odhcpd renews DHCPv6 client leases even after switching to stateless-only mode

[patrakov]

Long time ago, I have installed an OpenWRT router (TP-Link Archer C7 v2) in my network. Now it runs OpenWRT 19.07-rc2. By default, it came with a setup that combines stateless and stateful DHCPv6 on br-lan. Therefore, my desktop PC (which is almost never turned off) has obtained a DHCPv6 lease containing an address, 2a02:17d0:4a6:5700::dba, and keeps renewing it.

Today is December 30. Last time I reconfigured DHCP on December 10, to set it to stateless-only (although SLAAC-only would have worked as well). I have explicitly restarted odhcpd just now, just in case if I forgot to restart it back then.

Even though stateful DHCPv6 is disabled, when the desktop PC thinks that it needs to renew its DHCPv6 lease, the router renews it.

Proof:

root@Archer:~# /etc/init.d/odhcpd restart
root@Archer:~# ubus call dhcp ipv6leases
{
    "device": {
        "br-lan": {
            "leases": [

            ]
        }
    }
}
### waited until the desktop PC decided to renew the lease...
root@Archer:~# ubus call dhcp ipv6leases
{
    "device": {
        "br-lan": {
            "leases": [
                {
                    "duid": "0004ee68edcf72d2c8646b63ccaee52c6a16",
                    "iaid": 2128153145,
                    "hostname": "aep-haswell",
                    "accept-reconf": false,
                    "assigned": 3514,
                    "flags": [
                        "bound"
                    ],
                    "ipv6-addr": [
                        {
                            "address": "2a02:17d0:4a6:5700::dba",
                            "preferred-lifetime": 3210,
                            "valid-lifetime": 3570
                        }
                    ],
                    "valid": 2668
                }
            ]
        }
    }
}

Strangely, my laptop (which is off quite often) does not have a DHCPv6 lease - but well, the software there is not identical.

The /etc/config/dhcp file is:

config dnsmasq
    option domainneeded '1'
    option localise_queries '1'
    option local '/lan/'
    option domain 'lan'
    option expandhosts '1'
    option authoritative '1'
    option readethers '1'
    option leasefile '/tmp/dhcp.leases'
    option resolvfile '/tmp/resolv.conf.auto'
    option nonwildcard '1'
    option localservice '1'
    option enable_tftp '1'
    option tftp_root '/etc/tftpboot'
    option dhcp_boot 'netboot.xyz-undionly.kpxe'
    option rebind_protection '0'

config dhcp 'lan'
    option interface 'lan'
    option start '100'
    option limit '150'
    option dhcpv6 'server'
    option ra 'server'
    option ra_management '0'
    option force '1'
    option leasetime '72h'

config dhcp 'wan'
    option interface 'wan'
    option ignore '1'

config odhcpd 'odhcpd'
    option maindhcp '0'
    option leasefile '/tmp/hosts/odhcpd'
    option leasetrigger '/usr/sbin/odhcpd-update'
    option loglevel '4'

# ...and some "config host" entries.

I would expect the 2a02:17d0:4a6:5700::dba lease to go away automatically.

[patrakov]

Attaching pcapng file that illustrates the issue: initially odhcpd replies with NoBinding, but later accepts the client's idea about the lease. 922-927.pcapng.zip

[dedeckeh]

From R19.07 onwards odhcpd can be configured to refuse statefull DHCPv6 requests by setting dhcpv6_na and dhcpv6_pd to 0 in the dhcp lan config

[patrakov]

Is it possible to have both prefix delegation and no IPv6 addresses other than the statelessly autoconfigured ones?

[dedeckeh]

I don't completely understand your question; can you clarify the question a bit more in detail ?

[patrakov]

OK, let me reword. Your comment mentions setting dhcpv6_pd to 0, and I don't like that, because I have one client that does want a delegated prefix (for virtual machines). Is there any way around that? Or is it a protocol limitation, that a delegated prefix must come with an IPv6 address assigned through non-SLAAC mechanism, and if one client wants it, then everyone should get such address?

[dedeckeh]

No you can just set dhcpv6_na to 0 and odhcpd will not assign a statefull IPv6 address anymore while it will assign a delegated prefix

[patrakov]

Thanks, will test tomorrow.

[dedeckeh]

I will close this issue; feel free to re-open if the above described proposal does not fix your issue