Connection issues due to glibc returning IPv6 addresses on IPv4-only systems after update

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Environment

SUSE Linux Enterprise Server 11 Service Pack 4 (SLES 11 SP4)

Situation

On systems configured to disable the AF_INET6 address family by blacklisting the ipv6 kernel module, some programs can't connect to other systems any more that are having DNS address records for both IPv4 and IPv6 addresses. The issue occurs after a maintenance update of the glibc package to version 2.11.3-17.102.1.

Resolution

With few exceptions, this affects mainly older 3rd party programs that will request IPv6 address records from DNS, but fail if the inet6 address family isn't present on the systems. In this case it's recommended to remove the blacklist entry for the ipv6 kernel module or get a bug fix from the vendor of the 3rd party program.

In other cases, the program or component refusing to connect is explicitly requiring an IPv6 capable system and may need a configuration change if it isn't present, notably libtirpc which is required for NFS. For that service udp6 and tcp6 need to be disabled in /etc/netconfig.

Alternatively, IPv6 addresses can be removed from DNS in such an environment. Preferably, the ipv6 module should not be blacklisted.

Cause

The patch for glibc, that avoided IPv6 host address lookups when no IPv6 address family is present, had to be removed, as otherwise host lookups performed on IPv6-only systems may hang in certain valid and supported environments. The patch still being present on SLES 11 was an oversight. Any issues arising from the change were only hidden from view due to this patch.

Additional Information

Blacklisting the ipv6 kernel module disables all IPv6 support of the Linux kernel, including the inet6 address family. Having IPv6 addresses configured in a DNS zone without all machines accessing these hosts supporting IPv6 is a rather dubious situation to begin with. We strongly recommend to not blacklist the ipv6 kernel module.

Disclaimer

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