[CentOS] dhcpd frequent renewals

Folks

This might be the wrong place to ask, but I don't know where to turn.
My internal home network, including wireless, is controlled by a 
Centos6 server, which provides dhcpd services, along with NAT.  I 
have DHCPD configured with the addresses 192.168.155.200 through 
192.168.155.254 as the range for dynamic allocations.  The 
default-lease time is 1800 seconds, the maximum is 3600 seconds.

My windows clients, and even an ipad-mini behave nicely, asking for 
DHCP renewals once ever five minutes, or at about 80% of the default 
lease time, a behavior I can understand.  However, several of my 
guests, with their own iPads, I-watches, iPhones, connect to my 
network (via a wireless access point which does not do routing 
functions) and they're renewing once every 20-30 seconds.  In 
addition, these devices also loose connectivity for brief intervals, 
which seems to be roughly synchronized with dhcp renewal.  This last 
fact I deduce by doing "tail -f /etc/log/messages" and hearing them 
say "lost connection" at just about the same moment the DHCPREQUEST 
and DHCPACK statements show up.

It's difficult to believe that Apple IOS devices (all of which are 
running apple's latest) have a dhcp client problem not shared by 
windows or even linux hosts.

Does anyone have any clues?

David Kurn
San Francisco

DHCPD.CONF file is excerpted below:

----------------------------------------
ddns-update-style			none;

subnet 192.168.155.0 netmask 255.255.255.0 {
	authoritative;
	option routers 			192.168.155.2;
	option subnet-mask		255.255.255.0;
	option broadcast-address	192.168.155.255;
	option domain-name		"daku.org";
	option domain-name-servers	192.168.155.2;
	option netbios-name-servers	192.168.155.2;
	
	option time-offset		-28800;	# Pacific standard time
	
	range dynamic-bootp		192.168.155.200 192.168.155.254;
	default-lease-time		1800;
	max-lease-time			3600;
}

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