The Tool Desk
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Start by identifying which phase fails
SSH public-key login depends on several pieces lining up: the client must reach the intended server and account, select an identity it can use, and offer a valid proof of possession. The server must then find and authorize the corresponding public key under its active configuration and account policy. OpenBSD’s ssh(1) manual describes this division between client proof and server authorization.
Use the first error or log message to locate the failing layer. A timeout or connection refusal occurs before the user key can solve anything. If the connection proceeds but public-key authentication fails, inspect identity selection and server authorization instead.
1. Confirm the host, port, and account
Check that the command targets the correct hostname or configured alias, port, and remote username. A familiar host alias may expand to a different server or account than expected. If the connection cannot be established to the intended host, changing a key will not fix that earlier failure.
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Start with a verbose attempt, for example:
ssh -v user@hostReplace user and host with the intended remote account and host. Consult the manual for the SSH client installed on your system; options and output can vary by implementation and version. Keep hostnames, usernames, and diagnostic output private when sharing a report.
2. See which identities the client considers
Verbose output helps show whether the client attempts public-key authentication and which identities it considers. OpenSSH’s -v option increases diagnostic output; additional verbosity can provide more detail. The exact messages depend on client version and configuration.
If the client never offers the expected identity, check its effective configuration and the identity-selection rules before assuming the server rejected that key. OpenSSH client settings are documented in ssh_config(5). A host-specific setting or alias may direct the client to a different identity or authentication method.
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3. Check the local key file and its permissions
Verify that the private-key path named in your configuration or command exists and is readable by the client. OpenSSH documents private-key files and their corresponding public-key files, commonly distinguished by a .pub suffix. The client needs access to the private key to prove possession; the server receives and checks the public-key counterpart.
OpenSSH ignores private-key files that are accessible by others. Inspect the actual file and its permissions rather than applying a broad permission change. The recommended permissions and behavior can differ across operating systems and SSH implementations; use your installed client’s manual for exact guidance. Never share a private key or passphrase in a support forum or issue tracker.
4. Check whether the expected agent identity is available
An SSH agent holds identities for use by the client; it does not create a key automatically. OpenSSH’s ssh-agent(1) manual states, “The agent initially does not have any private keys.” Identities can be added with ssh-add, or loaded by the client when configured with AddKeysToAgent.
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If you expect to authenticate through an agent, confirm that your session can access the intended agent and that the required identity is loaded. This matters in shells, remote development environments, containers, and other contexts where the agent available to one process may not be available to another. Do not expose an agent socket or private-key material in public logs.
5. Verify the remote account and authorized-key source
Make sure the username is the account where the public key is authorized. A correct key offered for the wrong account will not authenticate as the intended user.
On the server, confirm that the account’s authorized-key source contains the matching public key. OpenSSH server configuration can set AuthorizedKeysFile to one or more paths, use paths relative to the user’s home directory, or set it to none. The default-looking path is not conclusive if the server has an override. See the OpenBSD Project’s sshd_config(5) manual.
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6. Inspect server permissions and access policy
A matching public key can still be refused because of filesystem permissions, ownership, or server policy. Check the actual home directory and authorized-key path, then review the effective server settings for the account. Relevant controls include:
PubkeyAuthentication, which governs whether public-key authentication is enabled.AllowUsers,DenyUsers,AllowGroups, andDenyGroups, which can restrict who may log in.AuthenticationMethods, which can require more than one authentication step.RevokedKeys, which can identify keys the server must reject.- Global and
Match-specific settings, which may give different accounts or connection conditions different rules.
Do not loosen permissions broadly as a troubleshooting shortcut. First establish which account, path, and active policy the server is using. Configuration names and behavior can vary across OpenSSH releases and vendor builds, so check the installed server’s manual and effective configuration.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.7. Use server logs when client output is not enough
Client verbosity can show what the client attempted; server-side diagnostics can explain why the server declined an offered key. If you administer the server, inspect its authentication logs and configuration. OpenSSH documents that server authentication diagnostics can be shown at DEBUG level or higher. Changing logging settings may require administrative access and should follow the server’s local operational practices.
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OpenSSH notes that the server may inform a client of errors that prevented public-key authentication from succeeding after authentication completes using a different method. Therefore, the absence of a detailed rejection reason in a client session does not prove that the key itself is malformed. If you do not administer the host, ask its administrator to check server-side evidence rather than repeatedly changing local keys.
8. Investigate algorithms or FIDO requirements only when indicated
Consider algorithm compatibility when the diagnostic output or key type points to negotiation or key-type rejection. FIDO-backed SSH keys are a specialized case: OpenSSH documents authenticator-hosted ECDSA and Ed25519 key types, and server controls such as touch-required and verify-required. These can require physical user presence or user verification, such as a PIN, and do not apply to ordinary non-FIDO key types. Confirm that the client, authenticator interface, operating system, and server policy support the particular setup before treating it as a likely cause.
What the evidence can tell you
| Evidence source | What it can help identify | Access needed |
|---|---|---|
| Client verbose output | Connection progress, identity selection, and whether public-key authentication is attempted. | Run a verbose SSH attempt from the client. |
| Server authentication diagnostics | Server-side reasons an offered public key or login attempt is rejected. | Server access or help from its administrator; OpenSSH documents debug diagnostics at DEBUG level or higher. |
These are complementary views, not substitutes: client output records the client’s actions, while server logs can expose the policy or authorization decision. OpenBSD manuals describe OpenSSH behavior; managed services, appliances, older releases, and other SSH clients may differ. Check the documentation for the versions actually in use.
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