Gitlab on premise > permission denied (ssh key login) - ssh

Question

I'm trying since hours to get the ssh login working.
The Server is on Debian 10 and the clients are on Linux Mint and Windows 10.
This is the error trying to login with:
ssh -Tvvv git#dev.example.io
debug1: Will attempt key: /home/herbert/.ssh/pubkey RSASHA256:LygJtb4xx3RHfaMr1wM3sgSjTnj59nNUFgnQZO/0xoo explicit agent
debug2: pubkey_prepare: done
debug3: send packet: type 5
debug3: receive packet: type 7
debug1: SSH2_MSG_EXT_INFO received
debug1: kex_input_ext_info: server-sig-algs=<ssh-ed25519,ssh-rsa,rsa-sha2-256,rsa-sha2-512,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521>
debug3: receive packet: type 6
debug2: service_accept: ssh-userauth
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug3: send packet: type 50
debug3: receive packet: type 51
debug1: Authentications that can continue: publickey
debug3: start over, passed a different list publickey
debug3: preferred publickey
debug3: authmethod_lookup publickey
debug3: remaining preferred:
debug3: authmethod_is_enabled publickey
debug1: Next authentication method: publickey
debug1: Offering public key: /home/herbert/.ssh/pubkey RSA SHA256:LygJtb4xx3RHfaMr1qM3sgSjTnj59nNUFgnQZO/0xoo explicit agent
debug3: send packet: type 50
debug2: we sent a publickey packet, wait for reply
debug3: receive packet: type 51
debug1: Authentications that can continue: publickey
debug2: we did not send a packet, disable method
debug1: No more authentication methods to try.
git#dev.example.io: Permission denied (publickey).
This is the ssh config file:
Host dev.example.io
HostName dev.example.io
PreferredAuthentications publickey
IdentityFile ~/.ssh/pubkey
PubkeyAcceptedKeyTypes +ssh-rsa
I have also tried "Fast lookup of authorized SSH keys in the database" to get things working, but they don't.
Maybe someone has an idea how to debug what is going wrong, for the moment I have no idea > it's a fresh gitlab installation and I have never had this problem before with an installation.
Thank you.
PS: Of cause > the rsa keys are correct on gitlab and they are also correct saved in the autohorized_keys file.

Answer 1

Whenever the SSH looks OK on client side, and yet is refused by the server side, assuming you have access to the server, you can consider debugging there.
On that Debian server:
stop the SSH daemon
relaunch it manually (using its full path)
make one connection from the client
look on the server stdout the output from the daemon
That is, on Debian server side:
sudo systemctl stop ssh.service
/usr/sbin/sshd -dd