Cannot resolve domain name: userserver.agni.lindenlab.com

I can't get into SL because the client is saying it cannot resolve the domain name, userserver.agni.lindenlab.com. Funny, I can ping it:

Pinging util.agni.lindenlab.com [66.150.244.151] with 32 bytes of data:

Reply from 66.150.244.151: bytes=32 time=79ms TTL=53
Reply from 66.150.244.151: bytes=32 time=80ms TTL=53
Reply from 66.150.244.151: bytes=32 time=79ms TTL=53
Reply from 66.150.244.151: bytes=32 time=79ms TTL=53

Ping statistics for 66.150.244.151:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 79ms, Maximum = 80ms, Average = 79ms

The two put together means there's probably a firewall involved.

http://secondlife.com/tiki/tiki-index.php?page=TechVerifyingProtocol

Same message happened to me. I logged off and logged back on and it was fine then. I have my Mac OSX firewall configured as well (based on the provided link Lee gave) and my DSL/router box did not have the firewall up when it did happen. Another one of those oddly behaviors. I think it caused from packet loss or a lousy ISP network or a hop somewhere in the route to the sim server. Try tracerouting to see what kind of results you get.

Oh, additional note. Try having the ping run continously and see if you catch any packet loss then as it is continously pinging the server. You may get a good 4 pings now but a random hiccup can jump in at any moment.

A continuous ping would be ping -t or you can specify the amount of pings to send with ping -n count.

No, it's not firewall or packet loss. I logged in, had to upgrade to the new version, did so, and after the update this started happening.

Actually, it probably is a firewall. Second Life says it can't reach the server; its network connection is totally blocked. The server is obviously online, and you can connect to it outside of Second Life (as evidenced by the ping); so, some program is blocking Second Life's network access.

This happens after updates because Second Life triggers most forms of Internet Worm Protection by changing its executable, thus blocking SL (even if you excluded it).

Now, if you aren't using firewall software, including the Windows Firewall, that's one thing. But if you do and you haven't reconfigured it, you owe it to yourself to give it a try.