|
|
Table of Contents
This chapter deals with NetBIOS over TCP/IP name to IP address resolution. If your MS Windows clients are not configured to use NetBIOS over TCP/IP, then this section does not apply to your installation. If your installation involves the use of NetBIOS over TCP/IP, then this chapter may help you to resolve networking problems.
NetBIOS over TCP/IP has nothing to do with NetBEUI. NetBEUI is NetBIOS over Logical Link Control (LLC). On modern networks it is highly advised to not run NetBEUI at all. Note also that there is no such thing as NetBEUI over TCP/IP the existence of such a protocol is a complete and utter misapprehension.
Many MS Windows network administrators have never been exposed to basic TCP/IP networking as it is implemented in a UNIX/Linux operating system. Likewise, many UNIX and Linux administrators have not been exposed to the intricacies of MS Windows TCP/IP-based networking (and may have no desire to be, either).
This chapter gives a short introduction to the basics of how a name can be resolved to its IP address for each operating system environment.
Since the introduction of MS Windows 2000, it is possible to run MS Windows networking without the use of NetBIOS over TCP/IP. NetBIOS over TCP/IP uses UDP port 137 for NetBIOS name resolution and uses TCP port 139 for NetBIOS session services. When NetBIOS over TCP/IP is disabled on MS Windows 2000 and later clients, then only the TCP port 445 is used, and the UDP port 137 and TCP port 139 are not.
When using Windows 2000 or later clients, if NetBIOS over TCP/IP is not disabled, then the client will use UDP port 137 (NetBIOS Name Service, also known as the Windows Internet Name Service, or WINS), TCP port 139, and TCP port 445 (for actual file and print traffic).
When NetBIOS over TCP/IP is disabled, the use of DNS is essential. Most installations that disable NetBIOS over TCP/IP today use MS Active Directory Service (ADS). ADS requires dynamic DNS with Service Resource Records (SRV RR) and with Incremental Zone Transfers (IXFR). Use of DHCP with ADS is recommended as a further means of maintaining central control over the client workstation network configuration.
The key configuration files covered in this section are:
/etc/hosts
/etc/resolv.conf
/etc/host.conf
/etc/nsswitch.conf
This file contains a static list of IP addresses and names.
127.0.0.1 localhost localhost.localdomain 192.168.1.1 bigbox.quenya.org bigbox alias4box
The purpose of /etc/hosts is to provide a
name resolution mechanism so users 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 decimal numbers that are separated by a dot (or period) for example, 168.192.1.1.
MAC addresses use 48 bits (or 6 bytes) and are typically represented as two-digit hexadecimal numbers separated by colons: 40:8e:0a:12:34:56.
Every network interface must have a MAC address. Associated with a MAC address may be one or more IP addresses. There is no relationship between an IP address and a MAC address; all such assignments are arbitrary or discretionary in nature. At the most basic level, all network communications take place using MAC addressing. Since MAC addresses must be globally unique and generally remain 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 Address Resolution Protocol (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 is a broadcast-oriented method that uses User Datagram Protocol (UDP) to send a request to all interfaces on the local network segment using the all 1s 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 minimum 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 a basic level of name
resolution can exist before any other method of name resolution
becomes available.
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 is the primary means by which the setting in
/etc/resolv.conf may be effected. It is a critical configuration file. This file controls
the order by which name resolution may proceed. The typical structure is:
order hosts,bind multi on
Both addresses should be returned. Please refer to the
man page for host.conf for further details.
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+ hesiod 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 assume a principal of speaking only when necessary.
Starting with version 2.2.0, Samba has Linux support for extensions to
the name service switch infrastructure so 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 (i.e., 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 its NetBIOS
machine name, as long as that machine is within the workgroup to
which both the Samba machine and the MS Windows machine belong.
MS Windows networking is predicated on the name each machine is given. This name is known variously (and inconsistently) as the “computer name,” “machine name,” “networking name,” “NetBIOS name,” or “SMB name.” All terms mean the same thing with the exception of “NetBIOS name,” which can also apply to the name of the workgroup or the domain name. The terms “workgroup” and “domain” are really just a simple 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 1-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.
Unique NetBIOS names and group names tables list typical NetBIOS name/service type registrations.
Table 28.1. 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 |
Table 28.2. Group Names
| WORKGROUP<03> | Generic name registered by all members of WORKGROUP |
| WORKGROUP<1c> | Domain cntrollers/netlogon servers |
| WORKGROUP<1d> | Local master browsers |
| WORKGROUP<1e> | Browser election service |
It should be noted that all NetBIOS machines register their own
names as per Unique NetBIOS names and group names. This is in vast contrast to TCP/IP
installations where the system administrator traditionally
determines 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 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 finds 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.
Whichever machine first replies, it 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 the use of just a password (known as share-level 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-level security in a workgroup environment, thus requiring the use of a username 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.
All MS Windows machines employ an in-memory buffer in which is stored the NetBIOS names and IP addresses for all external machines that machine has communicated with over the past 10 to 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 is shut down before the name is expired and flushed from the cache, then an attempt to exchange a message with that machine will be subject to timeout delays. Its name is in the cache, so a name resolution lookup will succeed, but the machine cannot respond. This can be frustrating for users but is a characteristic of the protocol.
The MS Windows utility that allows examination of the NetBIOS name cache is called “nbtstat.” The Samba equivalent is called nmblookup.
This file is usually located in MS Windows NT 4.0 or Windows 200x/XP in the directory
%SystemRoot%\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.
It typically looks like this:
# 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 computer names # (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 computer name. The address and the computer name # 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 effects 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 pre-loaded 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 addition the share "public" in the example below must be in the # LanMan Server 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 # pre-loaded, 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.
This file is usually located in MS Windows NT 4.0 or Windows 200x/XP in
the directory %SystemRoot%\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.
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 is dependent on how the NetBIOS Node Type parameter is configured. A Node Type of 0 means that NetBIOS broadcast (over UDP broadcast) is 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.
A WINS (Windows Internet Name Server) service is the equivalent 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.
For information about setting up Samba as a WINS server, read Network Browsing.
TCP/IP network configuration problems find every network administrator sooner or later. The cause can be anything from keyboard mishaps to forgetfulness to simple mistakes to carelessness. Of course, no one is ever deliberately careless!
“I can ping my Samba server from Windows, but I cannot ping my Windows machine from the Samba server.”
The Windows machine was at IP address 192.168.1.2 with netmask 255.255.255.0, the Samba server (Linux) was at IP address 192.168.1.130 with netmask 255.255.255.128. The machines were on a local network with no external connections.
Due to inconsistent netmasks, the Windows machine was on network 192.168.1.0/24, while the Samba server was on network 192.168.1.128/25 logically a different network.
A common cause of slow network response includes:
Client is configured to use DNS and the DNS server is down.
Client is configured to use remote DNS server, but the remote connection is down.
Client is configured to use a WINS server, but there is no WINS server.
Client is not configured to use a WINS server, but there is a WINS server.
Firewall is filtering out DNS or WINS traffic.
“The name of the Samba server was changed, Samba was restarted, and now the Samba server cannot be pinged by its new name from an MS Windows NT4 workstation, but it does still respond to pinging using the old name. Why?”
From this description, three things are obvious:
WINS is not in use; only broadcast-based name resolution is used.
The Samba server was renamed and restarted within the last 10 or 15 minutes.
The old Samba server name is still in the NetBIOS name cache on the MS Windows NT4 workstation.
To find what names are present in the NetBIOS name cache on the MS Windows NT4 machine, open a cmd shell and then:
C:\>nbtstat -nNetBIOS Local Name Table Name Type Status ------------------------------------------------ FRODO <03> UNIQUE Registered ADMINISTRATOR <03> UNIQUE Registered FRODO <00> UNIQUE Registered SARDON <00> GROUP Registered FRODO <20> UNIQUE Registered FRODO <1F> UNIQUE RegisteredC:\>nbtstat -c NetBIOS Remote Cache Name Table Name Type Host Address Life [sec] -------------------------------------------------------------- GANDALF <20> UNIQUE 192.168.1.1 240C:\>
In this example, GANDALF is the Samba server and FRODO is the MS Windows NT4 workstation. The first listing shows the contents of the Local Name Table (i.e., identity information on the MS Windows workstation), and the second shows the NetBIOS name in the NetBIOS name cache. The name cache contains the remote machines known to this workstation.