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:
A stand-alone 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
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.