Ling's Notes

Linux Permissions

2 min read

Question: What happens if I am on a server and then I ssh into this server again?

Nothing special or dangerous happens by itself; you just get another shell on the same machine.

Nothing special or dangerous happens by itself; you just get another shell on the same machine

You now have two nested sessions:

  • outer: your original SSH (or local) shell

  • inner: the SSH session created from the server to itself.

Processes in the inner session are separate from the outer one, with their own TTY, environment, history, etc.

Question: how to create a user on an archlinux server without sudo privilege but make it a member of docker group?

Create the New User

useradd -m -G docker -s /bin/bash newusername

Set a Password

passwd newusername

Verify Group Membership

id newusername
# Expected: uid=1001(newusername) gid=1001(newusername) groups=1001(newusername),999(docker)

Generate the Key Pair

ssh-keygen -t ed25519 -C "newusername@yourserver" -f /home/newusername/.ssh/id_ed25519 -N ""

Set Up authorized_keys

cat /home/newusername/.ssh/id_ed25519.pub >> /home/newusername/.ssh/authorized_keys

Fix Permissions

SSH is strict about permissions — it will silently refuse to use keys with wrong ownership or modes:

chown -R newusername:newusername /home/newusername/.ssh
chmod 700 /home/newusername/.ssh
chmod 600 /home/newusername/.ssh/authorized_keys
chmod 600 /home/newusername/.ssh/id_ed25519

Share the Private Key

ssh -i ~/.ssh/id_ed25519 newusername@your_server_ip

Setup Authorized Keys

Confirm authorized_keys matches that key

sudo -u deploy cat /home/deploy/.ssh/ed25519.pub
sudo -u deploy cat /home/deploy/.ssh/authorized_keys

Ensure authorized_keys contains the same line as ed25519.pub. If it doesn’t, append it:

sudo -u deploy sh -c 'cat ~/.ssh/ed25519.pub >> ~/.ssh/authorized_keys'
chmod 600 /home/deploy/.ssh/authorized_keys

Test SSH Access

ssh -i /path/to/private_key -p SSH_PORT SSH_USER@SSH_HOST

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