/etc/hosts is not evaluated after modification on Linux

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Dear colleagues, I am trying to find out, if a given hostname can be resolved and if not, I update a hosts file and check if the resolution works again. Following works fine on Windows and Solaris, but not on Linux:

#!/usr/bin/perl

use strict;
use warnings;

our $| = 1;

my $host_name = "test_host";
my $host_ip = "192.168.0.50";

#my $hosts = 'C:\Windows\System32\drivers\etc\hosts';
my $hosts = '/etc/hosts';

unless (defined gethostbyname $host_name) {
        print "Cannot find \"$host_name\"\n";

        print "Adding $host_name to $hosts ... ";
        open my $fh, ">>", $hosts or die "Cannot open \"$hosts\" for a
+ppending: $!\n";
        print $fh "$host_ip\t$host_name\n";
        close $fh;
        print "OK\n";

        print "Checking if $host_name is known now\n";

        my $packed_ip = gethostbyname $host_name || 0;

        print "--- $packed_ip ---\n";
}
[download]

If I start this on Linux (

SUSE Linux Enterprise Server 10 (x86_64)
VERSION = 10
PATCHLEVEL = 4

perl -v

This is perl, v5.8.6 built for i686-linux
[download]

) it does not work:

# /tmp/gethostbyname.pl
Cannot find "test_host"
Adding test_host to /etc/hosts ... OK
Checking if test_host is known now
--- 0 ---
#
[download]

Can anyone explain why and is there any workaround? Thanks, Vadim

Comment on /etc/hosts is not evaluated after modification on Linux Select or Download Code

Replies are listed 'Best First'.
Re: /etc/hosts is not evaluated after modification on Linux by Corion (Patriarch) on May 30, 2015 at 16:56 UTC
My guess is that on Linux, the call to `gethostbyname` is cached somewhere outside the resolver and the updated information does not reach your program. I don't know enough about Linux to know how to make the C library, or the resolver deliver you the updated information immediately.	[reply] [d/l]
Re: /etc/hosts is not evaluated after modification on Linux by philipbailey (Curate) on May 30, 2015 at 21:11 UTC
As Corion says, it sounds like a caching issue. On SLES, it is likely you have an nscd daemon running, which is responsible for caching this data. According to the documentation, running this command after changing /etc/hosts will invalidate the cache and should resolve your problem, I think: `sudo nscd -i /etc/hosts`	[reply] [d/l]
Re^2: /etc/hosts is not evaluated after modification on Linux by Anonymous Monk on May 31, 2015 at 06:22 UTC
Indeed, following helps: system("/usr/sbin/nscd", "-i", "hosts"); Thanks a lot! -Vadim	[reply]
Re: /etc/hosts is not evaluated after modification on Linux by afoken (Chancellor) on May 31, 2015 at 06:44 UTC
I am trying to find out, if a given hostname can be resolved and if not, I update a hosts file and check if the resolution works again. Very strange approach. It seems you are working in a network with private IP addresses, and have a nameserver that does not "know enough" about that network. Why don't you update the data the nameserver uses instead of messing with /etc/hosts, probably on many computers? Another question: How to you update records in /etc/hosts when needed, e.g. when `test_host` moves from 192.168.0.50 to 192.168.0.55? Do you touch each and every /etc/hosts file? And a third thing: There should be no underscore (_) in a hostname (see Wikipedia for the relevant RFCs). It may work, but only by chance. UPDATE: Maybe you don't have a nameserver at all? Maybe you are abusing `gethostbyname()` to find a hostname in `/etc/hosts`? If yes, consider using dnsmasq. It provides DNS and DHCP services (optionally also TFTP), and adding hostnames to the DNS service is as easy as editing /etc/hosts - literally, because you simply edit /etc/hosts on the machine running dnsmasq. dnsmasq "watches" /etc/hosts, so you don't have to do more than editing the file. Alexander -- Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)	[reply] [d/l] [select]
Re^2: /etc/hosts is not evaluated after modification on Linux by Anonymous Monk on May 31, 2015 at 09:02 UTC
Yes there is no nameserver in this particular example and /etc/hosts is used for resolution. How already pointed by philipbailey nscd is used for caching and if after updating of /etc/hosts in the script cache needs to be invalidated by using "nscd -i hosts"	[reply]
Re: /etc/hosts is not evaluated after modification on Linux by vinoth.ree (Monsignor) on May 30, 2015 at 17:36 UTC
Also look at Sys::Hostname module also. *All is well. I learn by answering your questions...*	[reply] [d/l]
Re: /etc/hosts is not evaluated after modification on Linux by Anonymous Monk on May 31, 2015 at 05:33 UTC
Unix systems tend to use NSS nowadays. If your `/etc/nsswitch.conf` contains a line like hosts: dns [!UNAVAIL=return] files then `/etc/hosts` isn't normally consulted at all. The move is towards more stringent lookups & authority, I suppose.	[reply]