Curious behaviour with expired/abandoned leases not being given out

[Christopher Causer]

Hi DHCP Users list,

May I please have some help understanding what is going on here? I have a pool that by some accounts is not full and in others it is and I don't understand why.

The logs indicate that it is pushing close to the limits of free addresses when balancing
2020-03-10T10:53:19+00:00 $PRIMARY      dhcpd[2147]: balanced pool 55dccc4d51e0 10.20.200.0/22  total 1008  free 2  backup 2  lts 0  max-misbal 1
2020-03-10T10:53:20+00:00 $SECONDARY dhcpd[3683]: balanced pool 555eee5eb1e0 10.20.200.0/22  total 1008  free 2  backup 2  lts 0  max-misbal 1

And this is confirmed when some people try to get leases
2020-03-10T10:54:22+00:00 $PRIMARY      dhcpd[3683]: DHCPDISCOVER from $MAC via 10.20.203.254: peer holds all free leases
2020-03-10T10:54:22+00:00 $SECONDARY dhcpd[2147]: DHCPDISCOVER from $MAC via 10.20.203.254: peer holds all free leases

However we run dhcpd-pools. The subnet is 43% utilized, but 100% "touch utilized":
$ dhcpd-pools
Ranges:
shared net name     first ip           last ip            max   cur    percent  touch   t+c  t+c perc     bu  bu perc
[snip]
All networks        10.20.200.1      - 10.20.203.240     1008   434     43.056    573  1007    99.901      1    0.099

What is happening here? I can see the touched percentage being high, but as I understand it that should not be denying any leases being given out. According to the dhcpd-pools man page, touched IPs are "
IP's which appear in the lease file, but who's leases have expired. A touched IP is either expired or abandoned." We have many subnets and there are only two with a touch percentage of ~100% and these are both subnets experiencing issues.

I've set the abandon-lease-time to 1800. Running ping against the subnet, sources from the DHCP server, I do not get anywhere close to 50% responses.

Any help gratefully received. Redacted config below. The leases file is too large and contains sensitive data that I don't think my employer would be willing to hand out unfortunately. I am happy to redact and share relevant snippets if required though.

# /etc/dhcp/dhcpd.confcd
-------------------------------------------------->8-------------
ddns-updates off;
ddns-update-style none;
authoritative;
log-facility local7;
local-address $IP;
server-identifier $IP;

key EXKEY {
    [snip]
};

omapi-port 7911;
omapi-key EXKEY;

include "/etc/dhcp/options.conf";

include "/etc/dhcp/failover.conf";

include "/etc/dhcp/nets.conf";
-------------------------------------------------->8-------------



## /etc/dhcp/options.conf
-------------------------------------------------->8-------------
option option-150 code 150 = array of ip-address;
-------------------------------------------------->8-------------



## /etc/dhcp/failover.conf
-------------------------------------------------->8-------------
failover peer " dhcp" {
    primary;
    address $THIS_ADDRESS;
    port 847;
    peer address $PEER_ADDRESS;
    peer port 848;
    max-response-delay 5;
    max-unacked-updates 10;
    load balance max seconds 3;
    # ONLY ON PRIMARY
    mclt 1800;
split 128;
# /ONLY ON PRIMARY

}
-------------------------------------------------->8-------------



## /etc/dhcp/nets.conf
# Very long, so snipped considerably
-------------------------------------------------->8-------------
abandon-lease-time 1800;
default-lease-time 900;
max-lease-time 900;
ping-check false;
one-lease-per-client true;
boot-unknown-clients true;
option domain-name-servers [snip];
option domain-name "[snip]";
option time-servers uk.pool.ntp.org;

subnet 10.20.200.0 netmask 255.255.252.0 {
    option routers 10.20.203.254;
    option subnet-mask 255.255.252.0;

    pool {
            deny dynamic bootp clients;
            failover peer "dhcp";
            range 10.20.200.1 10.20.203.240;
    }
}
-------------------------------------------------->8-------------


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