Timeouts when downloading resources via SSL/IPv6

I've got a strange issue here that started about a week ago. It caused some problems with my XenForo installation (see my support thread here), but further investigation provides more clues that the problem runs a bit deeper.

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To test, I used a plain old wget to download two images that the XenForo image proxy failed to load:

Code:

$ wget https://s-media-cache-ak0.pinimg.com/736x/86/ea/2b/86ea2bb8fc6e82c1db13e1b3cbf09742.jpg

--2015-08-03 18:47:59--  https://s-media-cache-ak0.pinimg.com/736x/86/ea/2b/86ea2bb8fc6e82c1db13e1b3cbf09742.jpg
Resolving s-media-cache-ak0.pinimg.com... 2600:1408:17:3a2::2781, 2600:1408:17:39e::2781, 2600:1408:17:3a0::2781, ...
Connecting to s-media-cache-ak0.pinimg.com|2600:1408:17:3a2::2781|:443... failed: Connection timed out.
Connecting to s-media-cache-ak0.pinimg.com|2600:1408:17:39e::2781|:443... failed: Connection timed out.
Connecting to s-media-cache-ak0.pinimg.com|2600:1408:17:3a0::2781|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 71070 (69K) [image/jpeg]
Saving to: “86ea2bb8fc6e82c1db13e1b3cbf09742.jpg”

100%[=========================================================================================================================>] 71,070      --.-K/s   in 0.03s 

2015-08-03 18:48:30 (2.31 MB/s) - “86ea2bb8fc6e82c1db13e1b3cbf09742.jpg” saved [71070/71070]

...and the second test...

Code:

$ wget https://upload.wikimedia.org/wikipedia/commons/c/ce/Alfa_Romeo_GTV_facelift_front.JPG

--2015-08-03 18:51:29--  https://upload.wikimedia.org/wikipedia/commons/c/ce/Alfa_Romeo_GTV_facelift_front.JPG
Resolving upload.wikimedia.org... 2620:0:861:ed1a::2:b, 208.80.154.240
Connecting to upload.wikimedia.org|2620:0:861:ed1a::2:b|:443... failed: Connection timed out.
Connecting to upload.wikimedia.org|208.80.154.240|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 442188 (432K) [image/jpeg]
Saving to: “Alfa_Romeo_GTV_facelift_front.JPG”

100%[=========================================================================================================================>] 442,188     2.35M/s   in 0.2s  

2015-08-03 18:51:44 (2.35 MB/s) - “Alfa_Romeo_GTV_facelift_front.JPG” saved [442188/442188]

Although both of the images do load eventually, it takes about 10 seconds or so to deal with those timeouts. Clearly, it's breaking XenForo because the PHP script is giving up...but why is it taking so long?

I noticed in the first request, it never falls back to IPv4 and still downloads the image, but in the second, it does fall back to IPv4. Note that I have never performed any special configurations for IPv6 for my Centminmod install. I have not made any recent changes to the server configuration in over a month, when I made the upgrade to PHP 5.6.10.

Both of the images can be loaded here just fine.

Any ideas as to what might be going on?

 

Thanks, you were exactly right!

My host (ReliableSite) confirmed and fixed an issue with one of their routers that was causing a delay in IPv6 traffic.

For anyone else who may be troubleshooting a similar problem, see my post here.