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can ping ONLY by HOSTNAME but not ip address

Posted on 2006-11-04
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Last Modified: 2008-09-16
Actually, I'm not actually even having this problem, but I just wanted to know how would it be possible for a computer to ping websites by hostname, but times out for its corresponding ip address.

Usually it's the other way around where you can ping by ip but not hostname, which would indicate a dns error that can sometimes be resolved by flushing the dns. Just curious to know what the technical explanation is if someone was only able to ping by hostname and not ip.
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Question by:CHSnake
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lrmoore earned 1000 total points
ID: 17874614
hostnames can sometimes resolve to multiple IP's and where dns will round-robin a request, one or more servers can be down and not answer an icmp ping so they can't be reached by IP address, but can by dns name.
It would be rare, but it is possible.
It could also be that there is a local hosts file entry for the host name where the system will resolve a hostname to IP address 1.2.3.4 which answers only if addressed by hostname, whereas an nslookup may resolve it to a different IP address that will not respond if you try to ping the IP address.

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by:IanTh
ID: 17874624
dns is a very robust service that virtually never fails. A local resolver corruption can cause this by having faulty dns data cached. The local resolver cache is a local dns system effectively
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by:jdunneuk
ID: 17875927
if you can ping a hostname and not a ip it could be that the actually server is using a nat or perhaps some adaptation of pat
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Expert Comment

by:QuietRoot
ID: 17875985
the only solution i can see is that the resolved dest ip is diff then the dst ip you're issuing.
and yeah, off course, the round-robin situation it's true as well.
i'm not shore it is possible to filter icmp traffic by checking if the Echo Request used a name server resolution process or was send with an ip address in the first place.

can u share an example?
cheers,
BC
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Author Comment

by:CHSnake
ID: 17876614
Ok, I just had an issue where someone was able to ping yahoo, but not its ip, where normally they can. This of course caused a no route situation. I just wasn't sure how that happened, and what the solution would've been.

I guess I should've checked to see if the same condition existed for other websites, to narrow down whether it was happening for all websites that respond to icmp, or if yahoo just happened to be having this issue at that time.
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Expert Comment

by:QuietRoot
ID: 17879674
i'm shore it's the second option :)
i'm sticking to my opinion here :D

it's the round robin situation that we're talking about and i see 2 ways of explaining this:
1) the first ping went to an ip add (ping [ip]) and the second time the dns resolved to another dst (ping [host]) due to the round robin process. and somehow you didn't noticed that there was 2 diff host addresses.
2) somehow after the replay from the first ping there was some kinda of network problem between you machine and the dst add :|.

anyway, as i recall, and i've mentioned this above, i am not familiar with any policy that can filter icmp traffic by checking if the package dst hostname was resolved before, in order to find the destination ip address, or not. :).
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