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Start by proving where the failure is. From a network independent of the server, test DNS, TCP ports, HTTP, and SSH. If only the website fails, inspect the web stack; if every port fails, use your provider’s serial, KVM, or virtual console before rebooting. Preserve logs and memory evidence first, then classify the incident as a service failure, resource exhaustion, boot problem, kernel crash, or infrastructure outage.
1. Establish whether the website, server, or network failed
A browser error does not prove that Linux crashed. DNS may point to the wrong address, a firewall may block one port, a reverse proxy may be unhealthy, or the host may be completely offline. Test from a different network or monitoring system so that a local router, VPN, or cached DNS result does not hide the actual boundary.
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Run independent reachability checks
- Resolve the hostname:
dig +short example.comornslookup example.com. Compare the returned address with the server address documented for the site. Check both the apex name andwwwname if both are used. - Test the web ports:
nc -vz example.com 80andnc -vz example.com 443. A refused connection means the host answered but no process accepted the connection; a timeout can indicate filtering, a dead host, or a broken route. - Request headers without downloading the whole page:
curl -I --connect-timeout 10 https://example.com/. Record the HTTP status, redirect target, TLS error, and response time. - Test administration separately:
nc -vz server-address 22and then an SSH attempt with verbose output,ssh -vvv user@server-address. Do not repeatedly retry credentials during an incident; rate limits and account lockouts can create another problem. - Check a second service, such as a health endpoint, DNS server, mail port, or provider monitoring panel. If HTTP, SSH, and other ports fail together, move to out-of-band access.
Save the exact time in UTC, source network, resolved addresses, command output, and screenshots. Also check the hosting provider’s status page and instance events for a maintenance action, hypervisor failure, attached-volume problem, or network policy change.
| Observed result | Likely boundary | Next action |
|---|---|---|
| DNS returns the wrong or no address | DNS or domain configuration | Check authoritative records, nameserver delegation, TTL, and recent DNS changes before touching Linux. |
| DNS works; TCP 80/443 fails; SSH works | Firewall, listener, reverse proxy, or web service | Inspect listening sockets, firewall rules, proxy status, and web-service logs. |
| HTTP fails; SSH and local services work | Application or web stack | Check the service unit, configuration validation, dependencies, certificates, and recent deploy. |
| SSH and HTTP both time out | Host, route, provider network, or kernel hang | Use serial/KVM/provider console and preserve evidence before rebooting. |
| Provider console is also unavailable | Infrastructure, power, hypervisor, or storage failure | Open a provider incident with timestamps and independent test results; avoid blind reboot loops. |
2. Preserve evidence before restarting
A reboot can restore availability while erasing the only clues to the cause. It may also overwrite volatile kernel messages, terminate a process whose memory state would explain an outage, or make the previous boot impossible to inspect when journald was not persistent. If customers are affected, document the first observed failure and the business decision to restore service, but capture what you can first.
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Make a short incident record
- First known failure time and the time each symptom was observed.
- Independent DNS, TCP, HTTP, and SSH results, including source locations.
- Provider events, instance status, console screenshots, and any scheduled maintenance.
- Recent deploys, configuration edits, package upgrades, kernel changes, certificate renewals, and traffic or workload changes.
- Current boot ID, previous boot ID if available, service status, kernel messages, memory, swap, disk, and inode readings.
- Location of any crash dump,
vmcore, or files under/var/crash.
If the host is reachable through SSH or a console, collect read-only evidence before changing services. Redirect command output to a timestamped directory on a filesystem with free space. Do not run destructive cleanup, rotate logs manually, or remove a suspected core file until it has been copied.
3. Use the console when SSH does not respond
SSH depends on the network stack, firewall, sshd, authentication, and a functioning scheduler. A failed SSH connection therefore does not distinguish a crashed kernel from a stopped daemon. A serial console, KVM, cloud emergency console, or provider rescue environment bypasses some of those layers.
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- Open the provider or physical console and record what is visible: login prompt, boot messages, kernel stack trace, emergency target, filesystem errors, or a completely frozen screen.
- If a shell is available, check whether the system is merely busy:
date; uptime; w; ps -eo state,pid,ppid,comm,wchan:32 --sort=state | head -40. Processes in uninterruptible sleep (D) alongside storage errors suggest an I/O stall rather than a normal web-service failure. - Capture
dmesg -Twhere permitted,free -h,vmstat 1 5,df -h, anddf -ih. Save output before attempting recovery. - Check whether the machine is accepting local commands, whether the root filesystem is read-only, and whether the system clock is plausible. A full filesystem can prevent services, logging, and SSH from starting.
- Only after evidence is collected, choose a graceful service restart, a controlled reboot, or provider escalation. A forced power cycle is a last resort because it can damage filesystems and eliminate volatile evidence.
4. Read systemd and journald at the failing layer
Begin with the service that should answer the failed request, then work down through dependencies. Replace the example unit with the actual name used by the distribution.
Current-boot and service-specific commands
systemctl status nginx.service(or the relevant unit) shows load state, recent exit status, the main process, and a short journal excerpt.journalctl -bdisplays messages from the current boot. systemd’s boot-debugging guidance recommends this view when investigating startup failures.journalctl -p err -bfilters the current boot to error and higher priorities. Treat it as a triage filter, not proof that every listed error caused the outage.journalctl -u nginx.service -bnarrows output to one service in the current boot.journalctl --since "2026-10-15 12:00" --until "2026-10-15 13:00"limits output to the incident window; use the actual UTC or local times consistently.systemctl --failedlists failed units, whilesystemctl list-dependencies --reverse nginx.servicehelps identify a dependency that prevented startup.ss -lntupshows which processes are listening. Compare the bound address and port with the firewall and proxy configuration.
Validate configuration before restarting. Typical examples are nginx -t, apachectl configtest, or the application’s documented validation command. A clean syntax check does not prove that a database, certificate, mount, secret, or upstream endpoint is available.
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Make journald survive a reboot
On systems using volatile storage, the previous boot’s journal disappears at shutdown. Create the persistent journal directory and flush the journal before an incident:
sudo mkdir -p /var/log/journal
sudo systemd-tmpfiles --create --prefix /var/log/journal
sudo journalctl --flush
Configure retention and monitor available space according to the distribution’s policy. Red Hat’s RHEL 10 recovery guidance documents persistent storage and journal filtering; settings can differ between releases, so check the installed systemd and distribution documentation rather than copying a retention value blindly.
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Run journalctl --list-boots. Boot 0 is the current boot, -1 is the immediately previous boot, and lower numbers are older boots. Then inspect a prior boot, for example journalctl -b -1 -p err, and review shutdown records with last -Fxn2 shutdown reboot. Red Hat cautions that previous-boot journal analysis is unavailable if persistence was not configured in advance. A missing prior boot is evidence of missing retention, not proof that a clean shutdown occurred.
5. Distinguish a failed service from a failed dependency
Web servers commonly depend on sockets, certificates, mounted filesystems, DNS, databases, queues, and upstream application workers. A service may be “active” while returning errors, or “failed” because a dependency failed first.
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Check the complete request path
- Confirm the expected listener with
ss -lntupand inspect the unit’s effective settings usingsystemctl cat service.service. - Check recent unit transitions:
journalctl -u service.service --since "30 min ago", including exit codes, restart loops, timeout messages, and permission errors. - Inspect mounts with
findmntand capacity withdf -handdf -ih. A full inode table can look like an application failure even when gigabytes remain. - Test local upstreams directly, such as
curl http://127.0.0.1:PORT/health, to separate proxy routing from application failure. - Check certificate validity and clock drift when TLS handshakes or signed tokens fail.
When a configuration or dependency is fixed, restart only the affected unit, watch its journal in a second console with journalctl -fu service.service, and verify the public request from an independent network. Do not repeatedly restart a unit in a loop; restart storms can hide the first error and increase load.
6. Check memory, swap, and other resource exhaustion
A host that exhausts physical memory and swap can become extremely slow because of reclaim and I/O pressure, stop responding to SSH, or invoke the kernel’s out-of-memory (OOM) killer. OOM messages often identify the process terminated, but the killed process is not necessarily the original leak or workload that consumed memory.
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Also record load average, disk latency, filesystem capacity, and container or cgroup limits. In a container, the host may have free RAM while the container’s memory limit is exhausted; inspect the applicable cgroup metrics and orchestrator events.
Choose a remedy based on the cause
- Reduce concurrency or workload when a legitimate traffic spike exceeds capacity.
- Fix a leak or runaway job when one process grows without bound; capture its configuration and version before restarting it.
- Increase RAM when the normal workload genuinely needs more working memory.
- Add swap only with an understanding of latency and storage behavior. Swap can provide an emergency buffer but is not a substitute for adequate RAM, and heavy swapping can make an already unhealthy host less responsive.
- Set service-level limits and alerts so a single worker, batch job, or container cannot consume all available memory.
After recovery, correlate the OOM timestamp with deploys, traffic, scheduled jobs, and application metrics. Preserve the earliest memory-pressure lines and surrounding journal context rather than copying only the final “killed process” line.
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7. Recover a system that fails during normal startup
If the kernel boots but the machine hangs or repeatedly fails while services start, use the console to enter a reduced systemd target. systemd’s recovery guidance recommends systemd.unit=rescue.target (also called runlevel 1) for this class of problem.
- At the boot loader, edit the Linux entry and append
systemd.unit=rescue.targetto the kernel command line for one boot. Use the provider console, not SSH. - Boot the entry and authenticate at the rescue shell. The environment intentionally starts fewer services.
- Read
journalctl -b,systemctl --failed, and the prior-boot journal if it exists. Identify the first failing unit and its dependency chain. - Correct the specific configuration, mount, permission, package, or dependency problem. Make a backup of edited files and validate syntax.
- Return to the normal target only after the failure is understood, then watch the journal while services start and verify the site externally.
If the root filesystem is damaged or cannot be mounted, stop at the rescue environment and use the provider’s recovery workflow or a trusted rescue image. Do not run filesystem repair against a mounted production filesystem.
8. Investigate a kernel panic or hard crash
A kernel crash dump is volatile RAM copied to disk when kernel execution is disrupted. Canonical’s Ubuntu documentation lists kernel panic, non-maskable interrupt, machine-check exception, hardware failure, and manual intervention among documented triggers. A panic differs from an ordinary service crash: user-space logs may stop abruptly, the console can show a stack trace, and the machine may reboot without a clean shutdown record.
Verify crash-dump readiness
- Run
kdump-config showon Ubuntu and inspectsystemctl status kdump-tools. - Confirm that the kernel command line reserves crash memory (the
crashkernelsetting) and that the configured dump destination has enough free space. - Look for crash artifacts under
/var/crashand verify their timestamps and ownership. Ubuntu notes that an unready state can result from insufficient reserved memory. - Preserve the console trace, previous-boot journal, kernel version, loaded modules, hardware or hypervisor events, and the complete dump path before attempting cleanup.
Do not trigger a test crash over SSH. Ubuntu warns that SysRq crash testing reboots the machine, can cause data loss under load, and disconnects SSH; testing belongs in an approved maintenance window on an idle or lightly loaded system with local or serial console access, backups, and a recovery plan. In particular, never run echo c > /proc/sysrq-trigger on a production host merely to see whether crash capture works.
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For unexplained recurring panics, compare kernel and firmware changes, hardware-machine-check records, storage errors, virtualization events, and the crash-dump analysis from the distribution’s supported tooling. A single panic line rarely establishes causality; the surrounding trace and environment matter.
9. Decide whether to reboot, restore, or escalate
| Recovery choice | Use when | Evidence and risk |
|---|---|---|
| Restart one service | A unit or application is isolated and the host is healthy. | Preserves most host evidence; may interrupt active requests. |
| Controlled reboot | The host is responsive but unrecoverably wedged at the service or resource layer. | Can restore service, but loses volatile state; capture journals and metrics first. |
| Rescue target | Normal startup fails because of a unit, mount, or configuration dependency. | Preserves the installed system while limiting services; requires console access. |
| Provider snapshot or restore | Filesystem or configuration damage is known and a recent, tested recovery point exists. | Can shorten downtime but may discard writes after the snapshot; verify data consistency. |
| Provider or specialist escalation | Console is unavailable, hardware or hypervisor events are suspected, or a kernel dump needs expert analysis. | Provide timestamps, boot IDs, logs, dump paths, and actions already taken; avoid repeated destructive attempts. |
The fastest apparent fix is not always the safest. A forced reboot can conceal a hardware, storage, or memory fault that will recur. If uptime is restored before the cause is known, keep the incident open, preserve the collected evidence, and schedule root-cause work.
10. Verify recovery instead of trusting a green status page
- Confirm the expected processes and listeners with
systemctl --failedandss -lntup. - Request the site from an independent network over both HTTP and HTTPS. Check redirects, TLS, representative dynamic pages, authentication, and a write path if safe.
- Exercise dependencies: database connectivity, queues, object storage, mounted volumes, and background workers.
- Watch logs and resource graphs for at least one normal traffic cycle. A service that starts and immediately enters a restart loop is not recovered.
- Record the recovery time, commands run, configuration changes, data validation, and remaining uncertainty in the incident record.
11. Prevent the next unexplained outage
- Persistent logs: Enable journald storage in
/var/log/journal, set retention appropriate to disk capacity, and verify thatjournalctl --list-bootsshows prior boots after a controlled restart. - Actionable monitoring: Alert on HTTP status and latency, DNS changes, SSH reachability, memory and swap pressure, disk and inode utilization, failed systemd units, and unexpected reboots. Alerting must reach a system independent of the server.
- Recovery points: Keep tested backups and provider snapshots. A backup that has never been restored is an assumption, not a recovery plan.
- Out-of-band access: Document serial, KVM, rescue, and provider-console paths, including who owns credentials and how emergency access is approved.
- Crash capture: Configure kdump where the distribution and hardware support it; reserve adequate crashkernel memory and dump storage, and verify readiness without inducing a production crash.
- Runbooks: Practice a maintenance-window exercise covering reachability tests, evidence collection, rescue boot, restoration, and customer communication.
- Change discipline: Record kernel, package, firewall, DNS, certificate, and application changes with timestamps so an outage can be correlated quickly.
A useful runbook makes the first five minutes repeatable: establish the failure boundary, open the console if necessary, preserve logs and resource data, classify the failing layer, then select the least destructive recovery action. That sequence gives you a better chance of restoring the website without sacrificing the evidence needed to stop the crash from returning.
Frequently Asked Questions
How can I tell whether Linux rebooted or actually crashed?
Use persistent journald and run journalctl --list-boots to compare boot IDs, then check journalctl -b -1 and last -Fxn2 shutdown reboot. A clean shutdown record suggests an orderly reboot; an abrupt end, panic trace, OOM messages, or a crash dump points toward a hard failure. Without persistent logs, the previous boot may not be recoverable.
What should I do first when SSH is completely unavailable?
Test DNS and TCP reachability from an independent network, then use the provider’s serial, KVM, or emergency console. Capture the console state and any available logs before rebooting. Do not assume an SSH timeout alone proves that the kernel crashed.
Where are Ubuntu kernel crash dumps stored?
Ubuntu documents crash artifacts under /var/crash. Check kdump-config show, systemctl status kdump-tools, crashkernel reservation, available dump storage, and the artifact timestamp.
Can adding swap fix an OOM outage?
Swap can provide additional virtual-memory capacity, but heavy swapping introduces latency and may leave the host unresponsive. Identify the consuming workload first; reduce it, fix leaks, or add RAM when appropriate, and use swap as a deliberate capacity decision rather than a universal cure.
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