Gentoo Archives: gentoo-user

From: Rich Freeman <rich0@g.o>
To: gentoo-user@l.g.o
Subject: Re: [gentoo-user] Strange behaviour of dhcpcd
Date: Fri, 31 Oct 2014 11:09:16
In Reply to: Re: [gentoo-user] Strange behaviour of dhcpcd by Marc Joliet
1 On Fri, Oct 31, 2014 at 6:47 AM, Marc Joliet <marcec@×××.de> wrote:
2 > Am Fri, 31 Oct 2014 07:52:54 +0100
3 > schrieb "J. Roeleveld" <joost@××××××××.org>:
4 >> On Tuesday, October 28, 2014 07:31:56 PM Marc Joliet wrote:
5 >> >
6 >> > - I don't know whether we have an IP block or not; I suspect not. At the
7 >> > very least, we didn't make special arrangements to try and get one.
8 >>
9 >> Then assume not. Most, if not all, ISPs charge extra for this. (If they even
10 >> offer it)
11 >
12 > That's what I thought :) .
13 >
15 Generally speaking you can't just attach a modem to your LAN and have
16 it act as a DHCP server. Your ISP probably will assign you dynamic
17 IPs, but they will not as a matter of policy assign you more than one
18 unless you pay for them. IPv4 address space is in short supply these
19 days.
21 I'm using FIOS and in my case the "modem" is in a box in the basement
22 and the ISP provides a router with the service. Whatever you plug
23 into the "modem" will obtain a DHCP lease for one routable IP. If you
24 do plug more than one device into the "modem" then the first device to
25 get the IP is the only one that will get an IP - the modem won't hand
26 out another unless it gets a DHCPRelease from the MAC that was issued
27 the original lease or until that lease expires, or until you call up
28 the ISP on the phone and get them to release it manually.
30 Another design would be to issue a new IP anytime a device asks for
31 one, but to silently cancel the lease of the last IP that was issued
32 and drop packets using it. For a single device being plugged in that
33 won't have any impact, and if for some reason you buy a new router and
34 plug it in you don't have to worry about your old router still having
35 a lease. This is less standards-compliant, but perhaps more
36 clueless-friendly.
38 In general, though, you really shouldn't be plugging your ISP's modem
39 into anything but a router for general use. In fact, I have the
40 router provided by my ISP configured as a bridge and running into
41 another router (FIOS uses MoCA over coax in the standard install and
42 I'm too lazy to run CatV and beg Verizon to reconfigure the modem to
43 use the RJ45 connection instead). Note that if you use an
44 ISP-provided router there is a good chance that they can essentially
45 VPN into your LAN. The last time I called up Verizon over a cablecard
46 issue they helpfully turned on DHCP on my router so that it started
47 competing with my DHCP server, and then I was wondering why PXE was
48 randomly failing. Now all they can do is disable bridge mode, which
49 will break my external connection and be a fairly obvious point to
50 troubleshoot.
52 --
53 Rich


Subject Author
Re: [gentoo-user] Strange behaviour of dhcpcd Marc Joliet <marcec@×××.de>