Start by identifying where SSH fails: a refusal or timeout usually means the client has not reached a usable SSH service, while Permission denied (publickey) means the server reached an authentication decision. Check the destination and port first; change keys only when the connection reaches authentication.
Identify the SSH failure stage
SSH first establishes a connection to a server and performs its initial protocol handshake and key exchange. Authentication happens afterward. This distinction helps narrow the next step: a refused connection or timeout points toward the destination, listener, or network path; a public-key denial points toward the login account or credentials.
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OpenSSH’s sshd listens for client connections. Its configured port defaults to 22, but a server can use another port. Confirm the actual host and port rather than assuming the default applies. See the OpenBSD sshd_config(5) manual and OpenBSD ssh_config(5) manual.
Confirm the host, account, and port
Run SSH in verbose mode to see where the client is connecting and how far it gets:
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ssh -v user@hostFor a server configured on a non-default port, specify that port explicitly:
ssh -v -p PORT user@hostReplace user, host, and PORT with the account, hostname or address, and port for your server. Look near the start of the verbose output for the destination and port. If either is wrong, correct it before investigating keys.
For GitHub, its documented diagnostic command is ssh -vT [email protected]; GitHub’s SSH username is git, not your personal GitHub username. That account name is specific to GitHub. Other hosted services and self-managed servers may require different usernames and port settings. See GitHub’s SSH troubleshooting guide.
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Fix “connection refused”
A refusal generally indicates that the connection could not be established at the destination and port the client tried. It is not a reason to regenerate keys first: key authentication takes place only after a connection reaches the SSH service.
- Check that the hostname or IP address is correct.
- Confirm the server’s actual SSH port. OpenSSH defaults to port 22, but an administrator may have configured another one.
- If you control the server, verify that the SSH daemon is installed, running, and listening on the intended interface and port.
- Check relevant host and cloud firewall rules, router or NAT forwarding, and network policy. The right commands and service manager depend on the operating system and provider.
If the server listens on a different port, use it in the client command and make sure applicable firewall and routing rules allow traffic to that port. OpenSSH documents the listener and port configuration in its server configuration manual.
Fix “connection timed out”
A timeout means the client did not complete the connection within the time allowed. Check that the address resolves to the intended host, that the port is correct, and that traffic is not being filtered or routed incorrectly by a local, corporate, or cloud firewall, proxy, or network policy.
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OpenSSH’s ConnectTimeout setting controls how long the client waits while establishing a connection and performing the initial protocol handshake and key exchange. A longer value can help if the endpoint is slow to respond, but it cannot make an unreachable host, incorrect port, or blocked route reachable. The setting is described in the OpenBSD ssh_config(5) manual.
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Fix “Permission denied (publickey)”
GitHub explains that a “Permission denied” error means the server rejected the connection. For Permission denied (publickey), check the account and key the client actually tried—not just whether a key file exists.
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- Verify the hostname and login account. For GitHub SSH, use
gitas the account name. On another server, use the account provisioned for SSH access. - See which identity the client offers. Run
ssh -v user@hostand inspect the identity-file and “Offering public key” lines, along with the authentication-method messages. If the key is not the one you intend to use, specify its private-key path with-i. - Check the key association or authorization. For GitHub, compare the fingerprint of the offered key with the public keys on the intended account. For a self-managed server, confirm that the corresponding public key is installed for the target account in its authorized keys.
- Check which user is running SSH. The agent and key available to one local user may not be available to another. GitHub notes that a key created as one user may not be used by a command run with
sudo. - If you administer the server, check its access rules. Review effective public-key authentication settings, applicable user or group allow/deny rules and
Matchblocks, the authorized-keys location, and the target user’s file ownership and permissions.
For GitHub, these commands test the default identity and then a specified private-key file:
ssh -vT [email protected]
ssh -i ~/.ssh/KEY-FILE -vT [email protected]Replace KEY-FILE with the actual private-key filename. To list keys available through the SSH agent, GitHub documents ssh-add -l -E sha256. Share or install only the public key; never paste or upload a private key. See GitHub’s guide to Permission denied (publickey).
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Server-administrator checks
If you control the host, validate the SSH daemon configuration before changing it:
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sudo sshd -t
sudo sshd -Tsshd -t checks configuration validity. sshd -T prints effective configuration; the manual also documents using -C to evaluate matching connection parameters. For deeper diagnosis, sshd -d runs the daemon in foreground debug mode, and -E can direct debug output to a file. These are administrator operations: do not replace or restart a live daemon, or change access controls, without a recovery path. Options and behavior can vary by operating system and package. See the OpenBSD sshd(8) manual.
When refusal or authentication denial persists, inspect the effective values for Port, ListenAddress, authentication methods, user and group access rules, Match conditions, and StrictModes. OpenSSH notes that the first obtained value is used for many settings, so included files and ordering can affect the result; check effective configuration instead of relying on a single line. StrictModes, enabled by default in the cited manual, checks user file modes and ownership.
MaxStartups and per-source penalties are advanced controls that can refuse or drop unauthenticated connections in some circumstances. They are worth investigating when the failure pattern suggests connection limits or source-based handling, not as a first-line fix for an individual user’s SSH problem. The OpenSSH server configuration manual documents these settings.
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Quick Recap
Choose the next check by what the failure affects
- One key or one user fails: check the login account, offered identity, key association, and account-specific server rules.
- All clients fail at the same host and port: prioritize the listener, server configuration, firewall, routing, or network policy.
- Only one network fails: investigate filtering, proxies, routing, and network policy on that path.
- The target is a hosted service: use that service’s documented SSH username, port, and supported connection options; do not transplant GitHub-specific values to another provider.
- You do not administer the server: provide its administrator with the exact host, port, command, error, and relevant verbose output, while removing secrets and sensitive details.
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