fix that adds interactive peers only as /128 routes, which did not prevent a brief recurrence the next day
   • Changed routing to always look first for machines that carry a host's ID, whatever kind of host it is

   Action items

   • Give the machines used to test and alert on 6PN non-local 6PN addresses
   • Make host IDs in 6PN addresses carry no weight in routing decisions, for gateway peers as well as machines
   • Move new peers and machines across the board to a better model than the current hack kept for existing gateway peers

   Lessons

   Not everyone at Fly.io knew that a worker's host ID could be reused by a host that is not a worker, because host ID assignment was not well documented, and the gateways had not been touched for a long time, so the problem stayed hidden until edges became gateways. All existing 6PN checks used machines on their original host, so they could not catch any problem with host ID reuse or stable 6PN addresses; host IDs need to stop influencing 6PN routing in general for this kind of incident not to recur.

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2. IL-0353 Fly.io edge proxies in US west coast regions were overloaded and flapping, with lazy loader delays of seconds

   Fly.io2026

   Symptoms

   On 28 May 2026, Fly.io's edges in US west coast regions began flapping, first mainly in SJC. Logs and metrics of fly-proxy, the userspace L4 and L7 load balancer on the edges, showed lazy loader latency of 500 ms to several seconds, so many new requests waited that long or longer, while proxy CPU usage and the inbound connection rate were not especially high.

   High latency

   Root cause

   Capacity exhaustion

   Component

   CDN and edge networkService discovery

   Contributing factor

   No capacity planningUnknown limitOverlapping failures

   Detection

   Fly.io noticed flapping in its US west coast regions, and proxy logs and metrics showed high lazy loader latency.

   Remediation

   • Deployed the proxy in SJC with a temporary mitigation for one app with very large state in Corrosion, which helped only briefly
   • Shifted Anycast traffic from SJC to LAX
   • Lowered Airtime's bandwidth limit, which reduced softirq CPU usage and host load average
   • Bounced the proxy and hard-killed all proxy processes pending shutdown, which cleared the lazy loader problem

   Action items

   • Provision significantly more edge capacity
   • Fixed the bug that let multiple proxy instances coexist
   • Changed Airtime so that, for now, it applies a much stricter limit when it is not the expected active proxy instance
   • Fixed load shedding in the lazy loader so request latency has a reasonable upper bound
   • Shard the in-memory service catalog and move to finer-grained locking, tested with Antithesis
   • Rework Airtime to react to overall system load instead of only the proxy
   • Look into better monitoring for when the proxy is not in its expected configuration

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