Cloudflare Linux, Networking & Scripting interview questions

Ordering and retransmission are what TCP adds, and both of them cost time. Naming something that deliberately gives them up, and saying why, is what turns a textbook answer into a real one.

The question that separates people who have debugged a hung rollout from people who have read about one. The interesting half is not the signal, it is PID 1.

A staple of every networking round, and the follow-up is always about TLS termination. Knowing which layer sees what decides most of the answer.

The oldest question in systems interviewing and still the best, because it is unbounded: every layer you can describe is another layer you could debug.

The question that reveals whether someone knows what load average actually counts on Linux. It is not a CPU metric, and that single fact resolves the whole thing.

The classic ephemeral port exhaustion question. TIME_WAIT is a familiar phrase and the useful facts are narrower: which side accumulates it, and why the popular fix is dangerous.

The scripting round in disguise. The constraint is the question: it exists to see whether you stream or whether you load, and everything else is about whether the script is safe to run twice.

Interviewers use this to sort memorised trivia from people who have chased a slow lookup. Three roles, one bit, and the caching rules most candidates repeat incorrectly.

The flags are the easy part. The scored part is proving you can capture on a saturated box without adding load, then read the file afterward like evidence.

The signature is unmistakable once seen: handshakes complete, then the connection freezes at the same byte count every time. Here is the probe sequence and the fixes, ranked.

Latency fine, loss low, link idle, yet a single copy takes hours. The arithmetic of bandwidth-delay product settles this question before any tool opens.

Five commands separate people who have actually built this from people who read about it once. Here is the working sequence, plus the gotchas that break it.

One round trip versus two is the headline, but the ops-relevant differences hide in what stays plaintext, how resumption works, and which middlewares the change broke.

tcpdump copies packets to userland; eBPF aggregates inside the kernel next to the event. Know which hook runs where, what a verifier log is telling you, and when XDP drops for free.

One CPU doing all the packet work is a topology failure, not a tuning mystery. RSS queues, MSI-X vectors, local_cpulist and irqbalance: the whole chain from symptom to pin.

Most throughput tests measure the tester, not the network. Single stream versus parallel, slow start, sender-side retransmits, and the CPU ceiling: how to produce a number you can act on.

Loss-based versus model-based congestion avoidance decides whether your WAN transfers fill the pipe or die on the first stray drop. Here is the operator's decision, not the RFC summary.

X-Forwarded-For, PROXY protocol v1 and v2, and the trust boundaries between them. Get the layer wrong and your rate limiting, geo rules and audit trails all key off the balancer's address.

Anycast turns BGP convergence into your failover mechanism. Sessions stick to the site they landed on, monitoring must go per-site, and a route you cannot withdraw cleanly is worse than DNS.

Five stages, and which one you are stuck in tells you what to fix. A machine that reaches a GRUB prompt has a different problem from one that hangs waiting on a filesystem that no longer exists.

The arithmetic takes thirty seconds to learn and gets asked constantly. What it is really testing is whether you have ever had to size a VPC that could not be resized later.

Most of what looks like used memory on a healthy Linux box is page cache doing its job. Knowing which column to read, and why df and du disagree, is half of Linux triage.

A growing backlog of sockets waiting for the application to close its side usually points at lifecycle handling. Here is the hunt from ss output to the pool timeout that caused it.

Most hand-written units boot in dev and die in production. The gap is Type selection, restart policy, and the drop-in workflow vendors expect you to respect.

A one-line mistake that locks you out of the obvious fix. The fast recovery teaches more about the permission model than any man page will.