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I noticed in my local webserver logs that the IPv6 address of my desktop changed after upgrading to Kubuntu 12.04.

inet6 addr: identical:identical:identical:identical:changed:changed:changed:changed/64 Scope:Global

Why did this happen, how can I avoid .htaccess rules from breaking during OS upgrades?

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It depends on how your address was configured in the first place. You can configure a static address if you don't want it to change. If you use DHCPv6 then it depends a lot on the DHCP server. If you use plain SLAAC (stateless autoconf) then it should remain stable as long as your MAC address of your network adapter is stable, and it you use SLAAC with privacy extensions than it is not stable by design.

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I'm not aware of setting up DHCPv6. How can I check for SLAAC? Still using the same PC, so the MAC address of the NIC is identical to the one before the upgrade. – jippie May 17 '12 at 9:38
I assume here that you have IPv6 enabled on the LAN where your desktop is connected. You are very probably using SLAAC if you haven't configured anything else. What you are seeing are probably the privacy extensions, but because you have hidden the real IPv6 addresses in your question I can't tell you for sure. Configuration details are on i.e. home.regit.org/2011/04/ipv6-privacy. Also look at /etc/sysconf – Sander Steffann May 17 '12 at 14:30
But are you sure you want to do this? You might just enable access from the whole /64. – Sander Steffann May 17 '12 at 14:32
All I know about IPv6 is that I should forget everything I know about IPv4. How can I be sure the /64 is not accessible from the internet? PC, (NAT) router and webserver are fully IPv6 enabled. The thing that worries me here is the 'Scope: Global' part. – jippie May 17 '12 at 22:24
You basically want a stateful firewall that only allows outbound connections but not incoming connections. Most home-routers have such a firewall by default, and decent business firewalls will offer this as well. – Sander Steffann May 24 '12 at 9:40

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