of the firewall capability has produced a tool
with significantly greater features than its predecessor.
ifconfig command
The ifconfig command is a
low-level command used to administer network connections on a
UNIX host. This command is used to configure network layer
address and DHCP operation interface design. ifconfig
can also be used to configure data-link layer addressing if the
network card has a configurable MAC address, and is used to
enable and disable specific interfaces. netstat
command
netstat on UNIX hosts operates in much
the same way as it does on Windows hosts. On UNIX hosts,
netstat also serves to provide the network engineer with
information on current network connections and sockets and can
also filter information displayed. route command
The
route command, not surprisingly, is used to add,
delete, and manage IP routing information on the UNIX host.
ip command
ip is a command available in
some newer UNIX variants. This command is a powerful unified
network configuration tool, and supports some unique
configuration capabilities not available in other tools. One
important capability of the ip tool is its support for
multiple protocols, including IPv4, IPv6, and IPX.
Telnet
This client can be used in the same way for
UNIX as for Windows hosts to help troubleshoot transport layer
connectivity issues. Another Telnet-like troubleshooting tool
is netcat.
Content 6.3
Troubleshooting Transport Layer Issues on Network Hosts
6.3.4 Common issues with NetBIOS networks There
are four common NetBIOS-related issues: - NetBIOS node
type misconfiguration
- NetBIOS workgroup mismatch
- NetBIOS scope mismatch
- Duplicate NetBIOS
names
These issues are usually a result of
misconfiguration by a user or network engineer. Note that
several of these problems can be caused by DHCP server
misconfiguration. One of the most common misconfigurations on a
small NetBIOS network is the NetBIOS node type. Recall from
previous content that there are four possible settings for the
NetBIOS node type: - B-node (Broadcast node)
- P-node (Peer-to-peer node)
- M-node (Mixed node)
- H-node (Hybrid node)
Obviously,
configuring the NetBIOS node type as peer-to-peer in a network
that does not have a NetBIOS name server stops the network from
functioning. Conversely, using broadcast-mode in a routed
network stops the hosts from accessing resources across the
router. The NetBIOS node type is usually set using a DHCP
server. Recall that DHCP can be used to configure more than
just a client IP address. DHCP can be used to set a number of
optional settings on the client. The settings are referred to
as DHCP options. DHCP options commonly used are shown in Figure
. Because Cisco routers can be configured as DHCP servers,
NetBIOS DHCP options need to be taken into account. Use the
commands netbios-name-server
name-server-address and netbios-node-type
type from DHCP-configuration mode to implement
NetBIOS DHCP options from a Cisco router-based DHCP server.
DHCP options 44 and 46 are commonly used in NetBIOS networks
with NetBIOS name servers. The NetBIOS node type can also be
set manually on Windows hosts. The registry of the Windows
machine must be edited in order to do this. Further information
on this can be found in knowledge-base article 160177 at
http://support.microsoft.com/. When two NetBIOS computers have
different values defined for the workgroup name, these
computers are unable to communicate. The workgroup name is
usually configured during the initial installation of the
computer operating system or when networking is being added to
the computer. The workgroup name is not case-sensitive. The
NetBIOS Scope ID is a left-over piece of NetBIOS technology
rarely used in modern networking. A NetBIOS Scope is a group of
computers that can only communicate with each other. Because
the NetBIOS Scope ID is blank by default, a single computer
misconfigured with a NetBIOS Scope ID is not able to
communicate with other hosts on the network. Because this is a
setting that is not often used, it can be quite difficult to
troubleshoot. Note that like the NetBIOS node type, the NetBIOS
Scope ID can also be controlled centrally from the DHCP server,
using DHCP option 47. Unlike the node type however, the Scope
ID can also be easily altered on the local machine through the
network interface GUI. When duplicate NetBIOS names exist in a
network, the duplicate hosts are unable to connect to the
network until they are configured with a unique NetBIOS name.
Duplicate NetBIOS names occur most often when there is no
structure to the naming scheme in a NetBIOS network. In small
networks, the lack of a structure poses no real issue. In
larger networks however, a system for ensuring unique NetBIOS
naming is strongly recommended. Note that the host name can be
set using DHCP option 12. This option is intended for use when
using DHCP reservations (where an IP address is specifically
reserved for a given MAC address), as it allows for full
host-specific network information to be controlled from a
single administrative interface. Obviously, using DHCP option
12 with dynamic DHCP clients would cause significant problems
on the network.
Content 6.3
Troubleshooting Transport Layer Issues on Network Hosts
6.3.5 Gathering NetBIOS information Several
tools are discussed in other modules for troubleshooting
network problems, such as ping, ipconfig, and
winipcfg. Some additional tools useful for gathering TCP
and NetBIOS information are: netstat is a command line utility that
displays information on protocol statistics and current TCP/IP
network connections. netstat supports IP, IPv6, ICMP,
ICMPv6, TCP, TCPv6, UDP, and UDPv6, and can be used to display
information for: - All connections and listening ports
running on the host
- Ethernet (Layer 2) statistics for
the host, such as numbers of bytes, packets, and errors
received and sent for the host.
- Port and address
information for connections to and from the host
- Which connections the host initiated
- Per-protocol
connection information
- Per-protocol statistics
- Routing table entries
netstat can also
be configured to re-query and re-display information at a
configured interval. By configuring netstat to re-query
at intervals and directing this output to a text file,
netstat can be used to build a profile of the behavior
of the host TCP/IP transport layer operations over time.
nbtstat is a command line utility that displays
information on protocol statistics and current NetBIOS
connections running over TCP/IP. Unlike netstat,
nbtstat is also able to interrogate remote machines for
connection information. Recall that NetBIOS uses a flat
namespace, and that accurate NetBIOS name-to-IP address mapping
is important for correct operation of a NetBIOS-based network.
The functions of nbtstat are centered around reporting
on and resetting the information in name tables on local
machines, remote machines, and central NetBIOS name servers.
Content 6.4 Troubleshooting Complex
Network Systems 6.4.1 Identifying complex
transport layer problems As a network increases in
complexity, the chances of network failure and sub optimal
performance increase. Problems that might have remained
undetected, in simpler networks, may combine to cause problems
in a more complex one. Complex network problems are usually
caused by the combination of smaller problems, which by
themselves appear to have no immediate impact on the network.
To resolve these problems, develop a profile of the symptoms
and use this as a starting point for troubleshooting
activities. Also keep in mind that some network problems can be