A. Name: Policy on the Operation of DHCP and BOOTP Servers on
B. Number: 20000530-dhcpserver
C. Author(s): M.Sirota, D.Kassabian, M.Muth (ISC Networking &
D. Status: [ ] proposed [ ] under review [x] approved [ ] rejected
[ ] obsolete
E. Date proposed: 2000-01-25
F. Date revised: 2005-01-10
G. Date approved: 2000-05-30
H. Current revision effective date: 2005-01-10
II. Authority and Responsibility
Information Systems and Computing's Networking organization is
responsible for the operation of PennNet (Penn's data networks) and
therefore has the authority and responsibility to specify requirements
for any devices connecting to PennNet. This authority extends to device
configuration management and information provided by configuration
servers, as incorrect information could adversely impact the operation
of other network-connected devices.
III. Executive Summary
This policy specifies the requirements for Dynamic Host
Configuration Protocol (DHCP) servers and related infrastructure
operating on PennNet. It also
provides "best practice" recommendations for server administrators.
The purpose of this policy is to specify the requirements and
limitations for DHCP server operation on PennNet. While DHCP servers
can provide a very efficient and convenient way to manage IP addresses
and other configuration information
for a group of network-connected devices, their use under certain
circumstances can cause significant problems (see VI. Risk of
VI. Risk of Non-compliance
Since DHCP works in part through broadcast client requests, it is easy
to see that within a single broadcast domain, only
one DHCP server should be configured to respond to requests. More than
one DHCP server operating on the broadcast domain could cause
results, including multiple and differing responses being returned to a
client having made a request, which in turn can result in unpredictable
client operation. For this reason, some coordination
among those operating local and central DHCP servers is essential.
- Broadcast Domain
- A broadcast domain is a subnet or collection of subnets on which
broadcasts are shared. A machine broadcasting packets can be heard
by any other machine within that broadcast domain.
On PennNet, a broadcast domain is typically a single IP subnet, but
that is not necessarily always the case. ISC Network Operations can
assist with determining the limits of your broadcast domain.
- DHCP or BOOTP Server
Any device that responds to valid client DHCP or BOOTP requests
on PennNet (henceforth called "server").
- Relay Agent
- Any device that forwards such requests along to
a server, usually residing on another subnet or broadcast domain.
Another problem has to do with providing service to a number of
different academic or administrative units sharing a broadcast domain.
may wish to use DHCP to make IP address leases available to its own
clients, but may find that address leases made available on the
may be offered to and accepted by clients from any or all other units
sharing the broadcast domain. This is
undesirable since the pool of address leases available to any one of
the units is both limited and the basis for network charges.
This policy applies to any device acting as a DHCP or BOOTP
server or relay agent on PennNet.
Restrictions on the operation of servers apply to all centrally
operated PennNet subnets. Subnets that are supported by organizations
other than the University's central networking organization are not
covered directly by this policy. Network users are advised to check
with their local LSP if uncertain.
VIII. Statement of policy
IX. Recommendations and Best Practices
- Anyone running a server or relay agent on PennNet
must register the server or relay agent with ISC Networking. ISC
Networking reserves the right to disallow a server
or relay agent if it would result in a conflict with another
serving the same broadcast domain.
- Authorized servers or relay agents may need to be shut down or
reconfigured at a later date, if another academic or administrative
unit sharing the broadcast domain finds a
- ISC Networking will not configure central relay agents to
point to any servers other than those centrally operated by ISC
Networking. Similarly, any relay agents must be configured to
point only to servers for which the server
administrator has granted permission for the traffic.
- DHCP service offered by ISC Networking is restricted to a subset
of possible DHCP functionality. Standard DHCP service provided on
centrally operated servers
includes no special options. Depending on various performance,
robustness and interoperability considerations, ISC Networking may be
able to provide customized options.
Fees associated with this service are published at
- All IP addresses handed out by a server must
be registered in accordance with the
Policy on the use of PennNet IP address space at
The following related practices are strongly recommended by ISC
- Servers running on shared broadcast domains (multiple academic or
administrative units sharing a broadcast domain) should be configured to
to uniquely known clients only, perhaps using
MAC address as an identifier.
Alternatively, a private subnet can be ordered from ISC
Networking for additional cost, thus isolating the clients and server
private broadcast domain.
- Servers must be configured with a thorough understanding of the
network topology in order to avoid conflict later. ISC Networking can
help to describe the network topology, but no support is offered for
any specific server configuration syntax.
- Lease lengths should be chosen carefully; they should be long
enough to avoid unneeded network traffic, but short enough to
avoid running out of leases. There is no default recommendation.
Some factors involved in the decision include
the size of the address range, the frequency with which machines come
and go, and the amount of traffic the server is prepared to handle.
- Relay agents should be configured to deliver packets
to the unicast IP address of the server(s) rather than
the broadcast address of the associated segment(s).
- The use of Dynamic BOOTP is discouraged because
address assignments last forever. DHCP should be used instead.
ISC Networking does not plan to actively police the network in an effort
to discover unregistered or misconfigured servers or relay
agents, but will act on those discovered during the normal
course of events in operating and/or troubleshooting the network.
Notification shall be made to the LSP and/or server administrator
for the area whenever possible and practical.
C. Remedy: Remedy will either be the suspension of DHCP
services from the non-compliant server, or removal of the server or
relay agent from the network until such time as the DHCP service
can be suspended or brought into compliance.
ISC Networking will offer assistance to the LSP for the area.
D. Financial Implications: Charges may be assessed
for time spent by ISC Networking in troubleshooting
problems attributable to a non-compliant or misconfigured server or
Please see the
Policy on Troubleshooting Charges for Violations of PennNet Policies
for information on additional fees that may be assessed to cover the
costs incurred in
troubleshooting related to violations of this policy.
Charges are also associated with
customized DHCP services, as specified at
E. Responsibility: Responsibility for remedy lies with
the network user. In the vast majority of cases, the area
LSP will have involvement in the implementation of the remedy.
F. Time Frame:
Non-compliant servers must be remedied immediately to reduce risk of
networking failures for other network users.
Please see the
Policy on Computer Disconnection from PennNet
Please see the Appeals section of the
Policy on Computer Disconnection from PennNet