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Today's Topics:
1. RE: Can you use DHCP to assign an address to the downstream
interface used by dhcrelay? (Renyk de'Vandre)
2. RE: Can you use DHCP to assign an address to the downstream
interface used by dhcrelay? (Vallevand, Mark K)
----------------------------------------------------------------------
Message: 1
Date: Wed, 28 Feb 2024 12:53:20 +0000
From: Renyk de'Vandre <r.k.devan...@bham.ac.uk>
To: Users of ISC DHCP <dhcp-users@lists.isc.org>
Subject: RE: Can you use DHCP to assign an address to the downstream
interface used by dhcrelay?
Message-ID:
<lnxp265mb1114dd55faa3cf3d05ad31b6f2...@lnxp265mb1114.gbrp265.prod.outlook.com>
Content-Type: text/plain; charset="utf-8"
Hi All,
Re: below
Just read this and it had me curious, can you explain why you don't want to or
can't set a static IP address on your dhcp relay interface? It would be good
to see your basic topology:
i.e.
DHCP SERVER <--> routing infrastructure <--> edge router <--> switched
infrastructure 1 <--> dhcprelay requiring dhcp lease for relay interface <-->
switched infrastructure 2 <--> client requiring dhcp lease
My thoughts were:
1. (what is the dhcprelay running on)
2. Are the switched infrastructure 1 and switched infrastructure 2 separate
i.e. broadcasts from 2 and 1 are separate?
If that was the case (just thinking out loud here) I first thought you could
you put an ip helper on the edge router which would listen for the broadcast
from the dhcprelay interface address and assign an address, however this would
not work because this would only work for assigning an address to the upstream
interface between edge router and switched infrastructure 1.... so I totally
agree below I can't see how this could be made to work.... and if the
switched infrastructure 1 and 2 are collapsed with the edge router and
dhcprelay on the same switched network then why do you need the dhcprelay
instead of using the edge router/vlan interface and ip helpers.... (all
conjecture here).
but back to my main thought, what is the reason for not wanting to assign the
static address on the dhcp-relay..
Ren
-----Original Message-----
From: dhcp-users <dhcp-users-boun...@lists.isc.org> On Behalf Of Simon
Sent: Friday, February 2, 2024 12:43 PM
To: Users of ISC DHCP <dhcp-users@lists.isc.org>
Subject: Re: Can you use DHCP to assign an address to the downstream interface
used by dhcrelay?
CAUTION: This email originated from outside the organisation. Do not click
links or open attachments unless you recognise the sender and know the content
is safe.
On 2 Nov 2023, at 17:13, Vallevand, Mark K <mark.vallev...@unisys.com> wrote:
> I have configured dhcrelay upstream and downstream interfaces and a server
> address.
> DHCP addresses are correctly relayed from machines on the downstream side to
> the upstream side and back.
> Rather than using a static address in the downstream interface, I would like
> to assign a DHCP address using dhclient to the downstream interface.
> Dhcrelay does not seem to handle this case. If the origin of the request is
> the actual downstream interface, it is not relayed.
> Is this possible? Can I use dhclient to get an address for the downstream
> interface used by dhcrelay?
Sorry, I?ve not had time to keep up here for a bit - I see this one seems to
have gone unanswered.
Unfortunately, you are in a catch-22 situation here. If the downstream
interface doesn?t have an IP address then presumably the relay cannot fill in
the GI-Addr field. If it can?t fill in the GI-Addr field, then the server can?t
locate the interface in order to offer an address.
To get around this situation, you?d need to configure the device in some way
before it could then use DHCP to get an address for the downstream - and once
you?ve set this up, you might as well just configure the interface.
Alternatively, you might be able to do some ?fudge? to get a subnet from the
server (similar to the way IPv6 Prefix Delegations can be done) and then use an
algorithm to configure the interface (e.g. pick the lowest usable address in
the subnet). The DHCP server would need some way of associating the request
with the device - and of course it needs the subnet details (such as router)
configured anyway in order to hand them to the clients.
However, I think your observation is a separate implementation specific
limitation. Because of the need to handle unaddressed packets (i.e. it needs to
receive and send broadcasts from/to the clients before they have an IP address)
I assume the relay agent shares code with the server in using a raw packet
interface rather than going through the IP stack. You can?t have two bits of
software doing this, and in any case running a client would result in a
broadcast packet OUT of the interface which would not be seen as an incoming
packet by the relay agent.
I assume not the answer you were looking for, but at least you now understand
why it isn?t working.
Simon
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------------------------------
Message: 2
Date: Wed, 28 Feb 2024 13:36:26 +0000
From: "Vallevand, Mark K" <mark.vallev...@unisys.com>
To: Users of ISC DHCP <dhcp-users@lists.isc.org>
Subject: RE: Can you use DHCP to assign an address to the downstream
interface used by dhcrelay?
Message-ID:
<co3pr07mb9271fe584c28ade87e0a5e85ea...@co3pr07mb9271.namprd07.prod.outlook.com>
Content-Type: text/plain; charset="utf-8"
Basically, it was a project requirement to allow a DHCP address on the
dhcp_relay interface.
Regards.
Mark K Vallevand
Unisys 651-635-7708 |
mark.vallev...@unisys.com<mailto:mark.vallev...@unisys.com>
[cid:image001.png@01DA6A18.D5725450]<http://www.unisys.com/>
From: dhcp-users <dhcp-users-boun...@lists.isc.org> On Behalf Of Renyk de'Vandre
Sent: Wednesday, February 28, 2024 6:53 AM
To: Users of ISC DHCP <dhcp-users@lists.isc.org>
Subject: RE: Can you use DHCP to assign an address to the downstream interface
used by dhcrelay?
Hi All, Re: below Just read this and it had me curious, can you explain why you
don't want to or can't set a static IP address on your dhcp relay interface? It
would be good to see your basic topology: i.?e. DHCP SERVER <--> routing
infrastructure
Hi All,
Re: below
Just read this and it had me curious, can you explain why you don't want to or
can't set a static IP address on your dhcp relay interface? It would be good
to see your basic topology:
i.e.
DHCP SERVER <--> routing infrastructure <--> edge router <--> switched
infrastructure 1 <--> dhcprelay requiring dhcp lease for relay interface <-->
switched infrastructure 2 <--> client requiring dhcp lease
My thoughts were:
1. (what is the dhcprelay running on)
2. Are the switched infrastructure 1 and switched infrastructure 2 separate
i.e. broadcasts from 2 and 1 are separate?
If that was the case (just thinking out loud here) I first thought you could
you put an ip helper on the edge router which would listen for the broadcast
from the dhcprelay interface address and assign an address, however this would
not work because this would only work for assigning an address to the upstream
interface between edge router and switched infrastructure 1.... so I totally
agree below I can't see how this could be made to work.... and if the
switched infrastructure 1 and 2 are collapsed with the edge router and
dhcprelay on the same switched network then why do you need the dhcprelay
instead of using the edge router/vlan interface and ip helpers.... (all
conjecture here).
but back to my main thought, what is the reason for not wanting to assign the
static address on the dhcp-relay..
Ren
-----Original Message-----
From: dhcp-users
<dhcp-users-boun...@lists.isc.org<mailto:dhcp-users-boun...@lists.isc.org>> On
Behalf Of Simon
Sent: Friday, February 2, 2024 12:43 PM
To: Users of ISC DHCP
<dhcp-users@lists.isc.org<mailto:dhcp-users@lists.isc.org>>
Subject: Re: Can you use DHCP to assign an address to the downstream interface
used by dhcrelay?
CAUTION: This email originated from outside the organisation. Do not click
links or open attachments unless you recognise the sender and know the content
is safe.
On 2 Nov 2023, at 17:13, Vallevand, Mark K
<mark.vallev...@unisys.com<mailto:mark.vallev...@unisys.com>> wrote:
> I have configured dhcrelay upstream and downstream interfaces and a server
> address.
> DHCP addresses are correctly relayed from machines on the downstream side to
> the upstream side and back.
> Rather than using a static address in the downstream interface, I would like
> to assign a DHCP address using dhclient to the downstream interface.
> Dhcrelay does not seem to handle this case. If the origin of the request is
> the actual downstream interface, it is not relayed.
> Is this possible? Can I use dhclient to get an address for the downstream
> interface used by dhcrelay?
Sorry, I?ve not had time to keep up here for a bit - I see this one seems to
have gone unanswered.
Unfortunately, you are in a catch-22 situation here. If the downstream
interface doesn?t have an IP address then presumably the relay cannot fill in
the GI-Addr field. If it can?t fill in the GI-Addr field, then the server can?t
locate the interface in order to offer an address.
To get around this situation, you?d need to configure the device in some way
before it could then use DHCP to get an address for the downstream - and once
you?ve set this up, you might as well just configure the interface.
Alternatively, you might be able to do some ?fudge? to get a subnet from the
server (similar to the way IPv6 Prefix Delegations can be done) and then use an
algorithm to configure the interface (e.g. pick the lowest usable address in
the subnet). The DHCP server would need some way of associating the request
with the device - and of course it needs the subnet details (such as router)
configured anyway in order to hand them to the clients.
However, I think your observation is a separate implementation specific
limitation. Because of the need to handle unaddressed packets (i.e. it needs to
receive and send broadcasts from/to the clients before they have an IP address)
I assume the relay agent shares code with the server in using a raw packet
interface rather than going through the IP stack. You can?t have two bits of
software doing this, and in any case running a client would result in a
broadcast packet OUT of the interface which would not be seen as an incoming
packet by the relay agent.
I assume not the answer you were looking for, but at least you now understand
why it isn?t working.
Simon
--
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Contact us at
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