AT&T mobile intercepting TCP sockets?

[jared at puck.nether.net (Jared Mauch)]

> On May 21, 2018, at 3:35 PM, Chris Adams <cma at cmadams.net> wrote:
> 
> I ran into an odd issue with access to a website I manage from AT&T
> mobile devices this weekend.  The website worked for everybody not on
> AT&T mobile, and AT&T mobile users could access other sites; the problem
> was just this combination.
> 
> Android and iOS phones, as well as a Linux system tethered to an Android
> phone, all had the same problem.  On the Linux system, I disabled IPv6
> in Firefox, and it could then connect.  Browsers got various "connection
> reset" type errors; on Linux, I could telnet to port 80 or 443, and it
> would connect and immediately close.
> 
> The site does have an IPv6 address, but I had missed getting the
> webserver to listen on IPv6 (my mistake).  Adding that looks to have
> solved the problem.
> 
> When I ran tcpdump on the server and had someone try to connect from
> their AT&T mobile iPhone, I saw three connection attempts a few tenths
> of a second apart (all refused by the server).
> 
> My question is this: is AT&T mobile intercepting the TCP socket (and
> not handling "connection refused" correctly)?  Is that a known thing?


Yes they are.  You can see this in test-ipv6.com it will report the proxy/Via
Header addition.

- jared