I believe debian's ssh server is recent enough for this technique which is dead easy.
First edit /etc/ssh/sshd_config with the following values
Code:
Subsystem sftp internal-sftp
Match Group sftponly
ChrootDirectory /home/%u
ForceCommand internal-sftp
AllowTcpForwarding no
You should put them at the bottom.
Now add him a user as normal, setting the shell to /bin/false and adding them to the sftponly group.
Next chown root:root /home/[username] and chmod 0755 /home/[username]
Restart ssh and you're done.
The only caveat is that the user won't be able to write to /home/[username] I suggest adding a public_html folder and let them write to that.
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