Re: [Full-disclosure] Gmail and China's GFW

[nix@xxxxxxxxxxxxxxxx]

> Gmail can be blocked by simply blocking Google's SSL certificate(s).
>
> When you visit Gmail, Google sends your browser its SSL Server
> Certificate. Without this certificate, no connection can be made.
>
> For example, running the following command, then browsing to
> <https://mail.google.com/>, will produce the following output:
>
> $ sudo ngrep -d eth0 -x 'Google Inc'
> ...
>   47 6f 6f 67 6c 65 20 49    6e 63 31 18 30 16 06 03    Google Inc1.0...
>   55 04 03 14 0f 6d 61 69    6c 2e 67 6f 6f 67 6c 65    U....mail.google
>   2e 63 6f 6d 30 81 9f 30    0d 06 09 2a 86 48 86 f7    .com0..0...*.H..
> ...
>
> This output shows part of Google's ASN.1 encoded X.509 certificate.
>
> Therefore, the following Linux firewall commands will stop any
> connection to Gmail:
>
> $ sudo iptables -I INPUT -m string --algo bm --hex-string
> '|476f6f676c6520496e63311830160603550403140f6d61696c2e676f6f676c652e636f6d30819f30|'
> -j DROP
>
> Try it, you will get a connection timeout:
>
> $ curl --connect-timeout 60 https://mail.google.com/
> curl: (28) SSL connection timeout
>
> The same applies for Twitter, Facebook... Much more efficient than
> DNS/IP blocking!
>

Thanks for the information. I was not actually aware of that you can block
with iptables using hex strings. Very nice.

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