This is just a short one. I currently working on a project which involves managing a server owned and hosted by multiple different companies. for me to be able to build the server i need to insist on certain things. one of these this things is a static ipv6 address.
Now many organisation use Router Advertisements to distribute the default gateway and some use the to dynamically allocate ip addresses. To my knowledge there is know way of providing a statically mapped address via RA’s. So you probably know where im going with this.
The network i was installing in today used RA’s to distribute both a prefix and a default route. I already have a number of systems that that receive a default route via RA; however known of these networks offers a prefix in the RA. At this point i could have probably contacted the operator and asked to stop sending out the prefix but where is the fun in that, there must be a way to accept the route and not the prefix.
We use centos so my first search was the [not so] amazingly documented network-scripts to see if there was a special flag i could get to achieve the behaviour i was after. I found no such variable. so i decided i would need to set the appropriate kernel parameter in a /sbin/ifup-local script.
The are a number of kernel parameters which we are interested in
accept_ra - BOOLEAN Accept Router Advertisements; autoconfigure using them. Possible values are: 0 Do not accept Router Advertisements. 1 Accept Router Advertisements if forwarding is disabled. 2 Overrule forwarding behaviour. Accept Router Advertisements even if forwarding is enabled. Functional default: enabled if local forwarding is disabled. disabled if local forwarding is enabled. accept_ra_defrtr - BOOLEAN Learn default router in Router Advertisement. Functional default: enabled if accept_ra is enabled. disabled if accept_ra is disabled. accept_ra_pinfo - BOOLEAN Learn Prefix Information in Router Advertisement. Functional default: enabled if accept_ra is enabled. disabled if accept_ra is disabled. accept_ra_rt_info_max_plen - INTEGER Maximum prefix length of Route Information in RA. Route Information w/ prefix larger than or equal to this variable shall be ignored. Functional default: 0 if accept_ra_rtr_pref is enabled. -1 if accept_ra_rtr_pref is disabled. accept_ra_rtr_pref - BOOLEAN Accept Router Preference in RA. Functional default: enabled if accept_ra is enabled. disabled if accept_ra is disabled.
The ones that are really important to use are
accept_ra #needs to be set to 1 accept_ra_defrtr #needs to be set to 1 accept_ra_pinfo #needs to be set to 0
When the network scripts run if IPV6_AUTOCONF=yes then accept_ra will be set to 1. this will also cause the other *_ra_* parameters to be set to 1 as documented above. so the only thing we really need to do is set accept_ra_pinfo to 0. this is where we use the /sbin/ifup-local script below.
#!/bin/sh DEVICE="$1" /sbin/sysctl -e -w net.ipv6.conf.$DEVICE.accept_ra_pinfo=0 >/dev/null 2>&1
Conclusion
if you want router advertisement default gateway but you want to ignore the prefix sent. Add IPV6_AUTOCONF=yes to your ifcfg-em? file and create /sbin/ifup-local with the above code.
Enjoy
b4ldr 