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<p class="MsoNormal"><span style="font-size:11.0pt">Basically, it was a project requirement to allow a DHCP address on the dhcp_relay interface.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:3.0pt;border:none;padding:0in"><span style="font-size:11.0pt;font-family:"Arial",sans-serif;color:#444444">Regards.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:3.0pt;border:none;padding:0in"><b><span style="font-size:11.0pt;font-family:"Arial",sans-serif;color:#444444"><o:p> </o:p></span></b></p>
<p class="MsoNormal" style="margin-bottom:3.0pt;border:none;padding:0in"><b><span style="font-size:11.0pt;font-family:"Arial",sans-serif;color:#444444">Mark K Vallevand</span></b><span style="font-size:11.0pt;font-family:"Arial",sans-serif;color:#444444"><o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:3.0pt"><span style="font-size:9.0pt;font-family:"Arial",sans-serif;color:#444444">Unisys 651-635-7708 |
</span><span style="font-size:9.0pt;font-family:"Arial",sans-serif"><a href="mailto:mark.vallevand@unisys.com"><span style="color:#0563C1">mark.vallevand@unisys.com</span></a><span style="color:#444444">
<o:p></o:p></span></span></p>
<p class="MsoNormal" style="margin-bottom:3.0pt"><span style="font-size:8.0pt;color:#2072BC"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-bottom:3.0pt"><a href="http://www.unisys.com/"><span style="font-size:11.0pt;color:windowtext;text-decoration:none"><img border="0" width="124" height="27" style="width:1.2916in;height:.2812in" id="Picture_x0020_49" src="cid:image001.png@01DA6A18.D5725450"></span></a><span style="font-size:11.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt">From:</span></b><span style="font-size:11.0pt"> dhcp-users <dhcp-users-bounces@lists.isc.org>
<b>On Behalf Of </b>Renyk de'Vandre<br>
<b>Sent:</b> Wednesday, February 28, 2024 6:53 AM<br>
<b>To:</b> Users of ISC DHCP <dhcp-users@lists.isc.org><br>
<b>Subject:</b> RE: Can you use DHCP to assign an address to the downstream interface used by dhcrelay?<o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-line-height-alt:.75pt"><span style="font-size:1.0pt;color:white">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
<o:p></o:p></span></p>
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<pre style="white-space:pre-wrap"><span style="font-size:11.0pt;font-family:"Arial",sans-serif">Hi All,<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">Re: below <o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">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:<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">i.e. <o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">DHCP SERVER <--> routing infrastructure <--> edge router <--> switched infrastructure 1 <--> dhcprelay requiring dhcp lease for relay interface <--> switched infrastructure 2 <--> client requiring dhcp lease<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">My thoughts were:<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">1. (what is the dhcprelay running on)<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">2. Are the switched infrastructure 1 and switched infrastructure 2 separate i.e. broadcasts from 2 and 1 are separate?<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">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).<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">but back to my main thought, what is the reason for not wanting to assign the static address on the dhcp-relay..<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">Ren<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">-----Original Message-----<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">From: dhcp-users <<a href="mailto:dhcp-users-bounces@lists.isc.org">dhcp-users-bounces@lists.isc.org</a>> On Behalf Of Simon<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">Sent: Friday, February 2, 2024 12:43 PM<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">To: Users of ISC DHCP <<a href="mailto:dhcp-users@lists.isc.org">dhcp-users@lists.isc.org</a>><o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">Subject: Re: Can you use DHCP to assign an address to the downstream interface used by dhcrelay?<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">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.<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">On 2 Nov 2023, at 17:13, Vallevand, Mark K <<a href="mailto:Mark.Vallevand@UNISYS.com">Mark.Vallevand@UNISYS.com</a>> wrote:<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">> I have configured dhcrelay upstream and downstream interfaces and a server address.<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">> DHCP addresses are correctly relayed from machines on the downstream side to the upstream side and back.<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">> Rather than using a static address in the downstream interface, I would like to assign a DHCP address using dhclient to the downstream interface.<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">> Dhcrelay does not seem to handle this case. If the origin of the request is the actual downstream interface, it is not relayed.<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">> Is this possible? Can I use dhclient to get an address for the downstream interface used by dhcrelay?<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">Sorry, I’ve not had time to keep up here for a bit - I see this one seems to have gone unanswered.<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">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.<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">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.<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">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.<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">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.<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">I assume not the answer you were looking for, but at least you now understand why it isn’t working.<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">Simon<o:p></o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">--<o:p></o:p></span></pre>
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<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">dhcp-users mailing list<o:p></o:p></span></pre>
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<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">-- <o:p></o:p></span></pre>
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<pre><span style="font-size:11.0pt;font-family:"Arial",sans-serif">dhcp-users mailing list<o:p></o:p></span></pre>
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