Frequently Asked Questions

Linux Security HOWTO
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# 13. Frequently Asked Questions

1. Is it more secure to compile driver support directly into the kernel, instead of making it a module?

   Answer: Some people think it is better to disable the ability to load device drivers using modules, because an intruder could load a Trojan module or a module that could affect system security.

   However, in order to load modules, you must be root. The module object files are also only writable by root. This means the intruder would need root access to insert a module. If the intruder gains root access, there are more serious things to worry about than whether he will load a module.

   Modules are for dynamically loading support for a particular device that may be infrequently used. On server machines, or firewalls for instance, this is very unlikely to happen. For this reason, it would make more sense to compile support directly into the kernel for machines acting as a server. Modules are also slower than support compiled directly in the kernel.

2. Why does logging in as root from a remote machine always fail?

   Answer: See Section 4.2. This is done intentionally to prevent remote users from attempting to connect via `telnet` to your machine as `root`, which is a serious security vulnerability, because then the root password would be transmitted, in clear text, across the network. Don't forget: potential intruders have time on their side, and can run automated programs to find your password. Additionally, this is done to keep a clear record of who logged in, not just root.

3. How do I enable shadow passwords on my Linux box?

   Answer:

   To enable shadow passwords, run `pwconv` as root, and `/etc/shadow` should now exist, and be used by applications. If you are using RH 4.2 or above, the PAM modules will automatically adapt to the change from using normal `/etc/passwd` to shadow passwords without any other change.

   Some background: shadow passwords is a mechanism for storing your password in a file other than the normal `/etc/passwd` file. This has several advantages. The first one is that the shadow file, `/etc/shadow`, is only readable by root, unlike `/etc/passwd`, which must remain readable by everyone. The other advantage is that as the administrator, you can enable or disable accounts without everyone knowing the status of other users' accounts.

   The `/etc/passwd` file is then used to store user and group names, used by programs like `/bin/ls` to map the user ID to the proper user name in a directory listing.

   The `/etc/shadow` file then only contains the user name and his/her password, and perhaps accounting information, like when the account expires, etc.

   To enable shadow passwords, run `pwconv` as root, and `/etc/shadow` should now exist, and be used by applications. Since you are using RH 4.2 or above, the PAM modules will automatically adapt to the change from using normal `/etc/passwd` to shadow passwords without any other change.

   Since you're interested in securing your passwords, perhaps you would also be interested in generating good passwords to begin with. For this you can use the `pam_cracklib` module, which is part of PAM. It runs your password against the Crack libraries to help you decide if it is too-easily guessable by password-cracking programs.

4. How can I enable the Apache SSL extensions?

   Answer:

   1. Get SSLeay 0.8.0 or later from �

   2. Build and test and install it!

   3. Get Apache source

   4. Get Apache SSLeay extensions from here

   5. Unpack it in the apache source directory and patch Apache as per the README.

   6. Configure and build it.

   You might also try ZEDZ net which has many pre-built packages, and is located outside of the United States.

5. How can I manipulate user accounts, and still retain security?

   Answer: most distributions contain a great number of tools to change the properties of user accounts.

   - The `pwconv` and `unpwconv` programs can be used to convert between shadow and non-shadowed passwords.

   - The `pwck` and `grpck` programs can be used to verify proper organization of the `passwd` and `group` files.

   - The `useradd`, `usermod`, and `userdel` programs can be used to add, delete and modify user accounts. The `groupadd`, `groupmod`, and `groupdel` programs will do the same for groups.

   - Group passwords can be created using `gpasswd`.

   All these programs are "shadow-aware" -- that is, if you enable shadow they will use `/etc/shadow` for password information, otherwise they won't.

   See the respective man pages for further information.

6. How can I password-protect specific HTML documents using Apache?

   I bet you didn't know about http://www.apacheweek.org, did you?

   You can find information on user authentication at http://www.apacheweek.com/features/userauth as well as other web server security tips from http://www.apache.org/docs/misc/security_tips.html

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