PDA

View Full Version : Resolving DNS the right way


Adesso
17th October 2007, 03:08 PM
Greetings .

After spending 2 solid days setting up my redhat server to dial into the Internet over pppoe and foward it over my second network card to a internal network, I have it running almost 100%

My only nut cracking problem now is that my DNS does not seem to always resolve.. With a normal nslookup to hotmail.com comes back immeditly with a reply.. no problem, but wget hotmail.com .. just sits there..

When I try wget on google.com.. all works fine..

Both nslookups work , but not both wget.... this is really puzzling me.. Any help would be appreciated.. :confused:

Adesso
31st October 2007, 12:00 PM
Solved the problem.. seems my static ip ended in 255, and this is not acceptable as a destination IP by windows server..

A change in IP, and all is working fine... What a nut cracker.. you klive and you learn I guess.. :D

Zotter
1st November 2007, 05:11 AM
Nice catch - now ya know 255 is broadcast addy in IPv4.

Thanks for sharing!

Adesso
6th November 2007, 01:43 PM
Yeh Zotter... I knew the 255 thing before the time, but I was really not thinking that A, my ISP would actually give me such an IP, and B that on non Microsoft servers it worked 100%..

This would mean that A an broadcast address as IP is legal, and B Microsoft server suck.. but the latter is a known fact..

So I am perplexed at to why some servers seems to allow this.. should this not be illegal ?? My ISP was also convinced that this IP was legal, but now I guess they think otherwise..

But since the problem is solved, I was just thinking/sharing loud out here..

A.Serbinski
6th November 2007, 05:29 PM
Without knowing your subnet mask and network address, it is impossible to say if it was correct to block your IP address or not. Regarding the broadcast address, it is NOT correct to say that "ending in 255" is the broadcast address. It is in fact the bit COMPLIMENT of the SUBNET MASK bitwise or'd with the NETWORK address.

For example, if the subnet mask is 255.255.0.0 and network address is 123.123.0.0, then the broadcast address is 123.123.255.255 and 123.123.123.255 is a perfectly acceptable IP address.

Take another example using subnet mask 255.255.255.128 and using a network address of 123.123.123.50, then the broadcast address will be 123.123.123.127 -- however, in this example, 123.123.123.255 is NOT an acceptable IP address since it is outside the range of the network, which allows IP addresses in the range of .51 through .126 inclusive.

The easiest way to think of this is the following;
Convert your subnet mask and IP address into BINARY. If the IP address has all "1"'s where the subnet mask has all "0"'s, then it is, in fact, the broadcast address. If the IP address has all "0"'s where the subnet mask has all "0"'s, then it is the network address. Anything in between is an acceptable IP address

Adesso
13th November 2007, 11:29 AM
:eek:

Thanks Serbinski for that explanation, I guess I will still be pondering this for a while, cause my IP knowledge is not all that great.. now try explaining that for IPv6 and I am sure you would be lost. :D

What was strange however was that it worked partially.. I could surf the web, only some servers did not agree with my IP. in particular hotmail.com and myfamily.com also faz.net.. but everything else was working fine.. This most likely points to the fact that somehow there subnets was defined differently.. I guess..

Just a question of interest.
If the server has a IP of 50 and the subnet ends in 127, then you said valid IP range from 51 to 126. If the server then changes to a IP of 10, would that increase the range to 10-126?? Does this mean that the subnet of 127 include all IP's from the server IP to 127. I was always thinking that a subnet 127 ment from 1 - 126 regardless of what the server IP is .. and if the range limitation as you said is true, would this not make it possible to attack a server from the lower than 50 IP side..Theoretically....

If only IPv6 was this easy.. :P Anyway.. this is getting off topic.. Thanks for the interseting read anyway.. (Freaky Binary explanation.. will try it when I have time... sounds cool)

Have a great day :)

Nagua - Hirara Travel Photos - Newport Travel Photos on Instagram