Mosaic User Authentication Tutorial


This tutorial surveys the current methods in
NCSA Mosaic and NCSA HTTPd for restricting access to documents. The tutorial also walks through setup and use of these methods.

Mosaic 2.0 and NCSA HTTPd allow access restriction based on several criteria:

This tutorial is based heavily on work done by Ari Luotonen at CERN and Rob McCool at NCSA. In particular, Ari wrote the client-side code currently in Mosaic 2.0, and Rob wrote NCSA HTTPd 1.3.

Tutorial Contents

Getting Started

Before you can explore access authorization, you need to install NCSA HTTPd 1.0a5 or later on a Unix machine under your control, or get write access to one or more directories in a filespace already being served by NCSA HTTPd. Other HTTP Servers also support access authentication, and some of the information presented here will help you understand how it works, but this tutorial is designed specifically for NCSA HTTPd.

NCSA HTTPd 1.5 has support for HTTP Basic Authentication (Basic), as well as the proposed Message Digest Authentication (MD5). Most, if not all, current browsers should support HTTP Basic Authentication, but not all browsers support MD5. Some browsers that do include NCSA Mosaic/X 2.7 and Spyglass Mosaic

General Information

There are two levels at which authentication information can be passed to the server: the global access configuration file and the per-directory configuration files. This tutorial primarily covers per-directory configuration. See the NCSA HTTPd documentation for information on global configuration.

Per-directory configuration means that users with write access to part of the filesystem that is being served (the Document Tree) can control access to their files as they wish. They need not have root access on the system or write access to the server's primary configuration files. Also, the per-directory configuration files are read and parsed by the server on each access, allowing run-time re-configuration. The global configuration files are only parsed on start-up or restart, which usually requires root authority. There is a speed penalty associated with using the per-directory configuration files, but that's the trade-off you have to take.

Access control for a given directory is controlled by a specific file in the directory with a filename as specified by the AccessFileName directive. The default filename is .htaccess

How Secure Is It?

In Basic HTTP Authentication, the password is passed over the network not encrypted but not as plain text -- it is "uuencoded." Anyone watching packet traffic on the network will not see the password in the clear, but the password will be easily decoded by anyone who happens to catch the right network packet.

So basically this method of authentication is roughly as safe as telnet-style username and password security -- if you trust your machine to be on the Internet, open to attempts to telnet in by anyone who wants to try, then you have no reason not to trust this method also.

In MD5 Message Digest Authentication, the password is not passed over the network at all. Instead, a series of numbers is generated based on the password and other information about the request, and these numbers are then hashed using MD5. The resulting "digest" is then sent over the network, and it is combined with other items on the server to test against the saved digest on the server. This method is more secure over the network, but it has a penalty. The comparison digest on the server must be stored in a fashion that it is retrievable. Basic Authentication stores the password using the one way crypt() function. When the password comes across, the server uudecodes it and then crypts it to check against the stored value. There is no way to get the password from the crypted value. In MD5, you need the information that is stored, so you can't use a one way hashing function to store it. This means that MD5 requires more rigorous security on the server machine. It is possible, but non-trivial, to implement this type of security under the UnixTM security model.

Basic ByPassword Authentication: Step By Step

This should help you set up protection on a directory via the Basic HTTP Authentication method. This method also uses the standard plaintext password file. If you have a large user base, NCSA HTTPd supports a DBM based password file for faster access.

So let's suppose you want to restrict files in a directory called turkey to username pumpkin and password pie. Here's what to do:

Create a file called .htaccess in directory turkey that looks like this:

AuthUserFile /otherdir/.htpasswd
AuthGroupFile /dev/null
AuthName ByPassword
AuthType Basic

<Limit GET>
require user pumpkin

Note that the password file will be in another directory (/otherdir).

AuthUserFile must be the full Unix pathname of the password file.

Also note that in this case there is no group file, so we specify /dev/null (the standard Unix way to say "this file doesn't exist").

AuthName can be anything you want. The AuthName field gives the Realm name for which the protection is provided. This name is usually given when a browser prompts for a password, and is also usually used by a browser in correlation with the URL to save the password information you enter so that it can authenticate automatically on the next challenge. Note: You should set this to something, otherwise it will default to ByPassword, which is both non-descriptive and too common.

AuthType should be set to Basic, since we are using Basic HTTP Authentication. Other possibilities for NCSA HTTPd 1.5 are PEM, PGP, KerberosV4, KerberosV5, or Digest. These other types of authentication will be discussed later.

In this example, only the method GET is restricted using the LIMIT directive. To limit other methods (particularly in CGI directories), you can specify them separated by spaces in the LIMIT directive. For example:

require user pumpkin
If you only use GET protection for a CGI script, you may be finding that the REMOTE_USER environment variable is not getting set when using METHOD="POST", obviously because the directory isn't protected against POST.

Create the password file /otherdir/.htpasswd

The easiest way to do this is to use the htpasswd program distributed with NCSA HTTPd. Do this:

htpasswd -c /otherdir/.htpasswd pumpkin

Type the password -- pie -- twice as instructed.

Check the resulting file to get a warm feeling of self-satisfaction; it should look like this:


That's all. Now try to access a file in directory turkey -- your browser should demand a username and password, and not give you access to the file if you don't enter pumpkin and pie. If you are using a browser that doesn't handle authentication, you will not be able to access the document at all.

Multiple Usernames/Passwords

If you want to give access to a directory to more than one username/password pair, follow the same steps as for a single username/password with the following additions:

Add additional users to the directory's .htpasswd file.

Use the htpasswd command without the -c flag to add additional users; e.g.:

htpasswd /otherdir/.htpasswd peanuts
htpasswd /otherdir/.htpasswd almonds
htpasswd /otherdir/.htpasswd walnuts

Create a group file.

Call it /otherdir/.htgroup and have it look something like this:

my-users: pumpkin peanuts almonds walnuts

... where pumpkin, peanuts, almonds, and walnuts are the usernames.

Then modify the .htaccess file in the directory to look like this:

AuthUserFile /otherdir/.htpasswd
AuthGroupFile /otherdir/.htgroup
AuthName ByPassword
AuthType Basic

<Limit GET>
require group my-users

Note that AuthGroupFile now points to your group file and that group my-users (rather than individual user pumpkin) is now required for access.

That's it. Now any user in group my-users can use his/her individual username and password to gain access to directory turkey.

Prepared Examples

Following are several examples of the range of access authorization capabilities available through Mosaic and NCSA HTTPd. The examples are served from a system at NCSA.

Simple protection by password.

This document is accessible only to user fido with password bones.

Important Note: There is no correspondence between usernames and passwords on specific Unix systems (e.g. in an /etc/passwd file) and usernames and passwords in the authentication schemes we're discussing for use in the Web. As illustrated in the examples, Web-based authentication uses similar but wholly distinct password files; a user need never have an actual account on a given Unix system in order to be validated for access to files being served from that system and protected with HTTP-based authentication.

Protection by password; multiple users allowed.

This document is accessible to user rover with password bacon and user jumpy with password kibbles.

Protection by network domain.

This document is only accessible to clients running on machines inside domain

Note for non-NCSA readers: The .htaccess file used in this case is as follows:

AuthUserFile /dev/null
AuthGroupFile /dev/null
AuthName ExampleAllowFromNCSA
AuthType Basic

<Limit GET>
order deny,allow
deny from all
allow from

Protection by network domain -- exclusion.

This document is accessible to clients running on machines anywhere but inside domain

Note for NCSA readers: The .htaccess file used in this case is as follows:

AuthUserFile /dev/null
AuthGroupFile /dev/null
AuthName ExampleDenyFromNCSA
AuthType Basic

<Limit GET>
order allow,deny
allow from all
deny from

For More Information

NCSA HTTPd Development Team / / 9-27-95

Casa de Bender