[CentOS] Diagnosing IPv6 routing

Thu Apr 30 22:08:19 UTC 2020
Chris Adams <linux at cmadams.net>

Once upon a time, Kenneth Porter <shiva at sewingwitch.com> said:
> 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.

Yeah, unfortunately things like that can happen, v4 or v6 (like I
couldn't get to a local TV station's website a little while ago from my
home connection, but could from elsewhere).

>From your traceroutes, it kind of looks like it's possible that it's
something on your gateway (but I'm not really sure).  Do you have any
IPv6 firewall running there?

One other note about mail on v6 - not only do you need to have a valid
reverse (with matching forward) DNS record, you probably need to do TLS
with a valid cert (Let's Encrypt is free and easy).

Chris Adams <linux at cmadams.net>