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Description of problem:
According to what I see in the ifup-post script changes in the "DNS2" variable are ignored.

As far as I can see in the upstream fedora repository Lukas already commited
a fix in da08727f3bbcc419096b160968aee83104bb7cf6

https://git.fedorahosted.org/cgit/initscripts.git/commit/?id=da08727f3bbcc419096b160968aee83104bb7cf6

Version-Release number of selected component (if applicable):
As far as I can see that bug is present in all 6.x and 7.x releases.

https://git.fedorahosted.org/cgit/initscripts.git/commit/?h=rhel7-branch&id=21ee08c718692c6a4ff32204767638f0c3aafa65

*** Bug 1364895 has been marked as a duplicate of this bug. ***

With initscripts-9.49.37-1.el7 I've:
ONBOOT=yes
TYPE=Ethernet
BOOTPROTO=dhcp
DEVICE=eth0
DNS2=8.8.4.4
PEERDNS=yes

then /etc/resolv.conf contains:
nameserver 8.8.4.4

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2016-2456.html