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List:       ipchains-list
Subject:    Re: 2 important questions
From:       Justin Shore <listuser () neo ! pittstate ! edu>
Date:       2001-02-20 23:09:08
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At 10:32 AM +0000 2/20/01, RANCHORDAS - JIGAR wrote:
>Hello there,
>
>1. Is DHCP (Dynamic Host Configuration Protocol) used only by
>ISPs to assign dynamic IP addresses to customers?  Is is used for
>another purpose?

It doesn't have to be used by ISPs.  My University uses it to 
staticly assign IPs to campus machines.  That way they can move users 
around in our 12 C's without touching the user's machines.  I 
personally don't like doing it that way but...   Where ever you 
NAT/MASQ, you could easily use DHCP.  Labs or dorms are good 
examples.  Keep a record of who (by MAC) was given what IP at what 
time and you then have your accountability.  It can be very useful 
for users that roam, say wireless laptop users or instructors that 
move from building to building with their laptop to teach a class. 
If you have a routed network, you've broken your campus up into 
physical pieces, usually buildings or floors if there are a high 
concentration of unique users.  When user Joe Blow moves from 
building A to building D to teach a class, he takes his laptop. 
You're currently assign Joe a static IP via DHCP that will only work 
in the building he's currently in.  Well, now he's in a nother 
building and wants to use his laptop on the network.  With DHCP and 
the help of your router, you can assign an IP helper address to an 
interface feeding a building.  When the router receives a DHCP 
request on that interface, it forwards it on the the IP helper 
address you defined.  I belive it also sets a header tag or maybe 
changes the content of the packet so the DHCP server knows which 
network the request came from (reason will be apparent in a second). 
My knowledge of the IP helper is logical because I haven't yet done 
it.  It works though.  The DHCP server receives the request and then 
decides whether or not to give that machine an IP.  This all depends 
on how you want the requests handled.  Maybe you want all requests 
assigned an IP.  Maybe you want to make sure each person gets a 
certain IP.  Maybe you just want to make sure you know about that 
person (ie recognize the requesting MAC) before you give it an IP. 
However you do it, it should also take into account the network that 
the node will be on because in most routed cases, that IP wil only 
work in that building.  The DHCP server decides to assign an IP to 
that node to it pick an IP that will work on the network that the 
request came from (and isn't already in use or reserved, ie it's in 
the pool of IPs you told the server it could use), and sends the 
directed reply back to that node.  Now Joe has an IP that will work 
in building D instead of his normal building, A.  Make sense?  Like I 
said, this is the logical way it would do it to the best of my 
knowledge.  Someday I'll actually get the powers that be to allow me 
to revamp our flat network.

>2. Using an ipchains script, is there any way to use a 'if statement'
>in the beginning to test whether networking is up and running in
>Linux?


# Source networking configuration.
. /etc/sysconfig/network

# Check that networking is up.
[ ${NETWORKING} = "no" ] && exit 0


Justin
-- 

--
Justin Shore                    Pittsburg State University
Systems Manager                 Kelce 157Q
Office of Information Systems   Pittsburg, KS 66762
Voice: (316) 235-4606           Fax: (316) 235-4545
http://www.pittstate.edu/ois/

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