Project Gutenberg is a site hosted by iBiblio that provides public domain ebooks. Unfortunately, the only people who can access the site are people who either have IPv4 only or have a native IPv6 address. People who get an IPv6 address via a tunnel, such as the one provided automatically by their ISP (for example, AT&T's 6rd tunnel) cannot access the Project Gutenberg site at all. Nor can they access the main iBiblio home page, which has the same problem. Changing my IPv4 and IPv6 DNS servers to Google DNS does not fix the issue. Is iBiblio breaking PMTUD by filtering ICMPv6 traffic? That could be the cause.
To be clear, I'm not affiliated with Project Gutenberg. I'm just someone who wants to access that site more easily, and I see from their sidebar that they're hosted by iBiblio and that the iBiblio site has the same problem.
Answer by Slinkwyde · Jul 30, 2016 at 03:03 PM
tracert www.gutenberg.org
Tracing route to gutenberg.org [2610:28:3090:3000:0:bad:cafe:47] over a maximum of 30 hops:
1 1 ms 1 ms 1 ms 2602:30a:c0f6:d94f:f6f2:6dff:fe95:27e0
2 6 ms 2 ms 1 ms 2602:30a:c0f6:d940::1
3 30 ms * 30 ms 2602:300:c533:1510::1
4 247 ms 183 ms 55 ms rlgnc404me3.ipv6.att.net [2001:1890:ff:ffff:12:122:127:141]
5 * Request timed out.
6 65 ms 67 ms 65 ms rtp7600-gw-to-rtpcrs-gw.ncren.net [2610:28:105:4::2]
7 66 ms 65 ms 65 ms 2610:28:105:1c::2
8 69 ms 66 ms 68 ms mk-to-ml-v1600.net.unc.edu [2610:28:3090:23::2]
9 67 ms 66 ms 65 ms 2610:28:3090:3000:0:bad:cafe:47
Trace complete.
tracert www.ibiblio.org
Tracing route to www.ibiblio.org [2610:28:3090:3000:0:bad:cafe:23] over a maximum of 30 hops:
1 2 ms 1 ms 1 ms 2602:30a:c0f6:d94f:f6f2:6dff:fe95:27e0
2 3 ms 1 ms 1 ms 2602:30a:c0f6:d940::1
3 30 ms * 30 ms 2602:300:c533:1510::1
4 55 ms 54 ms 55 ms rlgnc404me3.ipv6.att.net [2001:1890:ff:ffff:12:122:127:141]
5 * Request timed out.
6 67 ms 67 ms 66 ms rtp7600-gw-to-rtpcrs-gw.ncren.net [2610:28:105:4::2]
7 67 ms 66 ms 66 ms 2610:28:105:1c::2
8 66 ms 68 ms 65 ms mk-to-ml-v1600.net.unc.edu [2610:28:3090:23::2]
9 70 ms 69 ms 68 ms www.ipv6.ibiblio.org [2610:28:3090:3000:0:bad:cafe:23]
Trace complete.
Answer by Slinkwyde · Jul 30, 2016 at 03:13 PM
In a forum discussion I started before posting this question, there are now two people saying Project Gutenberg is working for them over their tunnel. One of them uses Hurricane Electric's tunnel, and the other hasn't yet said. I'm using AT&T's automatically provided 6rd tunnel and can't get to Project Gutenberg (or iBiblio) unless I turn off IPv6 or use an IPv4-based VPN (Private Internet Access).
https://www.reddit.com/r/ipv6/comments/4v8ncz/project_gutenberg_not_working_over_ipv6
Answer by Slinkwyde · Jul 31, 2016 at 10:22 AM
Since iBiblio is located at the University of North Carolina, do you think their campus IPv6 policy could be related to this problem? Specifically, the part where it says "We reserve the right to refuse any non-global or non-native (i.e. tunneled) IPv6 address." Also, the box labeled "Examples of IPv6 addresses that are discouraged:"
To be clear, I'm outside the UNC network. I'm in Houston on residential AT&T U-verse, using the 6rd tunnel automatically provided by AT&T.