Convert IP to hostname from a file with perl - perl

Question

I'm trying to convert IP's to hostnames via file using perl (without using built in Socket function).
My file looks like this:
192.168.1.1 firewall
192.168.2.4 wifi
192.168.3.10 switch
My code:
use strict;
use warnings;
my $input_dns_file='./file';
our %ip2host_hash;
sub read_dns_file()
{
open(DNS_FILE,'<',$input_dns_file) or die;
while ( my $line=<DNS_FILE> ){
my ($ip,$hostname) = split(/\s+/,$line,2);
$ip2host_hash{$ip} = $hostname;
}
The problem is that the hash always return the first line of the file. How do I fill the hash %ip2host so it can return every hostname when I enter the IP address?

Answer 1

I think what you are looking for is a script to extract a hostname from a list (on file), given the IP. This is simple. Here's a one-liner:
perl -wle '
$ip = shift;
%list = map split,<>;
print $list{$ip} || "$ip not found";
' 192.168.2.24 list_of_ips
-l will chomp the input and add a newline to print (for convenience). map will just split each element in the input list, so it fits with a hash structure. The last statement will print the hostname, if it is found, or an error if not.
Here's a script:
use strict;
use warnings;
my $ip = shift;
my $file = "list_of_ips";
open my $fh, '<', $file or die $!;
my %list = map split, <$fh>;
print $list{$ip} || "$ip not found\n";
What you may consider is using a short circuit, to return as soon as a match is found. This will be better from a performance point of view, especially with large input files.
while (<$fh>) {
my ($num, $host) = split;
if ($num eq $ip) {
print $host;
last;
}
}