Re: [gentoo-user] Strange behaviour of dhcpcd

[J. Roeleveld]

On Tuesday, October 28, 2014 07:31:56 PM Marc Joliet wrote:
> Am Tue, 28 Oct 2014 16:28:37 +0000
>
> schrieb Mick <michaelkintzios@×××××.com>:
> > On Monday 27 Oct 2014 23:44:58 Marc Joliet wrote:
> > > Hi list
> > >
> > > First off: this is a "fixed" issue, in that I don't see the behaviour
> > > anymore, so time is not of the essence ;) . I'm only looking for an
> > > explanation, or for comments from other people who experienced this.
> > >
> > > So the issue was some really strange behaviour on the part of dhcpcd.  I
> > > completed a move a few weeks ago and got an internet connection last
> > > Wednesday (using a local cable company, that is, using a cable modem
> > > connected to via ethernet). I reconfigured my system to use regular DHCP
> > > (a relief after the PPPoE mess in the dorm), but dhcpcd could not apply
> > > the default route; it *obtained* one, but failed with "if_addroute:
> > > Invalid argument". I tried it manually, to no effect: "ip route"
> > > complained about invalid arguments, and I think plain "route" said "file
> > > exists", but I'm not sure anymore (either way, the error messages were
> > > less than clear).  The funny thing is, I *could* set the default route,
> > > just not to the one advertised via DHCP, but to the x.y.z.2+ instead of
> > > x.y.z.1, which even gave me access to the internet part of the time.
> > >
> > > Now the funny thing is what fixed it:
> > >   *commenting out the entirety of /etc/dhcpcd.conf*
> > >
> > > Then dhcpcd ran with   default settings and could apply the default
> > > route.
> > > Even more bizarre is the fact that it kept working after uncommenting it
> > > again (and I track it with git, so I'm 100% sure I got it back to its
> > > original state). This leads me to believe that there was some
> > > (corrupted?)
> > > persistent state somewhere that got overwritten by starting dhcpcd after
> > > I
> > > commented out the file, but I have no clue where.
> > >
> > > Has anyone seen this sort of behaviour before, or anything similar to
> > > it?
> > > I searched for the error messages I was seeing, but couldn't find
> > > anything.  I was using gentoo-sources-3.15.9 (now I'm at 3.16.6) and
> > > dhcpcd 6.4.3 at the time, but also had the issue with dhcpcd 6.4.7, to
> > > which I could upgrade by using the aforementioned x.y.z.2 gateway.
> > > Perhaps
> > > it was a bug in the kernel? But that's just guessing.
> > >
> > > Regards,
> >
> > Since dhcpcd doesn't misbehave any more it would be difficult to check
> > what
> > was the cause of this problem.  You didn't say if the cable modem is
> > functioning as a router or as in a full or half bridge mode and if there
> > is a router between your PC and the modem that distributes IP addresses.
> > You also didn't say if the ISP has allocated an IP block or just a single
> > IP address.
> First off: thanks for the response.  Note that I have no clue about modems
> (other than that the modulate and demodulate signals), let alone cable
> modems and the wide variety of hardware out there. I also have no clue
> about the protocols involved (save for a tiny bit of IP and TCP/UDP).  Just
> so you know what to expect.
>
> Anyway, in answer to your queries:
>
> - I do not know for sure how the modem is configured, and whether it hands
>   out the addresses itself or whether these come from the other end of the
>   cable connection.  But from what I can observe it does *not* function as a
> router; it has *one* Ethernet connection, and that's it.  I did not test it
> in a bridged network, to see if it hands out addresses to multiple clients.
> Our ISP refers to it as a "LAN modem".

Sounds similar to what I've been using for the past 10+ years.

>   OK, I looked up more information:  It's a Thomson THG571, and the manual
> (I found a copy here:
>   http://www.kabelfernsehen.ch/dokumente/quicknet/HandbuchTHG570.pdf) refers
> to "Transparent bridging for IP traffic", and AFAICT makes no mention of
> routing.  It does explicitly say that it gets an IP address from the ISP,
> so I suspect that it acts as a bridge for all IP clients (like the "IP
> Client Mode" in Fritz!Box routers).  So it sounds to me that the DHCP
> packets likely come from a server beyond the router. Is this the half
> bridge mode you alluded to?

Not sure about half-bridge mode. But most cable-modems work in bridge-mode. 
(If they have more then 1 ethernet-port, they act as routers)

>   Oh, and there are two powerline/dLAN adapters in between (the modem is in
> the room next door), but direct connections between my computer and my
> brother's always worked, and they've been reliable in general, so I assume
> that they're irrelevant here.

Uh-oh... If you have multiple machines that can ask for a DHCP-lease, you 
might keep getting a different result each time it tries to refresh.

>   Furthermore, I found out the hard way that you *sometimes* need to reboot
> the modem when connect a different client for the new client to get a
> response from the DHCP server (I discovered this after wasting half a day
> trying to get our router to work, it would log timeouts during
> DHCPDISCOVER).  I didn't think it was the modem because when we first got
> it, I could switch cables around between my computer and my brother's and
> they would get their IP addresses without trouble.  *sigh*

That's a common flaw. These modems are designed with the idea that people only 
have 1 computer. Or at the very least put a router between the modem and 
whatever else they have.
Please note, there is NO firewall on these modems and your machine is fully 
exposed to the internet. Unless you have your machine secured and all unused 
services disabled, you might as well assume your machine compromised.

I once connected a fresh install directly to the modem. Only took 20 seconds 
to get owned. (This was about 9 years ago and Bind was running)

> - At the time there was no router, just the modem.  We now have a Fritz!Box
>   3270 with the most recent firmware, but we got it after I "solved" this
>   problem.
>
> - 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)

--
Joost