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DHCP Server

Hello Experts,

 

When I do ipconfig /all command in my command prompt I could see the DHCP servers address as below:

 

DHCP server: 1.1.1.5

 

But when i logged into my gateway and checked the helper-address are mentioned as different.

 

ip helper-address 10.237.4.161
ip helper-address 10.243.252.55
ip helper-address 10.247.25.67

I am confused by seeing this. I know that the 10X series are belong to dhcp server, then the 1.1.1.5 belongs to ?

 

Thanks

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VIP Mentor

Re: DHCP Server

Hello

I would say the dhcp server ip you see on your pc would be the dhcp server that allocated as your ip address Those the helper address that are defined on your router would only forward any client requests to any dhcp server that didn't reside on your own network.


Is that 1.1.1.5 reachable?



kind regards
Paul

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Re: DHCP Server

Yes Paul. Its is reachable.


Pinging 1.1.1.5 with 32 bytes of data:
Reply from 1.1.1.5: bytes=32 time=5ms TTL=128
Reply from 1.1.1.5: bytes=32 time=5ms TTL=128
Reply from 1.1.1.5: bytes=32 time=5ms TTL=128
Reply from 1.1.1.5: bytes=32 time=72ms TTL=128
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VIP Mentor

Re: DHCP Server

Hello

I would say that host is your current dhcp server



kind regards
Paul

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Re: DHCP Server

Okay thanks. Then the purpose of helper address is just to forward the traffic to 1.1.1.5?
The 10.x series helper address is also the dhcp server address right ?
Highlighted
VIP Advisor

Re: DHCP Server

Hi there,
A DHCP request is sent as a broadcast, so is either picked up by a listening host on the same broadcast domain/ subnet, or is forwarded to a host specified on an interface by the ip helper-address. It is then a race to see who responds first...

 

What does the running config of the L3 interface which this host is connected to look like. Is the address 1.1.1.5 part of the primary/ secondary ip address subnet?

 

cheers,
Seb.

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Re: DHCP Server

Hi Seb,
In sh run config I could just see the L3 Ip address, ip helper-address and default commands(no ip redirects, no ip unreachables, no ip proxy-arp)

My doubt here is what is the difference between those ip helper-address and the address shown in cmd prmpt
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VIP Advisor

Re: DHCP Server

There is something odd about your implementation... if the server 1.1.1.5 is not routed by the gateway interface of the host then I can only assume that DHCP requests going to the helper-addresses is being forwarded onto 1.1.1.5.

 

The DHCP OFFER packet contains a field SIADDR which should be set to the IP address of the server making the offer. This is the IP which appears in ipconfig /all (assuming the offer is accepted by the client). If SIADDR is different to the helper-address IPs then the initial DHCP DISCOVER packet must have been redirected.

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VIP Mentor

Re: DHCP Server


@Seb Rupik wrote:

There is something odd about your implementation... if the server 1.1.1.5 is not routed by the gateway interface of the host then I can only assume that DHCP requests going to the helper-addresses is being forwarded onto 1.1.1.5.


That is unless the client request doesn't have to be forwarded and the sever answers locally!

 

 



kind regards
Paul

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VIP Advisor

Re: DHCP Server

Agreed. Which is why I was trying to tease out the details of the local gateway IP setup.

 

Assuming the DHCP server did answer locally, it could reasonably have any IP address, but we know it responds to a ping. So either the host is in the same subnet as the server, or if not then its ping is sent to the gateway, which itself must have a secondary IP in the same subnet as 10.1.15. The gateway acting as a the router between two subnets but connected to the same broadcast domain.