[ale] /etc/hosts and caching

Fri Nov 4 09:38:48 EDT 2011

I feel fairly confident /etc/hosts is NOT cached because of a recent experience I had with NetBackup 7.1 which does its own caching (unbeknownst to me) that was causing issues when I was modifying /etc/hosts.   More fun was had when it turned out that HP-UX had a separate entry in nsswitch.conf for using the newer IPv6 lookup routines other than hosts line (luckily Linux doesn’t but I saw postings at the time that suggests Solaris does).   NBU 7.1 also uses those newer IPv6 routines whereas many of the OS level commands weren’t unless I was specifically trying to use IPv6.

You can check with something like ping or telnet after edit of /etc/hosts.   They’ll show you the IP they’re trying.

Of course getent hosts <hostname> will read /etc/hosts but I’m not sure that

I really find it hard to believe that an /etc/hosts lookup is causing you serious delays unless the drive /etc/hosts is on is doing things it shouldn’t.   For one thing the hosts lookup doesn’t have to make a network call like the DNS lookup would.

Of course we’re assuming you’re not using some other name service like NIS or NIS+ and that your nsswitch.conf has files before dns on the hosts line.

________________________________
From: ale-bounces at ale.org [mailto:ale-bounces at ale.org] On Behalf Of leam hall
Sent: Friday, November 04, 2011 8:31 AM
To: Atlanta Linux Enthusiasts
Subject: Re: [ale] /etc/hosts and caching

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<mailto: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<mailto:Damon at damtek.com>

From: ale-bounces at ale.org<mailto:ale-bounces at ale.org> [mailto: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<mailto: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<mailto:Damon at damtek.com>

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