DHCPv6 Option 18 (interface-id) relay problem

Andreas Hanke andreas.hanke at r-kom.de
Wed Nov 7 07:52:03 UTC 2012

Hello together,

I am working on a problem with my ISC DCHP Version 4.2.4-P2 server for IPv6 since a lot of days.
But I could not found a solution. So I hope someone could help me.

- Situation:
I have to discover the Host by an interface id to give him a fixed IP address.
This interface id will inserted by an DSLAM which is connected to a lot of DSL Modems.
The interface id is the identifier, on which physical port the DSL Modem is connected.
I like to use the host definition into my dhcp.conf:

host ws {
        host-identifier option dhcp6.interface-id "Interface-ID: DSLAM01 eth 2/1/01/21"
        fixed-address6 2001:db8:1:1::701;

- My setup:
Client PC with DSL Modem <-------------> DSLAM which is inserting a option 18 with the interface id <-----------> CISCO 7606-S Router which have a helper at the interface which is connected to the DHCP server. This helper forward the DCHP Message to the DHCP server and works as a relay <-----------> ISC DCHP server for IPv6

- Parts of my server configuration:
# Please see: http://www.ipamworldwide.com/dhcp-options/isc-dhcpv6-options.html
#This function logs the DHCPv6 client id, which means the Link Local Address (Option 1)
log(info,concat("Client-ID: " , binary-to-ascii(16,8,"-",option dhcp6.client-id)));
#This function logs the DHCPv6 interface id, which means the port of the DSLAM (Option 18)
log(info,concat("Interface-ID: " , option dhcp6.interface-id));

- My problem:
The ISC DHCP server could not discover the interface-id (Option 18), which is inserted by the DSLAM -> get no log message (see configuration).
The client-id (Option 1), which is inserted by the Client PC with DSL Modem works fine -> get log message (see configuration).

I tested it without CISCO and without DSLAM/DSL Modem direct connect over an Networkswitch and inserting the interface-id (Option 18) by the Client PC, that works -> get log message (see configuration).

After that I analysed the TCP packets. What I found out is, that the difference is that:
- Message type "RELAY-FORW (12)" with CISCO and DSLAM/DSL Modem:
<field name="dhcpv6.msgtype" showname="Message type: Relay-forw (12)" size="1" pos="62" show="12" value="0c"/>
- Message type "SOLICIT (1)" without CISCO and DSLAM/DSL Modem:
<field name="dhcpv6.msgtype" showname="Message type: Solicit (1)" size="1" pos="62" show="1" value="01"/>

Side information:
If I configured an ipv6 range without host definitions into my dhcp.conf the Client PC get in both situations an ip6 address.

Could you tell me if the message type is the problem and if yes what I have to change?
If no, what went wrong?

I have attached the both network packets with situation 1 "with CISCO and DSLAM" and situation 2 "without CISCO and DSLAM".

Thanks a lot for your help!



Andreas Hanke
IP Network Operations

andreas.hanke at r-kom.de<mailto:andreas.hanke at r-kom.de>

R-KOM Regensburger Telekommunikationsgesellschaft mbH & Co. KG,
Kommanditgesellschaft, Sitz Regensburg, Registergericht Regensburg,
HRA 5818
Persönlich haftende Gesellschafterin und Geschäftsführung:
R-KOM Regensburger Telekommunikationsverwaltungsgesellschaft mbH,
Sitz Regensburg, Registergericht Regensburg,
HRB 6764; Geschäftsführer: Dipl.-Inf. (FH) Alfred Rauscher
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