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gethostbyaddr returning null for CNAME records

P: n/a
Hello:

I am using gcc on RedHat Linux 9.

When I try to do a reverse dns query using gethostbyaddr, it works
fine for hosts that have a PTR record, but it returns null for hosts
that have a CNAME record. Any ideas how to work around this?

Here is an example:
dig -x 66.54.79.4
Gives me this answer:
4.79.54.66.in-addr.arpa. 35499 IN PTR
mail.JAMMConsulting.com.

And:
dig -x 64.61.26.100
Gives me this answer:
100.26.61.64.in-addr.arpa. 40544 IN CNAME 100.jfax.com.

I wrote this test program:
#include <netdb.h>
#include <netinet/in.h>
#include <sys/socket.h>

int main(argc, argv)
int argc;
char *argv[];
{
/* Perform the lookup */
/*unsigned long network_ip = htonl(0x42364f04); /* IP 66.54.79.4 */
unsigned long network_ip = htonl(0x403d1a64); /* IP 64.61.26.100 */

struct hostent *result;
result = gethostbyaddr((const void*)&network_ip, sizeof(unsigned
long), AF_INET);
if( result == NULL ) {
printf( "Hostentry is null\n" );
} else {
if( result->h_name == NULL ) {
printf( "Hostname is null\n" );
} else
printf( "Got back %s\n", result->h_name );
}
}

when I run it with the network ip for 66.54.79.4, I get back
mail.JAMMConsulting.com which is what I expected, but when I run it
with the network ip for 64.61.26.100, I get back a null result
pointer.

Any ideas how to work around this?

Thanks
Neil

--
Neil Aggarwal, JAMM Consulting, (972)612-6056, www.JAMMConsulting.com
FREE! Valuable info on how your business can reduce operating costs by
17% or more in 6 months or less! =>
http://newsletter.JAMMConsulting.com
Nov 14 '05 #1
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2 Replies


P: n/a
On 24 Feb 2004 22:18:44 -0800, ne**@JAMMConsulting.com (Neil Aggarwal)
wrote in comp.lang.c:
Hello:

I am using gcc on RedHat Linux 9.

When I try to do a reverse dns query using gethostbyaddr, it works
fine for hosts that have a PTR record, but it returns null for hosts
that have a CNAME record. Any ideas how to work around this?


[snip]

Ask in news:comp.os.linux.development.apps. The function you are
asking about is not part of the standard C library, it is a
platform-specific extension supplied by your compiler/OS combination,
so off-topic here.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++
http://www.contrib.andrew.cmu.edu/~a...FAQ-acllc.html
Nov 14 '05 #2

P: n/a
ne**@JAMMConsulting.com (Neil Aggarwal) writes:
When I try to do a reverse dns query using gethostbyaddr, it works
fine for hosts that have a PTR record, but it returns null for hosts
that have a CNAME record. Any ideas how to work around this?


Your question is outside the domain of comp.lang.c, which discusses
only the standard C programming language, including the standard C
library. This is a remarkably narrow topic compared to what many
people expect.

For your convenience, the list below contains topics that are not
on-topic for comp.lang.c, and suggests newsgroups for you to explore
if you have questions about these topics. Please do observe proper
netiquette before posting to any of these newsgroups. In particular,
you should read the group's charter and FAQ, if any (FAQs are
available from www.faqs.org and other sources). If those fail to
answer your question then you should browse through at least two weeks
of recent articles to make sure that your question has not already
been answered.

* OS-specific questions, such as how to clear the screen,
access the network, list the files in a directory, or read
"piped" output from a subprocess. These questions should be
directed to OS-specific newsgroups, such as
comp.os.ms-windows.programmer.misc, comp.unix.programmer, or
comp.os.linux.development.apps.

* Compiler-specific questions, such as installation issues and
locations of header files. Ask about these in
compiler-specific newsgroups, such as gnu.gcc.help or
comp.os.ms-windows.programmer.misc. Questions about writing
compilers are appropriate in comp.compilers.

* Processor-specific questions, such as questions about
assembly and machine code. x86 questions are appropriate in
comp.lang.asm.x86, embedded system processor questions may
be appropriate in comp.arch.embedded.

* ABI-specific questions, such as how to interface assembly
code to C. These questions are both processor- and
OS-specific and should typically be asked in OS-specific
newsgroups.

* Algorithms, except questions about C implementations of
algorithms. "How do I implement algorithm X in C?" is not a
question about a C implementation of an algorithm, it is a
request for source code. Newsgroups comp.programming and
comp.theory may be appropriate.

* Making C interoperate with other languages. C has no
facilities for such interoperation. These questions should
be directed to system- or compiler-specific newsgroups. C++
has features for interoperating with C, so consider
comp.lang.c++ for such questions.

* The C standard, as opposed to standard C. Questions about
the C standard are best asked in comp.std.c.

* C++. Please do not post or cross-post questions about C++
to comp.lang.c. Ask C++ questions in C++ newsgroups, such
as comp.lang.c++ or comp.lang.c++.moderated.

* Test posts. Please test in a newsgroup meant for testing,
such as alt.test.

news.groups.questions is a good place to ask about the appropriate
newsgroup for a given topic.

--
"Some programming practices beg for errors;
this one is like calling an 800 number
and having errors delivered to your door."
--Steve McConnell
Nov 14 '05 #3

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