I am troubleshooting DHCPv4 queries and trying to understand how the IAID values are calculated. According to the code in dhclient (dhc6.c - line 1715 to 2971) the IAID should be the last 4 (least significant) digits of the network card's MAC address. However, in all my packet dumps it shows up as some other value. I can force the IAID to become the last four digits of the MAC using dhclient -i eth0 however I don't understand where the initial value comes from?
Running ubuntu-18.04.1-live-server-amd64.iso on VMWare I get the following in my packet captures or DHCP requests:
Example 1 - default IAID value (IAID does not match MAC)
MAC: 00:0c:29:34:88:e5
DHCP Option 61: ff:2b:94:34:c1:00:02:00:00:ab:11:7b:e7:02:70:db:a5:9c:24
Example 2 - post dhclient -i (IAID does match)
MAC: 00:0c:29:34:88:e5
DHCP Option 61: ff:29:34:88:e5:00:01:00:01:23:b1:83:eb:00:0c:29:34:88:e5;
