Chapter 1. Introduction to Samba

Chapter 1. Introduction to Samba
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David Lechnyr

Unofficial HOWTO

<david@lechnyr.com>

April 14, 2003

Table of Contents

Background
Terminology
Related Projects
SMB Methodology
Epilogue
Miscellaneous

“ -"If you understand what you're doing, you're not learning anything." --- Anonymous -”

-Samba is a file and print server for Windows-based clients using TCP/IP as the underlying -transport protocol. In fact, it can support any SMB/CIFS-enabled client. One of Samba's big -strengths is that you can use it to blend your mix of Windows and Linux machines together -without requiring a separate Windows NT/2000/2003 Server. Samba is actively being developed -by a global team of about 30 active programmers and was originally developed by Andrew Tridgell. -

Background

-Once long ago, there was a buzzword referred to as DCE/RPC. This stood for Distributed -Computing Environment/Remote Procedure Calls and conceptually was a good idea. It was -originally developed by Apollo/HP as NCA 1.0 (Network Computing Architecture) and only -ran over UDP. When there was a need to run it over TCP so that it would be compatible -with DECnet 3.0, it was redesigned, submitted to The Open Group, and officially became -known as DCE/RPC. Microsoft came along and decided, rather than pay $20 per seat to -license this technology, to reimplement DCE/RPC themselves as MSRPC. From this, the -concept continued in the form of SMB (Server Message Block, or the "what") using the -NetBIOS (Network Basic Input/Output System, or the "how") compatibility layer. You can -run SMB (i.e., transport) over several different protocols; many different implementations -arose as a result, including NBIPX (NetBIOS over IPX, NwLnkNb, or NWNBLink) and NBT -(NetBIOS over TCP/IP, or NetBT). As the years passed, NBT became the most common form -of implementation until the advance of "Direct-Hosted TCP" -- the Microsoft marketing -term for eliminating NetBIOS entirely and running SMB by itself across TCP port 445 -only. As of yet, direct-hosted TCP has yet to catch on. -

-Perhaps the best summary of the origins of SMB are voiced in the 1997 article titled, CIFS: -Common Insecurities Fail Scrutiny: -

-Several megabytes of NT-security archives, random whitepapers, RFCs, the CIFS spec, the Samba -stuff, a few MS knowledge-base articles, strings extracted from binaries, and packet dumps have -been dutifully waded through during the information-gathering stages of this project, and there -are *still* many missing pieces... While often tedious, at least the way has been generously -littered with occurrences of clapping hand to forehead and muttering 'crikey, what are they -thinking? -

Terminology

- SMB: Acronym for "Server Message Block". This is Microsoft's file and printer sharing protocol. -
- CIFS: Acronym for "Common Internet File System". Around 1996, Microsoft apparently - decided that SMB needed the word "Internet" in it, so they changed it to CIFS. -
- Direct-Hosted: A method of providing file/printer sharing services over port 445/tcp - only using DNS for name resolution instead of WINS. -
- IPC: Acronym for "Inter-Process Communication". A method to communicate specific - information between programs. -
- Marshalling: - A method of serializing (i.e., sequential ordering of) variable data - suitable for transmission via a network connection or storing in a file. The source - data can be re-created using a similar process called unmarshalling. -
- NetBIOS: Acronym for "Network Basic Input/Output System". This is not a protocol; - it is a method of communication across an existing protocol. This is a standard which - was originally developed for IBM by Sytek in 1983. To exaggerate the analogy a bit, - it can help to think of this in comparison your computer's BIOS -- it controls the - essential functions of your input/output hardware -- whereas NetBIOS controls the - essential functions of your input/output traffic via the network. Again, this is a bit - of an exaggeration but it should help that paradigm shift. What is important to realize - is that NetBIOS is a transport standard, not a protocol. Unfortunately, even technically - brilliant people tend to interchange NetBIOS with terms like NetBEUI without a second - thought; this will cause no end (and no doubt) of confusion. -
- NetBEUI: Acronym for the "NetBIOS Extended User Interface". Unlike NetBIOS, NetBEUI - is a protocol, not a standard. It is also not routable, so traffic on one side of a - router will be unable to communicate with the other side. Understanding NetBEUI is - not essential to deciphering SMB; however it helps to point out that it is not the - same as NetBIOS and to improve your score in trivia at parties. NetBEUI was originally - referred to by Microsoft as "NBF", or "The Windows NT NetBEUI Frame protocol driver". - It is not often heard from these days. -
- NBT: Acronym for "NetBIOS over TCP"; also known as "NetBT". Allows the continued use - of NetBIOS traffic proxied over TCP/IP. As a result, NetBIOS names are made - to IP addresses and NetBIOS name types are conceptually equivalent to TCP/IP ports. - This is how file and printer sharing are accomplished in Windows 95/98/ME. They - traditionally rely on three ports: NetBIOS Name Service (nbname) via UDP port 137, - NetBIOS Datagram Service (nbdatagram) via UDP port 138, and NetBIOS Session Service - (nbsession) via TCP port 139. All name resolution is done via WINS, NetBIOS broadcasts, - and DNS. NetBIOS over TCP is documented in RFC 1001 (Concepts and methods) and RFC 1002 - (Detailed specifications). -
- W2K: Acronym for Windows 2000 Professional or Server -
- W3K: Acronym for Windows 2003 Server -

If you plan on getting help, make sure to subscribe to the Samba Mailing List (available at -http://www.samba.org). -

Related Projects

-There are currently two network filesystem client projects for Linux that are directly -related to Samba: SMBFS and CIFS VFS. These are both available in the Linux kernel itself. -

- SMBFS (Server Message Block File System) allows you to mount SMB shares (the protocol - that Microsoft Windows and OS/2 Lan Manager use to share files and printers - over local networks) and access them just like any other Unix directory. This is useful - if you just want to mount such filesystems without being a SMBFS server. -
- CIFS VFS (Common Internet File System Virtual File System) is the successor to SMBFS, and - is being actively developed for the upcoming version of the Linux kernel. The intent of this module - is to provide advanced network file system functionality including support for dfs (hierarchical - name space), secure per-user session establishment, safe distributed caching (oplock), - optional packet signing, Unicode and other internationalization improvements, and optional - Winbind (nsswitch) integration. -

-Again, it's important to note that these are implementations for client filesystems, and have -nothing to do with acting as a file and print server for SMB/CIFS clients. -

-There are other Open Source CIFS client implementations, such as the -jCIFS project -which provides an SMB client toolkit written in Java. -

SMB Methodology

-Traditionally, SMB uses UDP port 137 (NetBIOS name service, or netbios-ns), -UDP port 138 (NetBIOS datagram service, or netbios-dgm), and TCP port 139 (NetBIOS -session service, or netbios-ssn). Anyone looking at their network with a good -packet sniffer will be amazed at the amount of traffic generated by just opening -up a single file. In general, SMB sessions are established in the following order: -

- "TCP Connection" - establish 3-way handshake (connection) to port 139/tcp - or 445/tcp. -
- "NetBIOS Session Request" - using the following "Calling Names": The local - machine's NetBIOS name plus the 16th character 0x00; The server's NetBIOS - name plus the 16th character 0x20 -
- "SMB Negotiate Protocol" - determine the protocol dialect to use, which will - be one of the following: PC Network Program 1.0 (Core) - share level security - mode only; Microsoft Networks 1.03 (Core Plus) - share level security - mode only; Lanman1.0 (LAN Manager 1.0) - uses Challenge/Response - Authentication; Lanman2.1 (LAN Manager 2.1) - uses Challenge/Response - Authentication; NT LM 0.12 (NT LM 0.12) - uses Challenge/Response - Authentication -
- SMB Session Startup. Passwords are encrypted (or not) according to one of - the following methods: Null (no encryption); Cleartext (no encryption); LM - and NTLM; NTLM; NTLMv2 -
- SMB Tree Connect: Connect to a share name (e.g., \\servername\share); Connect - to a service type (e.g., IPC$ named pipe) -

-A good way to examine this process in depth is to try out -SecurityFriday's SWB program. -It allows you to walk through the establishment of a SMB/CIFS session step by step. -

Epilogue

“ -What's fundamentally wrong is that nobody ever had any taste when they -did it. Microsoft has been very much into making the user interface look good, -but internally it's just a complete mess. And even people who program for Microsoft -and who have had years of experience, just don't know how it works internally. -Worse, nobody dares change it. Nobody dares to fix bugs because it's such a -mess that fixing one bug might just break a hundred programs that depend on -that bug. And Microsoft isn't interested in anyone fixing bugs -- they're interested -in making money. They don't have anybody who takes pride in Windows 95 as an -operating system. -”

“ -People inside Microsoft know it's a bad operating system and they still -continue obviously working on it because they want to get the next version out -because they want to have all these new features to sell more copies of the -system. -”

“ -The problem with that is that over time, when you have this kind of approach, -and because nobody understands it, because nobody REALLY fixes bugs (other than -when they're really obvious), the end result is really messy. You can't trust -it because under certain circumstances it just spontaneously reboots or just -halts in the middle of something that shouldn't be strange. Normally it works -fine and then once in a blue moon for some completely unknown reason, it's dead, -and nobody knows why. Not Microsoft, not the experienced user and certainly -not the completely clueless user who probably sits there shivering thinking -"What did I do wrong?" when they didn't do anything wrong at all. -”

“ -That's what's really irritating to me." -”

-- -Linus Torvalds, from an interview with BOOT Magazine, Sept 1998 -

Miscellaneous

-This chapter is Copyright 2003 David Lechnyr (david at lechnyr dot com). -Permission is granted to copy, distribute and/or modify this document under the terms -of the GNU Free Documentation License, Version 1.2 or any later version published by the Free -Software Foundation. A copy of the license is available at http://www.gnu.org/licenses/fdl.txt. -