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

In reply to: Tom Pusateri : "IPv6 accept_rtadv for default route and prefix but force host portion of /64 address?"
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From: Roy Marples <roy_at_marples.name>
Date: Wed, 01 Oct 2025 14:33:23 UTC

Hi


You can use dhcpcd from ports for this, rather than the FreeBSD Kernel.


https://man.freebsd.org/cgi/man.cgi?query=dhcpcd.conf 



Example configuration in dhcpcd.conf


interface igb0
    slaac token ::123


Now, dhcpcd will generate SLAAC addresses using the token instead of the hardware address.


Fair warning, dhcpcd will take over some stuff on your router so you may want to limit it to some interfaces only and maybe some protocols only. See the fine man page for details.


Good luck!


Roy

 
 

 
 ---- On Mon, 29 Sep 2025 22:32:59 +0100  Tom Pusateri<pusateri@keehole.org> wrote ----


 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