Wednesday, April 3, 2013

DHCPv6: ISC MAC logging and Cisco relay agent configuration

DHCPv6 server MAC address logging

One of the differences between DHCPv6 and DHCPv4 is that it uses DUID as an identifier instead of a MAC address.

As you probably know, DUID's are for the majority of OS (ie Windows) based on a timestamp suffixed by a MAC address

For some of our internal systems we use a MAC as an identifier, for now we will also be  needing this for IPv6. The default ISC DHCPv6 daemon isn't logging a MAC address by default. One way to have it print out is by adding this to your DHCP config.

option dhcp6.macaddr code 193 = string;
option dhcp6.leased-address code 194 = string;
option dhcp6.macaddr = binary-to-ascii(16, 8, ":", suffix(option dhcp6.client-id, 6));
option dhcp6.leased-address = binary-to-ascii(16,16, ":", substring(suffix(option dhcp6.ia-na, 24),0,16));
log (info, concat ("Lease for ",config-option dhcp6.leased-address, " leased to ", config-option dhcp6.macaddr));

Above code will only work for DUID-LLT and DUID-LL (so not DUID-EN, but I don't know anyone using this at the moment)
More info over DUID on
Other great blogpost about DHCPv6 at

DHCPv6 relay configuration on Cisco equipment

Overview of the configuration we're using on our routers.

(1) We're using FE80:: as our IPv6 default gateway everywhere, seems to work for me for now ;) any best practices for this? (update: because of issues with linux and fe80:: (linux responds to fe80:: if it's specified on any link, this has now changed to FE80::1)

(2) Asking the hosts on the subnet to not do SLAAC please, we're asking you nicely.
(3) Letting the hosts know we're managing the config, that they must use DHCPv6.
(4) Just to be sure also set this flag, tell them to use DHCPv6 not only for getting an IP but also for getting e.g. DNS servers
(5) We're the boss on this subnet.
(6) relay DHCPv6 requests to our server and use (7) the loopback as a source for this.

We're using link-local addresses for routing, so if we don't specifiy an source-interface, the relay agents would try to use a link-local address which obviously can not be routed.

Interface vlan42
  ipv6 address FE80::1 link-local (1)
  ipv6 address 2001:0db8:100:4200::1/64
  ipv6 nd prefix default 2592000 604800 no-autoconfig (2)
  ipv6 nd managed-config-flag (3)
  ipv6 nd other-config-flag (4)
  ipv6 nd router-preference High (5)
  ipv6 dhcp relay destination 2001:0db8:0:40::547:1 (6)
  ipv6 dhcp relay source-interface Loopback0 (7)

interface Loopback0
 ipv6 address  2001:0db8:300::63/128


  1. big thanks for this post!


  3. Nice article. Thank you. Can you tell me is CISCO software ( works good at MAC?