In article <44m1u4$
...@corona.laa.com.au>,
bru
...@corona.laa.com.au (Bruce Lyon) wrote:
>Adrian Ball (adrian.b
...@bbsrc.ac.uk) wrote:
>: The subject says it all!
>check out latest bootp stuff on your nearest archie ...!
>: --
>: Adrian Ball Bio-Informatics Dept, IACR-Rothamsted
>: adrian.b...@bbsrc.ac.uk Harpenden, Herts
>: +44 1582 763133 - ext 2419 England
>: http://www.res.bbsrc.ac.uk/computing/staff/adrian.html AL5 2JQ
or try this
DHCP FAQ
Author
John Wobus, jmwo...@syr.edu (corrections welcome)
Date
8/15/1995
This file
http://web.syr.edu/~jmwobus/comfaqs/dhcp.faq.html
Questions
1. General
1. What is DHCP?
2. What is DHCP's purpose?
3. How is it different that BOOTP or RARP?
4. Why shouldn't clients assign IP numbers without the use of a
server?
5. Can DHCP support statically defined addresses?
6. Can a BOOTP client boot from a DHCP server?
7. Can a DHCP client boot from a BOOTP server?
8. Can a DHCP client update its DNS entry through DHCP?
9. When will the server to server protocol be defined?
10. Is there a DHCP mailing list?
11. In a subnetted environment, how does the DHCP server discover
what subnet a request has come from?
12. Where is DHCP defined?
13. Can DHCP support remote access?
14. What are the Gotcha's?
2. Info on Implementations
1. What freeware DHCP servers are available?
2. What commercial DHCP servers are available?
3. Which vendors of client software currently support DHCP?
4. What are the DHCP plans of major client-software vendors?
5. What Routers forward DHCP requests?
6. Which implementations support or require the broadcast flag?
Answers
1. General
1. What is DHCP?
DHCP stands for "Dynamic Host Configuration Protocol".
2. What is DHCP's purpose?
DHCP's purpose is to enable individual computers on an IP
network to extract their configurations from a server (the
'DHCP server') or servers, in particular, servers that have
no exact information about the individual computers until
they request the information. The overall purpose of this is
> to reduce the work necessary to administer a large IP
network.
3. How is it different that BOOTP or RARP?
DHCP is based on BOOTP and maintains some backward
compatibility. The main difference is that BOOTP was designed
for manual pre-configuration of the host information in a
server database, while DHCP allows for dynamic allocation of
network addresses and configurations to newly attached hosts.
Additionally, DHCP allows for recovery and reallocation of
network addresses through a leasing mechanism.
RARP is a protocol used by Sun and other vendors that allows
a computer to find out its own IP number, which is one of the
protocol parameters typically passed to the client system by
DHCP or BOOTP. RARP doesn't support other parameters and
using it, a server can only serve a single LAN. DHCP and
BOOTP are designed so they can be routed.
4. Why shouldn't clients assign IP numbers without the use of a
server?
It is theoretically possible for client-machines to find
addresses to use by picking an address out of the blue and
broadcasting a request of all the other client machines to
see if they are using them. Appletalk is designed around this
idea, and Apple's MacTCP can be configured to do this for IP.
However, this method of IP address assignment has
disadvantages.
1. A computer that needs a permanently-assigned IP number
might be turned off and lose its number to a machine
coming up. This has problems both for finding services
and for security.
2. A network might be temporarily divided into two
non-communicating networks while a network component is
not functioning. During this time, two different
client-machines might end up claiming the same IP
number. When the network comes back, they start
malfunctioning.
3. If such dynamic assignment is to be confined to ranges
of IP addresses, then the ranges are configured in each
desktop machine rather than being centrally
administered. This can lead both to hidden configuration
errors and to difficulty in changing the range. Another
problem with the use of such ranges is keeping it easy
to move a computer from one subnet to another.
5. Can DHCP support statically defined addresses?
Yes. At least there is nothing in the protocol to preclude
this and one expects it to be a feature of any DHCP server.
This is really a server matter and the client should work
either way.
6. Can a BOOTP client boot from a DHCP server?
A DHCP server can be written this way. Since DHCP was
developed after BOOTP, it would be logical for most server
developers to support this.
7. Can a DHCP client boot from a BOOTP server?
A DHCP client can be written this way, i.e. to treat a BOOTP
reply as an unending lease on the IP address.
8. Can a DHCP client update its DNS entry through DHCP?
There are options in DHCP through which a DHCP client can
request that its DNS entry be updated. DHCP clients will be
able to take advantage of planned enhancements to the DNS
protocol that will allow dynamic updates through the network.
(Note: as far as I can tell, the DNS needs no protocol update
since the server already tells the clients how long they can
use the information they receive; what is really needed is a
DNS server that can make fuller use of this feature and that
cooperates with a DHCP server, perhaps through the use of
some new "DHCP-server-to-DNS-server" protocol).
9. When will the server to server protocol be defined?
The DHC WG of the IETF is actively investigating the issues
in inter-server communication. The protocol should be defined
"soon".
10. Is there a DHCP mailing list?
There are several:
List Purpose
---- -------
host-c...@sol.eg.bucknell.edu General discussion
dhcp-b...@bucknell.edu DHCP bakeoffs
dhcp-i...@bucknell.edu Implementations
dhcp-se...@bucknell.edu Server to server protocol
Admin requests for the host-conf list should go to
host-conf-requ...@sol.eg.bucknell.edu; admin requests for
the other lists should go to lists...@bucknell.edu.
11. In a subnetted environment, how does the DHCP server discover
what subnet a request has come from?
DHCP client messages are sent to off-net servers by DHCP
relay agents, which are often a part of an IP router. The
DHCP relay agent records the subnet from which the message
was received in the DHCP message header for use by the DHCP
server.
12. Where is DHCP defined?
In Internet RFCs.
RFC1541
R. Droms, "Dynamic Host Configuration Protocol",
10/27/1993.
RFC1534
R. Droms, "Interoperation Between DHCP and BOOTP",
10/08/1993.
RFC1533
S. Alexander, R. Droms, "DHCP Options and BOOTP
Vendor Extensions", 10/08/1993.
13. Can DHCP support remote access?
PPP has its own non-DHCP way in which communications servers
can hand clients an IP address called IPCP (IP Control
Protocol) but doesn't have the same flexibility as DHCP or
BOOTP in handing out other parameters. Such a communications
server may support the use of DHCP to acquire the IP
addresses it gives out. This is sometimes called doing DHCP
by proxy for the client. I know that Windows NT's remote
access support does this.
A feature of DHCP under development (DHCPinform) is a method
by which a DHCP server can supply parameters to a client that
already has an IP number. With this, a PPP client could get
its IP number using IPCP, then get the rest of its parameters
using this feature of DHCP.
SLIP has no standard way in which a server can hand a client
an IP address, but many communications servers support
non-standard ways of doing this that can be utilized by
scripts, etc. Thus, like communications servers supporting
PPP, such communications servers could also support the use
of DHCP to acquire
...
read more »
