dns_client.pl

use strict;
use warnings;

use LWP::UserAgent;
use File::Slurp;
use POSIX qw//;

my $ua = LWP::UserAgent->new;

my $resolv_conf_localhost = <<EOF;
nameserver 127.0.0.1
search ap-northeast-1.compute.internal
EOF

my $resolv_conf_original = <<EOF;
nameserver 172.16.0.23
search ap-northeast-1.compute.internal
EOF

main: {
    $ua->get("http://google.co.jp");

    print("resolv.conf = localhost\n");
    write_file("/etc/resolv.conf", $resolv_conf_localhost);
    sleep(3);

    print("resolve from parent\n");
    $ua->get("http://google.co.jp");
    sleep(3);

    print("resolve from child\n");
    fork_and_req();
    sleep(3);

    write_file("/etc/resolv.conf", $resolv_conf_original);
};


sub fork_and_req {
    my $pid = fork();
    if ( $pid == -1 ) { 
        printf STDERR "fork failed: %s", $!
    }   
    elsif ( $pid == 0 ) { 
        $ua->get("http://google.co.jp");
        POSIX::_exit(POSIX::EXIT_SUCCESS);
    }
    else {
        wait();
    }
}