excellent thank you to JuniperHost and you Wiggens.
So here is what I ended up with:
> 1> use Net::DNS;
2>my $res = Net::DNS::Resolver->new;
3> my $query = $res->query("example.com", "NS");
4>if ($query != undef) {
5> foreach ($query->answer) {
6> if ($_->type eq 'NS' ) {
print $rr->nsdname, "\n";
8> }else {
10> warn "query failed: ", $res->errorstring, "\n";
11> }
12> }
13> }
So I can find the all these object methods in the perl docs listed below?
I have yet to read object orientation with Perl as I am still on
Programming Perl Chapter 2.
thanks again,
Derek B. Smith
OhioHealth IT
UNIX / TSM / EDM Teams
"Wiggins
d'Anconia"
<[EMAIL PROTECTED] To
.org> [EMAIL PROTECTED]
cc
03/01/2005 11:08 Perl Beginners <beginners@perl.org>
AM Subject
Re: Net::DNS
[EMAIL PROTECTED] wrote:
> All,
>
> Can someone explain this code line by line?
> Obviously I understand most of it but lines 4, 5, 6 and 10 I am fuzzy.
>
I can try :-).
> For line 4: if ($query) ..... if what? Here I would expect to see if (
-s
> $query ) or something like a return code/exit code value check like $?
>>
> 8.
> What is if ($query) saying? Is there a more beginner way? After line 3
> could I check the exit value then run a more obvious if statement?
>
Generally this is a check for definedness. Many modules as a convention
return a true value of some sort, in this case a "query object" from the
resolver -or- they return undef as an error indication. So to handle the
error you check to see if $query is defined (or true/false). This is
module specific, but generally the module *should* tell you how it
expects you to do error handling.
> For line 5: foreach my $rr, we are looking for a record string eq to
> NS(nameserver) from $query answer? But where does $rr, $_ and answer
come
> from?
$rr is where the loop temporarily stores the looping variable for the
foreach, you can pick whatever you want to name the variable. In this
case the list (that gets looped over) is special, and is generated as
the return value of 'grep'. grep itself takes a list and loops over that
list testing each element to see if it evaluates the conditional to
true. $_ is where grep temporarily assigns each of the values in the
list passed to it, which in this case is the return value from calling
the 'answer' method on the $query object. 'answer' is an object method
and should be documented in Net::DNS. So this says:
Loop over every value returned by calling 'answer' on $query, checking
to see if calling 'type' on that value is equivalent to the string
literal 'NS'. Loop over every value where that test is true temporarily
setting the value from 'answer' to $rr. Or give me every answer where
the type of answer is an 'NS'.
> This entire line is fuzzy to me. Is there a more beginner way ?
Always. :-)
I would
> probably write it like:
> foreach ($query->answer) {
> if ( $_ eq NS) {
This is not quite right since you need to handle the inner loop (grep)
test the same.
if ($_->type eq 'NS') {
> print $rr->nsdname, "\n";
> }
> }
>
>
> For line 10: $res->errorstring. Where does errorstring come from? Are
> there globals in this module that I am missing? I looked on CPAN and
did
> not find any.
>
'errorstring' is a method of $res (resolver object) and goes back to the
error checking I mention above. So if $query is undefined (aka an error
happened) then we know (by virtue of the docs for Net::DNS::Resolver)
that we can call errorstring and ask it what the error was.
> In general I am having trouble understanding and using this modules
> potential.
>
This is where a detailed reading of the documentation for a module's API
is critical. And it is probably the most important task in learning
programming. Anyone can string syntax together, but understanding how to
use someone else's code is far more difficult. Note that in some cases
you may have to look at the documentation for modules that this
particular module inherits from.
>
> 1> use Net::DNS;
> 2>my $res = Net::DNS::Resolver->new;
> 3> my $query = $res->query("example.com", "NS");
>
> 4>if ($query) {
> 5>foreach my $rr (grep { $_->type eq 'NS' } $query->answer) {
> 6> print $rr->nsdname, "\n";
> 7> }
> 8>}
> 9>else {
> 10> warn "query failed: ", $res->errorstring, "\n";
> 11> }
>
Good luck,
>
> thank you,
> Derek B. Smith
> OhioHealth IT
> UNIX / TSM / EDM Teams
>
>
>
You should check out Learning Perl Objects, References, and Modules from
O'Reilly, and/or the object documentation available with perl,
perldoc perlboot
perldoc perltoot
perldoc perltooc
perldoc perlbot
perldoc -f grep
perldoc Net::DNS
perldoc Net::DNS::Resolver
http://danconia.org
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