使用perl和Net:：DNS进行DNS检查

因此，在中，有一个小脚本（参见第173页），其目的是迭代检查DNS服务器，查看它们是否返回给定主机名的相同地址。然而，书中给出的解决方案仅在主机具有静态IP地址时有效。如果我想让它与具有多个关联地址的主机一起工作，我将如何编写此脚本

代码如下：

#!/usr/bin/perl
use Data::Dumper;
use Net::DNS;

my $hostname = $ARGV[0];
# servers to check
my @servers = qw(8.8.8.8 208.67.220.220 8.8.4.4);

my %results;
foreach my $server (@servers) {
    $results{$server} 
        = lookup( $hostname, $server );
}

my %inv = reverse %results; # invert results - it should have one key if all
                           # are the same
if (scalar keys %inv > 1) { # if it has more than one key
    print "The results are different:\n";
    print Data::Dumper->Dump( [ \%results ], ['results'] ), "\n";
}

sub lookup {
    my ( $hostname, $server ) = @_;

    my $res = new Net::DNS::Resolver;
    $res->nameservers($server);
    my $packet = $res->query($hostname);

    if ( !$packet ) {
        warn "$server not returning any data for $hostname!\n";
        return;
    }
    my (@results);
    foreach my $rr ( $packet->answer ) {
        push ( @results, $rr->address );
    }
    return join( ', ', sort @results );
}

---

我遇到的问题是，在返回多个地址的主机名上调用代码时出现了此错误，例如www.google.com：

***  WARNING!!!  The program has attempted to call the method
***  "address" for the following RR object:
***
***  www.google.com.    86399   IN  CNAME   www.l.google.com.
***
***  This object does not have a method "address".  THIS IS A BUG
***  IN THE CALLING SOFTWARE, which has incorrectly assumed that
***  the object would be of a particular type.  The calling
***  software should check the type of each RR object before
***  calling any of its methods.
***
***  Net::DNS has returned undef to the caller.

这个错误意味着我试图在CNAME类型的rr对象上调用address方法。我只想在“A”类型的rr对象上调用address方法。在上面的代码中，我没有检查以确保我正在调用类型为“A”的对象上的地址。我添加了这行代码（下一行除外），它可以工作：

my (@results);
foreach my $rr ( $packet->answer ) {
    next unless $rr->type eq "A";
    push ( @results, $rr->address );
}

除非rr对象的类型是“A”，否则这行代码将跳到从

$packet->answer>获取的下一个地址