For DNS Resolution testing purposes... I want to disable all DNS caches in my Windows 7
Still I keep seeing "DNS Lookup : 0 ms" for consecutive requests of the same domain.
I've tried the obvious "ipconfig /flushdns", and also stopping totally the service:
net stop dnscache
also this command makes the same effect:
net stop "DNS Client"
and also I know Browser are caching DNS Lookups for very short time. So I flush their caches, close-open the browser, or I open the same domain in different browsers (Firefox, chrome, chrome incognito, IE ) to bypass that DNS Cache.
So the first time, DNS Lookup time can be 25ms (using 8.8.8.8) but then the next DNS Lookup is cached somewhere in the system and time is 0ms. And only goes away if I wait around 3 to 5 minutes to repeat the request.
What can I do to force the system to resolve the DNS every single time, even it's 5 seconds between the same DNS request ?
Has anything to do with keep-alive or some kind of re-use of TCP connections by Windows ? It shouldn't, because I re-open the browser. But I'm out of ideas,
Could you shed some light on this issue?
Thank you
It sounds like your goal is to simulate a non-existent configuration that doesn't exist in the wild (since all clients have DNS caches). It's not entirely clear why that's an interesting configuration to test, but it is possible to do so.
As you mentioned, all browsers have DNS caches. Windows' DNS client itself has a cache. Any upstream proxy you might be using also has a DNS cache.
In this case, you are hitting two problems: First, Fiddler itself maintains a DNS cache. Second, Fiddler pools keep-alive connections to the server, regardless of whether you close your browser client or not.
As described in the Fiddler book, you can control the Fiddler DNS cache using the preference fiddler.network.timeouts.dnscache. The default value is 150000 (measured in milliseconds, so that's 2.5 minutes). You can set this value to 0 to prevent DNS caching.
In order to prevent reuse of connections, you can either hit CTRL+X in the Fiddler session list, or call the FiddlerApplication.oProxy.PurgeServerPipePool method as desired.
Related
I am having very strange network problems. I am on a domain where a few servers are located on a different subnet. I can ping these servers, dns look them up and remote desktop to them by IP-address. I however cannot find them when using:
net view \server
or
Try to access them via windows explorer.
The person next to me who has an identical machine and is on the same subnet has no problems, as a matter of fact, I am the only one in a 50 person company having this problem!
This wouldn't be so much of a problem except for the fact that my machine cannot use web services located on these servers, neither via HTTP or NET.TCP.
After trying everything I can find on the internet and some more (added a new network card, reset policies, etc.) I finally got WireShark to see what is going on. When doing net view \server I notice that the server never responds to "Session Setup Request" but it did respond to "Negotiate Protocol Request". So what could possibly cause the server never to responde to the Session Setup Request?
Here is the server side capture (Not same session)
OK I found out what this was by comparing my tcpip registry (HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters) with a machine that worked. What I noticed is that I had the following 2 entries
EnablePMTUBHDetect 0
EnablePMTUDiscovery 1
but the other machine didn't. By deleting these entries, everything started working!
This however is very strange because these happen to be the default values for there registry keys so I do not understand why having these entries cause such a problem.
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?
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...
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.
Assume you have one box (dedicated server) that's on 24 7 and several other boxes that are user machines that have unused bandwidth. Assume you want to host several web pages. How can the dedicated server redirect http traffic to the user machines. It is desirable that the address field in the web browser still displays the right address, and not an ip. Ie. I don't want to redirect to another web page, I want to tell the web browser that it should request the same web page from a different server. I have been browsing through the 3xx codes, and I don't think they are made for anything like this.
It should work some what along these lines:
1. Dedicated server is online all the time.
2. User machine starts and tells the dedicated server that it's online.
(several other user machines can do similarly)
3. Web browser looks up domain name and finds out that it points to dedicated server.
4. Web browser requests page.
5. Dedicated server tells web browser to repeat request to user machine
Is it possible to use some kind of redirect, and preferably tell the browser to keep sending further requests to user machine. The user machine can close down at almost any point of time, but it is assumed that the user machine will wait for ongoing transactions to finish, no closing the server program in the middle of a get or something.
What you want is called a Proxy server or load balancer that would sit in front of your web server.
The web browser would always talk to the load balancer, and the load balancer would forward the request to one of several back-end servers. No redirect is needed on the client side, as the client always thinks it is just talking to the load balancer.
ETA:
Looking at your various comments and re-reading the question, I think I misunderstood what you wanted to do. I was thinking that all the machines serving content would be on the same network, but now I see that you are looking for something more like a p2p web server setup.
If that's the case, using DNS and HTTP 30x redirects would probably be what you need. It would probably look something like this:
Your "master" server would serve as an entry point for the app, and would have a well known host name, e.g. "www.myapp.com".
Whenever a new "user" machine came online, it would register itself with the master server and a the master server would create or update a DNS entry for that user machine, e.g. "user123.myapp.com".
If a request came to the master server for a given page, e.g. "www.myapp.com/index.htm", it would do a 302 redirect to one of the user machines based on whatever DNS entry it had created for that machine - e.g. redirect them to "user123.myapp.com/index.htm".
Some problems I see with this approach:
First, Once a user gets redirected to a user machine, if the user machine went offline it would seem like the app was dead. You could avoid this by having all the links on every page specifically point to "www.myapp.com" instead of using relative links, but then every single request has to be routed through the "master server" which would be relatively inefficient.
You could potentially solve this by changing the DNS entry for a user machine when it goes offline to point back to the master server, but that wouldn't work without an extremely short TTL.
Another issue you'll have is tracking sessions. You probably wouldn't be able to use sessions very effectively with this setup without a shared session state server of some sort accessible by all the user machines. Although cookies should still work.
In networking, load balancing is a technique to distribute workload evenly across two or more computers, network links, CPUs, hard drives, or other resources, in order to get optimal resource utilization, maximize throughput, minimize response time, and avoid overload. Using multiple components with load balancing, instead of a single component, may increase reliability through redundancy. The load balancing service is usually provided by a dedicated program or hardware device (such as a multilayer switch or a DNS server).
and more interesting stuff in here
apart from load balancing you will need to set up more or less similar environment on the "users machines"
This sounds like 1 part proxy, 1 part load balancer, and about 100 parts disaster.
If I had to guess, I'd say you're trying to build some type of relatively anonymous torrent... But I may be wrong. If I'm right, HTTP is entirely the wrong protocol for something like this.
You could use dns, off the top of my head, you could setup a hostname for each machine that is going to serve users:
www in A xxx.xxx.xxx.xxx # ip address of machine 1
www in A xxx.xxx.xxx.xxx # ip address of machine 2
www in A xxx.xxx.xxx.xxx # ip address of machine 3
Then as others come online, you could add then to the dns entries:
www in A xxx.xxx.xxx.xxx # ip address of machine 4
Only problem is you'll have to lower the time to live (TTL) entry for each record down to make it smaller (I think the default is 86400 - 1 day)
If a machine does down, you'll have to remove the dns entry, though I do think this is the least intensive way of adding capacity to any website. Jeff Attwood has more info here: is round robin dns good enough?
I am using internet IE 7, IE 8
My application in running in dns fail over environment with primary and back up server. As the primary server is down, failover changes to the secondary server after 2 - 3 minutes.
But the problem is, the current opened page in IE is still sending requests to the primary server due to dns caching, which stores the IP of the primary server for default 30 minutes) and hanged.
This problem can be solved if we can clear or edit the dns cache with C# ASP.Net coding.
Thanks in advance for replying.
You cannot access a client machine's DNS Cache from your ASP.NET server or anything in the browser. It would be a huge security hole to either of those environments to do so.
The better approach if you're looking for DNS failover is to talk to your network administrator. Ask him/her to set the TTL for your DNS records to a number smaller than your failover time. This will increase the frequency with which the client machines refresh their caches (for your site only) and shorten their downtime in the event of a failover.
The negligible drawback is that it can increase (ever so slightly) their wait time for the site because they have to do DNS requests more often.
There is an undocumented api called DnsFlushResolverCache in dnsapi.dll, see this link for an example for how to use it from C# (not ASP.NET) .
I have the similar issue from JAva applet. JVM has bug which doesn't honor the System property to disable DNS caching. Any workarounds? Changing the java.security file works. But I am looking for a better solution.
Users connect to our webserver via https, and stay on a secured connection throughout their use of our service. A typical user session will establish a small handful of connections to the server (one or two).
There are a very small number of exceptions we are trying to track down. Particular users will intermittently have handfuls of hundreds of connections established. When we happen to catch the problem in the act, we can see the exchange of the SSL handshake, and from the perspective of the server, all appears to be in order. Yet we never observe a payload - the client instead connects on a new port and initiates a new handshake.
We do not have access to the client, and cannot observe the behavior from that side of the connection. Nor do we have a local scenario that can reproduce the problem.
It is our belief (though not confirmed) that the user agent is connecting to our server directly, and not through a proxy.
Does anybody recognize these symptoms? Can anyone suggest steps to further identify the problem?
Are there any patterns you can see to this traffic, aside from making many repeated requests?
For example, do the requests come from the same IP ranges? Possibly search engines or other spiders, or maybe from countries that you normally don't get users from, possibly indicating some sort of weird botnet or at least something you could block?
Do these rogue requests always negotiate to use a particular cipher suite, potentially indicating the client software?
Does it make any difference if you change the available cipher suites available for negotiation?
What server software are you using, and are there any firewalls within your network that could potentially be dropping some responses to the user?
i've seen a botnet flooding https sites being mentoned.
this is probably not your situation, but i thought i mention it.
i'm seeing chrome (12.0.742.60 beta) flooding my server with https connections, some half a dozen or more connections for a single static picture being served... as if it had an optimization to build up connections with ready https handshakes waiting for requests to send, and then after the page (file) has been served it closes them all.
on plain http i see only two connections (one extra for favicon.ico).