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. +