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 Feature: Worried When Solving Network Problems?

Why can't my computer reach the gateway?
Why can I ping this computer, but can't connect to it?
What does this ICMP error mean?
Why answers from this firewall seems to be random?
Why... ?, When... ?, What... ?
All these questions correspond to frequently encountered problems by people dealing with networks.

 (Submitted by Laurent Constantin Tue May 22, 2001 )

  

Still worried when solving network problems?

Why can't my computer reach the gateway?
Why can I ping this computer, but can't connect on it?
What does this ICMP error mean?
Why answers from this firewall seems to be random?
Why... ?, When... ?, What... ?
All these questions correspond to frequently encountered problems by people dealing with networks :
  • network administrators
  • network developers
  • network students
  • etc.

Answers to these questions are numerous and depend on the systems, devices, network topology and protocols used.

Most of the time when people want help, and ask questions, they should have been able to find the solution by themselves. Indeed, their problems were not caused by a technical malfunction of their network, but by a misunderstanding about their network.

The aim of this article is not to answer to common questions, but to remind us to write "learn more about my network" on our todo list.


On a network, computers can communicate using several protocols.
The most frequently encountered network architecture is IP over Ethernet, so we have to understand:
  • Ethernet
  • ARP (RARP)
  • IP
  • ICMP
  • UDP
  • TCP

For example, if we don't know the answer of these questions, we need to emphasize on ... :
  • What's the difference between Ethernet and Internet ? --+ Ethernet, IP
  • ARP obtains the MAC address associated with an IP address; true or false? --+ Ethernet, ARP
  • Most of the time, why is no one answering RARP requests? --+ RARP
  • Is IP encapsulted in Ethernet ? --+ Ethernet
  • What's the difference between a switch and a hub? --+ Ethernet
  • Which kind of packets can I sniff when network traffic is being switched? --+ Ethernet
  • Can we see a Ethernet address behind a router? --+ Ethernet, IP
  • An IP packet is up to 64kbytes, but Ethernet is often 1500 bytes. How can it be sent ? --+ IP
  • How does IP routing work? --+ IP
  • ICMP is often drawn inside IP, and TCP/UDP drawn on IP. But really isn't ICMP encapsulated the same way TCP and UDP are? --+ ICMP
  • Can a system send back an ICMP error to an ICMP packet? --+ ICMP
  • What's the difference between TCP and UDP? --+ UDP, TCP
  • How does a TCP handshake work? --+ TCP
  • What's the usage of bits SYN, ACK, FIN, RST, URG, and PSH? --+ TCP
  • Etc. Hundreds of questions could be written.

Information on these subjects can be found on the web. For example, we can seach for "TCP/IP tutorial" and several documents will be displayed.

The protocols are described in RFCs :

We might also use the free Linux tool lcrzoex to improve our skills.

Lcrzoex contains over 200 functionnalities to test an Ethernet/IP network (sniff, spoof, configuration, clients, servers, etc.) :
# lcrzoex
a - easy examples (new users, start here)
b - informations on this computer
c - informations on other computers
d - sniff
e - ethernet spoof
f - ip spoof
g - udp spoof
h - tcp spoof
i - icmp spoof
j - arp/rarp spoof
k - udp real clients
l - udp virtual clients
m - tcp real clients
n - tcp virtual clients
o - udp real servers
p - udp virtual servers
q - tcp real servers
r - tcp virtual servers
s - udp real multiclient servers
t - udp virtual multiclient servers
u - tcp real multiclient servers
v - complex examples
w - miscellaneous utilities

More information and the last version of lcrzoex is available at:
http://www.laurentconstantin.com/us/lcrzo/lcrzoex/ [main server]
http://go.to/laurentconstantin/us/lcrzo/lcrzoex/ [backup server]
http://laurentconstantin.est-la.com/us/lcrzo/lcrzoex/ [backup server]


In conclusion, if we can afford to spend time learning more about our network, it will help in solving common problems faster. Several documents, standards and tools can be used to improve our knowledge.


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