Debug systemd-resolved by link, search domain, and server

For systemd-resolved troubleshooting, the safest approach is a bounded operational change, not a command pasted without context. This runbook starts with effective state, shows the smallest candidate action, and finishes by repeating the real user or system path.

TL;DR: Systemd-resolved can maintain per-link DNS servers and routing domains, so /etc/resolv.conf alone may not describe the effective lookup path. Capture a baseline with resolvectl status, make the reviewed change only when the evidence matches, then verify with resolvectl status eth0 and keep the rollback ready.

Audience: operators who understand IP addresses and ports and can capture bounded network evidence without collecting payloads unnecessarily. This guide assumes familiarity with Linux networking and a change window appropriate to the system.

The direct answer

The intended link and routing domain select the expected server and return a consistent answer. That is the success condition for systemd-resolved troubleshooting; command completion by itself is not enough.

The important boundary is name resolution, route selection, neighbor reachability, firewall policy, transport state, and application listener. Systemd-resolved can maintain per-link DNS servers and routing domains, so /etc/resolv.conf alone may not describe the effective lookup path. If an observation does not identify which side of that boundary failed, collect a narrower observation before changing state.

How the mechanism works

For systemd-resolved troubleshooting, use this mental model: a connection moves from a name to an address, through route and policy lookup, into a transport socket, and finally to the process bound to the selected address and port. The model prevents a common mistake—treating configuration text, control-plane acceptance, process state, and end-user behavior as the same proof.

Follow four stages:

1. Observe: identify the exact host, object, version, owner, and active configuration.
2. Interpret: write the expected result, the abnormal result, and what would remain inconclusive.
3. Change: apply one reviewed action at the narrowest layer that contradicts the baseline.
4. Verify: repeat the original path and compare the same evidence, including adjacent safety controls.

Preflight and safety boundary

Preserve a second management path before changing routes, resolver state, interfaces, or firewall rules, and redact payloads and private topology from captures.

Before systemd-resolved troubleshooting, record UTC time, the current version or digest, the exact target, recent changes, and who owns the workload. The rollback for this runbook is: revert the per-link DNS setting or restart the owning network manager, then verify public and private lookup paths.

Do not continue if the target identity is ambiguous, the current state cannot be saved, the only recovery session would be at risk, or the proposed command affects more objects than the brief names.

Capture the read-only baseline

Run these commands one at a time. Replace example names and addresses deliberately; do not paste production secrets into a transcript.

resolvectl status
resolvectl query example.com
readlink -f /etc/resolv.conf
resolvectl statistics

Interpret the baseline before moving on:

• Expected: the intended link and routing domain select the expected server and return a consistent answer.
• Abnormal: a VPN or interface owns a more specific routing domain, the stub file is replaced, or upstream DNSSEC fails.
• Inconclusive: missing output can also mean the wrong context, permissions, namespace, log window, binary, or target. Prove those assumptions before treating absence as health.

Save the decisive output, exit status, and timestamp. Redact credentials, customer data, private topology, tokens, and complete environment dumps.

Apply the smallest candidate change

The following is state-changing example syntax, not an instruction to run it unchanged:

sudo resolvectl dns eth0 192.0.2.53

For systemd-resolved troubleshooting, the proposed change is acceptable only when the read-only baseline predicts its effect and the rollback is available. The key risk is: overriding global DNS can leak private names or bypass split-DNS policy.

Prefer an immutable artifact, validated configuration, dry-run, transaction, candidate object, or staged target when the tool supports one. Record the exact command and UTC time so later telemetry can be correlated to the change.

Verify the result from the outside in

resolvectl status eth0
resolvectl query example.com
dig example.com

Verification for systemd-resolved troubleshooting has three layers:

1. The control plane or command reports the intended effective state.
2. The process, resource, or data path reflects that state without a new pressure signal.
3. The original user-visible or dependent-system path succeeds from an independent vantage point.

If resolvectl status eth0 succeeds but the original path still fails, stop. The change may have repaired a local symptom while DNS, policy, routing, caching, dependency, or client state remains broken.

Failure branches

The baseline does not match this runbook

When a VPN or interface owns a more specific routing domain, the stub file is replaced, or upstream DNSSEC fails, do not force the candidate command. Return to identity and scope, compare a healthy peer only through effective settings, and name a new falsifiable mechanism.

The change succeeds but behavior does not

A successful sudo resolvectl dns eth0 192.0.2.53 proves that one interface accepted a request. It does not prove convergence, readiness, data compatibility, external routing, or client recovery. Re-run the same evidence at each downstream boundary.

The change makes the system worse

Execute the written rollback: revert the per-link DNS setting or restart the owning network manager, then verify public and private lookup paths. Preserve the failed candidate and relevant logs long enough to explain the outcome; do not destroy the evidence with broad cleanup or repeated restarts.

Operator checklist

• Confirm the exact target, context, identity, version, and active owner.
• Capture the read-only baseline and one disconfirming observation.
• Label sudo resolvectl dns eth0 192.0.2.53 as state-changing during review.
• Keep recovery access and rollback independent of the path being edited.
• Change one layer, record UTC time, and wait for its real convergence boundary.
• Verify the original path, adjacent controls, resource pressure, and persistence.
• Update the runbook when observed behavior differs from the source-reviewed model.

Investigate it in Tryssh

$ resolvectl status eth0
Expected: the intended link and routing domain select the expected server and return a consistent answer.

Tryssh can preserve this evidence loop and show a state-changing command for human approval. It does not make the operator's identity, recovery access, rollback, or platform authority decisions.

Evidence and review status

This systemd-resolved troubleshooting runbook was source-reviewed on 2026-07-29 against current first-party documentation. The commands are illustrative and use example targets. The page does not claim that the change was reproduced across every distribution, managed service, version, network, or workload.

Limitations and trade-offs

Overriding global DNS can leak private names or bypass split-DNS policy. Managed platforms may generate configuration, restrict privileges, replace local state, or expose a different control plane than the upstream project. Confirm the installed version and provider contract before applying a repair.

Search visibility is not proof of operational correctness. Treat this page as a decision aid, preserve independent recovery, and stop when the evidence contradicts its assumptions.

Related operator runbooks

Continue with curl --resolve vs --connect-to, ip route get, the Linux networking foundation guide, and the SSH hardening checklist.

Sources and further reading

• Systemd-resolved troubleshooting first-party reference
• Linux kernel networking documentation
• ss manual
• ip-route manual