Re: [tor-relays] botnet? abusing/attacking guard nodes

I don't quite understand the last calculation.

"if all 65535 connections on an IP were open" => I'm guessing you mean ports

"the biggest Tor Guard has 0.91% Guard probability" => percentage of all entries into the network handled by this guard

=> 0.91% of all user connections

but how many user connections are there at a time?

and then don't understand how probability and ports availability can be combined?

Please elaborate.

Thanks

On Mon, 18 Dec 2017 at 23:11 teor <teor2345@xxxxxxxxx> wrote:

> On 19 Dec 2017, at 08:38, Toralf Förster <toralf.foerster@xxxxxx> wrote:
>
> On 12/17/2017 10:24 PM, teor wrote:
>> Using 256 per IP is probably reasonable.
>
> Is this a rather arbitrary limit or does this limit fit the use of NATed addresses entirely ?

That's an arbitrary safe upper bound.

The number of active connections that can be NATed per IP address is
limited by the number of ports: 65535. (Technically, it's 65535 per
remote IP address and port, but most NATs don't have that much RAM
or bandwidth.)

Also, genuine users behind a NAT would likely have multiple Tor and
non-Tor connections open. And spare ports are needed for NAT to manage
port churn and the TCP delay wait state on connection close.

To be more precise:
* if all 65535 connections on an IP were open to the Tor network, and
* the biggest Tor Guard has 0.91% Guard probability[0], then
* it would expect to see 597 connections.

Feel free to do the sums for your own guard's probability.

(We are aware of the issue, and we are working on a more permanent fix.)

[0]: https://atlas.torproject.org/#details/9844B981A80B3E4B50897098E2D65167E6AEF127

T

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Tim Wilson-Brown (teor)

teor2345 at gmail dot com
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