[NBLUG/talk] substituting DHCP/DNS in cheap routers

Ron Wickersham rjw at alembic.com
Fri Dec 30 13:37:11 PST 2011

On Fri, 30 Dec 2011, Bob Blick wrote:

> At work we switched to Sonic.net's Fusion and had to get a more modern
> DSL modem, opted for a combo modem/router/wifi from Netgear, a DGN1000.
> Immediately almost everybody started noticing problems with name
> resolutions - "try again?" screens in Firefox, etc. Trying again always
> seems to work, but it's frustrating. A more adventurous user has
> switched his computer to fixed settings instead of DHCP and says he has
> no more problems.
> Switching to fixed settings on user computers isn't practical for most
> people though, so I was trying to figure out a solution, or at least a
> real diagnosis for this problem. If it really is just the DGN1000 I will
> s&*7can it and buy something else, but if I could substitute a different
> DHCP server temporarily it would be useful.
> The DGN1000 only has one setting for DHCP (on or off) and one for DNS
> ("get automatically" or "use these addresses", tried both choices).
> Setting DNS to "use these addresses" doesn't change the address sent out
> to user computers though, they still get the DGN1000 as their DNS and
> gateway.
> Would running a DHCP server on the network be a valid thing to try?
> Would it be hard to set up? Would it work for wifi users too? I can use
> a WRT54G or a real PC, whichever is easiest.

hi Bob,

one aspect of DHCP leases is that when you may change the configuration 
the new parameters (such as DNS) may only affect _new_ leases, depending
on the DHCP client, since continuing an existing lease after it expires
will just request permission from the DHCP server to continue using the 
previously issued IP address.

putting a separate DHCP server on the network is certainly ok to compare
the results with the built-in server.   just a reminder that you never
want to have two DHCP servers active on the same subnet, so make sure the
built-in server is disabled before activating the new one.

configuring the ISC DHCP server is straightforward, and often the conf
file is /etc/dhcpd.conf (but some os/distributions may put it in another
location).  when the ISC dhcp daemon software is installed the dhcpd.conf
file has commented out examples so it's not difficult to set up.

the dhcp daemon does not automatically discover a changed dhcpd.conf file,
you must manually stop and restart the dhcpd process (and it also doesn't
use the unix tradition of responding to a HUP signal to reread the conf

more advanced configuration options allow you to always assign a fixed
ip address (outside the range of the dynamically assigned block) to a
host with a given MAC address, for instance.  the man page for dhcpd.conf
will give lots of details if you want to do more complex configurations.

one other DHCP daemon is dnsmasq which does other things such as DNS server
so its a big little program.  it's configuration file also has examples,
but this file is considerably longer since it has options for the other
capabilities.  dnsmasq may be what you find in small distributions for
embedded router systems.


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