Improved TCP syncookie implementation

[Igor Sysoev]

On Thu, 14 Sep 2006, Ruslan Ermilov wrote:

> On Wed, Sep 13, 2006 at 10:31:43PM +0200, Andre Oppermann wrote:
>> Igor Sysoev wrote:
>>> Well, suppose protocol similar to SSH or SMTP:
>>>
>>> 1) the client calls connect(), it sends SYN;
>>> 2) the server receives SYN and sends SYN/ACK with cookie;
>>> 3) the client receives SYN/ACK and sends ACK;
>>> 4) the client returns successfully from connect() and calls read();
>>> 5) the ACK is lost;
>>> 6) the server does not about this connection, so application can not
>>>   accept() it, and it can not send() HELO message.
>>> 7) the client gets ETIMEDOUT from read().
>>>
>>> Where in this sequence client may retransmit its ACK ?
>>
>> Never.  You're correct.  There is no data that would cause a retransmit
>> if the application is waiting for a server prompt first.  I shouldn't
>> write wrong explanations when I'm tired, hungry and in between two tasks. ;)
>>
>> This problem is the reason why we don't switch entirely to syncookies
>> and still keep syncache as is.
>>
> Perhaps it would be a good idea to remove net.inet.tcp.syncookies_only
> then?  In any case, please don't forget to update the syncache(4) manpage
> to reflect your changes, and if you decide not to remove this sysctl,
> please add a warning of its potential to break a protocol.

I think that setting syncookies only not globally, but on per port basis,
say, for HTTP would be helpfull. Setting it for other protocols, e.g, SSH,
rsync, SMTP, IMAP, POP3 may break them.


Igor Sysoev
http://sysoev.ru/en/