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I was a big fan of OpenDNS dns service, but recently I found few bad things about their offerings. I strongly recommend to stay away from OpenDNS service.

All your search queries belongs to OpenDNS

OpenDNS redirects all your Google search queries though their servers. They captures your search query data and they forwards to real google.com domain. Here is a quick DNS lookup:
$ host www.google.co.in 208.67.220.220
Sample output:

Using domain server:
Name: 208.67.220.220
Address: 208.67.220.220#53
Aliases:
www.google.co.in is an alias for www.google.com.
www.google.com is an alias for google.navigation.opendns.com.
google.navigation.opendns.com has address 208.67.219.230
google.navigation.opendns.com has address 208.67.219.231

They may also do same for your email and other search engine.

Update: Dave has pointed out the reason why OpenDNS forwards google through their server. You can also turn on or off this feature from OpenDNS control panel.

OpenDNS is bad for server

Don't use them on your colocated server or vps server. They redirect web browser users or scripts accessing nonexistent domains to a page containing sponsored search results, ads, and a search form. The DNS protocol requires that a query for a nonexistent domain must return the "NXDOMAIN" error response. Here is a sample output:
$ host abcabcxyzxyz.com 208.67.220.220
Sample output:

Using domain server:
Name: 208.67.220.220
Address: 208.67.220.220#53
Aliases:
abcabcxyzxyz.com has address 208.67.219.132
Host abcabcxyzxyz.com not found: 3(NXDOMAIN)

This encourages spam as you will not able to filter out spam queries using their dns servers.

OpenDNS caching sucks

I contacted their support about my problem but never got any reply. Their server always returns two IP address for my nameserver:
$ host ns2.nixcraft.net 208.67.220.220
Sample output:

Using domain server:
Name: 208.67.220.220
Address: 208.67.220.220#53
Aliases:
ns2.nixcraft.net has address 74.86.48.98
ns2.nixcraft.net has address 74.86.48.98

I don't have 2 IP address for ns2.nixcraft.net.

I strongly recommend running your own dns cache server along with your ISP forwarding nameservers.

Thanks to ricko for pointing out OpenDNS issue in a chat room and elsewhere on the Internet.

Update: Fri Nov 5, 2010 by Vivek: OpenDNS no longer redirects Google search queries though their servers:

$ host www.google.co.in 208.67.220.220
Using domain server:
Name: 208.67.220.220
Address: 208.67.220.220#53
Aliases:
www.google.co.in is an alias for www.google.com.
www.google.com is an alias for www.l.google.com.
www.l.google.com has address 173.194.33.104

Updated for accuracy.