From: Koen Peetermans (K.Peetermans@chello.be)
Date: Sat Jul 03 2004 - 05:15:03 GMT-3
Ken,
Yes ; mac address is a component of client-id, but not as hexadecimal values
one-to-one.
What I mean is, if you take the ascii string "Cisco-MAC-Name" and represent
that as hexadecimal you will get the client-id.
A MAC address is six bytes, but when this get represented with a cisco
client-id, it will be presented as a 14 byte ASCII string, 3 groups of 4
bytes that are the ASCII presentation of the MAC address + 2 times the dot
"." ascii string.
If you take the characters in your debug :
63.6973.636f.2d
This converts to ascii
"cisco-" (without the quotes)
I don't have an hexeditor to verify this, but I'm quite sure it's correct
looking at the differences in hex values and ascii values ;-)
Like mentioned, with 12.3(8)T you can opt for a full binary client-id (no an
ASCII one presented as binary), you can even fake a Mickeysoft client-id and
class-id:
I include part of the config of my broadband internet connected interface :
interface FastEthernet0
mac-address 0008.bcce.bcce
ip dhcp client client-id hex 010008BCCEBCCE
ip dhcp client class-id MSFT98
ip address dhcp hostname PowerLaptop
Kind regards,
Koen.
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Kenneth Wygand
Sent: vrijdag 2 juli 2004 21:14
To: Koen Peetermans; ccielab@groupstudy.com
Subject: RE: Weird MAC in DHCP request
Koen,
Are you implying that with Cisco boxes, the MAC address should be a
component of the client-ID? It doesn't illustrate this in the debugs from
my original post (bottom of this email thread).
Thanks for your contribution!
Ken
________________________________
From: nobody@groupstudy.com on behalf of Koen Peetermans
Sent: Fri 7/2/2004 2:32 PM
To: ccielab@groupstudy.com
Subject: RE: Weird MAC in DHCP request
Tom is correct, I would like to add that cisco uses an ASCII text as
Client-ID in the following form: cisco-MAC ADDR-interface
For example:
cisco-0010.7b3c.2102-Et0
If you have a Ethernet broadband connection and your provider doesn't like
you putting a router on his network, you can now hide/change this client-id
among other dhcp things with IOS 12.3(8)T.
Do a search for "configurable dhcp client" on CCO for more information.
Kind regards,
Koen.
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Tom
Martin
Sent: vrijdag 2 juli 2004 20:15
To: ccielab@groupstudy.com
Cc: Kenneth Wygand
Subject: RE: Weird MAC in DHCP request
Ken,
The DHCP client ID is not the same thing as the MAC. Each are
represented by their own fields in the DHCP packet. The DHCP RFC
requires that the station ID always be the same for a given host, where
the MAC address be the L2 address of the interface requesting an
address. For PCs and servers, you'll see the that they are usually the
same since (assuming a single NIC is present) the MAC will uniquely
identify the host.
The important point is that although both fields often carry the same
value, they don't have to match. For routers with multiple LAN
interfaces, use of the MAC address of the DHCP interface does not
uniquely represent the device. Cisco devices use a client ID that
resembles a very long MAC, but the client ID and MAC are not related.
For what it's worth, the client ID doesn't even have to resemble a MAC
(it can be any arbitrary string)...
-- Tom
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Kenneth Wygand
Sent: Friday, July 02, 2004 1:14 PM
To: ccielab@groupstudy.com
Subject: Weird MAC in DHCP request
R7(e0/0)----------(e0/0)R8
R8 is requesting an address through DHCP from R7. The followind debug
output is observed on R7:
<SNIP>
00:03:32: DHCPD: DHCPDISCOVER received from client
0063.6973.636f.2d30.3030.352e
.3565.3663.2e63.3065.302d.4574.302f.30 on interface Ethernet0/0.
00:03:34: DHCPD: assigned IP address 78.78.78.2 to client
0063.6973.636f.2d30.30
30.352e.3565.3663.2e63.3065.302d.4574.302f.30.
00:03:34: DHCPD: Sending DHCPOFFER to client
0063.6973.636f.2d30.3030.352e.3565.
3663.2e63.3065.302d.4574.302f.30 (78.78.78.2).
00:03:34: DHCPD: child pool: 78.78.78.0 / 255.255.255.0 (POOL1)
00:03:34: DHCPD: pool POOL1 has no parent.
00:03:34: DHCPD: child pool: 78.78.78.0 / 255.255.255.0 (POOL1)
00:03:34: DHCPD: pool POOL1 has no parent.
00:03:34: DHCPD: broadcasting BOOTREPLY to client 0005.5e6c.c0e0.
00:03:34: DHCPD: DHCPREQUEST received from client
0063.6973.636f.2d30.3030.352e.
3565.3663.2e63.3065.302d.4574.302f.30.
00:03:34: DHCPD: Sending DHCPACK to client
0063.6973.636f.2d30.3030.352e.3565.36
63.2e63.3065.302d.4574.302f.30 (78.78.78.2).
00:03:34: DHCPD: child pool: 78.78.78.0 / 255.255.255.0 (POOL1)
00:03:34: DHCPD: pool POOL1 has no parent.
00:03:34: DHCPD: child pool: 78.78.78.0 / 255.255.255.0 (POOL1)
00:03:34: DHCPD: pool POOL1 has no parent.
00:03:34: DHCPD: broadcasting BOOTREPLY to client 0005.5e6c.c0e0.
</SNIP>
The MAC address of the requesting client is 0005.5e6c.c0e0 and R7 knows
that (as you can see from the debugs). But why the heck would the
initial request appear to come from the following MAC Address:
0063.6973.636f.2d30.3030.352e.3565.3663.2e63.3065.302d.4574.302f.30
Where does this address even come from? Is there any correlation to the
"real" MAC address?
Here is the "show ip dhcp bindings" table:
<SNIP>
r7#show ip dhcp binding
IP address Client-ID/ Lease expiration Type
'''''''''''''''''Hardware address
78.78.78.2 0063.6973.636f.2d30. Mar 02 1993 12:03 AM
Automatic
'''''''''''''''''3030.352e.3565.3663.
'''''''''''''''''2e63.3065.302d.4574.
'''''''''''''''''302f.30
</SNIP>
(cut/paste this into notepad to view table properly)
Thanks in advance,
Ken
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