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Re: [dhcwg] Regarding DHCP Discover with Dest Ip as subnet broadcast

Ted Lemon <mellon@fugue.com> Mon, 18 May 2020 16:46 UTC

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From: Ted Lemon <mellon@fugue.com>
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Date: Mon, 18 May 2020 12:46:05 -0400
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Cc: dhcwg@ietf.org
To: Venkataratnam Naidu <ratnameee@gmail.com>
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Subject: Re: [dhcwg] Regarding DHCP Discover with Dest Ip as subnet broadcast
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On May 18, 2020, at 12:23 PM, Venkataratnam Naidu <ratnameee@gmail.com> wrote:
> Thank you for the reply.  Dhcp client using 255.255.255.255 always works. But wanted to know the dhcp server behavior when it receives Discover/Request with below. And also would like to know any dhcp-client implementations using the "subnet broadcast" as destination ip.

If you don’t see the behavior called out explicitly in the spec, you should not count on there being any standard behavior.  No correctly implemented client will use the subnet broadcast address, because this will not work in all cases. Some client that isn’t able to use the link-local broadcast may use subnet broadcast and hope for the best, but you can assume that there will be cases where this definitely will not work. If the client tries to do anything other than DHCPINFORM this way, there is a good chance that the DHCP server will guess the wrong subnet and give an address that won’t work. Or it might work just fine. But using the 255.255.255.255 broadcast is required for broadcast packets, not optional.

The DHCP server is permitted to unicast its response, and a client with no address listening on 0.0.0.0 is expected to handle this correctly.

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