Network Configuration: Lecture 2 - Troubleshooting Network

System Administration

Module 2:  Lecture 2:

Network Configuration: Troubleshooting Networks

 Troubleshooting Networks

How to Determine Network Configuration or Hardware Problems
A network admin’s job is not just to set up a network, but maintain it

• Thus, if anything goes wrong, the admin should track down the problem somehow
• The problem may be a configuration error, which the admin should fix
• The problem may be a hardware failure, which can only be fixed by repairing or replacing the hardware
• Nevertheless, the admin must determine which hardware has failed, so this can happen

 A Typical Scenario :
A user calls up the network admin, saying their network connection is down

• For most non-technical users, even this clear of a statement of the problem might not be possible
• A network admin will eventually figure at least this much out from the user’s comments

Step 1: Test Local Connectivity

First, an admin will try to connect to a local computer
           cmd% ping frankie.csfac8.uwindsor.ca
          ping: unknown host frankie.csfac8.uwindsor.ca
This could mean DNS servers are unavailable

• The admin should check the DNS server addresses, and the servers

Next, the admin will make sure the IP address works
      cmd% ping 137.207.140.93
     Destination host unreachable.
     Destination host unreachable.
     Destination host unreachable.

This could mean no connectivity

• The admin should check the configuration settings (ipconfig, netconfig, network-admin)
• However, if the ping command responds from a router that a host is unreachable, the node may actually be unreachable, or the address may not be bound

Obviously, we have connectivity since the router sent a message to us

 How Ping Works

Ping works by sending an ICMP Echo Request to the destination. The destination sends back an ICMP Echo Reply to the host. The host records the round trip time, and repeats the process a few times.

 A Problem With Ping

 ICMP Echo Requests are often disabled in some networks

• This is due to denial of service attacks that use ICMP Echo Request/Reply

In these networks, ping will not work

 Step 2: Test Remote Connectivity

 First, an admin will try to connect to a local computer
              cmd% ping www.google.ca
             ping: unknown host www.google.ca

This could be a routing problem

• Most likely the routing problem is within the organization
• The network administrator should check the route using the traceroute (traceroute, tracert) command

Testing Routes

 The traceroute command can give the logical addresses of each router in a route

• It may also tell you if there is a problem with the route

In Windows (2000/XP):
           tracert www.google.ca

In Linux/Solaris:
          traceroute www.google.ca

 How Does Traceroute Work?

Traceroute sends either a UDP or ICMP Echo packet to the destination

• Initially, the hop limit is set to 1, which causes the first router to drop the packet
• Routers, however, will respond with an ICMP Time Exceeded error message, which contains the router’s IP address
• The process is repeated a few times to get a few round trip times
• The domain name of the IP address is found (for convenience of the network admin)
• Next, the process is repeated with a hop limit of 2
• Then 3, 4, etc. until the destination host is reached

 The Windows Network Diagnostic Commands

 Ipconfig

Ipconfig is a Console Command which can be issued to the Command Line Interpreter (or command prompt) to display the network settings currently assigned to any or all network adapters in the machine. This command can be utilized to verify a network connection as well as to verify your network settings.

 Netstat

Displays active TCP connections, ports on which the computer is listening, Ethernet statistics, the IP routing table, IPv4 statistics (for the IP, ICMP, TCP, and UDP protocols), and IPv6 statistics (for the IPv6, ICMPv6, TCP over IPv6, and UDP over IPv6 protocols). Used without parameters, netstat displays active TCP connections.

Tracert

The tracert command is used to visually see a network packet being sent and received and the amount of hops required for that packet to get to its destination.
Users with Microsoft Windows 2000 and Windows XP who need additional information network latency and network loss should also consider using the patching command.

Ping

Helps in determining TCP/IP Networks IP address as well as determine issues with the network and assists in resolving them.

Pathping

Provides information about network latency and network loss at intermediate hops between a source and destination. Pathping sends multiple Echo Request messages to each router between a source and destination over a period of time and then computes results based on the packets returned from each router.

Nslookup

Displays information that you can use to diagnose Domain Name System (DNS) infrastructure. Before using this tool, you should be familiar with how DNS works. The Nslookup command-line tool is available only if you have installed the TCP/IP protocol.






Module 2_Lecture 2 - Network Configuration - Troubleshooting Networks