# Security

This part of the document by Hans Lermen, <lermen@fgan.de> on Apr 6, 1997.

These are the hints we give you, when running dosemu on a machine that is (even temporary) connected to the internet or other machines, or that otherwise allows 'foreign' people login to your machine.

- Don't set the -s bit, as of dosemu-0.97.10 DOSEMU can run in lowfeature mode without the -s bit set. If you want fullfeatures for some of your users, just use the keyword `nosuidroot' in /etc/dosemu.users to forbid some (or all) users execution of a suid root running dosemu (they may use a non-suid root copy of the binary though).

- Use proper file permissions to restrict access to a suid root DOSEMU binary in addition to /etc/dosemu.users `nosuidroot'. ( double security is better ).

- NEVER let foreign users execute dosemu under root login !!! (Starting with dosemu-0.66.1.4 this isn't necessary any more, all functionality should also be available when running as user)

- Do not configure dosemu with the --enable-runasroot option. Normally dosemu will switch privileges off at startup and only set them on, when it needs them. With '--enable-runasroot' it would permanently run under root privileges and only disable them when accessing secure relevant resources, ... not so good.

- Never allow DPMI programms to run, when dosemu is suid root.

  (in /etc/dosemu.conf set 'dpmi off' to disable)

  It is possible to overwrite sensitive parts of the emulator code, and this makes it possible for a intruder program under DOS, who knows about dosemu internals (which is easy as you have the source) to get root access also on non dosemu processes. Because a lot of games won't work without, we allow creation of LDT-descriptor that span the whole user space.

  There is a 'secure' option in /etc/dosemu.conf, that allows to turn off creation of above mentioned descriptors, but those currently protect only the dosemu code and the stack, and may be some diabolical person finds a way to use the (unprotected) heap.

  Anyway, better 'secure on' than nothing.