ISC DHCP infinite lease time

Solution 1:

It is not mentioned explicitly in the manpage, but setting lease time to -1 in any of the options you mention,

default-lease-time -1;
max-lease-time -1;

is effectively disabling the expiry time of the leases, so their expiration will be effectively set "to infinity".

Solution 2:

You should not configure an infinite lease time. The reason of having DHCP is to have a central management and flexibility. Making the lease time infinite, you will kill the flexibility.

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Solution 3:

I would suggest specifying lease times in seconds. So where:

default-lease-time 600; This being ten minutes
max-lease-time 7200; This being two hours

try:

default-lease-time 86400; This being one day
max-lease-time 604800; This being one week

You could try 2592000 which is 30 days.

I wouldn't exceed that.