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Use OpenSSH’s ProxyJump to reach an internal SSH host through a bastion without forwarding your local authentication agent to that bastion. Agent forwarding does not hand over your private-key file, but someone who can access the forwarded agent socket may be able to authenticate as you with identities loaded in your agent. OpenSSH’s default for ForwardAgent is no; for a straightforward bastion route, leave it off and configure the destination with ProxyJump.
What is the risk of SSH agent forwarding?
SSH agent forwarding makes a connection to your local authentication agent available through the remote machine. That can be convenient when you connect from a server to another SSH host, but it also gives users with access to the forwarded socket a way to ask your agent to perform authentication operations.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesThe key distinction is between using an identity and extracting its key material. OpenBSD’s official OpenSSH client configuration manual says that a user who can bypass permissions on the forwarded Unix-domain socket can access the local agent through the connection. That user cannot obtain key material from the agent, but can perform operations that enable authentication using identities loaded in it. The manual’s warning is direct: “Agent forwarding should be enabled with caution.”
In practical terms, “the server can steal my SSH key” is not quite the right description. The manual does not say an attacker can copy your private key out of the agent; it describes the risk of using your agent to authenticate while access to the forwarded socket is available. That distinction matters, but it does not make forwarding harmless: an authentication performed with your identity can still be consequential.
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The risk depends on access to the forwarded socket, not simply on the fact that a host is called a bastion. A jump host is still a machine you connect to, so its security and the permissions governing access to it matter. ProxyJump changes the connection path so you do not need to make your local agent available on that intermediate host.
How does ProxyJump reach an internal host?
ProxyJump tells the SSH client to connect to a jump proxy first, then establish TCP forwarding from that proxy to the ultimate destination. The client reaches the private host through the bastion, rather than relying on a shell session on the bastion to start a second SSH connection using a forwarded local agent.
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This is a routing choice, not a blanket guarantee of security. It avoids forwarding your agent as part of this route, but it does not remove the need to authenticate the destination correctly, protect your client keys, maintain sound server configuration, or secure the bastion. The OpenBSD manual documents how the option works; it does not claim that ProxyJump eliminates every SSH risk.
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How do I configure ProxyJump through a bastion?
Add a host entry for the bastion and a separate entry for the private destination in your SSH client configuration. The example below uses illustrative hostnames, an example private IP address, and the username alice; replace them with values for your environment.
Host bastion
HostName bastion.example.net
User alice
Host private-host
HostName 10.0.0.15
User alice
ProxyJump bastion- Set up the bastion entry.
Host bastionis the alias used by the next entry.HostNameidentifies the bastion to contact, andUserspecifies the account for that connection. - Set up the destination entry.
Host private-hostis the alias you will type. ItsHostNameis the internal host’s address, and itsUseris the account you want on that host. - Point the destination at the jump host.
ProxyJump bastiondirects the client to use the bastion entry as the route to the private destination. - Connect using the destination alias. Run
ssh private-host. The client uses the destination settings and makes the connection through the configured jump host.
The example deliberately has two host entries. OpenBSD notes that configuration for the destination host is generally not applied to jump hosts. A separate Host bastion stanza is the direct way to set the bastion’s own hostname, user, and any other jump-host settings you need to specify. Do not assume that putting a setting under Host private-host configures the bastion too.
How do agent forwarding and ProxyJump differ?
Both approaches can be part of a workflow that reaches a machine behind an intermediary, but they expose different things to that intermediary. Agent forwarding exposes access to the local agent socket; ProxyJump routes the connection through a jump host.
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| Question | Agent forwarding | ProxyJump |
|---|---|---|
| What is made available to the intermediate host? | Access to the forwarded local authentication agent socket. | A route that connects through the jump host to the ultimate target; the local agent is not forwarded as part of this path. |
| What could a user with socket access do? | Use agent operations to authenticate with identities loaded in the agent; the manual says the user cannot obtain key material. | The cited manual describes ProxyJump as connecting to the jump proxy and establishing TCP forwarding to the target. This does not itself confer a guarantee against other risks. |
| Where do you configure the intermediary? | Forwarding is controlled by ForwardAgent, which defaults to no. |
Use a ProxyJump setting for the destination and a separate host entry for the jump host when it needs its own settings. |
For a standard bastion-to-private-host route, ProxyJump addresses the access problem without making agent forwarding part of the route. Forwarding can remain useful in other workflows, but its convenience should be weighed against the fact that remote socket access can let someone act through loaded identities.
What should I check if ProxyJump does not connect?
First check whether the connection fails while reaching the bastion or while reaching the destination through it. The distinction helps identify which host entry or part of the route needs attention.
- Check the destination alias. Confirm that the name after
sshmatches theHostpattern in your configuration, and that itsHostNameidentifies the intended private host. - Check the bastion entry independently. Confirm that the alias named by
ProxyJumphas its own matchingHostentry and the correct bastion hostname and user. Destination settings generally do not configure the jump host. - Check which account belongs on each host. The jump-host user and destination-host user are separate connection settings. They may be the same, as in the example, but they do not have to be.
- Check the route, not just the credentials. ProxyJump first connects to the jump proxy and then establishes TCP forwarding to the ultimate target. A successful connection to the bastion alone does not establish that the destination can be reached through it.
- Check that you have not enabled forwarding unnecessarily. OpenSSH’s documented default for
ForwardAgentisno. If your configuration explicitly enables forwarding, reconsider whether that is needed for this connection.
These checks follow the documented division of responsibilities in the client configuration: the destination has its own settings, the jump host may need its own settings, and the jump option defines the route between them. They do not diagnose every possible network, server, or authentication failure, which can depend on your environment.
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What does ProxyJump not protect you from?
ProxyJump is a way to reach a target without forwarding the local agent as part of that route; it is not a substitute for securing either endpoint or the intermediary. You still need to connect to the intended destination, use appropriate authentication, and trust the bastion to the extent required by your network design.
Nor should the difference be inflated into a measured claim that one method is universally safer under every threat model. The OpenBSD manual documents the agent-socket risk and the connection behavior of ProxyJump. It does not quantify how often abuse occurs or provide a controlled comparison of security outcomes. The useful, supported conclusion is narrower: agent forwarding makes the local agent accessible through a forwarded socket, while ProxyJump provides a route through the bastion without forwarding that agent as part of the path.
Frequently Asked Questions
Can ProxyJump use more than one bastion?
Yes. OpenBSD’s ssh_config(5) manual says ProxyJump accepts one or more jump proxies, including sequential jump proxies. Each intermediary still needs to be reachable in the order the route requires.
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Can I use a different SSH username or port for the bastion?
Yes. The manual allows optional user and port values for jump proxies. Set the bastion’s connection details separately from the destination’s details so the two connections use the intended accounts and ports.
Does ProxyJump mean the bastion is trusted with my private key?
The cited manual describes ProxyJump as connecting through the jump proxy and establishing TCP forwarding to the destination; it does not describe this as giving the bastion your private-key material. That does not make the bastion risk-free: secure the intermediary and verify that your SSH connections reach the intended hosts.
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