[solved] DHCPv6-PD how to configure length of delegated prefix

Installing and Using OpenWrt
1

I have two OpenWrt hosts host A an host B. Host A is upstream of host B. The interface of host A where host B is connected to has an address with prefix /56. Host A has odhcpd running as DHCPv6-Server and delegates a prefix of /62 to host B.

Now I would like Host B to get a /58 prefix instead. So I configured Host B with reqprefix '58'. The problem is, host B still gets a prefix of /62. I verified with tcpdump that host A indeed requests /58 and that it receives a /62-prefix.

There must be some configuration option that allows to specify the mininum prefix length a DHCPv6 client can get, but I can't find it anywhere. Or alternatively, there may be a way to statically configure per client the prefix delegated to it. I looked here https://openwrt.org/docs/techref/odhcpd#dhcpv6 and here https://openwrt.org/docs/guide-user/network/ipv6/configuration#slaac_and_dhcpv6 and here https://openwrt.org/docs/guide-user/base-system/dhcp#dhcp_pools , searched the forum, even looked in luci.

1 Like
2

It might help if you show the output of:

ubus call system board
ifstatus wan6

show the output of the ipv6-address and ipv6-prefix stanza redact the first 8 characters of the IPv6 address

3

The first router needs a value large enough with ip6assign and the other router needs to explicit request the PD size on its dhcpv6 client network stanza.

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4

Host A, ifstatus wan6

...
    "ipv6-address": [
        {
            "address": "fe80::ff:2ab",
            "mask": 128
        }
    ],
    "ipv6-prefix": [
        {
            "address": "2xxx:xxxx:xxxx:4000::",
            "mask": 50,
            "class": "wan6",
            "assigned": {
                "sub1": {
                    "address": "2xxx:xxxx:xxxx:4000::",
                    "mask": 56
                },
                "sub2": {
                    "address": "2xxx:xxxx:xxxx:4100::",
                    "mask": 56
                }
            }
        }
    ],
...

Host A, ifstatus sub2

...
    "ipv6-prefix-assignment": [
        {
            "address": "2xxx:xxxx:xxxx:4100::",
            "mask": 56,
            "local-address": {
                "address": "2xxx:xxxx:xxxx:4100::1",
                "mask": 56
            }
        }
    ],
...

Host B, ifstatus wan6

...
    "ipv6-address": [
        {
            "address": "2xxx:xxxx:xxxx:4100:yyyy:yyff:feyy:yyyy",
            "mask": 64
        },
        {
            "address": "2xxx:xxxx:xxxx:4100::242",
            "mask": 128
        }
    ],
    "ipv6-prefix": [
        {
            "address": "2xxx:xxxx:xxxx:4104::",
            "mask": 62,
            "class": "wan6",
            "assigned": {

            }
        }
    ],
...

Not sure how the output of ubus call system board is useful. It's running in qemu.

5

The first router has ip6assign confgured to 56. So in theory it should be able delegate prefixes from /57 upwards.

6

Correct sub2 has a prefix of 56 so it should be able to handout a prefix of 58 unless there are other clients which are requesting large prefixes, so that is something to check.
I have the same setup (not qemu but just regular routers) and my downstream router can request a PD of 58 with option reqprefix

So maybe other clients of sub2 requesting large prefixes (or a qemu quirk), I use 24.10.1 on both routers.

7

Use ifstatus with the UCI interface name to see details about an interface.

When a prefix of the requested size is not available it will assign a smaller one. I do not know exactly how that works though. Prefix delegation is not an integral kernel feature; OpenWrt specific code somewhere does it.

8

If you do not need the openwrt integration then dump udhcp6d and dnsmasq and use kea and bind. :person_shrugging:

9

Before 2023, the /62 delegated prefix length was hard-coded in odhcpd; you could not change it.

Now you can:

config dhcp 'pd'
    ...
    option dhcpv6_pd_min_len '60'

See https://git.openwrt.org/?p=project/odhcpd.git;a=commitdiff;h=52112643308bb02a3b4fa2894dd7d4340ba4a237

EDIT: This answer is based purely on code search. I have not tested whether it works.

4 Likes
10

Nice. Thanks for the finding! Something I need to check out too!

11

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