1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
|
To: iana@isi.edu
Subject: Request for three DHCP option numbers
Date: Mon, 11 Nov 1996 16:35:13 -0500
From: Ralph E. Droms <droms@charcoal.eg.bucknell.edu>
An option for FQDNs in DHCP options
<draft-ietf-dhc-fqdn-opt-01.txt>
Abstract
DHCP [DHCP] can be used to automate the process of configuring TCP/IP
host computers. However, some of the DHCP options carry IP addresses
rather than Fully Qualified Domain Names (FQDN). Use of IP addresses
constrains the DHCP client to use the addresses that were in use at
the time the client received its configuration information; these
addresses may change over time, (e.g., a server may be assigned a new
IP address), so that the IP addresses used by the client may become
invalid.
An alternative to passing IP addresses is to pass FQDNs instead of
(numeric) IP addresses. Doing this allows to defer binding between a
particular network entity (e.g., a server) and its IP address until
run time. As stated in [Carpenter:95], "Deferring the binding avoids
the risk of changed mapping between IP addresses and specific network
entities (due to changing addressing information). Moreover,
reliance on FQDNs (rather than IP addresses) also localizes to the
DNS the changes needed to deal with changing addressing information
due to renumbering."
This document defines a new DHCP option that allows the use of FQDNs
instead of IP addresses in DHCP options.
Definitions
The following defines the format of the FQDN option.
+----------+----------+
| Code | Length |
+----------+----------+---------+-----------+--------------------
|Subcode |Sublength | FQDN
+----------+----------+---------+-----------+--------------------
..................
+----------+----------+---------+-----------+--------------------
|Subcode |Sublength | FQDN
+----------+----------+---------+-----------+--------------------
The option consists of a Code and Length fields followed by a
variable number of <subcode, sublength, FQDN> triples.
The code, length, subcode, and sublength fields are all one octet
long. The FQDN field is of variable length.
The code value for this option is 89.
The length field specifies the total length (in octets) of all the
triples carried in the option.
For each subcode carried in the FQDN option, the IP address in the
option represented by the subcode is replace by a FQDN.
The sublength field shall be set to the length (in octets) of the
FQDN carried in the option. The FQDN field carries the FQDN itself.
|
