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RHEL 6 glibc IPv6 DNS resolution bug

Jon Jensen

By Jon Jensen
July 17, 2012

We ran into an unpleasant bug in a Red Hat Enterprise Linux 6 glibc update a couple of weeks ago. It since has made its way into CentOS 6 as well.

The problem manifested itself in our particular case with this error from the Postfix mailer:

Jun 29 01:55:23 sl37 kernel: smtp[7093]: segfault at 1 ip 00007ffc0e455596 sp 00007fff99948f60 error 6 in libresolv-2.12.so[7ffc0e449000+16000]

But it affects all DNS resolution on the host, not just for mail.

If you have any IPv6 resolvers at all listed in /etc/resolv.conf, all your DNS resolution is likely to be broken with this version of glibc:


To work around the problem, you can either:

  • Use only IPv4 DNS resolvers (comment out the IPv6 resolvers for now)
  • or downgrade to the previous version of glibc using yum downgrade

Red Hat is aware of the bug and you can track progress toward a resolution in Bugzilla bug #835090.

If you’re using IPv6, watch out for this! If not, you’re fine.

ipv6 linux redhat sysadmin


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