Re: Duplicating a coroutine

[William Ahern <william@...>]

On Thu, Jul 20, 2023 at 10:12:11PM -0400, Mouse wrote:
> >> Oddly, [www.inf.puc-rio.br] doesn't answer pings, and mtr to it gets
> >> to 139.82.59.22, which has forged rDNS, before failing.  Maybe the
> >> machine is sick?
> 
> > rDNS looks sane to me, though: [...]
> 
> I no longer recall precisely what I saw.  But, now, I see 139.82.16.205
> as fine; it reverse-resolves to ogum.inf.puc-rio.br, which
> forward-resolves to 139.82.16.205.
> 
> But 139.82.59.22 has forged rDNS now: it reverse-resolves to
> rdc3.rdc.puc-rio.br and nothing else, but rdc3.rdc.puc-rio.br
> forward-resolves to 139.82.181.59 and nothing else.

Ah, right, I forgot to check to what rdc3.rdc.puc-rio.br resolves. I get
what you do.

> (Whether "forged" is the most appropriate term in this case is debatable -
<snip>

I think the specific term is forward-confirmed reverse DNS (FCrDNS). But
it's what I assumed you meant by forged rDNS; I just didn't follow through
on the thought and query both ways.

I guess receiving TCP ACKs rules out TTL issues, unless maybe it's very near
the limit. Could maybe be a PMTU issue, especially considering that PUC
Rio's gateway seems to be dropping ICMP Echo packets. I bet you're routing
over a VPN tunnel. <...> And I think I just duplicated the issue--I can't
download the document over a static IPSec tunnel:

  # OpenBSD host using default public egress assigned by ISP DHCP
  $ ftp 'http://www.inf.puc-rio.br/~roberto/docs/MCC15-04.pdf'
  Trying 139.82.16.205...
  Requesting http://www.inf.puc-rio.br/~roberto/docs/MCC15-04.pdf
  100% |**************************************************|   224 KB    00:01    
  229648 bytes received in 1.35 seconds (165.19 KB/s)

  # Same host but using a static address tunneled through a VPS gateway
  $ ftp -s 45.77.X.Y 'http://www.inf.puc-rio.br/~roberto/docs/MCC15-04.pdf'
  Trying 139.82.16.205...
  Requesting http://www.inf.puc-rio.br/~roberto/docs/MCC15-04.pdf
  ^C

I had to interrupt the latter as it made no progress. I can download from
archive.org, though:

  # NB: using direct URL to PDF, not Wayback Machine's iframe'd page
  $ ftp -s 45.77.X.Y 'https://web.archive.org/web/20040706055406if_/http://www.inf.puc-rio.br:80/~roberto/docs/MCC15-04.pdf'
  Trying 207.241.237.3...
  Requesting https://web.archive.org/web/20040706055406if_/http://www.inf.puc-rio.br:80/~roberto/docs/MCC15-04.pdf
  100% |**************************************************|   224 KB    00:00    
  229648 bytes received in 0.17 seconds (1.21 MB/s)

On attempting a download I see the ICMP packets sent to ogum.inf.puc-rio.br
by the tunnel gateway:

  04:48:36.846789 45.77.A.B > 139.82.16.205: icmp: 45.77.X.Y unreachable - need to frag (mtu 1444)
  04:48:36.846796 45.77.A.B > 139.82.16.205: icmp: 45.77.X.Y unreachable - need to frag (mtu 1444)
  [...]

So it definitely seems like a PUC Rio network issue, presumably the
139.82.59.22/rdc3.rdc.puc-rio.br gateway dropping all ICMP packets,
including Fragmentation Needed, not just Echo packets.