RE: Can you use DHCP to assign an address to the downstream interface used by dhcrelay?

"Vallevand, Mark K" <[email protected]> Wed, 28 Feb 2024 13:36:26 +0000
Newsgroups gmane.network.dhcp.isc.dhcp-client
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Basically, it was a project requirement to allow a DHCP address on the dhcp=
_relay interface.



Regards.

Mark K Vallevand
Unisys 651-635-7708 | [email protected]<mailto:mark.vallevand@unisy=
s.com>

[cid:[email protected]]<http://www.unisys.com/>

From: dhcp-users <[email protected]> On Behalf Of Renyk de'V=
andre
Sent: Wednesday, February 28, 2024 6:53 AM
To: Users of ISC DHCP <[email protected]>
Subject: RE: Can you use DHCP to assign an address to the downstream interf=
ace 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 inte=
rface? It would be good to see your basic topology: i.=E2=80=8Ae. DHCP SERV=
ER <--> 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  separ=
ate i.e. broadcasts from 2 and 1 are separate?



If that was the case (just thinking out loud here) I first thought you coul=
d you put an ip helper on the edge router which would listen for the broadc=
ast from the dhcprelay interface address and assign an address, however thi=
s would not work because this would only work for assigning an address to t=
he 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 t=
he static address on the dhcp-relay..



Ren



-----Original Message-----

From: dhcp-users <[email protected]<mailto:dhcp-users-bounce=
[email protected]>> On Behalf Of Simon

Sent: Friday, February 2, 2024 12:43 PM

To: Users of ISC DHCP <[email protected]<mailto:[email protected]=
.org>>

Subject: Re: Can you use DHCP to assign an address to the downstream interf=
ace 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 cont=
ent is safe.





On 2 Nov 2023, at 17:13, Vallevand, Mark K <[email protected]<mailt=
o:[email protected]>> wrote:



> I have configured dhcrelay upstream and downstream interfaces and a serve=
r 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 l=
ike 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 downstrea=
m interface used by dhcrelay?



Sorry, I=E2=80=99ve 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 inte=
rface doesn=E2=80=99t have an IP address then presumably the relay cannot f=
ill in the GI-Addr field. If it can=E2=80=99t fill in the GI-Addr field, th=
en the server can=E2=80=99t locate the interface in order to offer an addre=
ss.



To get around this situation, you=E2=80=99d need to configure the device in=
 some way before it could then use DHCP to get an address for the downstrea=
m - and once you=E2=80=99ve set this up, you might as well just configure t=
he interface.

Alternatively, you might be able to do some =E2=80=9Cfudge=E2=80=9D 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 o=
f associating the request with the device - and of course it needs the subn=
et details (such as router) configured anyway in order to hand them to the =
clients.



However, I think your observation is a separate implementation specific lim=
itation. Because of the need to handle unaddressed packets (i.e. it needs t=
o receive and send broadcasts from/to the clients before they have an IP ad=
dress) 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=E2=80=99t =
have two bits of software doing this, and in any case running a client woul=
d 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 understa=
nd why it isn=E2=80=99t working.





Simon



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<p class=3D"MsoNormal"><span style=3D"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=3D"MsoNormal"><span style=3D"font-size:11.0pt"><o:p>&nbsp;</o:p></=
span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt"><o:p>&nbsp;</o:p></=
span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt"><o:p>&nbsp;</o:p></=
span></p>
<div style=3D"mso-element:para-border-div;border:none;border-top:solid #CCC=
CCC 1.0pt;padding:4.0pt 0in 0in 0in">
<p class=3D"MsoNormal" style=3D"margin-bottom:3.0pt;border:none;padding:0in=
"><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-serif;=
color:#444444">Regards.<o:p></o:p></span></p>
<p class=3D"MsoNormal" style=3D"margin-bottom:3.0pt;border:none;padding:0in=
"><b><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if;color:#444444"><o:p>&nbsp;</o:p></span></b></p>
<p class=3D"MsoNormal" style=3D"margin-bottom:3.0pt;border:none;padding:0in=
"><b><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if;color:#444444">Mark K Vallevand</span></b><span style=3D"font-size:11.0p=
t;font-family:&quot;Arial&quot;,sans-serif;color:#444444"><o:p></o:p></span=
></p>
</div>
<p class=3D"MsoNormal" style=3D"margin-bottom:3.0pt"><span style=3D"font-si=
ze:9.0pt;font-family:&quot;Arial&quot;,sans-serif;color:#444444">Unisys 651=
-635-7708 |
</span><span style=3D"font-size:9.0pt;font-family:&quot;Arial&quot;,sans-se=
rif"><a href=3D"mailto:[email protected]"><span style=3D"color:#056=
3C1">[email protected]</span></a><span style=3D"color:#444444">
<o:p></o:p></span></span></p>
<p class=3D"MsoNormal" style=3D"margin-bottom:3.0pt"><span style=3D"font-si=
ze:8.0pt;color:#2072BC"><o:p>&nbsp;</o:p></span></p>
<p class=3D"MsoNormal" style=3D"margin-bottom:3.0pt"><a href=3D"http://www.=
unisys.com/"><span style=3D"font-size:11.0pt;color:windowtext;text-decorati=
on:none"><img border=3D"0" width=3D"124" height=3D"27" style=3D"width:1.291=
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/p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt"><o:p>&nbsp;</o:p></=
span></p>
<div style=3D"border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in =
0in 0in">
<p class=3D"MsoNormal"><b><span style=3D"font-size:11.0pt">From:</span></b>=
<span style=3D"font-size:11.0pt"> dhcp-users &lt;[email protected]=
sc.org&gt;
<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 &lt;[email protected]&gt;<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>
</div>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<div>
<p class=3D"MsoNormal" style=3D"mso-line-height-alt:.75pt"><span style=3D"f=
ont-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 add=
ress on your dhcp relay interface? It
 would be good to see your basic topology: i.=E2=80=8Ae. DHCP SERVER &lt;--=
&gt; routing infrastructure
<o:p></o:p></span></p>
</div>
<div>
<p class=3D"MsoNormal" style=3D"mso-line-height-alt:.75pt"><span style=3D"f=
ont-size:1.0pt;color:white"><o:p></o:p></span></p>
</div>
<pre style=3D"white-space:pre-wrap"><span style=3D"font-size:11.0pt;font-fa=
mily:&quot;Arial&quot;,sans-serif">Hi All,<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">Re: below <o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">Just read this and it had me curious, can you explain why you don't wan=
t to or can't set a static IP address on your dhcp relay interface?&nbsp;&n=
bsp; It would be good to see your basic topology:<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">i.e. <o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">DHCP SERVER&nbsp; &lt;--&gt;&nbsp;&nbsp; routing infrastructure &lt;--&=
gt;&nbsp; edge router&nbsp; &lt;--&gt; switched infrastructure 1 &lt;--&gt;=
 dhcprelay requiring dhcp lease for relay interface&nbsp; &lt;--&gt;&nbsp; =
switched infrastructure 2 &lt;--&gt; client requiring dhcp lease<o:p></o:p>=
</span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">My thoughts were:<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">1.&nbsp; (what is the dhcprelay running on)<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">2.&nbsp;&nbsp; Are the switched infrastructure 1 and switched infrastru=
cture 2&nbsp; separate i.e. broadcasts from 2 and 1 are separate?<o:p></o:p=
></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">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 br=
oadcast 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=
....&nbsp;&nbsp;&nbsp; so I totally agree below I can't see&nbsp; how this =
could be made to work....&nbsp;&nbsp; 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 route=
r/vlan interface and ip helpers....&nbsp;&nbsp; (all conjecture here).<o:p>=
</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">but back to my main thought, what is the reason for not wanting to assi=
gn the static address on the dhcp-relay..<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">Ren<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">-----Original Message-----<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">From: dhcp-users &lt;<a href=3D"mailto:[email protected]=
">[email protected]</a>&gt; On Behalf Of Simon<o:p></o:p></s=
pan></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">Sent: Friday, February 2, 2024 12:43 PM<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">To: Users of ISC DHCP &lt;<a href=3D"mailto:[email protected]">d=
[email protected]</a>&gt;<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">Subject: Re: Can you use DHCP to assign an address to the downstream in=
terface used by dhcrelay?<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">CAUTION: This email originated from outside the organisation. Do not cl=
ick links or open attachments unless you recognise the sender and know the =
content is safe.<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">On 2 Nov 2023, at 17:13, Vallevand, Mark K &lt;<a href=3D"mailto:Mark.V=
[email protected]">[email protected]</a>&gt; wrote:<o:p></o:p></s=
pan></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">&gt; I have configured dhcrelay upstream and downstream interfaces and =
a server address.<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">&gt; DHCP addresses are correctly relayed from machines on the downstre=
am side to the upstream side and back.<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">&gt; Rather than using a static address in the downstream interface, I =
would like to assign a DHCP address using dhclient to the downstream interf=
ace.<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">&gt; Dhcrelay does not seem to handle this case.&nbsp; If the origin of=
 the request is the actual downstream interface, it is not relayed.<o:p></o=
:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">&gt; Is this possible?&nbsp; Can I use dhclient to get an address for t=
he downstream interface used by dhcrelay?<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">Sorry, I=E2=80=99ve 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=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">Unfortunately, you are in a catch-22 situation here. If the downstream =
interface doesn=E2=80=99t have an IP address then presumably the relay cann=
ot fill in the GI-Addr field. If it can=E2=80=99t fill in the GI-Addr field=
, then the server can=E2=80=99t locate the interface in order to offer an a=
ddress.<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">To get around this situation, you=E2=80=99d need to configure the devic=
e in some way before it could then use DHCP to get an address for the downs=
tream - and once you=E2=80=99ve set this up, you might as well just configu=
re the interface.<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">Alternatively, you might be able to do some =E2=80=9Cfudge=E2=80=9D to =
get a subnet from the server (similar to the way IPv6 Prefix Delegations ca=
n 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 w=
ay 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=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">However, I think your observation is a separate implementation specific=
 limitation. Because of the need to handle unaddressed packets (i.e. it nee=
ds to receive and send broadcasts from/to the clients before they have an I=
P 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=E2=80=
=99t 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=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">I assume not the answer you were looking for, but at least you now unde=
rstand why it isn=E2=80=99t working.<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">Simon<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if"><o:p>&nbsp;</o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">--<o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
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<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
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<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">-- <o:p></o:p></span></pre>
<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
if">ISC funds the development of this software with paid support subscripti=
ons. Contact us at <a href=3D"https://urldefense.com/v3/__https:/www.isc.or=
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<pre><span style=3D"font-size:11.0pt;font-family:&quot;Arial&quot;,sans-ser=
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