[c-nsp] TCP MSS CLAMPING issue

Gert Doering gert at greenie.muc.de
Sun Jan 23 12:37:10 EST 2022


On Sun, Jan 23, 2022 at 06:31:40PM +0100, james list wrote:
> thanks for the feedback.
> Firewall vendor reports this:
> " When
> SYN Cookies
>  is activated, the firewall does not honor the TCP options that the server
> sends because it does not know these values at the time that it proxies the
> SYN/ACK. Therefore, values such as the TCP server???s window size and MSS
> values cannot be negotiated during the TCP handshake and the firewall will
> use its own default values. In the scenario where the MSS of the path to
> the server is smaller than the firewall???s default MSS value, the packet
> will need to be fragmented.  "

It does not have to know what the server would send to always put in an
MSS option of its own...  (but of course the vendor would tell you 
"this is not our fault").

> Here we see the client seems not RFC compliant, since in RFC6691 (
> https://datatracker.ietf.org/doc/html/rfc6691#appendix-A) is written:
> "If an MSS option is not received at connection setup, TCP MUST  assume a
> default send MSS of 536 (576-40) [TCP:4]."
> As recap:
> 1) during no attack client send MSS 1460 with DF=1, server respond through
> MSS 1436 (due to GRE), client uses 1436, connection is established
> correctly with TLS exchange
> 2) during attack client send MSS 1460 with DF = 1, server (=firewall in
> this phase due to syn-challenge) respond without MSS, client uses 1460, TLS
> exchange is broken
> From my point of view, since RFC6691 state "MUST use 536", the customer is
> not compliant.

It's "the Internet".  Pointing at clients as being "non compliant" is
not going to fix your server's operation - otherwise, all this fiddling
with TCP/MSS would not even be necessary in the first place.

(Another option would be, of course, to fix your network :-) - so 1500
byte packets can get through, and no need to reduce the client's MSS)

"If was one thing all people took for granted, was conviction that if you 
 feed honest figures into a computer, honest figures come out. Never doubted 
 it myself till I met a computer with a sense of humor."
                             Robert A. Heinlein, The Moon is a Harsh Mistress

Gert Doering - Munich, Germany                             gert at greenie.muc.de
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