You are in luck, sort of. In one of our previous tirades ;-) about DHCP, I
sent the following message, which includes a DHCP Discover relayed by a
router near the end. I can't send you the trace file, however, because it
has some confidential stuff in it.
Here's my config. The client is on the 36.1.1.0 network.
I was sitting with my EtherPeek protocol analyzer on the 10.10.0.0 network.
I could see the DHCP Discover come through to 10.10.0.1 as long as I used
"ip helper-address 10.10.0.1."
charlotte#s run
Building configuration...
Current configuration:
!
version 11.0
service udp-small-servers
service tcp-small-servers
!
hostname charlotte
!
enable password xxxx
!
interface Ethernet0
ip address 10.10.0.2 255.255.255.0
!
interface Ethernet1
ip address 36.1.1.1 255.255.255.0
ip helper-address 10.10.0.1
!
interface Serial0
ip address 192.168.40.2 255.255.255.0
no fair-queue
!
interface Serial1
no ip address
shutdown
!
interface TokenRing1
no ip address
shutdown
!
interface TokenRing0
no ip address
shutdown
!
router ospf 100
network 192.168.0.0 0.0.255.255 area 2
network 10.10.0.0 0.0.255.255 area 0
network 36.1.1.0 0.0.0.255 area 2
!
line con 0
line aux 0
transport input all
line vty 0 4
password cisco
login
!
end
The DHCP Discover from the client that I captured might be informative for
people learning about how DHCP Relay works. Notice that the packet is a
unicast, rather than a broadcast. Also, notice at the IP layer that the
source address is the router, not the client's 0.0.0.0 address that you
normally see with DHCP. The router also put its address in the DHCP server
under "Gateway IP Address." The DHCP server needs to see this to know which
subnet the client's request came from.
Ethernet Header
Destination: 00:00:0C:05:3E:80
Source: 00:00:0C:00:2E:75
Protocol Type:0x0800 IP
IP Header - Internet Protocol Datagram
Version: 4
Header Length: 5 (20 bytes)
Type of Service: %00000000
Precedence: Routine, Normal Delay, Normal Throughput, Normal
Reliability
Total Length: 328
Identifier: 12800
Fragmentation Flags: %000 May Fragment Last Fragment
Fragment Offset: 0 (0 bytes)
Time To Live: 127
Protocol: 17 UDP
Header Checksum: 0xD998
Source IP Address: 36.1.1.1
Dest. IP Address: 10.10.0.1
No IP Options
UDP - User Datagram Protocol
Source Port: 68 Bootstrap (BOOTP Client)
Destination Port: 67 Bootstrap Protocol Server
Length: 308
Checksum: 0x3159
BootP - Bootstrap Protocol
Operation: 1 Boot Request
Hardware Address Type: 1 Ethernet (10Mb)
Hardware Address Length: 6 bytes
Hops: 0
Transaction ID: 678970121
Seconds Since Boot Start: 0
Flags: 0x0000
IP Address Known By Client: 0.0.0.0 IP Address Not Known By Client
Client IP Addr Given By Srvr: 0.0.0.0
Server IP Address: 0.0.0.0
Gateway IP Address: 36.1.1.1
Client Hardware Address: 00:E0:98:89:52:FA
Unused: 0x00000000000000000000
Server Host Name:
................ 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Boot File Name:
................ 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
DHCP - Dynamic Host Configuration Protocol
DHCP Magic Cookie: 0x63825363
Message TypeDHCP Option
Option Code: 53 Message Type
Option Length: 1
Message Type: 1 Discover
Client IdentifierDHCP Option
Option Code: 61 Client Identifier
Option Length: 7
Hardware Type: 1
Hardware Address: 00:E0:98:89:52:FA
Requested IP AddressDHCP Option
Option Code: 50 Requested IP Address
Option Length: 4
Address: 36.1.1.2
Host Name AddressDHCP Option
Option Code: 12 Host Name Address
Option Length: 8
String: MACTEAM.
Vendor Class IdentifierDHCP Option
Option Code: 60 Vendor Class Identifier
Option Length: 7
Option Data:
MSFT 98 4D 53 46 54 20 39 38
Parameter Request ListDHCP Option
Option Code: 55 Parameter Request List
Option Length: 9
Requested Option: 1 Subnet Mask
Requested Option: 15 Domain Name
Requested Option: 3 Routers
Requested Option: 6 Domain Name Servers
Requested Option: 44 NetBIOS (TCP/IP) Name Servers
Requested Option: 46 NetBIOS (TCP/IP) Node Type
Requested Option: 47 NetBIOS (TCP/IP) Scope
Requested Option: 43 Vendor Specific Information
Requested Option: 77 User Class Information
DHCP Option End
Option Code: 255 End
Extra bytes (Padding):
........... 00 00 00 00 00 00 00 00 00 00 00
Frame Check Sequence: 0x00000000
Priscilla
At 05:44 PM 10/5/01, Lists Wizard wrote:
>Hi Group,
>
>I am interested in the details (at the packet level) of DHCP communication
>between the client and a DHCP server when there is a router in the middle.
>Can any body provide me with sniffer trace of this kind of communication? I
>use SnifferPro.
>
>Thanks
>
>Lists Wizard
________________________
Priscilla Oppenheimer
http://www.priscilla.com
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=22292&t=22272
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