Saturday, January 20, 2007

Blog Blocked - But By Who?

We see various problem reports in Blogger Help Group.
  • My blog is gone.
  • My blog is half gone.
  • My blog is not found.
  • I'm being told that I'm not supposed to see my blog.
  • There's nothing there.
All of these complaints may, or may not, come from any of several problems.


Active Filtering
You may try to view your blog (or maybe go to Blogger, to setup your blog), and get
403 Forbidden
This is, most likely, a Blogger problem (or a problem with your host, if your blog is hosted outside Blogger).

You could get something like "403 Forbidden". It may say "Bad customer! Naughty!", or something similar. Some ISPs, possibly influenced by political pressure, are filtering some websites, or all of Blog*Spot.

There are two ways of filtering Internet traffic. Each have different technologies, and different results.
  1. Active filtering.
  2. Passive filtering.


Active filtering is what you probably think of, when we mention filtering. Your computer asks for a given web page. A router, somewhere between the computer and the web page server, looks at the IP address or URL, and decides whether this web page is on the no-no list or not.

If the web page in question is on the list ("Bad customer! Naughty!"), you may get a screen saying "You aren't permitted here", or similar, or it may simply drop the request. Your browser may eventually decide that you're not getting anything back, and may eventually display "Server not found", or it may simply display a white screen. No guarantees here.

If the web page isn't on the list, it passes the request to the web server, and eventually, you get your web page.

Simple.

Not.

The problem with active filtering is that all web page requests have to wait for validation. Each web page request (millions / second) coming through a given router has to be checked against a long list of no-no websites, before being passed onwards (or not). Can you say "latency"?

In order to do active filtering, an ISP has to have some very beefy routers. Screening each web page request is pretty complex. Lots of complaints from angry customers too.

Passive Filtering
So some ISPs use passive filtering. The routers don't bother with checking a request when it's first received - they just pass the request on to the destination. Then they pass a log record to the logging computers.

While the request is traveling to its destination, a logging computer checks the request against the no-no list. Also, logging computers examine incoming traffic, from distant servers, and look for naughty content.

If the request is on the list, or if naughty content is detected, (Bad customer! Naughty!), logging computers just send out a series of messages in two directions.
  • To the destination server, a message from the originating computer saying "Forget about it".
  • To the originating computer, a similar message from the destination server saying "Forget about it".
This is known technically as a "Reset" or "RST" message. A computer getting an RST simply drops that conversation. The browser on the client computer displays a white screen (what else could you do?), and goes about its business.

Passive filtering is much easier to run. All that the routers have to do is pass the traffic log to another computer, as usual. All ISPs watch what traffic they are passing. Just have logging computers that do more than report the traffic, they actively interfere with it.

So both active and passive filtering can generate a white screen. There are other possible reasons for the white screen, too.

Local Filtering
Maybe you (or the owner of the network that you're using) has decided that certain websites are a problem. There are several types of local filtering that can be used, and they, too, can present you with either a "Bad customer! Naughty!" message, or a white screen.

Network Problems
Besides intentional filtering, by your ISP or by your local network, you can also have conversation drops caused by network problems. Either a DNS problem, or an MTU setting problem, can cause the white screen. See my tutorial Identifying A DNS Problem In Your Internet Service for information.

Diagnosing The Problem
If you think that your website is subject to filtering by the Great Firewall, you can check by using a computer in China, provided by GreatFirewallOfChina.Org. You can read about the GreatFirewallOfChina.Org in their FAQ.

I tested the GreatFirewallOfChina.Org Test, against several websites. The results were predictable.

More Information

1 comment:

Anonymous said...

Thx man