Date: Mon, 26 Jan 2004 12:52:52 -0800 From: Mike <addymin@pacbell.net> To: Matthew Seaman <m.seaman@infracaninophile.co.uk> Cc: freebsd-questions <freebsd-questions@freebsd.org> Subject: Re: Configuring IP address aliasing Message-ID: <40157E24.3040600@pacbell.net> In-Reply-To: <20040126172114.GA48421@happy-idiot-talk.infracaninophile.co.uk> References: <401542F2.8000309@pacbell.net> <20040126172114.GA48421@happy-idiot-talk.infracaninophile.co.uk>
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Matthew Seaman wrote: > On Mon, Jan 26, 2004 at 08:40:18AM -0800, Mike wrote: > >>QUESTION: Why do I use a different netmask (255.255.255.255) for a IP >>alias on FreeBSD? Why isn't 255.255.255.0 used? > > > It's all about routing of outgoing packets. Unlike linux, you can't > use the route(8) command to set up a route to a directly attached > network. All of the local routing is controlled through ifconfig(8) > -- when you run: > > # ifconfig fxp0 inet 192.168.0.1 netmask 0xffffff00 broadcast 192.168.0.255 up > > amongst other things, that establishes that interface as the route to > the 192.168.0.0/24 network. It also says that the source address on > packets sent out of that interface will be 192.168.0.1 > > Now suppose you add an alias address on the same subnet: > > # ifconfig fxp0 add 192.168.0.2 netmask 0xffffffff broadcast 192.168.0.255 > > Since the netmask is a /32 it says this interface is the route to the > specific single address. It also means that the 192.168.0.2 address > is only used on outgoing packets from connections directly to the > alias address. That's a bit of a simplification, as one of the > principle uses for alias addresses is to support jail(8)ed processes, > but it holds in general. > > Could FreeBSD use exactly the same semantics as Linux or Solaris -- > yes, sure. But the FreeBSD way gives you a tad more control, and to > be honest, it's the way it is mostly because the people who wrote the > code thought that was the way it should be. Ah yes! What a great explanation. Thanks Matthew. Best Regards, Michael > > Cheers, > > Matthew >
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