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some students can't load images

some students can't load images

by Hal Sadofsky -
Number of replies: 3

We have two students with odd problems. This is 2 out of four hundred and some, so I don't think it is really an issue with our installation, nevertheless, I'd like to either not be having these problems, or to know what is causing them.

1) A student seems to be able to do everything except one thing: when there is a graph that says to click on it to get a larger image, she doesn't get the larger image.

On IE she sees a new window open, but no image, and IE tells her "Internet Explorer cannot display the webpage"

On Firefox she sees a new window, but no image and Firefox tells her "Network Timeout. The server at euclid.uoregon.edu is taking too long to respond."

My guess is some sort of firewall issue. She appears to have no problem on campus, only in her residence, and I noticed these images involve making a request to port 81. Perhaps her residence's firewall blocks outgoing requests to port 81?

Anyone seen anything like this? If so, any suggestions on dealing with it?

2) Another student can't (apparently) load graphs at all. He is using Firefox on a Macbook, so I can't even exercise my desire to blame Windows for this.

He doesn't get the little version of the graphs that are part of the problem page, or the larger version when he clicks on where the little graphs should be.

This problem doesn't affect him when he uses a computer on campus, only when he is using his computer at home.

Could this also be some sort of firewall issue?

thanks, Hal

In reply to Hal Sadofsky

Re: some students can't load images

by Michael Gage -
I suspect a firewall issue for sure in the second case. To save server overhead the graphs are being served by a light weight server: lighttpd on port 81 (or sometimes port 8000). Some firewalls will block that.

They can verify this by checking the courses such as
(they can log in as guests)

Hosted does not use lighttpd and only uses port 80. hosted2 uses lighttpd
and port 8000 to send the graphs. I suspect they will see the graphs on
hosted but not the graphs on hosted2.

The first case puzzles me more, but it might still involve the same sort of firewall issue.

Hope this helps diagnose the problem. Whether or how one can unblock those ports for their computers at home depends on the source of the firewall -- it might be as simple as changing some settings on their home wifi/router if they are using something like that or perhaps the firewall on their personal PC/Mac. That seems more likely than the idea that a cable modem company is blocking those ports.

In reply to Michael Gage

Re: some students can't load images

by Xiong Chiamiov -

... and a necro bump.

Shouldn't any alternate servers be running on :8080? Port 81 is unofficially reserved for TOR, and 8000 is a common port for Shoutcast and other internet radio streams, hence likely to be blocked by school or business firewalls. Besides, 8080 is the de facto alternate port for http anyways, so it's much less likely to cause problems like this.

Of course, this is something to discuss with your sysadmin (or rather, they should already know >.>, but many of us get forced into programmer/sysadmin/dba/tech support all in one).

In reply to Xiong Chiamiov

Re: some students can't load images

by Arnold Pizer -
The port that lighttpd uses and how WeBWorK interacts with lighttpd is configurable by the user. Explicit instructions for setting this up can be found in

Port 81 is the "default" port (if not using port 80) in the config file for lighttpd and for that reason the above instructions suggest using port 81 for lighttpd. However it only takes very simple edits to change that.

Some people think that port 81 is an alternate port for http. See e.g.
where they state

WebServer Alternate Port 81
When a user types in your IP address or Host Name in their browser, the browser will look at the IP address on port 80. If the Port is blocked, port 81 is used as an alternate port for hosting a website.

Some sites block high level ports, some block all but a very restricted set of ports, etc. If students are blocked from accessing stuff on port 81, I would first suggest trying to have the IT people open access to requests on port 81. If you have control of the WeBWorK server, you can configure lighttpd (any the associated WeBWorK requests) to use any available port. For example port 8080 if that is not blocked.

If students can only access stuff on port 80, then one can forgo the use of lighttpd (which will put a slightly heavier load on the server) and this can be done either system wide (change global.conf) or on a course by course basis be editing the course.conf file.