Saturday, October 01, 2011

Access weirdness to some websites

I am NOT a networking specialist. These are just some of my observations about a weird issue I went through recently.

I have a desktop and laptop at my home. I use my desktop for all the heavy-lift work. Import photos, process the RAW images, fine tune them, resize them for web and print and finally back them up - pretty much the whole workflow for getting images from the memory card off the camera and getting it ready for print. I'm a flickr user - I upload my work there. Ever since I upgraded my desktop to Windows 7, I was unable to even open up the flickr homepage. The browser (ANY browser - I tried with firefox (different versions), chrome and IE) would just get stuck with "Connecting to http://www.flickr.com...". The laptop I had was running Windows XP - things worked just fine on that! For many weeks, I could not figure it out. I just worked-around this issue by using my laptop to upload work to flickr. 

Recently I was fed up enough to sit and spend some time trying to figure out what the hell this issue was all about. Let me categorically state that I have still not figured out what causes this issue even now! I just got the thing to work on my desktop. I still have to do one extra step everytime I have to upload pictures to flickr! 

I started with the basics. 

  1. Does my DNS resolve the web address? Yes it does...
    > nslookup flickr.com
    Server:  resolver1.opendns.com
    Address:  208.67.222.222

    Non-authoritative answer:
    Name:    flickr.com
    Address:  68.142.214.24

  2. Am I able to ping the flickr server? Yes
    > ping flickr.com

    Pinging flickr.com [68.142.214.24] with 32 bytes of data:
    Reply from 68.142.214.24: bytes=32 time=322ms TTL=53
    Reply from 68.142.214.24: bytes=32 time=325ms TTL=53
    Reply from 68.142.214.24: bytes=32 time=330ms TTL=52
    Reply from 68.142.214.24: bytes=32 time=323ms TTL=53

    Ping statistics for 68.142.214.24:
        Packets: Sent = 4, Received = 4, Lost = 0 (0% loss)
    Approximate round trip times in milli-seconds:
        Minimum = 322ms, Maximum = 330ms, Average = 325ms
  3. I tried opening up the browser and directly using the IP address of flickr.com in the address bar. Does not work - the site is not that simple!

    We're sorry, Flickr doesn't allow embedding within frames.
    If you'd like to view this content, please click here.
  4. I vaguely did not search on google. I found that some people do have similar issue, but not with flickr but some other websites. The one suggestion in many of those cases that stood out as being common was to check MTU value and try a lower value. I checked that both in Windows and in my Netgear modem, the MTU was set to 1500. Now the thing to do was to find the correct MTU for flickr.com...

    > ping flickr.com -f -l 1500

    Pinging flickr.com [68.142.214.24] with 1500 bytes of data:
    Packet needs to be fragmented but DF set.
    Packet needs to be fragmented but DF set.
    Packet needs to be fragmented but DF set.
    Packet needs to be fragmented but DF set.

    Ping statistics for 68.142.214.24:
        Packets: Sent = 4, Received = 0, Lost = 4 (100% loss)

    Flickr does not like MTU 1500

    > ping flickr.com -f -l 1400

    Pinging flickr.com [68.142.214.24] with 1400 bytes of data:
    Reply from 68.142.214.24: bytes=1400 time=354ms TTL=52
    Reply from 68.142.214.24: bytes=1400 time=350ms TTL=53
    Reply from 68.142.214.24: bytes=1400 time=355ms TTL=52
    Reply from 68.142.214.24: bytes=1400 time=350ms TTL=53

    Ping statistics for 68.142.214.24:
        Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
    Approximate round trip times in milli-seconds:
        Minimum = 350ms, Maximum = 355ms, Average = 352ms

    MTU 1400 works, but can we get this to work with a higher MTU. Lower MTU means more overhead!

    Using a binary search between 1400 and 1500, I ended up with a value 1472 which flickr seemed to work with. I changed the MTU setting on my modem and voila, I am now able to upload pictures without any hassles.
I do however do not leave my MTU set at 1472 all the time since most other things work fine with MTU 1500. This is the minor step (reduce MTU to 1472 at Netgear modem) at the time I upload photos to flickr and revert it back to 1500 once I am done. 
 

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