Comments on: DNS Query Code in C with Linux sockets

By: bamless

bamless — Sun, 10 Jun 2018 16:39:33 +0000

I know that this post is really old but i thought i should comment anyway, to help people who will stumble on this while browsing the web. This code shouldn’t be used in any production code, because it violates c strict aliasing rules (more info: https://stackoverflow.com/questions/98650/what-is-the-strict-aliasing-rule?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa). When casting the pointer `buf` of unsigned chars to the struct DNS_HEADER (or in any other place that a cast like this happens) we’re invoking undefined behaviour. A better solution would be to use a union to alias the char buffer to a struct DNS_HEADER (this is supported by the c99 and c11 standards), or to use a simple memcpy to copy the contents of the struct into the buffer.

By: Iharob Al Asimi

Iharob Al Asimi — Tue, 16 Aug 2016 21:39:10 +0000

I really appreciate the effort. And it seems that you do understand how to perform a DNS query. But you write terrible c code. I suggest fixing it because it’s really bad.

By: AD

AD — Wed, 12 Nov 2014 16:39:00 +0000

isnt aa part in dns header should be after opcode ? In your code you are using standard query which is 0 and aa which is again set to 0, that’s why it is working fine i guess. reference : https://www.ietf.org/rfc/rfc1035.txt

Anyways thanks for snippet it is useful :)

By: Nick

Nick — Mon, 10 Nov 2014 11:49:00 +0000

Is there some way I can get the TTL value from the answers portion?? I tried the following but it returns 0 (ntohs(answers[i].resource->ttl) majority of the time.

By: Nick

Nick — Tue, 21 May 2013 11:27:00 +0000

Very nice article! Can you provide more information about what the ReadName functions does? In my opinion it’s the most important function from the entire program and it’s very poor explainded. Thank you!

By: cristian_gog

cristian_gog — Mon, 14 May 2012 09:06:27 +0000

In reply to Razvan. yeah .. is not implemented, & there are some problems interpreting the message when MX is used ( because of the whitespaces i think )

By: Razvan

Razvan — Wed, 09 May 2012 07:47:37 +0000

In reply to Razvan. Couldn't edit the post and i replied to it.Seems all is working but the PTR. I seem to not receive the information i need when i try to fetch the domain for an ip address , a little help with this ?

By: Razvan

Razvan — Sat, 05 May 2012 10:05:51 +0000

Please ignore my previous comment,i saw that you also posted a linux version.I want to ask you , how do you fetch dns records for , lets say CNAME or PTR ?

By: Rajaona

Rajaona — Thu, 12 Apr 2012 17:02:03 +0000

In reply to Tylkas.

Thanks soo much …
Easily Understandable Code….
can you also provide the code for gethostbyaddr() function (in same pattern without using netdb.h library)

By: ching

ching — Fri, 24 Feb 2012 11:00:24 +0000

i do not understand this part…
if(*reader>=192)
324 {
325 offset = (*reader)*256 + *(reader+1) – 49152; //49152 = 11000000 00000000 ;)
326 reader = buffer + offset – 1;
327 jumped = 1; //we have jumped to another location so counting wont go up!
328 }
329 else
330 {
331 name[p++]=*reader;
332 }
333
334 reader = reader+1;
335
336 if(jumped==0)
337 {
338 *count = *count + 1; //if we havent jumped to another location then we can count up
339 }
340 }
341
342 name[p]=”; //string complete
343 if(jumped==1)
344 {
345 *count = *count + 1; //number of steps we actually moved forward in the packet
346 }
347

rest of program is understood…
please help

By: Tylkas

Tylkas — Thu, 09 Feb 2012 14:29:53 +0000

But I don’t understand this moment:

~$ ./client
Enter Hostname to Lookup : asdqwe
Resolving asdqwe
…
Answer Records : 1
Name : asdqwe has IPv4 address : 67.215.65.132

Why 67.215.65.132 ?

By: Tylkas

Tylkas — Thu, 09 Feb 2012 13:58:15 +0000

Nice code, thank you =]