dhcpd doesn't acknowledge dhcp requests

[dave c]

I don't recall seeing a line like this in your posted configurations:

interfaces "em2.112"

Dhcpd needs to be able to tell the difference between default vlan and an 802.1q tagged vlan
that is residing on a virtual interface. The only way to do that is to tell it to listen to the
alternative interface.

I believe it will also wish to know the definition of the default vlan and have it described as
a network in the configs. It's been years since I've setup vlan tagged dhcp, but do recall I
needed to enable the vlans in the configs before it would answer on them.

This page: http://serverfault.com/questions/412544/dhcp-server-for-multilple-vlans discusses
someone else who had a similar issue with tagged vlans not getting answered.

Hope this helps...

Dave

On 6/11/15 12:58, Sean McMurray wrote:

> Yes. That is tagged vlan 112 on interface em2.
> dhtest fails if I include a -v 112 to tell it about vlan 112.
> What are the problems with tagged vlans?
>
> On 06/11/2015 04:00 AM, Graham Clinch wrote:
>> Hi,
>>
>>> #tcpdump -vv -i em2.112
>>                   ^^^^^^^
>> Is that tagged vlan 112 on interface em2? - I'd not spotted that previously.  I've never tried
>> it personally, but my recollection of posts to this list is 'dhcpd and tagged vlans don't mix
>> well'.
>>
>> Does dhtest fail if run from a different device?  If so, I guess it's really going over the
>> (presumably untagged) loopback interface.
>>
>> Are you able to present vlan 112 untagged?
>>
>> Graham
>
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[Sean McMurray]

On 06/11/2015 11:39 AM, dave c wrote:
> I don't recall seeing a line like this in your posted configurations:
>
> interfaces "em2.112"
The man page does not document any "interfaces" directive. Rather, it
says to include the names of interfaces on the command line.
> Dhcpd needs to be able to tell the difference between default vlan and
> an 802.1q tagged vlan that is residing on a virtual interface. The
> only way to do that is to tell it to listen to the alternative interface.
Including em2.112 on the command line does not help. dhcpd still does
not respond to dhcp requests on vlan 112.
How is including this command line supposed to tell dhcpd that the
interface is tagged and to do anything different than when it
auto-discovers the interface based on subnet declarations?
> I believe it will also wish to know the definition of the default vlan
> and have it described as a network in the configs. It's been years
> since I've setup vlan tagged dhcp, but do recall I needed to enable
> the vlans in the configs before it would answer on them.
There is not default vlan. They are all tagged.
I'm not sure what you mean by "enable the vlans in the configs."

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[Sean McMurray]

I finally got it worked out. I'll document what worked for me here in
case it might be helpful to somebody in the future.

In order for dhcpd to recognize and respond to dhcp requests on a vlan,
I had to do two things:
1. Specifically enumerate the interfaces as dhcpd command line arguments
     In my case, dhcpd em2.112
2. Set the REORDER HEADER flag
     This can be done with vconfig like so: vconfig set_flag em2.112 1 1
     I made it permanent by adding it to the ifcfg script on my Redhat
host using the REORDER_HDR=1 option

After these two changes, the dhcpd daemon began responding to dhcp
requests on vlan 112.

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[Patrick Trapp]

Well done, sir. Thanks for sharing the solution.

________________________________________
From: [hidden email] [[hidden email]] on behalf of Sean McMurray [[hidden email]]
Sent: Friday, June 12, 2015 3:19 PM
To: Users of ISC DHCP
Subject: Re: dhcpd doesn't acknowledge dhcp requests

I finally got it worked out. I'll document what worked for me here in
case it might be helpful to somebody in the future.

In order for dhcpd to recognize and respond to dhcp requests on a vlan,
I had to do two things:
1. Specifically enumerate the interfaces as dhcpd command line arguments
     In my case, dhcpd em2.112
2. Set the REORDER HEADER flag
     This can be done with vconfig like so: vconfig set_flag em2.112 1 1
     I made it permanent by adding it to the ifcfg script on my Redhat
host using the REORDER_HDR=1 option

After these two changes, the dhcpd daemon began responding to dhcp
requests on vlan 112.

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dhcp-users mailing list
[hidden email]
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