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Thank you for the report Norman.

I think the problem is not in dhclient.
I'm going to switch the component to initscripts and here is the explanation:

If the ISC client for DHCPv4 receives no response to its broadcast message
in the INIT state, it continues retransmitting DHCPDISCOVER messages
for some configurable period of time and than it either
exits (when running dhclient with -1 parameter)
or becomes a daemon (when running without -1 parameter).

If the ISC client for DHCPv6 receives no response to its Solicit message, it
continues retransmitting Solicit messages for some configurable period of
time and than it either exits (when running dhclient with -1 parameter) or
continues running in foreground (when running without -1 parameter).

When the client for DHCPv6 is started (without -1) during system startup and it
receives no response to its Solicit messages it doesn't "give up" and
holds up the system indefinitely.

I can ask upstream (ISC) if this is a bug or it is intentional,
but I think this is not important for this bug report.

What I think is important is that ifup-eth starts dhclient for DHCPv4 by default (when PERSISTENT_DHCLIENT=yes is not used) with -1 parameter.

So I suggest to start dhclient for DHCPv6 also with -1 parameter
so it will behave like the dhclient for DHCPv4.

There's also -nw parameter. man dhclient says:
"-nw    Become a daemon process immediately (nowait) rather than waiting until an IP address has been acquired."
This also solves the problem,
but I think the "-1" parameter is what we need.

Created attachment 408599 [details]
suggested patch

Merged without the echos.

VERIFIED as fixed in initscripts-9.03.17-1.el6. 

service network restart now works fine and does not hang

NEW package initscripts-9.03.17-1.el6:
:: [   PASS   ] :: Running '(service network restart; echo PASS) &> /tmp/tmp.8mhQCoT3iP &'
:: [   PASS   ] :: Running 'sleep 5m'
:: [   PASS   ] :: File '/tmp/tmp.8mhQCoT3iP' should contain 'PASS'


OLD package initscripts-9.03.5-1.el6:
:: [   PASS   ] :: Running '(service network restart; echo PASS) &> /tmp/tmp.AVpvaDDujA &'
:: [   PASS   ] :: Running 'sleep 5m'
:: [   FAIL   ] :: File '/tmp/tmp.AVpvaDDujA' should contain 'PASS'

Red Hat Enterprise Linux 6.0 is now available and should resolve
the problem described in this bug report. This report is therefore being closed
with a resolution of CURRENTRELEASE. You may reopen this bug report if the
solution does not work for you.