Listening on interface without IP address

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Listening on interface without IP address

Miloslav Hůla
Hi,

we maintain about 200 VLANs with ISC DHCP server, relay running on
central box.

But the relay is somehow broken and I'm thinking how to bypass that.
Unfornutaly, we have no free IPv4 addresses for 2 DHCP servers.

Is it possible to configure ISC DHCP server to listen on given interface
but without IPv4 address on it?

No subnet declaration for eth0.376 (no IPv4 addresses).
** Ignoring requests on eth0.376.  If this is not what
    you want, please write a subnet declaration
    in your dhcpd.conf file for the network segment
    to which interface eth0.376 is attached. **

Somehow specify in dhcpd.conf where eth0.376 belongs to?

shared-network VLAN-376 {
        ... for interface eth0.376
}

Milo
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Re: Listening on interface without IP address

John W. Blue
What is not free?  GUI or RFC1918 address space?

John

Sent from Nine

From: Miloslav Hůla <[hidden email]>
Sent: Thursday, June 13, 2019 7:11 AM
To: [hidden email]
Subject: Listening on interface without IP address

Hi,

we maintain about 200 VLANs with ISC DHCP server, relay running on
central box.

But the relay is somehow broken and I'm thinking how to bypass that.
Unfornutaly, we have no free IPv4 addresses for 2 DHCP servers.

Is it possible to configure ISC DHCP server to listen on given interface
but without IPv4 address on it?

No subnet declaration for eth0.376 (no IPv4 addresses).
** Ignoring requests on eth0.376.  If this is not what
    you want, please write a subnet declaration
    in your dhcpd.conf file for the network segment
    to which interface eth0.376 is attached. **

Somehow specify in dhcpd.conf where eth0.376 belongs to?

shared-network VLAN-376 {
        ... for interface eth0.376
}

Milo
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Re: Listening on interface without IP address

Simon Hobson
In reply to this post by Miloslav Hůla
Miloslav Hůla <[hidden email]> wrote:

> we maintain about 200 VLANs with ISC DHCP server, relay running on central box.
>
> But the relay is somehow broken and I'm thinking how to bypass that. Unfornutaly, we have no free IPv4 addresses for 2 DHCP servers.
>
> Is it possible to configure ISC DHCP server to listen on given interface but without IPv4 address on it?
>
> No subnet declaration for eth0.376 (no IPv4 addresses).
> ** Ignoring requests on eth0.376.  If this is not what
>   you want, please write a subnet declaration
>   in your dhcpd.conf file for the network segment
>   to which interface eth0.376 is attached. **
>
> Somehow specify in dhcpd.conf where eth0.376 belongs to?
>
> shared-network VLAN-376 {
> ... for interface eth0.376
> }

What could work would be something like this :

shared-network VLAN-376 {
  subnet a.b.c.d ... {
    range a.b.c.e ; };
  subnet 10.1.120.0 ... {};
}

And give the DHCP server an IP address in the 10.1.120.0/24 subnet.
The server will operate using the 10.1.120.0 subnet, and the shared-network statement will make it associate the a.c.b.d subnet with the same interface. You will need to setup routing so that unicast packets from the client at a.b.c.e can reach the server at 10.1.120.nn. This could be either by adding a 10.1.120.xx address to the router on VLAN 376, or by adding a route via some other route - for example routing to 10.0.0.0/15 to some other interface on the DHCP server. For the latter, the DHCP server will need to be listening on that interface.

Ideally - just fix the relay agent !

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Re: Listening on interface without IP address

Miloslav Hůla
Dne 2019-06-13 v 19:36 Simon Hobson napsal(a):

> Miloslav Hůla <[hidden email]> wrote:
>
>> we maintain about 200 VLANs with ISC DHCP server, relay running on central box.
>>
>> But the relay is somehow broken and I'm thinking how to bypass that. Unfornutaly, we have no free IPv4 addresses for 2 DHCP servers.
>>
>> Is it possible to configure ISC DHCP server to listen on given interface but without IPv4 address on it?
>>
>> No subnet declaration for eth0.376 (no IPv4 addresses).
>> ** Ignoring requests on eth0.376.  If this is not what
>>    you want, please write a subnet declaration
>>    in your dhcpd.conf file for the network segment
>>    to which interface eth0.376 is attached. **
>>
>> Somehow specify in dhcpd.conf where eth0.376 belongs to?
>>
>> shared-network VLAN-376 {
>> ... for interface eth0.376
>> }
>
> What could work would be something like this :
>
> shared-network VLAN-376 {
>    subnet a.b.c.d ... {
>      range a.b.c.e ; };
>    subnet 10.1.120.0 ... {};
> }
>
> And give the DHCP server an IP address in the 10.1.120.0/24 subnet.
> The server will operate using the 10.1.120.0 subnet, and the shared-network statement will make it associate the a.c.b.d subnet with the same interface. You will need to setup routing so that unicast packets from the client at a.b.c.e can reach the server at 10.1.120.nn. This could be either by adding a 10.1.120.xx address to the router on VLAN 376, or by adding a route via some other route - for example routing to 10.0.0.0/15 to some other interface on the DHCP server. For the latter, the DHCP server will need to be listening on that interface.

Thank you! That's a great idea. Actually we already have VLANs with more
subnets, so it should work.

> Ideally - just fix the relay agent !

Sure, but it is a long story. Bug is reported.
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Re: Listening on interface without IP address

Miloslav Hůla
>> What could work would be something like this :
>>
>> shared-network VLAN-376 {
>>    subnet a.b.c.d ... {
>>      range a.b.c.e ; };
>>    subnet 10.1.120.0 ... {};
>> }
>>
>> And give the DHCP server an IP address in the 10.1.120.0/24 subnet.
>> The server will operate using the 10.1.120.0 subnet, and the
>> shared-network statement will make it associate the a.c.b.d subnet
>> with the same interface. You will need to setup routing so that
>> unicast packets from the client at a.b.c.e can reach the server at
>> 10.1.120.nn. This could be either by adding a 10.1.120.xx address to
>> the router on VLAN 376, or by adding a route via some other route -
>> for example routing to 10.0.0.0/15 to some other interface on the DHCP
>> server. For the latter, the DHCP server will need to be listening on
>> that interface.
>
> Thank you! That's a great idea. Actually we already have VLANs with more
> subnets, so it should work.

I deployed it for a first VLAN and it works, even without private range
routing. Only I have to set:

server-identifier ...server-public-ip...;

for subnet with public IP range in shared-network VLAN-376.

First DISCOVER, OFFER, REQUEST, ACK are done by broadcast via eth0.376
(interface with private IP), following REQUST, ACK are done by routed
unicast (via. server public IP) and so as release.

Many thanks again.

Milo
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