Integrating MS Windows networks with Samba

Agenda

To identify the key functional mechanisms of MS Windows networking -to enable the deployment of Samba as a means of extending and/or -replacing MS Windows NT/2000 technology.

We will examine:

Name resolution in a pure Unix/Linux TCP/IP - environment -
Name resolution as used within MS Windows - networking -
How browsing functions and how to deploy stable - and dependable browsing using Samba -
MS Windows security options and how to - configure Samba for seemless integration -
Configuration of Samba as:
1. A stand-alone server
2. An MS Windows NT 3.x/4.0 security domain member -
3. An alternative to an MS Windows NT 3.x/4.0 Domain Controller -

Name Resolution in a pure Unix/Linux world

The key configuration files covered in this section are:

/etc/hosts
/etc/resolv.conf
/etc/host.conf
/etc/nsswitch.conf

`/etc/hosts`

Contains a static list of IP Addresses and names. -eg:

	127.0.0.1	localhost localhost.localdomain
-	192.168.1.1	bigbox.caldera.com	bigbox	alias4box

The purpose of /etc/hosts is to provide a -name resolution mechanism so that uses do not need to remember -IP addresses.

Network packets that are sent over the physical network transport -layer communicate not via IP addresses but rather using the Media -Access Control address, or MAC address. IP Addresses are currently -32 bits in length and are typically presented as four (4) decimal -numbers that are separated by a dot (or period). eg: 168.192.1.1

MAC Addresses use 48 bits (or 6 bytes) and are typically represented -as two digit hexadecimal numbers separated by colons. eg: -40:8e:0a:12:34:56

Every network interfrace must have an MAC address. Associated with -a MAC address there may be one or more IP addresses. There is NO -relationship between an IP address and a MAC address, all such assignments -are arbitary or discretionary in nature. At the most basic level all -network communications takes place using MAC addressing. Since MAC -addresses must be globally unique, and generally remains fixed for -any particular interface, the assignment of an IP address makes sense -from a network management perspective. More than one IP address can -be assigned per MAC address. One address must be the primary IP address, -this is the address that will be returned in the ARP reply.

When a user or a process wants to communicate with another machine -the protocol implementation ensures that the "machine name" or "host -name" is resolved to an IP address in a manner that is controlled -by the TCP/IP configuration control files. The file -/etc/hosts is one such file.

When the IP address of the destination interface has been -determined a protocol called ARP/RARP is used to identify -the MAC address of the target interface. ARP stands for Address -Resolution Protocol, and is a broadcast oriented method that -uses UDP (User Datagram Protocol) to send a request to all -interfaces on the local network segment using the all 1's MAC -address. Network interfaces are programmed to respond to two -MAC addresses only; their own unique address and the address -ff:ff:ff:ff:ff:ff. The reply packet from an ARP request will -contain the MAC address and the primary IP address for each -interface.

The /etc/hosts file is foundational to all -Unix/Linux TCP/IP installations and as a minumum will contain -the localhost and local network interface IP addresses and the -primary names by which they are known within the local machine. -This file helps to prime the pump so that a basic level of name -resolution can exist before any other method of name resolution -becomes available.

`/etc/resolv.conf`

This file tells the name resolution libraries:

The name of the domain to which the machine - belongs -
The name(s) of any domains that should be - automatically searched when trying to resolve unqualified - host names to their IP address -
The name or IP address of available Domain - Name Servers that may be asked to perform name to address - translation lookups -

`/etc/host.conf`

/etc/host.conf is the primary means by -which the setting in /etc/resolv.conf may be affected. It is a -critical configuration file. This file controls the order by -which name resolution may procede. The typical structure is:

	order hosts,bind
-	multi on

then both addresses should be returned. Please refer to the -man page for host.conf for further details.

`/etc/nsswitch.conf`

This file controls the actual name resolution targets. The -file typically has resolver object specifications as follows:

	# /etc/nsswitch.conf
-	#
-	# Name Service Switch configuration file.
-	#
-
-	passwd:		compat
-	# Alternative entries for password authentication are:
-	# passwd:	compat files nis ldap winbind
-	shadow:		compat
-	group:		compat
-
-	hosts:		files nis dns
-	# Alternative entries for host name resolution are:
-	# hosts:	files dns nis nis+ hesoid db compat ldap wins
-	networks:	nis files dns
-
-	ethers:		nis files
-	protocols:	nis files
-	rpc:		nis files
-	services:	nis files

Of course, each of these mechanisms requires that the appropriate -facilities and/or services are correctly configured.

It should be noted that unless a network request/message must be -sent, TCP/IP networks are silent. All TCP/IP communications assumes a -principal of speaking only when necessary.

Samba version 2.2.0 will add Linux support for extensions to -the name service switch infrastructure so that linux clients will -be able to obtain resolution of MS Windows NetBIOS names to IP -Addresses. To gain this functionality Samba needs to be compiled -with appropriate arguments to the make command (ie: make -nsswitch/libnss_wins.so). The resulting library should -then be installed in the /lib directory and -the "wins" parameter needs to be added to the "hosts:" line in -the /etc/nsswitch.conf file. At this point it -will be possible to ping any MS Windows machine by it's NetBIOS -machine name, so long as that machine is within the workgroup to -which both the samba machine and the MS Windows machine belong.

Name resolution as used within MS Windows networking

MS Windows networking is predicated about the name each machine -is given. This name is known variously (and inconsistently) as -the "computer name", "machine name", "networking name", "netbios name", -"SMB name". All terms mean the same thing with the exception of -"netbios name" which can apply also to the name of the workgroup or the -domain name. The terms "workgroup" and "domain" are really just a -simply name with which the machine is associated. All NetBIOS names -are exactly 16 characters in length. The 16th character is reserved. -It is used to store a one byte value that indicates service level -information for the NetBIOS name that is registered. A NetBIOS machine -name is therefore registered for each service type that is provided by -the client/server.

The following are typical NetBIOS name/service type registrations:

	Unique NetBIOS Names:
-		MACHINENAME<00>	= Server Service is running on MACHINENAME
-		MACHINENAME<03> = Generic Machine Name (NetBIOS name)
-		MACHINENAME<20> = LanMan Server service is running on MACHINENAME
-		WORKGROUP<1b> = Domain Master Browser
-
-	Group Names:
-		WORKGROUP<03> = Generic Name registered by all members of WORKGROUP
-		WORKGROUP<1c> = Domain Controllers / Netlogon Servers
-		WORKGROUP<1d> = Local Master Browsers
-		WORKGROUP<1e> = Internet Name Resolvers

It should be noted that all NetBIOS machines register their own -names as per the above. This is in vast contrast to TCP/IP -installations where traditionally the system administrator will -determine in the /etc/hosts or in the DNS database what names -are associated with each IP address.

One further point of clarification should be noted, the /etc/hosts -file and the DNS records do not provide the NetBIOS name type information -that MS Windows clients depend on to locate the type of service that may -be needed. An example of this is what happens when an MS Windows client -wants to locate a domain logon server. It find this service and the IP -address of a server that provides it by performing a lookup (via a -NetBIOS broadcast) for enumeration of all machines that have -registered the name type *<1c>. A logon request is then sent to each -IP address that is returned in the enumerated list of IP addresses. Which -ever machine first replies then ends up providing the logon services.

The name "workgroup" or "domain" really can be confusing since these -have the added significance of indicating what is the security -architecture of the MS Windows network. The term "workgroup" indicates -that the primary nature of the network environment is that of a -peer-to-peer design. In a WORKGROUP all machines are responsible for -their own security, and generally such security is limited to use of -just a password (known as SHARE MODE security). In most situations -with peer-to-peer networking the users who control their own machines -will simply opt to have no security at all. It is possible to have -USER MODE security in a WORKGROUP environment, thus requiring use -of a user name and a matching password.

MS Windows networking is thus predetermined to use machine names -for all local and remote machine message passing. The protocol used is -called Server Message Block (SMB) and this is implemented using -the NetBIOS protocol (Network Basic Input Output System). NetBIOS can -be encapsulated using LLC (Logical Link Control) protocol - in which case -the resulting protocol is called NetBEUI (Network Basic Extended User -Interface). NetBIOS can also be run over IPX (Internetworking Packet -Exchange) protocol as used by Novell NetWare, and it can be run -over TCP/IP protocols - in which case the resulting protocol is called -NBT or NetBT, the NetBIOS over TCP/IP.

MS Windows machines use a complex array of name resolution mechanisms. -Since we are primarily concerned with TCP/IP this demonstration is -limited to this area.

The NetBIOS Name Cache

All MS Windows machines employ an in memory buffer in which is -stored the NetBIOS names and IP addresses for all external -machines that that machine has communicated with over the -past 10-15 minutes. It is more efficient to obtain an IP address -for a machine from the local cache than it is to go through all the -configured name resolution mechanisms.

If a machine whose name is in the local name cache has been shut -down before the name had been expired and flushed from the cache, then -an attempt to exchange a message with that machine will be subject -to time-out delays. i.e.: Its name is in the cache, so a name resolution -lookup will succeed, but the machine can not respond. This can be -frustrating for users - but it is a characteristic of the protocol.

The MS Windows utility that allows examination of the NetBIOS -name cache is called "nbtstat". The Samba equivalent of this -is called "nmblookup".

The LMHOSTS file

This file is usually located in MS Windows NT 4.0 or -2000 in C:\WINNT\SYSTEM32\DRIVERS\ETC and contains -the IP Address and the machine name in matched pairs. The -LMHOSTS file performs NetBIOS name -to IP address mapping oriented.

It typically looks like:

	# Copyright (c) 1998 Microsoft Corp.
-	#
-	# This is a sample LMHOSTS file used by the Microsoft Wins Client (NetBIOS
-	# over TCP/IP) stack for Windows98
-	#
-	# This file contains the mappings of IP addresses to NT computernames
-	# (NetBIOS) names.  Each entry should be kept on an individual line.
-	# The IP address should be placed in the first column followed by the
-	# corresponding computername. The address and the comptername
-	# should be separated by at least one space or tab. The "#" character
-	# is generally used to denote the start of a comment (see the exceptions
-	# below).
-	#
-	# This file is compatible with Microsoft LAN Manager 2.x TCP/IP lmhosts
-	# files and offers the following extensions:
-	#
-	#      #PRE
-	#      #DOM:<domain>
-	#      #INCLUDE <filename>
-	#      #BEGIN_ALTERNATE
-	#      #END_ALTERNATE
-	#      \0xnn (non-printing character support)
-	#
-	# Following any entry in the file with the characters "#PRE" will cause
-	# the entry to be preloaded into the name cache. By default, entries are
-	# not preloaded, but are parsed only after dynamic name resolution fails.
-	#
-	# Following an entry with the "#DOM:<domain>" tag will associate the
-	# entry with the domain specified by <domain>. This affects how the
-	# browser and logon services behave in TCP/IP environments. To preload
-	# the host name associated with #DOM entry, it is necessary to also add a
-	# #PRE to the line. The <domain> is always preloaded although it will not
-	# be shown when the name cache is viewed.
-	#
-	# Specifying "#INCLUDE <filename>" will force the RFC NetBIOS (NBT)
-	# software to seek the specified <filename> and parse it as if it were
-	# local. <filename> is generally a UNC-based name, allowing a
-	# centralized lmhosts file to be maintained on a server.
-	# It is ALWAYS necessary to provide a mapping for the IP address of the
-	# server prior to the #INCLUDE. This mapping must use the #PRE directive.
-	# In addtion the share "public" in the example below must be in the
-	# LanManServer list of "NullSessionShares" in order for client machines to
-	# be able to read the lmhosts file successfully. This key is under
-	# \machine\system\currentcontrolset\services\lanmanserver\parameters\nullsessionshares
-	# in the registry. Simply add "public" to the list found there.
-	#
-	# The #BEGIN_ and #END_ALTERNATE keywords allow multiple #INCLUDE
-	# statements to be grouped together. Any single successful include
-	# will cause the group to succeed.
-	#
-	# Finally, non-printing characters can be embedded in mappings by
-	# first surrounding the NetBIOS name in quotations, then using the
-	# \0xnn notation to specify a hex value for a non-printing character.
-	#
-	# The following example illustrates all of these extensions:
-	#
-	# 102.54.94.97     rhino         #PRE #DOM:networking  #net group's DC
-	# 102.54.94.102    "appname  \0x14"                    #special app server
-	# 102.54.94.123    popular            #PRE             #source server
-	# 102.54.94.117    localsrv           #PRE             #needed for the include
-	#
-	# #BEGIN_ALTERNATE
-	# #INCLUDE \\localsrv\public\lmhosts
-	# #INCLUDE \\rhino\public\lmhosts
-	# #END_ALTERNATE
-	#
-	# In the above example, the "appname" server contains a special
-	# character in its name, the "popular" and "localsrv" server names are
-	# preloaded, and the "rhino" server name is specified so it can be used
-	# to later #INCLUDE a centrally maintained lmhosts file if the "localsrv"
-	# system is unavailable.
-	#
-	# Note that the whole file is parsed including comments on each lookup,
-	# so keeping the number of comments to a minimum will improve performance.
-	# Therefore it is not advisable to simply add lmhosts file entries onto the
-	# end of this file.

HOSTS file

This file is usually located in MS Windows NT 4.0 or 2000 in -C:\WINNT\SYSTEM32\DRIVERS\ETC and contains -the IP Address and the IP hostname in matched pairs. It can be -used by the name resolution infrastructure in MS Windows, depending -on how the TCP/IP environment is configured. This file is in -every way the equivalent of the Unix/Linux /etc/hosts file.

DNS Lookup

This capability is configured in the TCP/IP setup area in the network -configuration facility. If enabled an elaborate name resolution sequence -is followed the precise nature of which isdependant on what the NetBIOS -Node Type parameter is configured to. A Node Type of 0 means use -NetBIOS broadcast (over UDP broadcast) is first used if the name -that is the subject of a name lookup is not found in the NetBIOS name -cache. If that fails then DNS, HOSTS and LMHOSTS are checked. If set to -Node Type 8, then a NetBIOS Unicast (over UDP Unicast) is sent to the -WINS Server to obtain a lookup before DNS, HOSTS, LMHOSTS, or broadcast -lookup is used.

WINS Lookup

A WINS (Windows Internet Name Server) service is the equivaent of the -rfc1001/1002 specified NBNS (NetBIOS Name Server). A WINS server stores -the names and IP addresses that are registered by a Windows client -if the TCP/IP setup has been given at least one WINS Server IP Address.

To configure Samba to be a WINS server the following parameter needs -to be added to the smb.conf file:

	wins support = Yes

To configure Samba to use a WINS server the following parameters are -needed in the smb.conf file:

	wins support = No
-	wins server = xxx.xxx.xxx.xxx

where xxx.xxx.xxx.xxx is the IP address -of the WINS server.

How browsing functions and how to deploy stable and -dependable browsing using Samba

As stated above, MS Windows machines register their NetBIOS names -(i.e.: the machine name for each service type in operation) on start -up. Also, as stated above, the exact method by which this name registration -takes place is determined by whether or not the MS Windows client/server -has been given a WINS server address, whether or not LMHOSTS lookup -is enabled, or if DNS for NetBIOS name resolution is enabled, etc.

In the case where there is no WINS server all name registrations as -well as name lookups are done by UDP broadcast. This isolates name -resolution to the local subnet, unless LMHOSTS is used to list all -names and IP addresses. In such situations Samba provides a means by -which the samba server name may be forcibly injected into the browse -list of a remote MS Windows network (using the "remote announce" parameter).

Where a WINS server is used, the MS Windows client will use UDP -unicast to register with the WINS server. Such packets can be routed -and thus WINS allows name resolution to function across routed networks.

During the startup process an election will take place to create a -local master browser if one does not already exist. On each NetBIOS network -one machine will be elected to function as the domain master browser. This -domain browsing has nothing to do with MS security domain control. -Instead, the domain master browser serves the role of contacting each local -master browser (found by asking WINS or from LMHOSTS) and exchanging browse -list contents. This way every master browser will eventually obtain a complete -list of all machines that are on the network. Every 11-15 minutes an election -is held to determine which machine will be the master browser. By the nature of -the election criteria used, the machine with the highest uptime, or the -most senior protocol version, or other criteria, will win the election -as domain master browser.

Clients wishing to browse the network make use of this list, but also depend -on the availability of correct name resolution to the respective IP -address/addresses.

Any configuration that breaks name resolution and/or browsing intrinsics -will annoy users because they will have to put up with protracted -inability to use the network services.

Samba supports a feature that allows forced synchonisation -of browse lists across routed networks using the "remote -browse sync" parameter in the smb.conf file. This causes Samba -to contact the local master browser on a remote network and -to request browse list synchronisation. This effectively bridges -two networks that are separated by routers. The two remote -networks may use either broadcast based name resolution or WINS -based name resolution, but it should be noted that the "remote -browse sync" parameter provides browse list synchronisation - and -that is distinct from name to address resolution, in other -words, for cross subnet browsing to function correctly it is -essential that a name to address resolution mechanism be provided. -This mechanism could be via DNS, /etc/hosts, -and so on.

MS Windows security options and how to configure -Samba for seemless integration

MS Windows clients may use encrypted passwords as part of a -challenege/response authentication model (a.k.a. NTLMv1) or -alone, or clear text strings for simple password based -authentication. It should be realized that with the SMB -protocol the password is passed over the network either -in plain text or encrypted, but not both in the same -authentication requets.

When encrypted passwords are used a password that has been -entered by the user is encrypted in two ways:

An MD4 hash of the UNICODE of the password - string. This is known as the NT hash. -
The password is converted to upper case, - and then padded or trucated to 14 bytes. This string is - then appended with 5 bytes of NULL characters and split to - form two 56 bit DES keys to encrypt a "magic" 8 byte value. - The resulting 16 bytes for the LanMan hash. -

You should refer to the Password Encryption chapter in this HOWTO collection -for more details on the inner workings

MS Windows 95 pre-service pack 1, MS Windows NT versions 3.x -and version 4.0 pre-service pack 3 will use either mode of -password authentication. All versions of MS Windows that follow -these versions no longer support plain text passwords by default.

MS Windows clients have a habit of dropping network mappings that -have been idle for 10 minutes or longer. When the user attempts to -use the mapped drive connection that has been dropped, the client -re-establishes the connection using -a cached copy of the password.

When Microsoft changed the default password mode, they dropped support for -caching of the plain text password. This means that when the registry -parameter is changed to re-enable use of plain text passwords it appears to -work, but when a dropped mapping attempts to revalidate it will fail if -the remote authentication server does not support encrypted passwords. -This means that it is definitely not a good idea to re-enable plain text -password support in such clients.

The following parameters can be used to work around the -issue of Windows 9x client upper casing usernames and -password before transmitting them to the SMB server -when using clear text authentication.

	passsword level = integer
-	username level = integer

By default Samba will lower case the username before attempting -to lookup the user in the database of local system accounts. -Because UNIX usernames conventionally only contain lower case -character, the username level parameter -is rarely even needed.

However, password on UNIX systems often make use of mixed case -characters. This means that in order for a user on a Windows 9x -client to connect to a Samba server using clear text authentication, -the password level must be set to the maximum -number of upper case letter which could appear -is a password. Note that is the server OS uses the traditional -DES version of crypt(), then a password level -of 8 will result in case insensitive passwords as seen from Windows -users. This will also result in longer login times as Samba -hash to compute the permutations of the password string and -try them one by one until a match is located (or all combinations fail).

The best option to adopt is to enable support for encrypted passwords -where ever Samba is used. There are three configuration possibilities -for support of encrypted passwords:

Use MS Windows NT as an authentication server

This method involves the additions of the following parameters -in the smb.conf file:

	encrypt passwords = Yes
-	security = server
-	password server = "NetBIOS_name_of_PDC"

There are two ways of identifying whether or not a username and -password pair was valid or not. One uses the reply information provided -as part of the authentication messaging process, the other uses -just and error code.

The down-side of this mode of configuration is the fact that -for security reasons Samba will send the password server a bogus -username and a bogus password and if the remote server fails to -reject the username and password pair then an alternative mode -of identification of validation is used. Where a site uses password -lock out after a certain number of failed authentication attempts -this will result in user lockouts.

Use of this mode of authentication does require there to be -a standard Unix account for the user, this account can be blocked -to prevent logons by other than MS Windows clients.

Make Samba a member of an MS Windows NT security domain

This method involves additon of the following paramters in the smb.conf file:

	encrypt passwords = Yes
-	security = domain
-	workgroup = "name of NT domain"
-	password server = *

The use of the "*" argument to "password server" will cause samba -to locate the domain controller in a way analogous to the way -this is done within MS Windows NT.

In order for this method to work the Samba server needs to join the -MS Windows NT security domain. This is done as follows:

On the MS Windows NT domain controller using - the Server Manager add a machine account for the Samba server. -
Next, on the Linux system execute: - smbpasswd -r PDC_NAME -j DOMAIN_NAME -

Use of this mode of authentication does require there to be -a standard Unix account for the user in order to assign -a uid once the account has been authenticated by the remote -Windows DC. This account can be blocked to prevent logons by -other than MS Windows clients by things such as setting an invalid -shell in the /etc/passwd entry.

An alternative to assigning UIDs to Windows users on a -Samba member server is presented in the Winbind Overview chapter in -this HOWTO collection.

Configure Samba as an authentication server

This mode of authentication demands that there be on the -Unix/Linux system both a Unix style account as well as an -smbpasswd entry for the user. The Unix system account can be -locked if required as only the encrypted password will be -used for SMB client authentication.

This method involves addition of the following parameters to -the smb.conf file:

## please refer to the Samba PDC HOWTO chapter later in 
-## this collection for more details
-[global]
-	encrypt passwords = Yes
-	security = user
-	domain logons = Yes
-	; an OS level of 33 or more is recommended
-	os level = 33
-
-[NETLOGON]
-	path = /somewhare/in/file/system
-	read only = yes

in order for this method to work a Unix system account needs -to be created for each user, as well as for each MS Windows NT/2000 -machine. The following structure is required.

Users

A user account that may provide a home directory should be -created. The following Linux system commands are typical of -the procedure for creating an account.

	# useradd -s /bin/bash -d /home/"userid" -m "userid"
-	# passwd "userid"
-	  Enter Password: <pw>
-	  
-	# smbpasswd -a "userid"
-	  Enter Password: <pw>

MS Windows NT Machine Accounts

These are required only when Samba is used as a domain -controller. Refer to the Samba-PDC-HOWTO for more details.

	# useradd -s /bin/false -d /dev/null "machine_name"\$
-	# passwd -l "machine_name"\$
-	# smbpasswd -a -m "machine_name"

Conclusions

Samba provides a flexible means to operate as...

A Stand-alone server - No special action is needed - other than to create user accounts. Stand-alone servers do NOT - provide network logon services, meaning that machines that use this - server do NOT perform a domain logon but instead make use only of - the MS Windows logon which is local to the MS Windows - workstation/server. -
An MS Windows NT 3.x/4.0 security domain member. -
An alternative to an MS Windows NT 3.x/4.0 - Domain Controller. -