Cisco Routers Disrupted by Cloudflare DNS Shift

Key Points:

A recent change in DNS records by Cloudflare caused many Cisco routers to go offline, highlighting the fragility of some enterprise network operations.
The issue was caused by a small coding change that was consistent with industry standards, but exposed weaknesses in Cisco’s DNS client implementations.
Analysts warn that this type of incident is likely to happen again due to the mismatch between the speed of cloud providers and the slowness of hardware vendors.

A recent incident involving Cloudflare and Cisco routers has brought attention to the fragility of some enterprise network operations. According to sources, a small coding change made by Cloudflare, which was perfectly consistent with industry standards, caused many Cisco routers to go offline. This change, which affected the ordering of DNS records, was rolled out globally and resulted in Cisco routers entering fatal reboot loops. Analysts say that this incident highlights the fragility of some enterprise network operations, particularly when it comes to DNS client implementations.

Robert Kramer, vice president/principal analyst for Moor Insights & Strategy, explained that the issue was caused by Cisco’s DNS client implementations, which assumed a certain sequence of DNS records instead of parsing the full response. This assumption, which is common in infrastructure gear, led to Cisco routers crashing with core dumps. Kramer noted that this incident is not a Cisco mistake, but rather a result of the common assumptions made by many DNS clients.

Networking consultant Yvette Schmitter, CEO of the Fusion Collective consulting firm, said that the Cloudflare change exposed Cisco’s architectural fragility. She explained that Cisco’s firmware was unable to handle the change in DNS record ordering, leading to fatal reboot loops. Schmitter also noted that Cisco has not released a public advisory or patch to address the issue, leaving enterprises to implement workarounds that disable DNS functionality on network infrastructure.

Analysts warn that this type of incident is likely to happen again due to the mismatch between the speed of cloud providers and the slowness of hardware vendors. Sanchit Vir Gogia, chief analyst at Greyhound Research, noted that cloud providers optimize for deployment speed and uptime percentages, rather than failure isolation. This can lead to complex systems that are prone to unforeseen failures.

The incident has also highlighted the importance of infrastructure reliability and DNS behavior. Kramer noted that traditional monitoring tools are not built to catch issues like this, and that health checks may not detect problems until it’s too late. He recommended that enterprises limit direct external DNS lookups from embedded or edge devices and route them through internal resolvers that can normalize responses.

Gogia also warned that fixing this problem may not be simple, and that secondary DNS redundancy may not be enough to guarantee protection. He noted that diversity is key, and that enterprises should consider routing DNS through internal resolvers or forwarders to insulate themselves from behavioral changes. This incident serves as a reminder of the importance of planning for the long term and architecting for resilience, particularly when it comes to Microsoft Azure and Windows Server environments, which rely heavily on DNS and cloud infrastructure. By prioritizing infrastructure reliability and DNS behavior, enterprises can reduce the risk of downtime and data loss, and ensure that their Microsoft-based systems remain secure and stable.