DHCP/ BOOTP

Given in RFC2131

First when device connects to a network, it sends Discover message. It is sent as a Broadcast. It uses Bootstrap Protocol which uses UDP. The ports are always fixed to 68 and 67 respectively for client and server.

SRC: 0.0.0.0 DST: 255.255.255.255

The DHCP servers on the n/w replies with Offers. It is also a broadcast since device doesn’t have IP address. Whichever server replies first, then, other servers will not reply (race condition).

After this, it sends a Request to one DHCP server and it is sent as a broadcast even though it knows the destination IP so that other DHCP servers can know that it chose a different DHCP server and thus, can free the IP.

Now, the server first pings the IP address that it is going to allocate so that it can be sure that the IP has not been assigned statically.If address is used, server sends NAC and the client should DORA again. If no ping reply, only then, it sends an Acknowledgement with an IP address for the client. Thus, client gets its own IP address.

Note that now even other DHCP servers will send an ACK to our IP to tell that we are okay to use it. It will be unicast since we got our IP address.

After getting the IP address, the host sends an ARP for the assigned IP (it is gratuitous ARP since it does ARP for its own IP address. Here, it uses the source IP as 0.0.0.0). If it gets a reply, then, it sends a DHCP Declined to the server since another device already has the same IP address. (It is not done by the DHCP server since the DHCP server can be on different subnet also, hence it is done by the client.)

Note: that both the client and server will use the same Transaction ID.


After the lease time is expired, client sends a DHCP Renew message to DHCP server. The server sends back a ACK and client can keep the IP address for another 90 days.


The DHCP ACK will have details like Lease Time, DNS Server IP address, Subnet Mask, etc…


DHCP RELAY:

We know that DHCP request is sent as broadcast, but it cannot cross routers and vlans. Here, we have to place a dhcp server for each subnet and vlan which is not scalable. Thus, we use DHCP relay.