How to fix PTY allocation request failed on channel 0 in SSH

If you're performing a non-interactive operation (like git clone, file transfer, or running a remote command), the PTY allocation warning may be harmless. Try suppressing the TTY request:

ssh -T user@hostname command
git clone --depth 1 ssh://[email protected]/user/repo.git

The -T flag tells SSH not to allocate a pseudo-terminal, which is appropriate for non-interactive sessions. If the command completes successfully, the original error was just a warning.

If you have console or root access to the problematic server, verify that /dev/pts is properly mounted:

mount | grep pts

You should see output like: devpts on /dev/pts type devpts (rw,nosuid,noexec,...)

If the output is missing or shows incorrect options, remount it:

sudo umount /dev/pts
sudo mount -t devpts devpts /dev/pts

Verify again with mount | grep pts. The mount should now show proper devpts mounting.

Check the SSH server configuration file for PTY-related settings:

sudo grep -i permitty /etc/ssh/sshd_config

The output should show PermitTTY yes (the default). If it shows PermitTTY no or the line is missing, edit the config:

sudo nano /etc/ssh/sshd_config

Look for the PermitTTY line and ensure it is set to yes, or add it if missing:

PermitTTY yes

Save the file and reload SSH:

sudo systemctl reload ssh
# or on some systems:
sudo systemctl reload sshd

If the server is experiencing many open SSH sessions, it may have run out of available pseudo-terminals. Check current PTY usage:

ls /dev/pts/ | wc -l

This shows how many PTY sessions are currently allocated. Linux systems typically support up to 256. If you're near or at the limit, identify and kill idle sessions:

# List all process trees with TTY
ps aux | grep sshd

# Kill idle SSH sessions (use with caution)
ssh user@hostname pkill -f 'sshd:.*@pts'

If PTY exhaustion is chronic, investigate applications that may be leaving PTY handles open without closing them.

If the SSH server uses chroot jails (common in restricted setups), ensure the chroot environment has proper device filesystem access:

sudo ls -la /var/chroot/user/dev/pts 2>/dev/null

If the directory doesn't exist or has wrong permissions, create it:

sudo mkdir -p /var/chroot/user/dev/pts
sudo chmod 755 /var/chroot/user/dev/pts

Verify your sshd_config has the chroot directive:

sudo grep -i chrootdirectory /etc/ssh/sshd_config

Then reload SSH:

sudo systemctl reload sshd

Some servers, containers, or appliances deliberately don't support interactive SSH sessions. If commands still work with ssh -T or direct commands, the restriction is intentional. To get an interactive shell:

- For Docker containers: Use docker exec -it container_name bash instead
- For Kubernetes pods: Use kubectl exec -it pod_name -- /bin/bash
- For cloud VMs: Use the cloud provider's web console or serial console
- For appliances: Check documentation for provided management interfaces

If you absolutely need interactive SSH on a container or appliance, consult the provider's documentation for supported access methods.