Implementing CIFS: The Common Internet File System

CIFS is a network filesystem plus a set of auxiliary services supported by a bunch of underlying protocols. Any and all of these various bits have been called CIFS, which leaves us with a somewhat muddy definition. To make things easier, we'll start by saying that CIFS is "Microsoft's way of doing network file sharing," and work out the details as we go on.

The name "CIFS," of course, is an acronym. It stands for C ommon I nternet F ile S ystem, a title which deserves a bit of dissection.

Common

The term has a variety of connotations , but we will assume that Microsoft was thinking of common in the sense of commonly available or commonly used . All Microsoft operating systems have had some form of CIFS networking available or built in, and there are implementations of CIFS for most major non-MS operating systems as well.

Unfortunately, there is not yet a specification for CIFS that is complete, correct, authoritative , and freely available. Microsoft defines CIFS by their implementations and, as we shall see, their attempts at documenting the complete suite have been somewhat random. This has an adverse impact on the commonality of the system.

Internet

At the time that the "CIFS" name was coined many people felt that Microsoft was late to the table regarding the exploitation of the Internet. As will be described further on, the naming scheme they used back then (based on a piece of older LAN technology known as NetBIOS) doesn't scale to large networkscertainly not the Internet. The idea that CIFS would become an Internet standard probably came out of the work that was being done to redesign Microsoft's networking products for Windows NT5 (now known as Windows 2000 or W2K). Under W2K, CIFS can use the Domain Name System (DNS) for name resolution.

File System

CIFS allows you to share directories, files, printers, and other cool computer stuff across a network. That's the filesystem part. To make use of these shared resources you need to be able to find and identify them, and you also need to control access so that unauthorized folk won't fiddle where they shouldn't. This means that there is a hefty amount of administrivia to be managed, so CIFS file sharing comes surrounded by an entourage. There are separate, but intertwined protocols for service announcement, naming, authentication, and authorization. Some are based on published standards, others are not, and most have changed over the years .

0.2.1 A Recipe for Protocol Soup

The filesharing protocol at the heart of CIFS is an updated version of the venerable S erver M essage B lock (SMB) protocol, which dates back to the mid-1980s. The new name first appeared around 1996/97 when Microsoft submitted draft CIFS specifications to the I nternet E ngineering T ask F orce (IETF). Those drafts have since expired , and more recent documentation made available by Microsoft comes encumbered with confusing (and pointless) licensing restrictions.

The SMB protocol was originally developed to run over NetBIOS ( Net work B asic I nput O utput S ystem) LANs. This is a nasty little skeleton in the CIFS closet. Until W2K, NetBIOS support was required for SMB transport. The machine and service names visible in the Windows "Network Neighborhood" are, basically, NetBIOS addresses.

With Windows 3.11 (Windows for Workgroups), Microsoft introduced a service announcement and location system called the Browse Service. This service maintains the list of available file and print services that is presented via the Network Neighborhood (named "My Network Places" in newer Windows products). Also with Windows 3.11 Microsoft introduced the "workgroup" concept. Workgroups simplified network management by organizing servers and services into administrative groups. Microsoft expanded upon the workgroup concept under Windows NT to create NT Domains. ^[1]

^[1] The terms "NT Domain" and "W2K Domain" will be used to distinguish Microsoft's authentication/authorization domains from Domain Name System (DNS) domains.

As if that were not enough, there are also several SMB "dialects." These roughly correspond to major OS product releases or updates from Microsoft, and each adds extensions to the core SMB protocol. In their IETF CIFS draft, Microsoft presented an SMB dialect that was independent of NetBIOS, and W2K does include such a beast . As part of the split with NetBIOS, W2K also offers new name resolution, service announcement, authentication, and authorization mechanismsall based, more or less, upon Internet standards.

Don't worry. Like most complex problems, this can all be understood by breaking it down into little pieces and studying each one in turn . The whole is not so terrible once you understand the parts .