When attempting to access StackOverflow, I'm timing out when when retrieving content (CSS, JS, etc.) from the CDN. Occasionally it makes it through, but very rarely. Also, this does not just affect me, but a large campus of individuals. Here's the tail of a traceroute:

 7  xe2-0.cr01.sjc01.mzima.net (  3.757 ms  3.743 ms  3.712 ms
 8  te0-1.cr1.lax1.us.packetexchange.net (  14.686 ms  14.598 ms  14.596 ms
 9 (  11.034 ms  11.035 ms  11.020 ms
10 (  10.424 ms  10.424 ms  10.410 ms

While not reflected in the traceroute above, there is a long delay between hops 8 and 9 on my end. We're talking ~30 second delay. This is reflected in the ping:

PING sstatic.stackexchange.netdna-cdn.com ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=51 time=10.4 ms
64 bytes from icmp_seq=2 ttl=51 time=11.1 ms
64 bytes from icmp_seq=3 ttl=51 time=10.7 ms
64 bytes from icmp_seq=4 ttl=51 time=10.9 ms
64 bytes from icmp_seq=5 ttl=51 time=10.7 ms

--- sstatic.stackexchange.netdna-cdn.com ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 58022ms
rtt min/avg/max/mdev = 10.425/10.805/11.147/0.242 ms

While each of the individual times is good, take a look at the total time for 5 roundtrips (~60 seconds, or 20 seconds a ping).

Given that this appears to be between hops 8 and 9, it would appear to be further upstream than my network. Is there any chance this is a configuration issue with SO or their CDN?

Picture of firefox attempting to load CSS from CDN:

  • possible duplicate of cdn.sstatic.net is slowing down initial page loads Commented Jul 6, 2011 at 23:52
  • I'm hitting the same datacenter (LA), but am closer physically (Mountain View, CA). So possibly a similar issue, but I'm not getting slow downs, I'm getting no response.
    – Michael
    Commented Jul 6, 2011 at 23:54
  • I've seen the issue you're seeing as well. Sometimes the browser gives up waiting I think. I also see it on slashdot occasionally. I wonder if they're using the same CDN... Commented Jul 6, 2011 at 23:56

3 Answers 3


Would you happen to be using Ubuntu? If so try this:

Open up a Terminal, Login as Root, backup sysctl.conf, Input timestamps off command & reload sysctl

sudo su -

cd /etc/

cp sysctl.conf sysctl.conf.bak

echo "net.ipv4.tcp_timestamps=0" >> sysctl.conf && sysctl -p
  • 3
    jdorfman is one of our tech support contacts at NetDNA (the CDN provider)
    – balpha StaffMod
    Commented Jul 8, 2011 at 5:08
  • 1
    I am glad this helped. A little history on this. When I was working at Mahalo, we used mzima/packetexchange (now acquired by GTT) as our upstream provider. We switched to NetDNA from EdgeCast and all of our developers who ran Ubuntu where having this same issue. One of the developers (who is now the Data Systems Architect) figured this out. His name is John Watson, you can find him on Twitter: @wizputer. Basically the kid is a genius. Thanks John!
    – jdorfman
    Commented Jul 13, 2011 at 16:04
  • Is this an actual misconfiguration of the client system? Or a problem on your side not being able to handle certin TCP packages?
    – balpha StaffMod
    Commented Jul 13, 2011 at 21:41
  • @balpha I believe it is a Ubuntu 10.* issue.
    – jdorfman
    Commented Jul 14, 2011 at 23:21
  • Forgot to mention this. To prevent this from happening in the future we set this in sysctl: net.ipv4.tcp_tw_recycle = 0
    – jdorfman
    Commented Feb 19, 2013 at 19:16

For future reference: Here's a list of things that can help the support people at the CDN to debug issues like this:

  1. Sample CDN file (that is having issues), e.g. http://cdn.sstatic.net/rpg/all.css?v=c9c55a2b82c5
  2. Your IP address
  3. A traceroute to the CDN, i.e. sstatic.stackexchange.netdna-cdn.com
  4. A traceroute to the Origin, that is sstatic.net
  5. Which datacenter you are hitting (go to http://debug-02.netdna-cdn.com/, it will tell you "You are hitting the NetDNA XYZ Datacenter")

So please include as much information as possible when reporting CDN problems.

If you're uncomfortable with posting some of this publicly here on meta, you can also email it to [email protected].

  • In step 4, do you indeed (still) want a traceroute to sstatic.net, not cdn.sstatic.net? (Or: is the above still up-to-date?)
    – Arjan
    Commented Jul 31, 2013 at 10:29
  • Ah, Oded commented this might be outdated indeed. Work to do! ;-)
    – Arjan
    Commented Aug 11, 2013 at 15:08
  • ...and maybe you also want to know if people are on IPv6? (Then maybe listing ipv6-test.com can help, which shows "When both protocols are available, your browser uses ...", if CloudFlare has no debugging page that easily shows those details.)
    – Arjan
    Commented Aug 11, 2013 at 17:37
  • 6. Do you see a CAPTCHA when requesting the URL directly? ;-)
    – Arjan
    Commented Mar 18, 2014 at 17:40

you can check this repo: StackoverflowCDN, it is a chrome extension to abort long time downloading that css/js file of sstatic cdn, instead load local one.

  • 1
    But how would updates of the JavaScript be handled?
    – Arjan
    Commented Mar 18, 2014 at 17:37
  • so far, need to update js/css file in this chrome extension manually...
    – Bao
    Commented Mar 19, 2014 at 15:45

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