It is very silly but the default on the ifup-eth script tells dhclient ( the program that obtains a DHCP IP address if you have selected DHCP in your ifcfg-eth* config file) to EXIT / QUIT if the first attempt to obtain a lease fails.
No amount of dhclient.conf settings will fix this because if dhclient is started with -1 (which it is by default) then dhclient will quit.
This is obviously very bad for MOST cases. Say for example you have a power outage or you initially power on the system, if for some reason the link takes a few more seconds to come up, dhclient has probably already quit being unable to obtain a lease the first time.
So the option to set in your ifcfg-eth0 config file to solve the dhclient persistent issue:
The difference in how dhclient is started now looks like this:
/sbin/dhclient -H hostname -q -lf /var/lib/dhclient/dhclient-eth0.leases -pf /var/run/dhclient-eth0.pid eth0
If you don't have the option above you will see a "-1" which indicates that it would quit if the first lease attempt fails:
/sbin/dhclient -H hostname -1 -q -lf /var/lib/dhclient/dhclient-eth0.leases -pf /var/run/dhclient-eth0.pid eth0
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