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A friend had a computer with the following symptoms:

  1. the command ping cnn.com (or for any other domain) gave the message Ping: cannot resolve cnn.com: Unknown host

  2. the command ping 8.8.8.8 (or for any other IP) gave the message Ping: sendto: Host is down

We were both on the same network (Oxford University's 'OWL', which you have to use VPN with to visit non-Oxford sites and avoid getting redirected to an Oxford page asking you to log on. So on my Mac I looked at [System Preferences > Network > Advanced button > DNS] and copied the 'DNS Server' and 'Search Domain' that showed there when I connected to OWL (it seemed they were auto-detected). I then added them to the same place on my friend's computer, which had other DNS servers and no Search Domain. That fixed things.

Why was this? In general, how can I diagnose problems like 1 & 2 above.

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migrated from serverfault.com Aug 8 '12 at 16:11

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3 Answers 3

up vote 2 down vote accepted

It sounds like your friend's DNS settings were incorrect in the context of your "OWL" network and not reachable from within that network, especially if your search domain is set to something that can't be reached from there. Checking the DNS server IPs and search domain (as you did) are the best ways to determine this.

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Ta, what exactly is a search domain? –  tog22 Aug 8 '12 at 21:06
    
tog22: a domain within which DNS queries are expected to reside, most commonly your current network/domain. For example: if your FQDN were myhost.example.com, your search domain should by default be example.com, meaning others on your network would be able to look you up by your hostname alone without the domain (i.e., myhost). In OS X and other *NIX systems, this is set in /etc/resolv.conf with the search directive (search example.com). –  deesto Aug 9 '12 at 15:41
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To identify if it is a DNS issue you can use NSLOOKUP command. This is an explicit instruction to query DNS server about the target system details with hostname or hostip as input.

And incase if your DNS is correct but Host is unreachable you can use traceroute command get the exact hop that is creating issue.

Some explanation is available at below link

http://guide.netfronts.com/Advanced/Ping_and_Nslookup_with_Mac_OS_X.htm

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The DNS server and search domain are provided by the DHCP server on your local network.

Use tcpdump or wireshark and look at the DHCP/BOOTP replies.

With windows, you can go to your network properties and remove "automatic DNS" http://windows.microsoft.com/is-is/windows/change-tcp-ip-settings#1TC=windows-7

With Linux, you can change your client configuration (depend which one you use) or add script to check the DNS address in /etc/resolv.conf

http://www.zyxware.com/articles/2009/03/01/stop-your-dns-servers-from-changing-after-every-reboot-while-using-dhcp

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