I discovered that IPv6 is sort of working when I got an email rejection
from Comcast for not having an IPv6 PTR record. I discovered I could telnet
to port 25 on their MX server over IPv6! I then found I could tracroute6 to
them, but I couldn't to my Linode VPS in Fremont. It gets to the data
center and stops. Going the other way, my Linode can traceroute6 almost to
my AT&T-hosted server. Neither can reach the open port 25 on the other, but
both can reach mx1.comcast.net via IPv6.
I dropped the firewall ("systemctl stop firewalld", confirmed with
"ip6tables -L -v -n") on my Fremont server and still can't traceroute6 or
ping6 it. Nor can I do so from my home system on Comcast.
What else could be blocking ping6, traceroute6, and telnet to port 25 on
IPv6?
>From AT&T host to Linode host:
[root at saruman ~]# traceroute6 2600:3c01::f03c:91ff:febc:e22d
traceroute to 2600:3c01::f03c:91ff:febc:e22d
(2600:3c01::f03c:91ff:febc:e22d), 30 hops max, 80 byte packets
1 2001:1890:1837:5b00::1 (2001:1890:1837:5b00::1) 3.230 ms 3.181 ms
3.137 ms
2 2001:1890:c05:2f3a::ee6e:61cd (2001:1890:c05:2f3a::ee6e:61cd) 6.906 ms
6.901 ms 6.870 ms
3 ok3ca81crs.ipv6.att.net (2001:1890:ff:ffff:12:123:6:98) 8.305 ms
8.254 ms 8.258 ms
4 sffca21crs.ipv6.att.net (2001:1890:ff:ffff:12:122:110:65) 14.199 ms
14.164 ms 14.153 ms
5 sffca401igs.ipv6.att.net (2001:1890:ff:ffff:12:122:114:5) 9.861 ms
10.163 ms *
6 2001:1890:1fff:41e:192:205:32:222 (2001:1890:1fff:41e:192:205:32:222)
9.789 ms 8.078 ms 7.365 ms
7 linode-ic-342731-sjo-b21.c.telia.net (2001:2000:3080:1ac4::2) 6.788 ms
6.327 ms 6.262 ms
8 2600:3c01:3333:4::2 (2600:3c01:3333:4::2) 7.294 ms 7.728 ms 7.705 ms
9 * * *
>From Linode to AT&T:
[root at linode2 ~]# traceroute6 2001:1890:1837:5b01::100
traceroute to 2001:1890:1837:5b01::100 (2001:1890:1837:5b01::100), 30 hops
max, 80 byte packets
1 2600:3c01::8678:acff:fe0d:79c1 (2600:3c01::8678:acff:fe0d:79c1) 0.783
ms 2600:3c01::8678:acff:fe0d:a641 (2600:3c01::8678:acff:fe0d:a641) 0.834
ms 0.972 ms
2 2600:3c01:3333:4::1 (2600:3c01:3333:4::1) 0.645 ms 0.646 ms
2600:3c01:3333:2::1 (2600:3c01:3333:2::1) 0.508 ms
3 ix-ae-67-0.tcore1.sqn-san-jose.ipv6.as6453.net (2001:5a0:1000:500::a1)
1.059 ms 2001:550:2:3::d9:1 (2001:550:2:3::d9:1) 1.967 ms
ix-ae-67-0.tcore1.sqn-san-jose.ipv6.as6453.net (2001:5a0:100
4 be2063.ccr21.sjc01.atlas.cogentco.com (2001:550:0:1000::9a36:1a1)
2.340 ms 2001:1890:1fff:415:192:205:37:69
(2001:1890:1fff:415:192:205:37:69) 3.209 ms 4.412 ms
5 sffca21crs.ipv6.att.net (2001:1890:ff:ffff:12:122:114:6) 7.353 ms
6.769 ms 5.379 ms
6 ok3ca81crs.ipv6.att.net (2001:1890:ff:ffff:12:122:110:66) 4.785 ms
6.158 ms 6.073 ms
7 2001:1890:ff:ffff:12:122:119:125 (2001:1890:ff:ffff:12:122:119:125)
3.401 ms 3.406 ms 3.469 ms
8 ok3ca81crs.ipv6.att.net (2001:1890:ff:ffff:12:122:110:66) 8.444 ms
8.415 ms 2001:1890:c05:2f3a::116e:61cd (2001:1890:c05:2f3a::116e:61cd)
5.816 ms
9 2001:1890:ff:ffff:12:122:119:125 (2001:1890:ff:ffff:12:122:119:125)
4.688 ms 4.607 ms 4.565 ms
10 * * 2001:1890:c05:2f3a::116e:61cd (2001:1890:c05:2f3a::116e:61cd)
6.732 ms
11 * * *