DHCPREQUEST flooding

[Alex Moen]

I am running a 4.2.5 ISC DHCP server (up-to-date via Centos 7 
repository) for our ISP business.  We have around 7000 subscribers; most 
with an el-cheapo router, a few with no router at all.  Most of our 
customers are using a variant of Linksys router (Linksys, Cisco-Linksys, 
Belkin, etc) because that is what we provide if they ask for a router. 
However, this issue is not only a Linksys issue, as we are also seeing 
PCs exhibiting the same behavior.

The issue is that we have a fairly large number of devices (around 300) 
that are issuing DHCPREQUESTs at extremely short intervals (the worst, a 
few second apart).  In the last 6 hours, some of these devices have 
REQUESTed over 2000 times.  They are all being ACKed.

Is this a common problem that everyone sees, or do I have a config 
issue?  This has actually been going on for a long, long time, and I am 
just tired of the large log file sizes.  Since we're an ISP, we have to 
keep our logs for a few years time, so the log file size can become an 
issue.

A typical network stanza looks like:

         subnet 76.10.94.0 netmask 255.255.254.0 {
         pool {
               authoritative;
               range 76.10.94.20 76.10.95.200;
               min-lease-time 129600;
               max-lease-time 259200;
               default-lease-time 259200;
               option subnet-mask 255.255.254.0;
               option broadcast-address 76.10.95.255;
               option routers 76.10.94.1;
               }
         }

Thanks for any input!!

Alex