Thread: DHCPd questions
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Old 11th October 2013
J65nko J65nko is offline
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Join Date: May 2008
Location: Budel - the Netherlands
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I cannot answer these questions completely, but the OpenBSD dhclient(8) states:

Code:
In order to keep track of leases across system reboots and server
restarts, dhclient keeps a list of leases it has been assigned in the
/var/db/dhclient.leases.<IFNAME> file.  IFNAME represents the network
interface of the DHCP client (e.g. em0), one for each interface.  On
startup, after reading the dhclient.conf(5) file, dhclient reads the
leases file to refresh its memory about what leases it has been assigned.

Old leases are kept around in case the DHCP server is unavailable when
dhclient is first invoked (generally during the initial system boot
process).  In that event, old leases from the dhclient.leases.<IFNAME>
file which have not yet expired are tested, and if they are determined to
be valid, they are used until either they expire or the DHCP server
You could monitor the DHCP traffic with tcpdump(8) to see how other clients behave.

Code:
# tcpdump -eni fxp0 -s1500 -vvv port 67 or port 68

12:52:21.542162 00:1f:33:f1:ff:f9 ff:ff:ff:ff:ff:ff 0800 342: 192.168.222.20.68 > 255.255.255.255.67: [udp sum ok] xid:0xb6a32e84 ether 00:19:db:47:b0:4c vend-rfc1048 DHCP:REQUEST HN:"hercules" RQ:192.168.222.20 PR:SM+BR+DG+DN+NS+HN [tos 0x10] (ttl 16, id 0, len 328)

12:52:21.543071 00:08:c7:05:ca:0b 00:1f:33:f1:ff:f9 0800 348: 192.168.222.10.67 > 192.168.222.20.68: [udp sum ok] xid:0xb6a32e84 Y:192.168.222.20 S:192.168.222.10 ether 00:19:db:47:b0:4c file "pxeboot" vend-rfc1048 DHCP:ACK SID:192.168.222.10 LT:86400 SM:255.255.255.0 DG:192.168.222.10 DN:"utp.xnet" NS:192.168.222.10 RN:43200 RB:75600 TFTP:"hercules" [tos 0x10] (ttl 16, id 0, len 334)
Because of the -vv option the output is quite readable.

RE: doubling network users from 250 to 500

Having so many clients in one subnet or broadcast domain usually is not such a good idea. You can use subnets and still use a central DHCP server, by setting up dhcrelay(8) in each subnet.
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