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Quick reference

Many different ways to resolve an IP to a hostname in Perl

Some different ways to look up hostnames.

Notes:

  • getnameinfo() gets a hostname and a service name, so it's not exactly the same as gethostbyaddr()
  • although the docs imply you need to enter a port, that's just to get the local service name. If you set it as undef then you can ignore any service

# 1, old deprecated way

use Socket; # gethostbyaddr, inet_aton, AF_INET

sub ip_address_to_host {
    my ( $self, $ip_address ) = @_;
    my ($hostname) = gethostbyaddr(
        inet_aton($ip_address),
        AF_INET,
    );
    return $hostname;
}

# 2, newer better way

use Socket qw(AF_INET inet_pton getnameinfo sockaddr_in);

sub ip_address_to_host {

     my ( $self, $ip_address ) = @_;

    my $port = undef;
    my $socket_address = sockaddr_in($port, inet_pton(AF_INET, $ip_address));

    my $flags = 0;
    my $xflags = 0;
    my ($error, $hostname, $servicename) = getnameinfo($socket_address, $flags, $xflags);

    return $hostname;
}

# 3, best way - that only uses DNS and not /etc/hosts first

use Net::DNS;

sub ip_address_to_host {

     my ( $self, $ip_address ) = @_;

    my $res = Net::DNS::Resolver->new;
    my $target_ip = join('.', reverse split(/\./, $ip_address)).".in-addr.arpa";
    my $query = $res->query("$target_ip", "PTR") // return;
    my $answer = ($query->answer)[0] // return;
    my $hostname = $answer->rdatastr;

    return $hostname;
}

# 4, the code golf way (no validation) - by Mark B

 perl -le 'use Net::DNS::Resolver; print ((Net::DNS::Resolver->new()->query("10.232.32.158","PTR")->answer)[0]->ptrdname);'