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General
DHCP Flag
Roaming Flag
Default Node DHCP Options
Additional DHCP Options
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General
The DHCP (Dynamic Host Configuration Protocol) and BootP
(Bootstrap Protocol) pass network configuration information to a host
during startup. In NetDB, DHCP is configured on Node and Network records.
For Node records, a unique hardware address must be entered to configure
DHCP.
DHCP is also described in Internet RFCs 2131 and 2132.
Note that BootP is a subset of DHCP commands.
DHCP information is updated from NetDB approximately every
ten (10) minutes.
DHCP Flag (Node Record)
If the DHCP box is checked and a hardware address is entered,
the campus DHCP servers will reply to a DHCP request from
that hardware address. Default DHCP options will be returned. If specified in
the NetDB record, additional DHCP options will also be returned.
Roaming Flag (Node Records)
Roaming DHCP was implemented to allow machines to move to
other networks without requiring manual configuration of IP addresses,
etc. If the Roaming box is checked, Roaming DHCP is enabled for that hardware
address. If the machine now moves to another network that has available
roaming DHCP addresses, the DHCP server will give one of these addresses
to the mobile machine.
Default Node DHCP Options
The default Node DHCP options at Stanford are
IP address, subnet mask, gateway, domain name and domain name servers. The option numbers
come from RFCs 2131 and 2132.
| DHCP Option |
Option
Number |
Description |
| IP Address |
50 |
returns IP address assigned to requesting hardware address
|
| Subnet mask |
1 |
returns 255.255.0.0, in general |
| Gateway |
3 |
for 171.64 addresses, returns 171.64.1.1
for 171.65 addresses, returns 171.65.1.1
|
| Domain name |
15 |
returns domain name assigned to the Node |
| Domain name servers |
6 |
171.64.1.234 and 171.67.1.234 returned in random order |
Additional DHCP Options
In NetDB, the following additional DHCP options may be set for Node interfaces,
Networks or address spaces. Note: Use with caution as improper use may cause networking
problems. Enter DHCP options one per line, using the syntax <dhcp option> =
<value>.
| DHCP Option |
Option
Number |
Node
Interface |
Net |
Address
Space |
Description |
| next-server |
|
Y |
Y |
Y |
boot/config server |
| filename |
|
Y |
N |
Y |
boot/config file - e.g. thin clients |
| ntp-servers |
42 |
Y |
Y |
Y |
Network Time Protocol (NTP) server(s) |
| time-servers |
4 |
Y |
Y |
Y |
RFC 868 time server(s) |
| tftp-server-name |
66 |
Y |
Y |
Y |
TFTP boot/config server |
| bootfile-name |
67 |
Y |
N |
Y |
boot/config file - e.g. thin clients |
| log-servers |
7 |
Y |
Y |
Y |
|
| host-name |
12 |
Y |
N |
N |
|
| domain-name |
15 |
Y |
Y |
Y |
e.g., stanford.edu |
| domain-name-servers |
5 |
Y |
Y |
Y |
DNS servers |
| netbios-name-servers |
44 |
Y |
Y |
Y |
WINS servers |
| netbios-node-type |
46 |
Y |
Y |
Y |
1=B, 2=P, 4=M, 8=H |
| option-144 |
144 |
Y |
N |
Y |
for HP printers and Nortel phones |
| max-lease-time |
|
Y |
Y |
Y |
|
| default-lease-time* |
51 |
Y |
Y |
Y |
lease time for interface IPs (2 days) |
| dynamic-lease-time* |
51 |
N |
N |
Y |
lease time for dynamic IPs (42 minutes) |
| permit† |
|
N |
N |
Y |
see below |
*DHCP lease time options
There are two default lease durations in NetDB - one for statically
assigned (interface) IP addresses and one for dynamically assigned
(address space) IP addresses. The default for statically assigned
addresses is 2 days and for dynamically
assigned addresses it's 42 minutes.
The default-lease-time option sets the lease duration for
statically assigned addresses. The lease time can be set for an entire
network, an address space, or just an interface. The lease time for
dynamically assigned addresses will not be affected.
The dynamic-lease-time option sets the lease duration for
dynamically assigned addresses. This option can only be used on address
spaces because it only affects the lease time for address space
addresses.
When setting these options the value is in seconds. For example
dynamic-lease-time=600 sets the lease duration for
dynamically assignable address to 10 minutes.
†permit DHCP option
The permit DHCP option controls which clients can lease dynamically
assignable IP addresses, dynIPs, on a per subnet basis. By default, the
DHCP service leases dynIPs to roaming clients, i.e., nodes with the
roaming flag set. So any address space without the permit option
essentially has permit=roaming. Adding the permit
option to an address space completely overrides that default.
At this time there's only one other valid value for the permit
option: unknown-clients. Setting
permit=unknown-clients allows unknown clients to obtain
dynIPs in a particular address space. This explicit permit option
overrides the default, so only unknown clients can get dynIPs.
To allow both unknown clients and roaming clients to obtain dynIPs simply
combine values - permit=unknown-clients,roaming or
permit=roaming,unknown-clients.
Note that the set of unknown clients isn't just nodes not listed in NetDB,
but also includes nodes in NetDB without the DHCP flag set, and nodes in
NetDB in the Unknown state.
To summarize:
| |
| Permit Value | Clients that will get dynamically assigned leases
| (default) | Roaming clients
| unknown-clients | Only unknown clients (no roaming clients)
| unknown-clients,roaming | Unknown clients and roaming clients
| | | |
| | |
Unknown clients include:
- nodes not registered in NetDB
- nodes in NetDB without DHCP flag set
- nodes in NetDB in the "Unknown" state
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For more information, submit a ticket to HelpSU.
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