forwarding DHCP packets via a router.

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forwarding DHCP packets via a router.

Rajeev Bansal
Hi ,

 I have a specific requirement, where I would like to forward a dhcp discover/request/renew/release packets from a client via a router to another subnet. I can't use a dhcrelay for this purpose, because in my scenario both interface of my router (client side and dhcp server side) will not have IPv4 address assigned. Yes that means, I am trying to forward the dhcp packets to different link without my client knowing that it is communicating over different Ethernet link.

For example, lets say my router has eth0 and eth1 two interface and there is a laptop connected to eth0 and there is server running on the LAN on which eth1 is connected. Now I want to forward my client's dhcp requests from eth0 to eth1 and response from eth1 to eth0 seamlessly. Please note eth0 and eth1 both doesn't have any IP address assigned. 

I don't think dhcrelay will fit in my requirement, can someone suggest some way to do that. 

Thanks,
-Rajeev

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Re: forwarding DHCP packets via a router.

Gero Palacio
Hi there,

I think you can use a bridge to accomplish what you're looking for. If I understand correctly, you have 2 different LAN segments (one on each router's interface) and you need the client's interface connected to the DHCP server interface as it was in the same LAN. You can do that using a bridge. 

You should keep in mind your network topology though and if it's what you want for your network.. I'm just think in the simplest scenario were you have CLIENT <-> ROUTER <-> SERVER.

Hope this helps!

On Mon, Jul 13, 2015 at 2:15 PM, Rajeev Bansal <[hidden email]> wrote:
Hi ,

 I have a specific requirement, where I would like to forward a dhcp discover/request/renew/release packets from a client via a router to another subnet. I can't use a dhcrelay for this purpose, because in my scenario both interface of my router (client side and dhcp server side) will not have IPv4 address assigned. Yes that means, I am trying to forward the dhcp packets to different link without my client knowing that it is communicating over different Ethernet link.

For example, lets say my router has eth0 and eth1 two interface and there is a laptop connected to eth0 and there is server running on the LAN on which eth1 is connected. Now I want to forward my client's dhcp requests from eth0 to eth1 and response from eth1 to eth0 seamlessly. Please note eth0 and eth1 both doesn't have any IP address assigned. 

I don't think dhcrelay will fit in my requirement, can someone suggest some way to do that. 

Thanks,
-Rajeev

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Re: forwarding DHCP packets via a router.

Simon Hobson
In reply to this post by Rajeev Bansal
Rajeev Bansal <[hidden email]> wrote:

>  I have a specific requirement, where I would like to forward a dhcp discover/request/renew/release packets from a client via a router to another subnet. I can't use a dhcrelay for this purpose, because in my scenario both interface of my router (client side and dhcp server side) will not have IPv4 address assigned. Yes that means, I am trying to forward the dhcp packets to different link without my client knowing that it is communicating over different Ethernet link.

Can you clarify things a bit more ? Is there IPv4 connectivity between client and server ? If not then it won't work.

A few details that come to mind ...

Firstly, the relay agent does *not* have to be in a router, it can be any device on the same network segment (broadcast domain) as the client. So you can put a relay agent on any convenient system.

Both client and relay agent must have IPv4 connectivity to the server. It doesn't matter how (could be VPN, 4 in 6 tunnel, ...), but they need to be able to exchange unicast packets.

For the ISC relay agent, there is a restriction (due to historical design decisions) that all interfaces used by the relay agent must be broadcast capable (in practical terms, look like an ethernet interface). This also applies to the server unless you compile without raw sockets support - which would mean it couldn't handle clients on the directly attached network.
I don't know if a 4 in 6 tunnel (with an endpoint on the relay agent/server machine) will work for this.

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Re: forwarding DHCP packets via a router.

Leandro
In reply to this post by Rajeev Bansal
Bridge ?

On 13/07/15 14:15, Rajeev Bansal wrote:
Hi ,

 I have a specific requirement, where I would like to forward a dhcp discover/request/renew/release packets from a client via a router to another subnet. I can't use a dhcrelay for this purpose, because in my scenario both interface of my router (client side and dhcp server side) will not have IPv4 address assigned. Yes that means, I am trying to forward the dhcp packets to different link without my client knowing that it is communicating over different Ethernet link.

For example, lets say my router has eth0 and eth1 two interface and there is a laptop connected to eth0 and there is server running on the LAN on which eth1 is connected. Now I want to forward my client's dhcp requests from eth0 to eth1 and response from eth1 to eth0 seamlessly. Please note eth0 and eth1 both doesn't have any IP address assigned. 

I don't think dhcrelay will fit in my requirement, can someone suggest some way to do that. 

Thanks,
-Rajeev


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Re: forwarding DHCP packets via a router.

Simon Hobson
Leandro <[hidden email]> wrote:

> Bridge ?

A bridge will do it - but ONLY if it's OK to have a flat IPv4 network. Ie, the network as seen by IPv4 devices would have no router at all, the client and server would appear directly connected.

A bridge is (in this context) just another name for a switch.

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