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Date: Fri, 7 Sep 2001 02:47:46 +0100
From: Jamie Lokier <lk@tantalophile.demon.co.uk>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: notion of a local address [was: Re: ioctl SIOCGIFNETMASK: ip alias bug 2.4.9 and 2.2.19]
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H. Peter Anvin wrote:
> In autofs, I use the following technique to determine if the IP number
> for a host is local (and therefore vfsbinds can be used rather than
> NFS mounts):
> 
> connect a datagram socket (which won't produce any actual traffic) to
> the remote host with INADDR_ANY as the local address, and then query
> the local address.  If the local address is the same as the remote
> address, the address is local.

Nice.  Gives false negatives in some cases (e.g. a local tunnel) but
that doesn't really matter.

I've considered this technique for deciding whether it's safe to use the
MIT-SHM extension with X:

Open a SHM segment; write fairly secure random data into it; ask the X
server to connect to that segment and read it back as an image over the
X protocol; check whether they match.

If the X server reports an error, it's remote.  If it finds a SHM
segment it may still be remote, hence the random data check.

-- Jamie