[ale] /etc/hosts and caching

Fri Nov 4 18:42:32 EDT 2011

What is important to note about that example is that ping, like most
applications that do network lookups, will only do the lookup once at the
beginning. For that reason my example required that I put single ping calls
in a loop.
On Nov 4, 2011 5:48 PM, "Richard Bronosky" <Richard at bronosky.com> wrote:

> As with must things on Linux, the answer is "It depends on how your
> system is configured." This is pretty easily tested on any system
> though.
> ~$ sudo date # this is to prime my sudo password timeout
> Password:
> Fri Nov  4 17:47:12 EDT 2011
> ~$ host google.com
> google.com has address 74.125.159.106
> ~$ host bronosky.com
> bronosky.com has address 174.143.204.116
> ~$ (sleep 5; sudo tee -a /etc/hosts <<< "174.143.204.116 google.com")
> & (while true; do ping -q -c 1 google.com | head -1; sleep 1; done)
> [1] 4874
> PING google.com (74.125.159.106): 56 data bytes
> PING google.com (74.125.159.106): 56 data bytes
> PING google.com (74.125.159.106): 56 data bytes
> PING google.com (74.125.159.106): 56 data bytes
> PING google.com (74.125.159.106): 56 data bytes
> 174.143.204.116 google.com
> PING google.com (174.143.204.116): 56 data bytes
> PING google.com (174.143.204.116): 56 data bytes
> PING google.com (174.143.204.116): 56 data bytes
> PING google.com (174.143.204.116): 56 data bytes
> PING google.com (174.143.204.116): 56 data bytes
> ^C
> [1]+  Done                    ( sleep 5; sudo tee -a /etc/hosts <<<
> "174.143.204.116 google.com" )
>
> Is that quick enough for you?
>
>
> On Fri, Nov 4, 2011 at 8:30 AM, leam hall <leamhall at gmail.com> wrote:
> > I use it as it is turned on everywhere I've worked. I've not seen any
> data
> > that suggests it is a problem, but I'd be happy to see some facts.
> >
> > Leam
> >
> > On Fri, Nov 4, 2011 at 8:24 AM, Chesser.Damon <
> Damon.Chesser at suntrust.com>
> > wrote:
> >>
> >> Thanks for the reply, that is how I thought it worked also.  As for
> nscd,
> >> someone on this list said “Don’t.  Use. It.”
> >>
> >>
> >>
> >> Any other opinions on it?
> >>
> >>
> >>
> >> Damon at damtek.com
> >>
> >>
> >>
> >> From: ale-bounces at ale.org [mailto:ale-bounces at ale.org] On Behalf Of
> leam
> >> hall
> >> Sent: Friday, November 04, 2011 8:17 AM
> >> To: Atlanta Linux Enthusiasts
> >> Subject: Re: [ale] /etc/hosts and caching
> >>
> >>
> >>
> >> Good morning Damon!
> >>
> >>
> >>
> >> As far as I can tell by editing /etc/hosts on a RHEL 4 box, the IP
> >> addresses are not cached longer than it takes me to edit the file. I
> believe
> >> your server will alwyas look in /etc/hosts anyway, per the
> >> /etc/nsswitch.conf file. The normal hosts line is something like:
> >>
> >>
> >>
> >>    hosts:      files dns
> >>
> >> There is the program nscd and it's config file, ncsd.conf. This
> configures
> >> some caching for you but I *think* it's more DNS related than affecting
> >> /etc/hosts.
> >>
> >> Hope that helps.
> >>
> >>
> >>
> >> Leam
> >>
> >> On Fri, Nov 4, 2011 at 7:40 AM, Chesser.Damon <
> Damon.Chesser at suntrust.com>
> >> wrote:
> >>
> >> This dovetails into building a DNS caching server I asked earlier this
> >> week.  The issue is we are seeing latency in our application and a
> possible
> >> (but only possible, not proven) issue might be host lookups.  I figured
> that
> >> an entry into /etc/hosts would be faster than a WAN DNS lookup
> especially
> >> since the IP is static.  Someone was concerned with disk reads and that
> >> becoming a bottleneck.  Someone else pointed out that /etc/hosts file
> was
> >> cached.
> >>
> >>
> >>
> >> This started a google search by me to find out if that was true or not.
> >> Totally inconclusive.  Some have reported issues with not being able to
> get
> >> the Linux box to re-read the hosts file after a change was committed
> short
> >> of a reboot or init restart.  Others have said just make the change and
> it
> >> shows up.  I have not found any documentation saying whether it was
> cached
> >> or not.  Any smart guys know the answer or can provide any
> documentation on
> >> that?  It’s kind of funny, you think you KNOW something until someone
> says
> >> “Prove it”.
> >>
> >>
> >>
> >> Damon at damtek.com
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
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