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DHCPd using wrong interface

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Dan Egli

DHCPd using wrong interface

I am completely stumped on how to solve this one. I have a machine with
two NICs. It's my gateway machine between my home private LAN and the
internet. The gateway machine has enp0s3 with an address of 10.0.2.15 as
it's world facing address (yes, my ISP wants to run everything through
NAT. Don't ask me why). The LAN address range is `192.168.10.0/24 on
enp0s8. I configured dhcpd to completely ignore anything coming from
10.0.0.0/8, and I even have enp0s8 listed on the command line (and NOT
enp0s3). Yet when dhcpd tries to send update requests to bind, it keeps
using the 10.0.2.15 address, which bind properly refuses to listen to.
How do I make dhcpd send messages from enp0s8's 192.168.10.2 address vs
the 10.0.2.15 address?

--
Dan Egli
From my Test Server

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glenn.satchell

Re: DHCPd using wrong interface

Hi Dan

Which bind server is it trying to update? Those updates are sent as
unicast packets and are subject to normal routing rules. Perhaps it's
trying to update the ISP's DNS server?

Can you use tcpdump to see what the destination IP address is?

regards,
Glenn

On 2020-12-19 18:23, Dan Egli wrote:

> I am completely stumped on how to solve this one. I have a machine
> with two NICs. It's my gateway machine between my home private LAN and
> the internet. The gateway machine has enp0s3 with an address of
> 10.0.2.15 as it's world facing address (yes, my ISP wants to run
> everything through NAT. Don't ask me why). The LAN address range is
> `192.168.10.0/24 on enp0s8. I configured dhcpd to completely ignore
> anything coming from 10.0.0.0/8, and I even have enp0s8 listed on the
> command line (and NOT enp0s3). Yet when dhcpd tries to send update
> requests to bind, it keeps using the 10.0.2.15 address, which bind
> properly refuses to listen to. How do I make dhcpd send messages from
> enp0s8's 192.168.10.2 address vs the 10.0.2.15 address?

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Dan Egli

Re: DHCPd using wrong interface

On Tue, Dec 22, 2020 at 07:07:49PM +1100, [hidden email] wrote:
> Hi Dan
>
> Which bind server is it trying to update? Those updates are sent as
> unicast packets and are subject to normal routing rules. Perhaps it's
> trying to update the ISP's DNS server?
>
> Can you use tcpdump to see what the destination IP address is?

That may be workable. I'm actually away from the machines until much later tonight. The only question I have is how to use tcpdump as I've never used it before. I take it from reading the man page that I just run tcpdump while a machine is obtaining a lease on an IP?

Sorry, I'm still learning a lot.

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glenn.satchell

Re: DHCPd using wrong interface

Hi Dan,

Typically something like this, replace eth0 with the interface you want
to trace packets on. If you have wireshark you can use that instead.
There's all sorts of filtering you can apply, but if traffic is light
this will be sufficient.

tcpdump -i eth0

regards,
Glenn

On 2020-12-23 02:43, Dan Egli wrote:

> On Tue, Dec 22, 2020 at 07:07:49PM +1100, [hidden email]
> wrote:
>> Hi Dan
>>
>> Which bind server is it trying to update? Those updates are sent as
>> unicast packets and are subject to normal routing rules. Perhaps it's
>> trying to update the ISP's DNS server?
>>
>> Can you use tcpdump to see what the destination IP address is?
>
> That may be workable. I'm actually away from the machines until much
> later tonight. The only question I have is how to use tcpdump as I've
> never used it before. I take it from reading the man page that I just
> run tcpdump while a machine is obtaining a lease on an IP?
>
> Sorry, I'm still learning a lot.
>
> _______________________________________________
> ISC funds the development of this software with paid support
> subscriptions. Contact us at https://www.isc.org/contact/ for more
> information.
>
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