[CentOS] Diagnosing IPv6 routing

Thu Apr 30 18:17:08 UTC 2020
Kenneth Porter <shiva at sewingwitch.com>

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  * * *