Increased daemon startup time after combining subnets

Akey, Michael Mike.Akey at oregonstate.edu
Thu Apr 4 23:44:15 UTC 2019


After combining 4 subnet statements into one monolithic subnet statement our DHCP servers take more than double the time to process the leases database file on startup (from about 1 minute to over 3).  These subnets were already quite large /17's but combined make a single /15.  Normally we wouldn't make a subnet and pool that large but the usual rules don't apply to this network - it's for wireless clients and broadcast traffic is limited by network gear.

Having little knowledge of how dhcpd handles the leases file on startup, I assumed that merely combining these four subnets and pools into one single subnet and pool would have little effect on the operation of dhcpd.  I'm not adding any significant about of potential leases, so what is causing the additional startup time?  Any ideas?

Here is a before and after config snippet:

# Before

subnet 10.248.0.0 netmask 255.255.128.0 {
    default-lease-time 3600;
    max-lease-time 3600;
    option routers 10.248.0.1;
    option subnet-mask 255.255.128.0;
    pool {
        # Pool Statements
        failover peer "dhcp";
        deny dynamic bootp clients;
        # Allow statements
        range 10.248.0.128 10.248.127.254;
    }
}

subnet 10.248.128.0 netmask 255.255.128.0 {
    default-lease-time 3600;
    max-lease-time 3600;
    option routers 10.248.128.1;
    option subnet-mask 255.255.128.0;
    pool {
        failover peer "dhcp";
        deny dynamic bootp clients;
        range 10.248.128.128 10.248.255.254;
    }
}

subnet 10.249.0.0 netmask 255.255.128.0 {
    default-lease-time 3600;
    max-lease-time 3600;
    option routers 10.249.0.1;
    option subnet-mask 255.255.128.0;
    pool {
        failover peer "dhcp";
        deny dynamic bootp clients;
        range 10.249.0.128 10.249.127.254;
    }
}

subnet 10.249.128.0 netmask 255.255.128.0 {
    default-lease-time 3600;
    max-lease-time 3600;
    option routers 10.249.128.1;
    option subnet-mask 255.255.128.0;
    pool {
        failover peer "dhcp";
        deny dynamic bootp clients;
        range 10.249.128.128 10.249.255.254;
    }
}


# Afterward

subnet 10.248.0.0 netmask 255.254.0.0 {
    default-lease-time 3600;
    max-lease-time 3600;
    option routers 10.248.0.1;
    option subnet-mask 255.254.0.0;
    pool {
        failover peer "dhcp";
        deny dynamic bootp clients;
        range 10.248.0.128 10.249.255.254;
    }
}


Mike Akey
Systems Engineer, IT Infrastructure
University Information and Technology | Oregon State University
541-737-4948 | uit.oregonstate.edu


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