Web Magazine for Information Professionals

A Knight's Tale: Networked CD-ROM Redirectors

Jon Knight describes how MSCDEX.EXE and networked CD-ROM redirectors can introduce difficulties when using Windows 95 and NT to provide access to library CD-ROMs.

Many libraries have collections of CD-ROMs which they must deliver to their patrons in order to provide a well rounded information service. In many academic libraries and some larger public and company libraries some portion of the CD-ROM discs on offer are made available over local area computer networks. These networked CD-ROMs can then be used by the library’s patrons from workstations distributed throughout the library and indeed organisation.

In order to provide network access to the CD-ROMs, there must be some form of networked CD-ROM server placed on the network. Currently there are a large number of companies offering such products, with a wide range of facilities and price tags. In many cases these servers run on commodity PC hardware and use either proprietary protocols or extensions to existing network operating systems such as Novell Netware.

When CD-ROMs are used on standalone PCs they have traditionally had to use a small extension to the DOS operating system called the Microsoft CD-ROM Extensions or MSCDEX.EXE. MSCDEX.EXE is a small piece of software that can redirect calls to certain drive letters into commands to the CD-ROM drive. The software provides a consistant interface to any number of different types of drive and even allows different sorts of drive to be connected to the same machine at the same time. Many of the commercial networked CD-ROM server products have therefore simulated the Microsoft MSCDEX.EXE program with their own CD-ROM redirectors which must be loaded in place of the MSCDEX.EXE program in order to provide the workstation with access to the networked CD-ROMs.

Unlike MSCDEX.EXE itself, these redirectors are often capable of being unloaded once the user has finished using a networked CD-ROM product, which allows a library to “mix and match” multiple servers from different vendors. Note that machines with local CD-ROM drives that load MSCDEX.EXE can often have problems with redirector products because once loaded, MSCDEX.EXE can not easily be unloaded. Solutions to this problem include providing multiple boot time options (i.e. run with or without a local CD-ROM) and trying to ‘hide’ MSCDEX.EXE whilst the network redirectors are running. The first of these solutions works but is annoying for the end user whereas the latter doesn’t require a reboot but does not always work cleanly.

Problems with Windows 95/NT and large campus networks

With the advent of Windows 95 and Windows NT the need to use MSCDEX.EXE has been removed from the operating system. Both of these operating systems come with built-in driver support for a large number of CD-ROM drives. Whilst this has proved useful on machines with local CD-ROM drives attached, it can cause problems when machines with these operating systems are used with older networked CD-ROM servers that require the use of MSCDEX.EXE style redirectors. Either the redirector will refuse to load because it can detect Windows95/NT’s built in MSCDEX.EXE clone code or it can sometimes cause the operating system to crash or become unstable.

Another problem with using redirector software in a network environment is the need to install the redirector software onto all of the client machines prior to allowing them to attach to the CD-ROM service. Whilst this is achievable with the PCs directly under the control of the library systems team, if the CD-ROMs are to be used on PCs spread throughout the organisation which are managed by a variety of department’s IT staff, then the task becomes much more difficult. Ideally what is required is a method of accessing the networked CD-ROMs without the use of the redirector software.

Networking without redirectors

Some commercial networked CD-ROM support products have provided access to the networked CD-ROMs without a redirector for some time. One such example is the SCSI Express product from MDI which bolts onto a standard Novell Netware server. SCSI Express provides management tools for CD-ROM towers and/or jukeboxes and allows the workstations to access the discs by simply mapping to tbem as though they were a normal Novell CD-ROM drive.

The SCSI Express system works well for many CD-ROM products and allows CD-ROMs to be accessed by DOS, Windows 3.1, Windows 95 and Windows NT clients. A few CD-ROM products do cause problems though: these discs have code built into them to detect the MSCDEX.EXE (or equivalent) redirector in order to access the CD-ROM directly rather than via a mapped drive. Although SCSI Express does come with a “fake” redirector, this often does not fool these products. Luckily these products are becoming rarer and rarer and amongst the networked CD-ROM products in the Pilkington Library at - level access. These include DOS versions of Clarinet Clearview based products such as RAPRA, CITIS and Ergonomic Abstracts, the UKOP disc and the DOS version of FAME. Even with these products the newer Windows versions may be able to operate without the redirector in place (though you might need two discs held in different servers if you need to provide access to both DOS and Windows versions with and without redirectors respectively).

Another option for providing access to CD-ROMs via Netware is to make use of one of the free UNIX like operating systems such as Linux or FreeBSD and the freely available MARS NWE Netware server software. The CD-ROMs can be mounted as normal Linux filesystems and can then be exported as Novell Netware volumes by the MARS NWE software.

The advantages of this method of providing access to CD-ROMs over the network are not only that the operating system and server software are free, but also that the UNIX-like operating environment makes it easy to mount and dismount discs, requires relatively low spec hardware to offer a basic service and also can host other file services (such as providing access to the CD-ROMs for Macintoshes using the netatalk package or to Windows boxes using their native SMB networking by installing Samba).

A more radical product that some libraries have used with some success is Citrix’s WinFrame product. This allows any Windows application to be run on the server and merely have a thin client display the output on the user’s workstation. As the server environment is under control of the systems administrator all necessary drivers, fonts, DLL, etc. can be loaded on there for the CD-ROM products. Once the WinFrame software is loaded on the client PCs, no further configuration or installations are required on them. All new CD-ROM installations and upgrades are done directly on the server. The major downside of the software is currently the cost, although that factor is likely to change now that Microsoft have announced that Windows NT 5.0 will contain a modified form of the underlying technology licensed from Citrix.


The number of CD-ROM products that need redirectors that clone the behaviour of MSCDEX.EXE is becoming lower and lower. With the introduction of technologies such as WinFrame, the prospects for successfully providing networked access to a wide range of CD-ROM products over large campus networks is looking brighter than it has for some time. Coupled with the low cost options available by using the free UNIX look-alike operating systems as servers, there appears to be a solution for every budget and situation.

Author details

Jon Knight
ROADS Technical Developer
Email: jon@net.lut.ac.uk
URL: http://www.roads.lut.ac.uk/People/jon.html
Tel: o1509 228237