Re: IPv6 accept_rtadv for default route and prefix but force host portion of /64 address?

Van: Tom Pusateri <pusateri@keehole.org>
Datum: maandag, 29 september 2025 23:32
Aan: "net@freebsd.org" <net@FreeBSD.org>
Onderwerp: IPv6 accept_rtadv for default route and prefix but force host portion of /64 address?
> 
> Is there a way to change the configuration in /etc/rc.conf to get the prefix from the router advertisement but fix the host portion to something like ::123 so that I can change network cards in the server and never have to worry about the IPv6 address changing?
> 
> I have a DMZ interface on a FreeBSD router with a prefix delegation from my provider I assign to a downstream interface. I have another FreeBSD server on the DMZ network and would like it to have a fixed address allocated out of the assigned prefix from the router.
> 
> The router (also FreeBSD) is running rtadvd providing SLAAC and router advertisements.
> 
> The /etc/rc.conf for the DMZ server currently looks like this:
> 
> ifconfig_igb0_ipv6="inet6 2605:1:2:3::123/64 accept_rtadv”
> 
> This works fine and manually assigns the address out of the assigned prefix range as configured on the router.
> 
> It also assigns a second IPv6 address via SLAAC that I don’t use.
> 
> ifconfig output looks like this:
> 
> igb0: flags=1008843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST,LOWER_UP> metric 0 mtu 1500
>     options=4e527bb<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,VLAN_HWCSUM,TSO4,TSO6,LRO,WOL_MAGIC,VLAN_HWFILTER,VLAN_HWTSO,RXCSUM_IPV6,TXCSUM_IPV6,HWSTATS,MEXTPG>
>     ether ac:1f:6b:1a:04:c0
>     inet 1.2.3.123 netmask 0xfffffff8 broadcast 1.2.3.127
>     inet6 fe80::ae1f:6bff:fe1a:4c0%igb0 prefixlen 64 scopeid 0x1
>     inet6 2605:1:2:3::123 prefixlen 64
>     inet6 2605:1:2:3:ae1f:6bff:fe1a:4c0 prefixlen 64 autoconf pltime 604800 vltime 2592000
>     media: Ethernet autoselect (1000baseT <full-duplex>)
>     status: active
>     nd6 options=23<PERFORMNUD,ACCEPT_RTADV,AUTO_LINKLOCAL>
> 
> I get the correct upstream next hop for a default route:
> 
> % netstat -nra6
> Routing tables
> 
> Internet6:
> Destination                       Gateway                       Flags         Netif Expire
> ::/96                             link#3                        URS             lo0
> default                           fe80::207:43ff:fe31:7078%igb0 UG             igb0
> 
> 
> The problem with this is that the IPv6 prefix is configured on the router and configured on the server.
> 
> Thanks,
> Tom
> 
> 
>  
> 
> 
> 


Hi,

I think DHCPv6 could help you here. In IPv6 the address via DHCP is not connected to the MAC address directly, but to a DUID, which is something similar to the hostuuid. AFAIK it should be stable between hardware changes. The details might be important, read something like this https://metebalci.com/blog/a-note-on-dhcpv6-duid-and-prefix-delegation/. I think in my dhcpv6-client I can hardcode the DUID also if needed.

Another option could be a new feature in 16-CURRENT: ifconfig stableaddr which could help in getting a stable address, although I don't know if that is also stable if you change the network card. The author of the feature might be able to explain more about that.
See https://cgit.freebsd.org/src/commit/?id=31ec8b6407fdd5a87d70265762457c67ce618283.

Regards,
Ronald.