Re: [CFR] reflect resolv.conf update to running application

From: Matthew Reimer (mreimer_at_vpop.net)
Date: 08/22/05

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    To: arch@freebsd.org
Date: Mon, 22 Aug 2005 13:18:46 -0700

    Robert Watson wrote:

    >
    > On Sat, 20 Aug 2005, Hajimu UMEMOTO wrote:
    >
    >> Our resolver reads resolv.conf once, and never re-read it. Recent
    >> OpenBSD changed to re-read resolv.conf when it is updated. I believe it
    >> is useful specially for mobile environment. So, I made a patch for our
    >> resolver. Please review it.
    >
    > Two concerns:
    >
    > (1) Has anyone characterized the significant of the cost of doing a stat()
    > for every DNS lookup for a significant workload? Does it matter?
    > Most performance-critical network paths don't do name lookups in
    > order to prevent indefinite stalls in lookup, but doing, say, 1,000
    > additional stats a second is not a small issue.
    >
    > (2) By reading the configuration file more frequently and more quickly
    > after a change, we increase the chances of a race condition in which
    > the resolve reads a partially written resolv.conf file during an
    > update. Does this happen in practice? I've always been very leery
    > of re-reading configuration files automatically based on a
    > time-stamp, as updates to files are not atomic at all.

    How about just stat'ing resolv.conf if more than X seconds has passed since
    the last stat, rather than for every lookup?

    Matt
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