Agent SSH server should use a consistent key over workspace restarts #16490
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networking
Area: networking
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This was referenced Feb 18, 2025
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ThomasK33
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Feb 21, 2025
Fixes: #16490 The Agent's SSH server now initially generates fixed host keys and, once it receives its manifest, generates and replaces that host key with the one derived from the workspace ID, ensuring consistency across agent restarts. This prevents SSH warnings and host key verification errors when connecting to workspaces through Coder Desktop. While deterministic keys might seem insecure, the underlying Wireguard tunnel already provides encryption and anti-spoofing protection at the network layer, making this approach acceptable for our use case. --- Change-Id: I8c7e3070324e5d558374fd6891eea9d48660e1e9 Signed-off-by: Thomas Kosiewski <tk@coder.com>
aslilac
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Feb 21, 2025
Fixes: #16490 The Agent's SSH server now initially generates fixed host keys and, once it receives its manifest, generates and replaces that host key with the one derived from the workspace ID, ensuring consistency across agent restarts. This prevents SSH warnings and host key verification errors when connecting to workspaces through Coder Desktop. While deterministic keys might seem insecure, the underlying Wireguard tunnel already provides encryption and anti-spoofing protection at the network layer, making this approach acceptable for our use case. --- Change-Id: I8c7e3070324e5d558374fd6891eea9d48660e1e9 Signed-off-by: Thomas Kosiewski <tk@coder.com>
aslilac
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Feb 27, 2025
Fixes: #16490 The Agent's SSH server now initially generates fixed host keys and, once it receives its manifest, generates and replaces that host key with the one derived from the workspace ID, ensuring consistency across agent restarts. This prevents SSH warnings and host key verification errors when connecting to workspaces through Coder Desktop. While deterministic keys might seem insecure, the underlying Wireguard tunnel already provides encryption and anti-spoofing protection at the network layer, making this approach acceptable for our use case. --- Change-Id: I8c7e3070324e5d558374fd6891eea9d48660e1e9 Signed-off-by: Thomas Kosiewski <tk@coder.com>
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Summary
We should enhance the agent to use the same SSH key each time the workspace is restarted, so that users of Coder VPN (#14859) can SSH into their workspaces with no extra config steps.
I think it is acceptable to just use a hardcoded SSH key, since the Wireguard protocol routes packets based on the tunnel crypto keys, so anti-spoofing of the workspace is handled at this lower layer.
Background
Today we handle end user SSH connections with the Coder CLI, either directly with the
coder sshcommand, or indirectly by instructing the SSH process to startcoderas a proxy command. In both these circumstances, we are able to configure the SSH client to ignore the fact that our workspace SSH server uses a different key each time the workspace is restarted. This is very non-standard, and a default configured SSH client will reject reconnection to the same workspace after a restart.e.g.
If we want Coder VPN users to connect over SSH without special config, we need to use a consistent key.
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