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Put each server’s connection details in ~/.ssh/config. Give every server (or server group) a Host alias, set its real address with HostName, select the login account with User, and point IdentityFile to the matching private key. Add IdentitiesOnly yes when you want to prevent other keys loaded in ssh-agent from being offered.
What this setup changes
OpenSSH reads per-user settings from ~/.ssh/config before it connects. A Host line selects a configuration block. The word after Host is an alias you type at the shell; it does not have to be a DNS name. HostName supplies the real DNS name or IP address, User supplies the remote account, and IdentityFile names the private key used for authentication.
For example, ssh production can connect to prod.example.com as deploy with one key, while ssh staging connects to a different address with a different key. You stop adding -i to every command, and the intended key is visible in one auditable file.
Prerequisites before editing the file
- Have a private-key file for each trust boundary, such as
~/.ssh/id_ed25519_productionand~/.ssh/id_ed25519_staging. - Install the corresponding public key in the correct account on each Linux server. A key file on your workstation is not enough unless the server account accepts its public half.
- Use an OpenSSH client and the same user account whose home directory contains the configuration. The file is per user, so a configuration in one account’s home directory does not automatically apply to another account.
- Back up an existing configuration before making changes. SSH configuration is line-oriented; a typo can affect every connection that matches a block.
Protect the directory and private keys with restrictive permissions. A common baseline is chmod 700 ~/.ssh for the directory and chmod 600 ~/.ssh/id_ed25519_production ~/.ssh/id_ed25519_staging ~/.ssh/config for private files and the configuration. Never paste private-key contents into tickets, chat, documentation, or shell history.
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Create one explicit block per server
-
Create or open the per-user file:
mkdir -p ~/.ssh chmod 700 ~/.ssh ${EDITOR:-vi} ~/.ssh/config -
Add a block for each server. This example uses explicit aliases and one key per host:
Host production HostName prod.example.com User deploy IdentityFile ~/.ssh/id_ed25519_production IdentitiesOnly yes Host staging HostName staging.example.com User deploy IdentityFile ~/.ssh/id_ed25519_staging IdentitiesOnly yes -
Save the file and set its permissions:
chmod 600 ~/.ssh/config -
Connect with the alias rather than the long command:
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How OpenSSH interprets the entries
Host production is the selector. It creates a short, stable command name. HostName prod.example.com is the destination actually resolved by the client. User deploy avoids relying on your local username. IdentityFile ~/.ssh/id_ed25519_production points to the private key that should authenticate to that server.
The two blocks can use the same remote account while still selecting different keys. They can also use different accounts; change the User line in each block when, for example, one host requires admin and another requires deploy. Keep the alias descriptive enough that you can identify its environment without opening the file.
Stop ssh-agent from trying unrelated keys
An authentication agent can hold several identities and offer them to an SSH server. If the server rejects too many incorrect offers, the connection can fail before the intended key is tried. IdentitiesOnly yes tells the client to use only the identity and certificate files configured for that host, even when the agent offers additional identities.
That option is why the example uses both IdentityFile and IdentitiesOnly yes. It does not unload keys from the agent, erase them, or change agent behavior for other host blocks. It limits what this connection asks the agent and client to present.
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When strict selection is appropriate
- Use it when production, staging, customer, or personal servers have separate keys and the boundaries matter.
- Use it when an agent contains many keys and a server enforces a small authentication-attempt limit.
- Use it when you want a failed connection to indicate that the configured key is wrong, rather than silently trying a collection of unrelated keys.
When a sequence is intentional
There are cases where a server legitimately accepts more than one key or where you are migrating from an old key to a new one. In that situation, configure the sequence deliberately and understand that each identity may be tried. Do not add extra files merely as a precaution; every additional identity increases the number of credentials considered for that host.
Group servers with Host patterns
Host accepts patterns, so a single block can cover a group of servers. An asterisk matches any sequence of characters and a question mark matches one character. This block applies to matching names:
Host *.prod.example.com
User deploy
IdentityFile ~/.ssh/id_ed25519_production
IdentitiesOnly yesPatterns are useful when a fleet shares an account and key policy. If the servers are named api.prod.example.com, web.prod.example.com, and worker.prod.example.com, the block avoids repeating the same settings.
Pattern lists and exclusions
Comma-separated pattern lists combine alternatives. A pattern can be negated by putting ! before it. For example:
Host *.example.com,!legacy.example.com
User deploy
IdentityFile ~/.ssh/id_ed25519_current
IdentitiesOnly yesThe positive pattern selects the broader group, while the negated pattern excludes the legacy host. A list containing only negated patterns does not create a positive match; include at least one positive pattern when you want the block to apply.
When to prefer explicit aliases
Use an explicit alias when you need a stable short command, when a server is reached through different names, or when a broad pattern could accidentally include a host with a different trust boundary. An alias also makes reviews easier: a reviewer can see exactly which key is selected for production without mentally evaluating a fleet-wide wildcard.
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Understand multiple IdentityFile directives
IdentityFile is additive rather than a simple last-value-wins setting. The OpenSSH manual states that multiple IdentityFile directives add to the list of identities tried. This means the following block intentionally creates a sequence:
Host migration
HostName migrate.example.com
User deploy
IdentityFile ~/.ssh/id_ed25519_new
IdentityFile ~/.ssh/id_ed25519_old
IdentitiesOnly yesThe client can try the new key and then the old key. If you want strict one-key-per-server behavior, keep one IdentityFile and set IdentitiesOnly yes. Adding a second line does not replace the first one.
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IdentityFile accepts ~ and documented tokens that expand when a connection is made. The available tokens in this context include:
| Token | Expansion | Example use |
|---|---|---|
%d |
Local home directory | %d/.ssh/keys/%h |
%h |
Remote host | ~/.ssh/keys/%h |
%n |
Original host name supplied to the client | ~/.ssh/keys/%n |
%p |
Port | ~/.ssh/keys/%h-%p |
%r |
Remote user | ~/.ssh/keys/%r@%h |
%u |
Local user | ~/.ssh/keys/%u/%h |
Tokenized paths reduce repetition when your filenames follow a predictable scheme. Explicit filenames are easier to audit when servers have different trust boundaries, because the key choice is visible without calculating several expansions. Choose the style that makes accidental cross-environment use least likely.
Do not confuse HostKeyAlias with the user key
HostKeyAlias controls the name OpenSSH uses when looking up and storing a server’s host key. The option is useful for tunneling or for multiple servers that appear under one network host. It changes host-key database naming and validation; it does not select the private key used to authenticate your user.
Use IdentityFile for user authentication and HostKeyAlias only when you need a separate host-key name. Changing the latter will not fix a private-key or account-selection problem.
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Verify the effective settings before troubleshooting authentication
On OpenSSH clients that support the configuration-dump mode, run:
ssh -G productionThe output shows the options after the client has evaluated the Host blocks and aliases. Check that the resolved hostname, user, identityfile, and identitiesonly values match the server you intended. To focus on those fields on a typical Linux workstation:
ssh -G production | grep -E '^(hostname|user|identityfile|identitiesonly) 'Repeat the check for staging and for any wildcard-matched name. If the command reports an unexpected host or key, fix the matching Host block before changing server-side permissions.
Choose the setup that fits your workflow
| Setup | Repeatability | Risk of selecting the wrong key | Auditability | Behavior with ssh-agent | Portability |
|---|---|---|---|---|---|
Explicit alias with one IdentityFile and IdentitiesOnly yes |
High; use the same short command every time | Low when aliases and filenames are distinct | High; each server has a visible block | Only the configured identity and certificate files are used | Copy the config and the required keys to each workstation, then adjust paths |
| Wildcard group with one key | High for hosts that follow the naming pattern | Medium if the pattern is broader than the trust boundary | Medium; the applicable block must be inferred from the name | Deterministic when IdentitiesOnly yes is present |
Works where host naming and key paths are consistent |
Command-line ssh -i |
Low; the option must be repeated or scripted | Medium; a typo or stale path can select the wrong file | Limited to shell history, scripts, or the command being reviewed | Adding -i does not by itself prevent other agent identities from being offered |
Portable when the same path exists, but every command must carry the option |
Several IdentityFile lines |
High when a deliberate fallback sequence is required | Higher because each listed identity may be tried | Medium; the order must be understood | Combine with IdentitiesOnly yes to limit offers to that list |
Requires every listed file on the destination workstation |
Troubleshoot the common failure modes
The alias connects to the wrong server
Run ssh -G alias and inspect hostname. The word after Host is only a selector; it does not override HostName. Correct the destination in the matching block and test again.
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Inspect the resolved user value. A missing User line causes the client to use the local account name, which may not exist on the server. Add the required account to that host’s block.
The server reports too many authentication failures
Set IdentitiesOnly yes alongside the intended IdentityFile. This prevents unrelated agent identities from being offered. If the block contains several IdentityFile lines, remember that all of them are additive and may be tried.
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The intended key is not being used
Check the exact path shown by ssh -G, including capitalization and the expanded home directory. Confirm that the private file exists, is readable by your account, and corresponds to a public key installed for the selected User. A correct path cannot authenticate if the server account has a different public key.
A wildcard block does not apply
Compare the name supplied to ssh with the pattern. A wildcard such as *.prod.example.com matches names in that form; an unrelated alias such as production requires its own explicit block. If you used a negated pattern, ensure the same pattern list also contains a positive match.
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Host-key records are associated with names. If you intentionally need a different lookup name for tunneling or multiple servers on one host, use HostKeyAlias. Do not change it as a substitute for selecting the correct user key; it affects host-key validation, not user authentication.
The configuration works on one workstation but not another
The file and private keys are local to each user account. Copy the configuration through a secure channel, install only the keys that workstation needs, and update paths if home directories differ. Re-run ssh -G on the second machine to verify the resolved values instead of assuming the two environments are identical.
Maintain the configuration safely
- Use unique, descriptive aliases such as
production,staging, andbackup-eurather than ambiguous names. - Keep production and non-production keys in separate files and reference each file from only the intended host block.
- Use
IdentitiesOnly yesfor strict per-server selection, especially when an agent contains several identities. - Use wildcard blocks only for servers that genuinely share the same account and key policy; add explicit aliases for exceptions.
- Remember that multiple
IdentityFilelines add identities. Review the complete list whenever a connection unexpectedly tries more than one key. - Keep
~/.ssh, the configuration, and private keys protected with restrictive permissions, and never disclose private-key material. - After every substantial edit, validate each important alias with
ssh -Gbefore attempting a production change.
A practical final configuration
The following combines explicit aliases for sensitive environments with a wildcard for a uniform production subdomain:
Host production
HostName prod.example.com
User deploy
IdentityFile ~/.ssh/id_ed25519_production
IdentitiesOnly yes
Host staging
HostName staging.example.com
User deploy
IdentityFile ~/.ssh/id_ed25519_staging
IdentitiesOnly yes
Host *.prod.example.com
User deploy
IdentityFile ~/.ssh/id_ed25519_production
IdentitiesOnly yesUse ssh production and ssh staging for the named systems. Use the real matching hostname for another production subdomain. If one production host needs a different key or account, give it an explicit alias and configuration instead of relying on the broad pattern.
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