Debugging DHCPv6-PD

[oofnik]

@oofnik

Cannot mix IA for the same IAID 

I found a clue in the source of dhcpcd5 from 2018, when this error message is removed:

dhcp6: IAID is now unique for IA type

I am using the latest dhcpcd5 from Ubuntu 16.04, which is from before this commit. I will try a more modern version of dhcpcd5 with IANA:1 for both IA_NA and IA_PD and see what happens.

[oofnik]

UPDATE: solved

IA_PD works fine with dhcpcd5 version 7.0.8 with IAID:1 for both request types.

The server replies unless I specifically request option 67 (prefix exclude). When requesting this option with dhcpcd5 , the server does not send a reply, whereas with odhcp6c the server's reply must have contained something to confuse odhcp6c not to request a prefix at all.

I checked this with odhcp6c by running with the following options:

odhcp6c -s /lib/netifd/dhcpv6.script -Ntry -P0 -t120 -R -r23 eth1

Lo and behold, the reply went out asking for IA_PD as expected.

So it appears the ISP server does not support option 67 (prefix exclude), and the IAID uniqueness issue was a red herring.

I have modified my configuration to pass -R (equivalent uci config option defaultreqopts '0', undocumented):

config interface 'wan6'                  
        option ifname 'eth1'             
        option proto 'dhcpv6'            
        option reqprefix 'auto'          
        option reqaddress 'try'          
        option defaultreqopts '0'             
        option reqopts '23 24'

I will update the wiki so that this very important option is documented!

Thanks all, especially @trendy, for your effort in helping me solve this issue.

EDIT: remove options 82 and 83 from reqopts

[trendy]

Nice catch, that was a tricky one.

[dedeckeh]

Probably the contents of option 67 in the DHCPv6 advertise will not be correct and as result odhpc6c will drop the IA_PD option as the sanity check will fail.
Can you share a binary dump of option 67 as I cannot find this in the initial DHCPv6 packet dumps ?

[oofnik]

@dedeckeh Sure. I can send you the full packet captures with and without option 67, but since the data contains my prefix I'd prefer to send it via DM. Hope that's alright.