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I've got a /64 of IPv6 addresses available to one of my servers (Ubuntu 12.04). I'm binding them like this:

auto eth0
    iface eth0 inet static

    iface eth0 inet6 static
    address xxxx::2
    netmask 64
    gateway xxxx::1

    pre-up echo 0 > /proc/sys/net/ipv6/conf/eth0/autoconf
    pre-up echo 0 > /proc/sys/net/ipv6/conf/eth0/accept_ra
    pre-up echo 0 > /proc/sys/net/ipv6/conf/eth0/accept_ra_defrtr
    pre-up echo 0 > /proc/sys/net/ipv6/conf/eth0/accept_ra_pinfo
    pre-up echo 0 > /proc/sys/net/ipv6/conf/eth0/accept_ra_rtr_pref

    up /sbin/ifconfig eth0 inet6 add xxxx::3/64
    # ... snip ...
    up /sbin/ifconfig eth0 inet6 add xxxx::25/64

This works, but applications are all using xxxx::25 for their outgoing requests unless specifically told otherwise. If I add xxxx::26 in my /etc/network/interfaces, they all start using that. It seems like they just use whatever the highest number is.

How do I specify a certain address to be used as the default? In my case, I happen to want to use the lowest address (xxxx::2), but I'd really like to know how to specify a particular one, should the need arise in the future.

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up vote 1 down vote accepted

You can solve this by modifying the default route. Every route in Linux has the option to specify the default source address. If you specify the xxxx::3 address in your route then that one will be used by default:

/sbin/ip -6 route del default
/sbin/ip -6 route add default via xxxx::1 src xxxx::3

If you show the routing table you will see the result:

/sbin/ip -6 route
default via xxxx::1 dev eth0  src xxxx::222  metric 1024

One thing to watch out for is that IPv6 addresses are in a tentative state until duplicate address detection has been performed. Linux will refuse to use a tentative address as the default source address. This means that you might need to add a short sleep before adding this route so that the address has time to come out of the tentative state and become usable.

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This works great! Thanks for the tip about tentative addresses, too; that would have probably taken me a while to figure out why it would work when run manually but not on system boot. – Dan Mar 22 '14 at 12:06
I just wrote a script that waits for given addresses to leave the tentative state. Might be useful in init scripts: – Sander Steffann Mar 22 '14 at 17:36

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