Re: [Qemu-devel] [PATCH 6/6] vnc: track & limit connections

[Gonglei]

On 2014/10/17 14:38, Daniel P. Berrange wrote:

> On Fri, Oct 17, 2014 at 02:34:07PM +0800, Gonglei wrote:
>> On 2014/10/16 18:46, Gerd Hoffmann wrote:
>>
>>>   Hi,
>>>
>>>>> I try to prevent that by dropping the *oldest* connection, so you have a
>>>>> chance to connect even if a unprivileged attacker tries to use up all
>>>>> connection slots.
>>>>
>>>> Lets say the limit is 5. The bad guy has 5 open idle connections.
>>>> The good guy opens a new one and pushes off one of the bad guy's
>>>> connections. Fine so far. The bad guy though can simply open 5 more
>>>> connections and he'll push the good guy's connection off again.
>>>
>>> Correct.  It can't fully prevent the attack, but makes it harder to pull
>>> off.  Just having $limit idle connects isn't enough any more, the bad
>>> guy has to constantly bomb qemu with vnc connect requests, hoping this
>>> kicks out the good guy before it managed to authenticate.  The chances
>>> for the good guy are a bit better and it is also more likely that the
>>> attack sets off alarms in network monitoring.
>>>
>>
>> Hi,
>>
>> Happy that I don't miss this patch series and conversation.
>> I have a approach to prevent the brute force attack (which
>> had been tested in my team). Firstly, we must set password for
>> vnc server for security. Secondly, the DoS may bomb qemu
>> with vnc connect requests, trying to decrypt password at present.
> 
> Note that VNC passwords offer no meaningful level of security.
> 

Encryption algorithms?

> If you want security for VNC you must *always* use the TLS extension
> or the SASL extension, or both. These offer proven cryptographically
> strong authentication protocols.
> 
>> If we set the max trying times, and then
>> There are some concepts:
>>  - INTERVAL_TIME: a time window that user can connect vnc server
>>  - REJECT_TIME: the time of reject any connection
>>  - MAX_TRY_TIMES: the times that user can connect vnc server in 
>> INTERVAL_TIME,
>>    if attach the MAX_TRY_TIMES, the server will lock, any user can not 
>> connect again
>>    before REJECT_TIME attached. The old connected client will not be 
>> influenced.
> 
> How are you defining "user" in this description. Do you mean "Source IP 
> address"
> here ? Or any client connection ? Or something else ?
> 

I mean "any client connection". :)

Best regards,
-Gonglei