Intermittent 'the remote name could not be resolved'?

Question

I have an ASP.NET application that I use to read the contents of a web page by a HttpWebRequest frequently. There's no problem with the remote address and my application is always working fine.

While I don't change anything, sometimes (about once a day) I get this error:

the remote name could not be resolved.

Why a previously resolved DNS name sometimes fails to be resolved?

Answer 1

The intermittent nature of this is going to be extremely difficult to resolve and it's going to take a configuration change instead of a code solution. (hint: read everything ;)

I would guess that the remote servers DNS is set to expire pretty often. Probably daily or maybe even every 12 hours or so. This is the TTL (time to live) setting. Admins sometimes set this to an artificially low level if they need the ability to quickly move the site to a new server.

You can determine how often it expires by going to a command prompt and running:

nslookup
set debug
www.theserverdomain.com

At the top of this will be a section that says "AUTHORITY RECORDS:" with an item under it that says "ttl".

Now, (and I'm making an educated guess here), what's probably happening when you query your DNS server to resolve that host name your server will have this value cached.

However, once it expires the your server will have to contact another server upstream to get the ip address resolution, called DNS forwarding. If there are a lot of hops between yours and the remote server OR if one of the DNS servers between the sites is overloaded then it could timeout and send back the message you are receiving.

If this is true then the ONLY thing you can do is hardcode the DNS and IP address combination in your web servers hosts file. This is usually at C:\Windows\System32\drivers\etc and is a file named "hosts". There is an example on how to properly edit this within the file itself.

Once you create the host mapping in that file, your web server will no longer have to contact the DNS server to perform name resolution and it won't matter what the TTL is set to.

The only danger here is if they move the web site to a new IP address. At which point you could simply update your hosts file again...

Answer 2

The first thing I would check is if DNS is no longer correctly configured or malfunctioning.

Try (from a Windows command line)

nslookup MyDnsNameHere

and see if you get the IP you would expect.