Re: [gentoo-user] Strange behaviour of dhcpcd

[Rich Freeman]

On Fri, Oct 31, 2014 at 6:47 AM, Marc Joliet <marcec@×××.de> wrote:
> Am Fri, 31 Oct 2014 07:52:54 +0100
> schrieb "J. Roeleveld" <joost@××××××××.org>:
>> On Tuesday, October 28, 2014 07:31:56 PM Marc Joliet wrote:
>> >
>> > - I don't know whether we have an IP block or not; I suspect not.  At the
>> > very least, we didn't make special arrangements to try and get one.
>>
>> Then assume not. Most, if not all, ISPs charge extra for this. (If they even
>> offer it)
>
> That's what I thought :) .
>

Generally speaking you can't just attach a modem to your LAN and have
it act as a DHCP server.  Your ISP probably will assign you dynamic
IPs, but they will not as a matter of policy assign you more than one
unless you pay for them.  IPv4 address space is in short supply these
days.

I'm using FIOS and in my case the "modem" is in a box in the basement
and the ISP provides a router with the service.  Whatever you plug
into the "modem" will obtain a DHCP lease for one routable IP.  If you
do plug more than one device into the "modem" then the first device to
get the IP is the only one that will get an IP - the modem won't hand
out another unless it gets a DHCPRelease from the MAC that was issued
the original lease or until that lease expires, or until you call up
the ISP on the phone and get them to release it manually.

Another design would be to issue a new IP anytime a device asks for
one, but to silently cancel the lease of the last IP that was issued
and drop packets using it.  For a single device being plugged in that
won't have any impact, and if for some reason you buy a new router and
plug it in you don't have to worry about your old router still having
a lease.  This is less standards-compliant, but perhaps more
clueless-friendly.

In general, though, you really shouldn't be plugging your ISP's modem
into anything but a router for general use.  In fact, I have the
router provided by my ISP configured as a bridge and running into
another router (FIOS uses MoCA over coax in the standard install and
I'm too lazy to run CatV and beg Verizon to reconfigure the modem to
use the RJ45 connection instead).  Note that if you use an
ISP-provided router there is a good chance that they can essentially
VPN into your LAN.  The last time I called up Verizon over a cablecard
issue they helpfully turned on DHCP on my router so that it started
competing with my DHCP server, and then I was wondering why PXE was
randomly failing.  Now all they can do is disable bridge mode, which
will break my external connection and be a fairly obvious point to
troubleshoot.

--
Rich