Re: DHCP server that itself gets an IP address by DHCP

From	Reco <recoverym4n@gmail.com>
Newsgroups	linux.debian.user
Subject	Re: DHCP server that itself gets an IP address by DHCP
Date	2017-08-24 11:40 +0200
Message-ID	<uhWMh-4E5-7@gated-at.bofh.it> (permalink)
Organization	linux.* mail to news gateway

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	Hi.

In-Reply-To: <20170824074515.y4z2ummdigk2fcbn@kazuki.local>

On Thu, Aug 24, 2017 at 04:45:15PM +0900, Mark Fletcher wrote:
> Is there any clever way to pass through the name server settings 
> the DHCP server provides, so that if the ISP should change its name 
> server IP addresses in the future, my local DHCP server would pass along 
> the new addresses when next asked?
> 
> In other words, instead of specifying the name server addresses 
> explicitly in the dhcp.conf file, is there a way to specify that they 
> should be taken from the host the DHCP server is running on?

Somewhat hackish, but straightforward way to achieve this is to redirect
DNS requests from your LAN to correct DNS. Something like this should do
the trick:

iptables -t nat -A OUTPUT -i <LAN Port> -p udp --dport 53 \
-j DNAT --to-destination <ISP DNS>:53

iptables -t nat -A OUTPUT -i <LAN Port> -p tcp --dport 53 \
-j DNAT --to-destination <ISP DNS>:53

iptables -A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT

iptables -A FORWARD -p udp --dport 53 -j ACCEPT

iptables -A FORWARD -p tcp --dport 53 -j ACCEPT


PS Being in the similar situation I said 'screw it' and installed
caching DNS alongside with DHCP on a firewall. It simplified that setup
immensely.

Reco

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Re: DHCP server that itself gets an IP address by DHCP Reco <recoverym4n@gmail.com> - 2017-08-24 11:40 +0200
  threading, was Re: DHCP server that itself gets an IP address by DHCP David Wright <deblis@lionunicorn.co.uk> - 2017-08-24 15:30 +0200
    Re: threading, was Re: DHCP server that itself gets an IP address  by DHCP Reco <recoverym4n@gmail.com> - 2017-08-24 17:10 +0200
  Re: DHCP server that itself gets an IP address by DHCP Pascal Hambourg <pascal@plouf.fr.eu.org> - 2017-08-24 22:30 +0200
    Re: DHCP server that itself gets an IP address by DHCP Greg Wooledge <wooledg@eeg.ccf.org> - 2017-08-24 22:40 +0200
      Re: DHCP server that itself gets an IP address by DHCP Mark Fletcher <mark27q1@gmail.com> - 2017-08-25 00:40 +0200
        Re: DHCP server that itself gets an IP address by DHCP Greg Wooledge <wooledg@eeg.ccf.org> - 2017-08-25 14:20 +0200
          Re: DHCP server that itself gets an IP address by DHCP Mark Fletcher <mark27q1@gmail.com> - 2017-08-25 15:20 +0200
            Re: DHCP server that itself gets an IP address by DHCP Greg Wooledge <wooledg@eeg.ccf.org> - 2017-08-25 16:00 +0200
    Re: DHCP server that itself gets an IP address by DHCP Reco <recoverym4n@gmail.com> - 2017-08-24 22:40 +0200
      Re: DHCP server that itself gets an IP address by DHCP Mark Fletcher <mark27q1@gmail.com> - 2017-08-25 00:30 +0200
        Re: DHCP server that itself gets an IP address by DHCP Reco <recoverym4n@gmail.com> - 2017-08-25 10:20 +0200
        Re: DHCP server that itself gets an IP address by DHCP Greg Wooledge <wooledg@eeg.ccf.org> - 2017-08-25 14:20 +0200

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