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Description of problem: This is a slightly odd problem, and it may just need a documentation pointer. On Linux, IPv6 addresses are actually bound internally to the "lo" interface. This can be seen with "ip -6 route show table local": local 2001:630:12:... via :: dev lo proto none metric ... If you (erroneously?) do: ifdown lo ifup lo ...this kills IPv6 in a very confusing way; the (non-loopback) IPv6 addresses /128 routes are stripped off the "lo" interface and not re-added. IPv6 connectivity is broken, but: ip addr ifconfig ip -6 route ...all appear to show working connectivity. The only way to fix it is a subsequent re-start of the physical interface, to re-acquire the IPv6 autoconfig addresses. There are comments in the kernel source saying "Longer term, all of the dependencies ipv6 has upon the loopback device and it being up should be removed": http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=net/ipv6/addrconf.c;h=5b189c97c2fc16d28961955673c1fa94f3df21ad;hb=e6f597a1425b5af64917be3448b29e2d5a585ac8#l4705 ...but obviously that's a (very?) long-term fix. I'm by no means certain this is a RedHat bug, or what the fix would be if it were. Arguably it makes no sense to actually set "link down" on loopback, so maybe modify the initscripts to instead flush addresses; or print a warning as the interface goes down? In case anyone is curious; we discovered this because we were adding /128 IPs to loopback, which are routed to the box by static routing from our network. This allows e.g. DNS & SMTP servers to be on well-known IPs that can move with the box regardless of the underlying network and anycast routed (in the traditional sense, rather than the new IPv6-specific anycast stuff) Version-Release number of selected component (if applicable): AFAIK all versions of RHEL and Fedora are affected. How reproducible: Always Steps to Reproduce: 1. Test ipv6 connectivity is working 2. Do "ifdown lo", "ifup lo" 3. Test ipv6 connectivity is broken Actual results: IPv6 connectivity breaks when loopback interfaces goes down and is not restored when loopback comes back Expected results: IPv6 connectivity would only depend on the interface via which connectivity is present. Additional info:
I am sorry, but it is now too late in the RHEL-5 release cycle. RHEL-5.10 (the next RHEL-5 minor release) is going to be the first production phase 2 [1] release of RHEL-5. Since phase 2 we'll be addressing only security and critical issues. [1] https://access.redhat.com/support/policy/updates/errata/