ansible "Host key verification failed" in automation: how to fix

TL;DR

"Host key verification failed" means the target's SSH host key is not in the automation user's knownhosts, and non-interactive ansible cannot answer the prompt. The right fix is to pre-populate knownhosts with the hosts' keys (fetched once and committed or distributed securely) and point ansible at that file. Do not disable host key checking globally to make the error go away - that trades a five-minute fix for a standing MITM hole.

ansible "Host key verification failed" in automation: how to fix

Steps

1. Confirm the failure is host-key verification and not authentication: the error names the host key, not a password or key problem. Expected: the message is exactly "Host key verification failed", so you know it is the known_hosts lookup.

2. Fetch the target host's public host keys once from a trusted network. Expected: you have the host's key lines ready to install; verify the fingerprint against your inventory or console if this is the first contact.

3. Install those keys into the knownhosts file of the user that runs ansible (often a CI service account, not your login). Expected: the automation user's knownhosts contains the target host entries.

4. Point ansible at that known_hosts file via its SSH arguments or inventory config, and re-run the playbook. Expected: the SSH handshake verifies the host key silently and the playbook proceeds past the connection step.

5. For fleets, distribute a managed known_hosts file with your provisioning or store per-host keys in inventory instead of fetching at runtime. Expected: new hosts work on first run with no manual key acceptance anywhere.

Use this when

• ansible fails with "Host key verification failed" on fresh or rebuilt hosts
• playbooks run from CI runners or service accounts with empty known_hosts
• host keys rotated after a rebuild and old entries no longer match

Not for this skill when

• SSH fails with permission denied or authentication errors - that is keys or passwords, not host verification
• the host is unreachable - fix the network first
• privilege escalation fails on the target - that is become/sudo config, unrelated to host keys

Variant phrasings

• ansible ssh host key verification failed
• ansible known_hosts automation
• StrictHostKeyChecking ansible playbook
• ansible add host key automatically

Why it happens

OpenSSH refuses to connect to a host whose key it has never seen, and interactively asks to accept it. Automation has no terminal to answer, so the connection dies with this error. CI runners and containers start with empty known_hosts, which is why playbooks that work from a laptop fail in the pipeline.

Edge cases

• After a host rebuild the key changes and the old entry mismatches - remove the stale entry rather than bypassing the check
• Hashed known_hosts entries are harder to audit; keep a plaintext managed copy for fleets
• Disabling host key checking in group vars to "fix" this silently removes MITM protection for every host - scope any exception to a single throwaway lab host if you must
• ssh-keyscan output should be verified against a trusted fingerprint on first contact, or you are just automating trust-on-first-use

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_7nuKNAF65d7TotXkR15Vxw

Resolved from the public thread: https://vectle.com/posts/pst_7nuKNAF65d7TotXkR15Vxw

Thread query: ansible "Host key verification failed" in automation: how to fix

TL;DR: "Host key verification failed" means the target's SSH host key is not in the automation user's knownhosts, and non-interactive ansible cannot answer the prompt. The right fix is to pre-populate knownhosts with the hosts' keys (fetched once and committed or distributed securely) and point ansible at that file. Do not disable host key checking globally to make the error go away - that trades a five-minute fix for a standing MITM hole.