Re: Broken IPv6 / IPv4 configuration, or Gmail brokenness?

[Tim Woodall <debianuser@woodall.me.uk>]

On Sun, 22 Jan 2023, Celejar wrote:

> On Sun, 22 Jan 2023 16:28:55 -0500
> Dan Ritter <dsr@randomstring.org> wrote:
>
> > Celejar wrote:
> > > Hello,
> > >
> > > My Debian Sid system, including its networking system, has been
> > > working fine for a while. Recently, Gmail has not been working properly
> > > on the system: sending (via SMTP with SSL) times out, and receiving
> > > (via POP3 and IMAP) takes abnormally long. Today I finally made some
> > > efforts to understand what's going on. Here's what I know /
> > > understand / have been able to establish:
> > >
> > > 1) My ISP provides me with only IPv4, not IPv6.
> > >
> > > 2) Trying to send email through Gmail using "smtp.gmail.com", via
> > > Sylpheed and Swaks, times out without getting anywhere. For example:
> > >
> > > $ swaks -t celejar@gmail.com -s smtp.gmail.com:587 -tls -a LOGIN
> > > Username: celejar
> > > Password: ********
> > > === Trying smtp.gmail.com:587...
> > > *** Error connecting to smtp.gmail.com:587:
> > > *** 	IO::Socket::INET6: connect: timeout
> > Your system expects to be able to use IPv4 and IPv6. Google
> > handles both. Your ISP does not. DNS returns both IPv4 A records
> > and IPv6 AAAA records for smtp.gmail.com.
> >
> > In this particular case, you should change your system
> > preference to IPv4 first.
>
> Shouldn't this be included somewhere prominently in the Debian
> documentation, in the form of a Big Fat Warning that the standard
> dual-stack condiguration used by Debian can cause serious breakage if
> one's ISP doesn't support IPv6? (I'd be happy to add a warning to, say,
> https://wiki.debian.org/DebianIPv6, if I thought I understood the
> issues well enough to get it right.)
I think, although I'm not 100% sure as I now have fully working IPv6
everywhere, that your problem might be that something is providing a
default route.

If the box trying to connect to gmail knows there's no route then it
will use IPv4. If it thinks there's a route it will use it.

At a guess, your router is sending out RA messages that say the router
is a default route and then dropping packets when you actually try to
send.

radvdump will let you see who is broadcasting advertisements.

For completeness, in case you're using radvd, the magic setting to
disable the default route is
  AdvDefaultLifetime 0;

Easy when you know but I remember many frustrations trying to work out
how to disable the default route while still providing specific routes.