Date: Mon, 18 Feb 2008 21:55:50 +0000 From: Nick Barnes <Nick.Barnes@pobox.com> To: Bill Moran <wmoran@collaborativefusion.com> Cc: freebsd-net@freebsd.org Subject: Re: Multiple default routes on multihome host Message-ID: <38548.1203371750@thrush.ravenbrook.com> In-Reply-To: <20080218163618.5e6672d3.wmoran@collaborativefusion.com> from Bill Moran <wmoran@collaborativefusion.com> of "Mon, 18 Feb 2008 16:36:18 -0500"
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At 2008-02-18 21:36:18+0000, Bill Moran writes: > In response to Nick Barnes <Nick.Barnes@pobox.com>: > > > I have a multi-home host: more than one IP address. The addresses are > > in separate subnets but run over the same ethernet segment (this is a > > temporary situation while I switch an office network over from one > > network provider to another). > > > > I want packets from address A1 to be sent via gateway G1, but packets > > from address A2 to be sent via gateway G2. > > > > How do I do this? Can I just have more than one default route? I'm > > remote from the machine in question, so I don't want to tinker with > > the default route until I'm sure of the answer. > > You can't have multiple default routes. The fact that you want to is > an indicator of incorrect network design, although it could be an > artifact of the interim setup while you migrate things around. > > I would suggest you ask yourself (and possibly the list) _why_ you think > multiple default routes is necessary ... what is it that you're hoping > to accomplish. I'm guessing your looking for some sort of redundancy, > in which case something like CARP or RIP is liable to be the correct > solution. I agree that this is probably my inexperience showing. I have an office network which is switching leased line, from provider P1 to provider P2. I have a /25 from P1 and a (different) /24 from P2. I am doing the migration a few machines at a time: move a little, test a little, etc. I am dual-homing each host for a short period while I am switching it over. The dual-homing works just fine, over a shared ethernet segment, except for the fact that I can only have one default route. This means that I am sending packets from an address given to me by P2 to P1's router (my existing default route). As an experimental matter, today, this does in fact work - these packets are getting to their destinations, via P1 - but it looks a heck of a lot like spoofing and I am half-expecting the wrath of P1 to descend on me. Either that or for them to silently stop routing the packets. I would rather send packets from the P2 subnet addresses to the P2 router, while the packets from the P1 subnet addresses keep going to the P1 router. Apparently I can do this with some IPFW cunning, but that seems like overkill for what seems like it ought to be a common problem. If I were in the office, I would gird my loins for a single hard session on all the consoles, to do all the config changes at once, abandoning the P1 addresses. As it is, doing it remotely, I'm being a little more tentative. Nick B
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