CNT-2311-Chapter 8 Notes

Ifup and ifdown command *(pg. 408)

-The ifup command bring the interface up

-The ifdown command bring the interface down

Example of ifup/ifdown *(pag.408)

“ifup eth0”

–bring up the interface eth0“ifdown eth0”.

–bring down the interface eth0- useful to quickly to take down a interface or

bring it up, because you do not need to remember all the detail of the IP

address, route and so on.

Hostnames *(pg. 409)

There are 2 way to configure the hostname

-On DNS

-On you local computer

Domain Name System (DNS) *(pg. 409)

- “is a distributed database computers that converts between IP addresses and

hostnames”.

- The DNS server just sit and listen for other computers on the network to send

request.

- The server then send the request out to other DNS server if it cannot find it

in it cache.

- If the server found what the request is asking for then it will send a request

back to the computer telling it where to go.

Diagnosing Network Connections *(pg. 412)

- There are a few command that will help diagnose a problem.

- They are ping, traceroute, tracepath, netstat, and tcpdump.

Ping command *(pg. 412)

- The ping command is a very basic network test, which send ICMP packet to the

system you name, hostnames, or ip address and wait for a reply

- In Linux it send the packet every second until you press Ctrl+C key to stop

the stroke.

Traceroute command *(pg. 412-413)

- This command will sends a series of three test packets to each computer

between your system and a specified target system

Tracepath command *(pg. 413)

- This command is a lot similar to traceroute

Checking Network Status

-A useful diagnostic tool is Netstat

-Netstat can be used in place of many other tools.

-It also shows information that is not easily found in other ways.

Netstat options

-Interface information: use netstat –interface or -i parameter to see information about

network interfaces. This is similar to what ifconfig command displays

-Routing information: use netstat –route or -r parameter to see the routing table. This

output is similar to what the route command shows

-Masquerade information: use netstat –masquerade or –M to see information about

connections mediated by Linux’s NAT features . This is a good way to stretch limited

IPv4 addresses.

-Program use: use netstat –program or –p parameter to show information about programs that

are using network connections.

-All connections: use netstat –all or –a parameter to display information about the ports

that server programs open to listen for network connections.

tcpdump

-Advanced network troubleshooting tool.

-tcpdump is a packet sniffer that intercepts network packets and log them on the screen.

-Useful tool to verify that computer is receiving data from other computers

-This is also useful to examine the data in its raw form, which can be helpful if you

-understand protocols implementation details allowing you to spot problems.

-Be aware that packet sniffers can be used by individuals to capture passwords sent over

the network.

-You must use tcpdump in root mode.

-Once you enter tcpdump it summarizes what it is doing and prints lines, one for each packet

it monitors.

-Lines consist of time stamps, stack identifiers, origin system name, IP addresses and

port, destination system name and packet-specific information.

-Once tcpdump starts it dose not stop so you must press Ctrl+C to terminate it.

tcpdump options

- A: displays packet contents in ASCII

- D: Displays a list of interfaces tcpdump can listen to.

- n: Shows all addresses numerically.

- v: Shows additional packet information.

- w file: Used to write captured packets to the specified file.

-You could consult tcpdump’s man page for details and for additional options.

Additional Networking Tools

-Other than networking diagnostic programs you could use some common programs as debugging tools.

-One of the most helpful tools may be Telnet a program and protocol tool that is mainly used as

a remote login tool.

-Telnet is considered a poor choice as a remote login protocol due to the fact that it

is entirely unencrypted.

-A good rule to follow is that you should remove Telnet server from your system and never use

the telnet client program. SSH is a safer alternative to Telnet described in chapter 10.

Using Telnet to Debug Network Protocols

-If you include a port number after the remote hostname, the program connects to the

specified port allowing you to interact with the server.

-To use telnet this way you must know a lot of information about the protocol. Without

the knowledge of the protocol you can still use Telnet to test if a server is running or not.

-If you try to connect and get a connection refused error message you know that a remote

server is not running or is inaccessible. (This may be because a firewall is blocking

the connection)

-If the result is Escape character this means that the server is running, even though it

could not be working correctly.

-This test only works for protocols that us TCP. UDP tools will not connect.

Using Telnet to Debug Network Protocols continued

-Sometimes File Transfer Protocols (FTP) can be a helpful tool.

-This program allows you to transfer files between systems, as its name suggests.

-To use this tool type the program name followed by the FTP servers name, which will then

show you a login prompt, allowing you to issue FTP commands.