You want all the users on your Unixish system to be able to authenticate themselves over the Web using their already-assigned usernames and passwords.
Set up a realm using mod_auth and name /etc/passwd as the AuthUserFile:
<Directory "/home"> AuthType Basic AuthName HomeDir AuthUserFile /etc/passwd Require valid-user Satisfy All </Directory>
We must stress that using system account information for web authentication is a very bad idea, unless your site is also secured using SSL. For one thing, any intruder who happens to obtain one of your users' credentials not only can access the protected files over the Web, but can actually log onto your system where it's possible to do significant damage. For another, web logins don't have the same security controls as most operating systems; over the Web, an intruder can keep hammering away at a username with password after password without the system taking any defensive measures; all mod_auth will do is record a message in the Apache error log.However, most operating systems will enter a paranoid mode and at least ignore login attempts for a while after some number of failures.
If you still want to do this, either because you consider the risk acceptable or because it doesn't apply in your situation, the httpd.conf directives in the Solution will do the trick. The syntax and order of the fields in a credential record used by mod_auth happens (and not by accident) to match the standard layout of the /etc/passwd lines. mod_auth uses a simple text file format in which each line starts with a username and password and may optionally contain additional fields, with the fields delimited by colons. For example:
mod_auth ignores any additional fields after the password, which is what allows the /etc/passwd file to be used. Note that the password in the example is encrypted.
You can manage Apache mod_auth credential files with the htpasswd utility, but don't use this utility on the /etc/passwd file! Use the normal account maintenance tools for that.
Note that this technique will not work if shadow passwords are in use, because the password field of /etc/passwd contains nothing useful in that situation. Instead, the passwords are stored in the file /etc/shadow, which is readable only by root, while Apache runs as an unprivileged user. Furthermore, most modern Unixish operating systems use the /etc/shadow means of user authentication by default.