Why is ssh agent forwarding not working?

Solution 1:

It turns out my key was not in the agent, and this fixed it:

OS X:

ssh-add -K

Linux/Unix:

ssh-add -k

You can list loaded keys using:

ssh-add -l

ssh-add -L # for more detail

Solution 2:

Another possible reason is connection sharing: one might already be logged in on the other host without agent forwarding and connection sharing enabled. The second login with ssh -A (or equivalently specified in the config file) via the shared connection will silently ignore the -A flag. Only after completely logging out or disabling connection sharing for second login, the agent forwarding will work.

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Solution 3:

1. Check if your ~/.ssh/id_rsa ~/.ssh/id_dsa ~/.ssh/id_ecdsa files have the correct permissions which should be owned by your user and be chmoded 600.

2. Check that you have the correct public key on pupeno/.ssh/authorized_keys on b1, and check if authorized_keys has a line break at the end of the key.

3. Check if you have ssh-agent running, try to load keys via ssh-add

4. Try GSSAPI-based authentication and forwarding with ssh -K

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Solution 4:

I had problem with sshd server rejecting agent forwarding request because of no space left in /tmp. This was because sshd needs to create socket in /tmp. Cleaning disk up resolved my issue.

ssh -v said back then:

debug1: Remote: Agent forwarding disabled: mkdtemp() failed: No space left on device

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Solution 5:

For the benefit of other googlers who also arrived at this question:

Incorrect whitespace in a ~/.ssh/config file can also cause some head scratching.

I recently helped out one of my co-workers who had this:

# incorrect
host foobar ForwardAgent yes

instead of this:

# correct
host foobar
  ForwardAgent yes

I've also run into instances where missing indentation of the directives under the list of hosts made a difference to functionality, even though it's not supposed to.