Setup Secure SSH Proxy Account

No need to enable full login shell if the only intended usage is proxying.

Create user

useradd -m -s /sbin/nologin proxy

Setup ssh

cd /home/proxy
mkdir .ssh
touch .ssh/authorized_keys
chown -R proxy:proxy .ssh
chmod 700 .ssh
chmod 640 .ssh/authorized_keys

Set further restrictions if necessary. In /etc/ssh/sshd_config, add

Match User proxy
        X11Forwarding no

Add some user key

echo "some SSH key" >> .ssh/authorized_keys

Restart sshd

systemctl restart sshd

To start a SOCKSv5 proxy on localhost:8080, execute

ssh -D 8080 proxy@proxyhost -N

and leave it running.