Linux & Networking Interview Questions for DevOps

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 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.

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.

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.

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.

Two different resources produce the same errno, and the second one is invisible unless you know to look. One command distinguishes them.

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.

A beginner question with a senior follow-up hiding in it. Once you know a hard link is a second name for an inode, log rotation and backup behaviour both stop being mysterious.

Everyone recites the four letters. The scoring answer walks each packet across the wire and knows the places the exchange silently dies.

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 two-syscall duet behind every process on the box. The follow-ups live in what survives the transition, and candidates who skip that fail the next question.

A routine cloud operation with three layers that can quietly disagree. The strong answer is an ordered procedure you could run during business hours, without a reboot.

The answer is yes, and then the real question starts. Asymmetric routing and silent drops punish anyone who treats the routing tables as one flat list.

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.

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.

Two authentications happen in every SSH session, and the one usually left out is the server proving itself to you. Knowing who proves what, and where your agent socket ends up, is the scored material.

Every ops engineer schedules jobs; few can say which shell cron uses, where stdout lands, or why percent signs delete half a command. Those gaps are exactly what interviews test.

The swap-used graph is the least informative memory metric on the box. Here is what swappiness actually weights, which numbers to watch instead, and the policy worth defending.

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.

Everything top, htop and your APM agent display is derived from procfs anyway. Knowing the raw files turns any bare SSH session into a monitoring console.

Enterprise loops pair these topics because they test L2 literacy plus vendor coordination together. The scored insight is that LACP spreads flows, not packets, and needs the switch to agree.

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.

Everyone knows ulimit -n. The interview question is which process inherits which value, why limits.conf did nothing for your service, and how you size the ceiling honestly.

SIGKILL writes no stack trace and the application cannot defend itself. The verdict is already sitting in the kernel ring buffer, and reading it precisely is the skill being priced.

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.

Table full is the symptom everyone knows. The interview answer lives one layer down: per-state timers, the five-day ESTABLISHED default, and what conntrack -S says about hash pressure.

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.

Multiplexing removes application-level queuing and hands the problem to TCP. Here is where the blocking really lives, the loss math behind it, and the four levers that help.

A reset at a suspiciously round number is a fingerprint, not a bug. Matching client keepalives to the lowest timer in the path is the fix; here is the full ladder of knobs.

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.

The answer that lands is about one epoll loop per core, not config snippets. Master versus worker duties, the arithmetic of worker_connections, and the limits people forget to raise.

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.

A and AAAA are the easy part. The bites live at the zone apex, inside 255-byte TXT chunks, in CAA blocking your renewals, and in SRV records half the internet ignores.

Patching is easy; surviving the rollout is the interview. Staged rings with health gates, what livepatching can and cannot buy you, and the rollback plan you write before starting.

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.

Sets instead of thousand-rule chains, one engine for both IP families, atomic ruleset swaps, and a compatibility shim with sharp edges. Migration risk lives in coexistence and tooling assumptions, not syntax.

Volatile by default on many distros, size-capped by silent policy, binary on disk. Know the storage modes, the vacuum knobs, and how journald feeds central logging.

Copy-on-write freezes a point-in-time view while the database keeps writing. Fat snapshots overflow into invalid backups and thin pools exhaust into frozen volumes: size both deliberately.

Absolute caps versus latency-target protection, systemd IO attributes versus raw io.max, and why ionice lies to you. The staff-level answer assigns the job its own slice with intent.

Transparent versus explicit hugepages is a control question, not a size question. Fork-heavy stores suffer THP copy-on-write amplification while large-resident databases gain TLB reach.

VRRP heartbeats, market data feeds, mDNS noise and cluster chatter all ride multicast, and all of it assumes a switched LAN. Why clouds drop it and what replaces it.