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I'm connecting from machine M1 to machine M2 using ssh (to the same user on the other machine). I should also mention the user shares the same key on both machines. With password authentication, everything works fine; not so with public-key authentication; I've ensured ~/.ssh/authorized_keys on M2 has the RSA key as authorized, but still - ssh falls back to password authentication. I get the following with ssh -vvv:

debug2: key: /home/joeuser/.ssh/id_rsa (0x7f42679e8200),
debug2: key: /home/joeuser/.ssh/id_dsa ((nil)),
debug2: key: /home/joeuser/.ssh/id_ecdsa ((nil)),
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug3: start over, passed a different list publickey,password,keyboard-interactive
debug3: preferred publickey,keyboard-interactive,password
debug3: authmethod_lookup publickey
debug3: remaining preferred: keyboard-interactive,password
debug3: authmethod_is_enabled publickey
debug1: Next authentication method: publickey
debug1: Offering RSA public key: /home/joeuser/.ssh/id_rsa
debug3: send_pubkey_test
debug2: we sent a publickey packet, wait for reply
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug1: Trying private key: /home/joeuser/.ssh/id_dsa
debug3: no such identity: /home/joeuser/.ssh/id_dsa: No such file or directory
debug1: Trying private key: /home/joeuser/.ssh/id_ecdsa
debug3: no such identity: /home/joeuser/.ssh/id_ecdsa: No such file or directory
debug2: we did not send a packet, disable method

I should mention that I am able to connect using public-key authentication from other machines (not with the same key).

What are the potential reasons for key-based authentication failing in this case?

Note: The machines are both SLES (SuSE Linux Enterprise Server) 11.

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  • 2
    Read the log file of the sshd. It should contain a clear reason why it rejects your ssh key. Aug 30 '20 at 15:32
  • 1
    You need to check the logs on the server to find out why this failed. Aug 30 '20 at 16:27
  • also double check the authorized_keys file content vis-a-vis the key your client is offering. Run ssh-keygen -l -f FILE against each file, replacing FILE. Output should be the same. (If the authorized_keys file contains other keys it will have more lines but one of them should match the output of running this on your id_rsa.
    – user512619
    Sep 3 '20 at 7:18
  • @sitaram: Of course the key is there, otherwise this question wouldn't make sense...
    – einpoklum
    Sep 3 '20 at 22:25
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Check the basics:

  1. id_rsa and id_rsa.pub exist on both M1 and M2
  2. id_rsa has permission 600 (i.e only the owner can read-write) on both M1 and M2
  3. authorized_keys file has key pasted as a single line (no line break)
  4. Permission of authorized_keys is 600
  5. Typically, permission on my .ssh folder is 600 (default)
  6. Check the permission of each folder /home all way up to .ssh
  7. I know you want to use RSA, but try DSA key and see if it works. If it does, then we'll have zeroed in SSH and RSA config.
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The error you get is

/home/joeuser/.ssh/id_dsa: No such file or directory

Make sure this file exists, contains the private key that corresponds to the public key that you've added, belongs to joeuser and has 600 user permissions:

sudo chown joeuser /home/joeuser/.ssh/id_dsa
sudo chmod 600 /home/joeuser/.ssh/id_dsa

You should also try to explicitly define the private key like this:

ssh -i ~/.ssh/id_rsa user@remote.example.com

If you are not sure if this is the right key then I'd recommend to create a new RSA key pair

ssh-keygen -b 4096

and add the content of the public key ~/.ssh/id_rsa.pub to the authorized_keys file of the remote server. Make sure that you don't overwrite an existing private key that you still need to login to other servers!

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  • But I want to use the RSA key (which is the key I have): Offering RSA public key: /home/joeuser/.ssh/id_rsa is that not good enough?
    – einpoklum
    Aug 30 '20 at 10:26
  • And does that file exist? Because the error message you get says it doesn't.
    – digijay
    Aug 30 '20 at 10:37
  • The RSA key file exists, but I've never created a DSA key file so obviously it doesn't.
    – einpoklum
    Aug 30 '20 at 10:55
  • Try to use ssh agent. You add ssh private key and use it for a log in. To chech if a agent has a key inside use ssh-add -L Aug 30 '20 at 12:19

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