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From: jmwobus@gamera.syr.edu (John M. Wobus)
Newsgroups: comp.protocols.tcp-ip,comp.protocols.tcp-ip.ibmpc,comp.dcom.lans.misc,comp.os.ms-windows.networking.tcp-ip,comp.os.os2.networking.tcp-ip,comp.sys.mac.comm
Subject: DHCP FAQ
Date: 2 Nov 1995 16:59:00 GMT
Organization: Syracuse University, Syracuse
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DHCP FAQ
Author
John Wobus, jmwobus@syr.edu (corrections welcome)
Date
11/2/1995
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. Can a DHCP server back up another DHCP server?
10. When will the server to server protocol be defined?
11. Is there a DHCP mailing list?
12. In a subnetted environment, how does the DHCP server discover
what subnet a request has come from?
13. Where is DHCP defined?
14. Can DHCP support remote access?
15. Can a client have a home address and still float?
16. 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. What Routers include DHCP servers?
7. Which implementations support or require the broadcast flag?
8. How can I run Windows 95 without a DHCP server?
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?
No. There has been some discussion about adding this ability
to DHCP.
(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. Can a DHCP server back up another DHCP server?
This is the purpose of the "server to server protocol" (see
next question). I know of no other way that you can keep a
"hot" spare server in synch with your production server.
However, it is possible that some server vendors have
addressed this issue with their own features.
10. 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".
11. Is there a DHCP mailing list?
There are several:
List Purpose
---- -------
host-conf@sol.eg.bucknell.edu General discussion
dhcp-bake@bucknell.edu DHCP bakeoffs
dhcp-impl@bucknell.edu Implementations
dhcp-serve@bucknell.edu Server to server protocol
Admin requests for the host-conf list should go to
host-conf-request@sol.eg.bucknell.edu; admin requests for
the other lists should go to listserv@bucknell.edu. Archives
for the host-conf list are stored at
ftp.bucknell.edu://pub/dhcp/.
12. 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.
Note: a DHCP relay agent is the same thing as a BOOTP relay
agent, and the latter phrase is more commonly used.
13. 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.
14. 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 the IP addressees to give out.
I am not currently aware of any way in which DHCP can support
client-computers served solely by PPP or SLIP. Such a
computer doesn't have the IEEE-style MAC address that DHCP
requires to act as its key to determining which
client-computer is which within the same subnet.
Communications servers that acquire IP numbers for their
clients via DHCP run into the same roadblock in that they
have just one MAC address, but need to acquire more than one
IP address. One way such a communications server can get
around this problem is through the use of a set of unique
pseudo-MAC addresses for the purposes of its communications
with the DHCP server. Another way (used by Shiva) is to use a
different "client ID type" for your hardware address. Client
ID type 1 means you're using MAC addresses. However, client
ID type 0 means an ASCII string.
15. Can a client have a home address and still float?
There is nothing in the protocol to keep a client that
already has a leased or permanent IP number from getting
a(nother) lease on a temporary basis on another subnet (i.e.,
for that laptop which is almost always in one office, but
occiasionally is plugged in in a conference room or class
room). Thus it is left to the server implementation to
support such a feature. I've heard that Microsoft's NT-based
server can do it.
16. What are the Gotcha's?
o A malicious user could make trouble by putting up an
unofficial DHCP server.
# The immediate problem would be a server passing out
numbers already belonging to some computer yielding
the potential for two or more "innocent bystander"
nodes ending up with the same IP number. Net result
is problems using the nodes, possibly intermittent
of one or the other is sometimes turned off.
# A lot of problems are possible if a renegade server
manages to get a client to accept its lease
offering, and feeds the client its own version of
other booting parameters. One scenario is a client
that loads its OS over the network via tftp being
directed to a different file (possibly on a
different server), thus allowing the perpetrator to
take over the client. Given that boot parameters
are often made to control many different things
about the computers' operation and communication,
many other scenarios are just as serious.
Note that BOOTP has the same vulnerabilities.
o The "broadcast flag": DHCP includes a way in which
client implementations unable to receive a packet with a
specific IP address can ask the server or relay agent to
use the broadcast IP address in the replies (a "flag"
set by the client in the requests). The definition of
DHCP states that implementations "should" honor this
flag, but it doesn't say they "must". Some Microsoft
TCP/IP implementations used this flag, which meant in
practical terms, relay agents and servers had to
implement it. A number of BOOTP-relay-agent
implementations (e.g. in routers) handled DHCP just fine
except for the need for this feature, thus they
announced new versions stated to handle DHCP.
o Some of the virtual LAN schemes, i.e., those that use
the packet's IP number to decide which "virtual LAN" a
client-computer is on for the purposes of TCP/IP, don't
work when using DHCP to dynamically assign addresses.
DHCP servers and relay agents use their knowledge of
what LAN the client-station is on to select the subnet
number for the client-station's new IP address whereas
such switches use the subnet number sent by the
client-station to decide which (virtual) LAN to put the
station on.
o There have been servers that are inflexible as to the
list of configuration parameters they were able to
serve. If your client requires certain parameters, you
could find such a server unusable.
o Routers are sometimes configured so that one LAN on one
port has multiple network (or subnet) numbers. When the
router is relaying requests from such a LAN to the DHCP
server, it must pass along as IP number that is
associated with one of the network (or subnet) numbers.
The only way the DHCP server can allocate addresses on
one of the LAN's other network (or subnet) numbers is if
the DHCP server is specifically written to have a
feature to handle such cases, and it has a configuration
describing the situation.
o I hate to cast wide suspicions, but I've heard
occasional word on client DHCP implementations that do
not implement the entire protocol. Doing so requires
that the software module be able to wake up again after
a specified period of time and "renew the lease", i.e.,
ask to continue using the IP number. This is at least
one feature of DHCP that is very hard to implement in
some simpler systems.
o The knowledge that a particular IP number is associated
with a particular node is often used for various
functions. Examples are: for security purposes, for
network management, and even for identifying resources.
Furthermore, if the DNS's names are going to identify IP
numbers, the numbers, the IP numbers have to be stable.
Dynamic configuration of the IP numbers undercuts such
methods. For this reason, some sites try to keep the
continued use of dynamically allocatable IP numbers to a
minimum.
o There are a number of issues regarding the patched bootp
servers. These have been reported to re DD2.4.3:
# 'When run from inetd, I had problems with "Could
not bind port" and DHCP request failure. I don't
know why, and the problem went away when bootpd is
run as a daemon.'
# 'Unless you set "dl" to some value in the bootptab
file, the DHCP lease time, renewal time and
prebinding time will be rubbish, which will cause
occasional renewal problems.'
2. Info on Implementations
1. What freeware DHCP servers are available?
(This is not necessarily a complete list)
950415 Bootp server:
Bootp 2.4.3 (not DHCP, but with the "DHCP patches" mentioned
below, can handle DHCP requests)
ftp://ftp.mc.com/pub/bootp-2.4.3.tar.Z
950425 Bootp server version 2.4.3 with "samba" DHCP patches
(does static allocation of IP addresses)
http://www.sghms.ac.uk/~mpreston/bootp_dhcp.tar.Z
(within http://www.sghms.ac.uk/~mpreston/tools.htm")
950630 WIDE Project:
Akihiro Tominaga (tomy@sfc.wide.ad.jp)
WIDE Project
Keio Univ.
Japan
ftp://sh.wide.ad.jp/WIDE/free-ware/dhcp/dhcp-1.2.1.tar.gz
Check Archie for dhcp-1.2.1 because lots of sites distribute it.
950706 "samba" DHCP patches for bootp server:
(does static allocation of IP addresses)
ftp://nimbus.anu.edu.au:/pub/tridge/samba/contributed/DHCP.patch
(note: I've heard that the patched server will crash if it receives
one particular optional packet, the DHCP Release packet)
950711 Patched bootp server supporting DHCP-based "automatic" allocation:
(gives addresses dynamically, but never takes them away)
ftp://ftp.ntplx.net/pub/networking/bootp/bootp-DD2.4.3.tar.gz
2. What commercial DHCP servers are available?
(This is not necessarily a complete list)
950425 Silicon Graphics
950613 NetWare/IP 2.1 will NOT support DHCP but support for enhanced
bootp will be provided. I'm guessing this means DHCP-format
packets, but no address leasing.
950714 FTP Software (Services OnNet Product)
http://www.ftp.com/mkt_info/services.html
950714 Sun (SolarNet)
http://www.sun.com/cgi-bin/show?sunsoft/Products/Networking-products/pro
ducts/pcadmin.html
950714 Microsoft Windows NT
http://www.microsoft.com/NTServer/
http://www.microsoft.com/BackOffice/techbriefs/tech1000.htm
950714 Hewlett Packard HP-UX
950802 Process Software: server for OpenVMS
http://www.process.com/
950828 Novell: I heard a rumor that they will deliver a server by the
end of 1995.
950828 James Drews (drews@engr.wisc.edu)
of U Wisconsin is working on an NLM which he plans to sell commercially.
It is available for beta test at http://www.engr.wisc.edu:80/~drews/dhcp
/
950906 IBM: included in Warp Server which is in beta
951010 Wollongong: included in next release of PathWay for OpenVMS which is in
beta
951010 TGV: DHCP/BOOTP server will be included in Multinet for VMS v3.5.
951101 Competitive Automation's JOIN (415-321-4006): SunOS4.x,
Solaris2.x and DECOSF3.x,4.x DHCP/BOOTP servers; HP-UX planned.
http://www.join.com/
3. Which vendors of client software currently support DHCP?
(This is not necessarily a complete list)
950417 Shiva: proxy client for remote users (in Lanrovers and Netmodems)
950421 Microsoft: Windows for Workgroups
950425 Sun
950425 Silicon Graphics
950425 Hewlett-Packard
950502 NetManage: Chameleon 4.5
950630 Beame & Whiteside Software: resells Dirk Koeppen EDV-Beratungs-GmbH's
TCP/IP BOOT-PROM
950705 Microsoft: MS-TCP/IP 3.11a & MS-TCP/IP 3.11b
950711 Microsoft: Windows NT 3.5
950711 Microsoft: Windows for Workgroups 3.11a
950711 Frontier Technologies(800-929-3054): in SuperTCP for Windows
http:www.frontiertech.com
info@frontiertech.com
950712 Beame & Whiteside(800-720-7151): BW-Connect NFS for DOS & Windows
950725 IBM: a future release of AIX
950728 Sun: PCNFS for Windows
950801 FTP Software: for DOS and Windows (included in PC/TCP OnNet and
PC/TCP networking software; note: the DOS client utilizes DHCP
queries/responses to get an IP address, but does not track its
lease and renew when it should; however, the Windows client is
true DHCP. FTP has stated that the DHCP client the upcoming
OnNet 2.0 and PC/TCP 4.0 releases will perform lease renewal
properly).
http://www.ftp.com/
950802 Wollongong: PathWay Access ver 3.2 (Windows)
http://www.twg.com/
950802 WRQ: Reflection Network Series products (version 5) for Windows
http://www.wrq.com/
950814 Competitive Automation(415-321-4006): SunOS4.x, Solaris2.x and
DECOSF3.x,4.x clients
950906 IBM: included in Warp Server which is in beta
950915 Stampede: included in Remote Office Gold
951005 Apple: "Open Transport" included with PowerPC PCI Macintoshes.
951010 TGV: will be included in MultiNet for Windows V1.2
951011 Dirk Koeppen EDV-Beratungs-GmbH: TCP/IP DHCP Boot ROMs (TCP/IP
BOOT-PROM) www.dunkel.de/desoft
4. What are the DHCP plans of major client-software vendors?
Apple MacOS
MacTCP's successor, Open Transport, supports DHCP.
As of 7/5/95, Open Transport is included with the
Macintosh 9500. Version 1.1 of Open Transport will
ship as a separate product for 68030, 68040, and
PowerPC Macintoshes before the end of 1995.
Microsoft Windows95
will support it and will not support BOOTP.
Novell LAN Workplace for DOS
has plans for client support later in 1995.
IBM OS/2
will support it; I have no news on when or what
version.
5. What Routers forward DHCP requests?
(This is not necessarily a complete list).
Note that in general, these routers probably already had
BOOTP forwarding, but lacked the support for the BOOTP
broadcast flag (see "broadcast flag" under What are the
Gotcha's? above).
Cisco
(from Cisco FAQ) Routers running GSYS version
9.21(4) and 10.0(3) as well as later releases.
Wellfleet/Bay
(from Wellfleet FAQ) DHCP is supported by enabling
BOOTP support (with transmission and/or reception
as needed).
3Com Netbuilder
Version 7.2 software can support DHCP relaying
through the use of its generic UDP Helper service.
Version 8.0 and later officially supports DHCP.
Xyplex
Word is that release 5.1 slated for first quarter
1996 will support it.
ALANTEC
The switches' "router" function has have been
handling BOOTP forwarding since around 1993.
Support for the broadcast flag introduced in a
maintenance release of 2.5 of their software and is
in version 2.6 and later.
6. What Routers include DHCP servers?
DHCP requires disk storage (or some other form of reliable
non-volatile storage), making the task of DHCP service
compatible with servers but incompatible with dedicated
routers. There are a number of server types that can be
configured to both route and serve DHCP, but no dedicated
routers.
7. Which implementations support or require the broadcast flag?
The broadcast flag is an optional element of DHCP, but a
client which sets it works only with a server or relay that
supports it.
o Clients
Microsoft Windows NT
DHCP client support added with version 3.5
sets the broadcast flag. Version 3.51 and
later no longer set it. The exception is in
the remote access support: it sets the flag
when it uses DHCP to acquire addresses to
hand out to its PPP clients.
tcp/ip-32 for Microsoft Windows for Workgroups (WFW)
Version 3.11a sets it, but version 3.11B
doesn't.
Microsoft Windows 95
Does not set the broadcast flag.
8. How can I run Windows 95 without a DHCP server?
Not really a DHCP question, but it has been asked a lot,
particularly by sites for which changing from BOOTP
represents a lot of work. Some choices:
o Use no server at all for the Windows 95 clients: set the
addresses in each client's setup.
o Install a non-Microsoft TCP/IP stack for Windows 95 that
supports BOOTP.
o Switch from your current BOOTP server to one that
supports both BOOTP and DHCP.
A Document that addresses this question is the Windows 95tm
Networking FAQ,
http://www-leland.stanford.edu/~llurch/win95netbugs/faq.htm
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