Re: [PATCH] fix small DoS on connect() (was Re: BUG: Unusual TCP Connect() results.)

On Sun, Jun 12, 2005 at 11:13:23PM +1000, Herbert Xu wrote:

On Sun, Jun 12, 2005 at 02:32:53PM +0200, Willy Tarreau wrote:

quoted

but it's not the case (although the naming is not clear). So if the remote
end was the one which sent the SYN-ACK, it will clear its session. If it has
been spoofed, it will ignore the RST because in turn, the SEQ will not be
within its window.

This is what should happen:

1) client A sends SYN to server B.
2) attcker C sends spoofed SYN-ACK to client A purporting to be server B.
3) client A sends RST to server B.

Agreed till here.

The RST packet is sent by client A using its sequence numbers.  Therefore
it will pass the sequence number check on server B.

4) server B resets the connection.

No, precisely the RST sent by A will take its SEQ from C's ACK number.
This is why B will *not* reset the connection (again, tested) if C's ACK
was not within B's window.

Cheers,
Willy