How to request specific IA Prefix in DHCPv6 Solicit?

edofullin · October 22, 2021, 10:02am

Hi, unfortunately my ISP decided to assign dynamic IPv6 prefixes, making IPv6 difficult to configure across multiple VLANs and messing up internal IPs as well.

Talking to them they said that their dhcpv6 server honors DHCPv6 IA_PD option (if possible), that could help in always getting the same IPv6 from them.

Unfortunately though it seems the only options that are configurable from the config file are the IAID and the Prefix Length, not the Prefix address which stays at zero.

Is there any 'hacky' way to set this?

This is how it works on MikroTik, for example:

https://imgur.com/ANImKTX

https://imgur.com/K25HT6A

As you can see the Prefix Address is set, on OpenWRT it is not (or I don't know where to set it)

mikma · October 22, 2021, 11:27am

Odhcp6c which is the default DHCPv6 client always uses an empty prefix address in solicit messages anyway.

github.com

openwrt/odhcp6c/blob/94adc8bbfa5150d4c2ceb4e05ecd1840dfa3df08/src/dhcpv6.c#L379


      
          
          		ia_pd = alloca(n_prefixes * (sizeof(struct dhcpv6_ia_hdr) + sizeof(struct dhcpv6_ia_prefix)));
          
          		for (size_t i = 0; i < n_prefixes; i++) {
          			struct dhcpv6_ia_hdr hdr_ia_pd = {
          				htons(DHCPV6_OPT_IA_PD),
          				htons(sizeof(hdr_ia_pd) - 4 +
          				      sizeof(struct dhcpv6_ia_prefix) * !!request_prefixes[i].length),
          				request_prefixes[i].iaid, 0, 0
          			};
          			struct dhcpv6_ia_prefix pref = {
          				.type = htons(DHCPV6_OPT_IA_PREFIX),
          				.len = htons(sizeof(pref) - 4),
          				.prefix = request_prefixes[i].length
          			};
          			memcpy(ia_pd + ia_pd_len, &hdr_ia_pd, sizeof(hdr_ia_pd));
          			ia_pd_len += sizeof(hdr_ia_pd);
          			if (request_prefixes[i].length) {
          				memcpy(ia_pd + ia_pd_len, &pref, sizeof(pref));
          				ia_pd_len += sizeof(pref);
          			}

edofullin · October 22, 2021, 11:59am

That's really unfortunate.

Thanks for your answer, maybe I'll try adding support in the future.

trendy · October 22, 2021, 1:21pm

Check if you can utilize the other two available dhcp6 clients to fit your needs.

edofullin · October 23, 2021, 11:40am

Thanks! I'll try those.

The interface must be set to 'unmanaged' on LuCi right?
It would be nice to be able to set that parameter via the UI, it could come handy with many provider.

edofullin · October 23, 2021, 5:37pm

While it seems to work with dhcpcd it now breaks MAP-T (used by my ISP) which obviously doesn't work with other DHCPv6 clients.

It might be quicker trying to compile a custom version of odhcp6c with IA_PD support rather then trying to modify map-t to get DHCP options from other dhcp clients.