Awesome article. Haven't seen such a detailed and clear discription of the topic.

Hi Varun, Thank you for your compliment :-)

super details thanks a  lot for a great article...

I ever r seen this much clear explanatioin on dhcp relay agent. Thank you :)

This is by far the best explanation i have seen on DHCP.. thank a ton !!!

Best article written on DHCP to the date.

Thank you for your compliment. I'm glad you liked it. 

excellent!!! a crystal clear explanation with enormous details... verymuch appreciated... hats off...

Wonderful article !! Thanks for the insight, it's been helpful in understanding DHCP

The document is pretty helpful! Thanks for sharing such an enormous details article.

Excellent document. Very useful.Thanks for sharing

good explanation Thanks

Very, very gooood. Big Thanks !!!

omg! this is super work.

the best i ever seen

thanks !!

you did a well job but smothing you need to edit in relp port it should be 67

wonderful article...Really appriciated..Great Thanks...

Excellent Article.

Is it possible to know which editing tool you have used to draw such a beautiful detailed explanaions with respect to protocol ?

We mainly use Microsoft Visio as a drawing tool.

How does the the relay agent know the DHCP server IP and MAC?

The DHCP server IP should be configured on relay agent,

but MAC address of the DHCP server does not need to be known by the relay agent.

The source port and destination port used between router and DHCP server must be 67 in both direction.

Nice work, this gives a good explanation. 

Very informative article !! A quick note though, I think the source port from the relay agent should be 67 itself otherthan 68 whent the relay agent unicasts the packets to the Server. Correct me if i am wrong.

Very useful article on DHCP relay agent 

I've been surfing internet to get packet level explaination specifially with DHCP Server located on a different network, this is the best explaination I have ever came across.

- Great Job.  I can imagine your efforts and time behind this.

Cheers from Seattle, USA

It is possible to havce the same good lesson about DHCPv6 messages and if it possible understand better the transport between an client with DHCPv6 request over an IPv4 network backbone; the DHCP server is dual-stack ? Thank you too much

max@massimilianosbaraglia.it

Very well explained and easiest way with artifacts.

"GOOD WORK"

Wow supper ....

Vice nice explanation.

Can we configure any device except (router/switch) as DHCP relay agent?

If  yes, then can we configure one port as DHCP client and other port as DHCP relay agent on this device (this device neither  a router nor a switch)?  

wonderful explination

Was looking for information on how DHCP identifies it came from which subnet? Nice Explaination. Thanks for taking time and efforts.

really nice content

Hi, The DHCPDISCOVER and DHCPREQUEST packets used as SIP the IP: 1.1.1.254, not the 100.1.1.254

Hi , If between Router (DHCP Relay) and DHCP Server , there is a Firewall , does it work?

In this case , Does the client Get IP address?

Hi , If between Router (DHCP Relay) and DHCP Server , there is a Firewall , does it work?

In this case , Does the client Get IP address?

Dear Johannes ,

I'm writing you as a fellow German bloke and wanted to ask whether you could guide mit to an authoritative source which states, what the default Source IP Address of a DHCPv4 Discover / Request from a client via Relay Agent is.

I'm asking this because refering to my tests Cisco Routers in this regard behave differently to other Network manufacturers:

Behaviour with Cisco Router:

                                      eth0:192.168.1.1        eth1:10.0.0.1                     10.0.0.254
DHCP-Client  -------------   Cisco Router and IPv4 Relay Agent   --------------- DHCP-Server
sip 0.0.0.0                  --->          relayed with       ---->       sip: 192.168.1.1
dip 255.255.255.255                                                         dip:10.0.0.254

Behaviour with other Router systems:
                                 eth0:192.168.1.1          eth1:10.0.0.1                         10.0.0.254
DHCP-Client  -------------   Centos as Router / OpenWrt  ------------------------ DHCP-Server
sip 0.0.0.0                  --->          relayed with    ------>   sip: 10.0.0.1
dip 255.255.255.255                                                     dip:10.0.0.254

Tested in a virtual environment (Hyper-V)
Screenshots of Wireshark traces could be supplied if needed.

Question is: Unicast from Relay to DHCP by default IP of DHCP-side interface or client-side interface?

I found amongst others the following sources stating that default should be DHCP-sided interface:
1. https://docs.commscope.com/bundle/fastiron-08030-l3guide/page/GUID-44165432-627D-48C8-87BF-8D304FEA253C.html
2. https://documentation.nokia.com/srlinux/23-3/books/interfaces/dhcp-relay.html

2. states that:
"...The DHCP relay agent relays the DHCP Discover message toward the DHCP server (unicast). If configured to do so, information is added for the circuit ID and remote ID sub-options in DHCP option 82. The relayed packet is unicast toward the DHCP servers with the following values:

    SIP = outgoing interface IP address by default. If the source-address is configured, the relayed packet instead has SIP = configured source-address..."

Reference to the relevant RFC and in case for dhcrelay daemons option for "configured source-address" would be nice.

kind regards
Andreas, Munich