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| 6.3 |  | Troubleshooting Transport Layer Issues on Network Hosts |
| 6.3.3 |  | Gathering transport layer information on UNIX machines |

There are many variants of the UNIX operating system. The most common are SunOS, FreeBSD, SCO, and Linux. All of these operating systems have tools that can be used to troubleshoot network issues. These tools have been covered in greater detail elsewhere in this curriculum and are mentioned here in the context of troubleshooting options for transport layer issues on UNIX hosts.

Note that while these tools often have the same name and purpose, the command line options required to use them vary greatly from platform to platform. Consult the online manual of the local operating software, using the command man [tool_name], to get more information about the command line options of the tool to use.

It is important to note that almost all newer versions of UNIX variants support native packet-filtering firewall features. When troubleshooting transport layer problems with UNIX hosts, network engineers should be aware of the possibility that the problem could be caused by the local machine firewall configuration.

There are three common commands used to configure UNIX firewall features:

- On older UNIX variants using kernel version 2.0, use the ipfwadm command
- On UNIX variants using kernel version 2.2, use the ipchains command
- On newer UNIX variants using kernel version 2.4 or later, use the iptables command

Each new version 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.