Re: Experimental Privacy Functions and TCP SYN Payloads

[Yuchung Cheng]

On Wed, Feb 12, 2014 at 3:35 AM, Daniel Borkmann <borkmann@xxxxxxxxxxxxx> wrote:
> (please cc netdev)
>
> On 02/12/2014 11:25 AM, Quinn Wood wrote:
>>
>> If program on host A spoofs the source address of an outgoing IPv4 packet
>> then
>> places that address in the first 32 bits of a UDP payload, a program on
>> host B
>> that is aware of these behaviors can still reply to the program on host A.
>> [1]
>>
>> Continuing with this approach the program on host A could encrypt the UDP
>> pay-
>> load in a way that the program on host B can decrypt, and effectively
>> reduce
>> the ability of others in the wide network to passively determine who host
>> A is
>> sending transmissions to while simultaneously ensuring the program on host
>> B
>> can respond to the program on host A. [2]
>>
>> I'm uncertain how to proceed if I want to use TCP for stateful
>> connections.
>> The requirement of a handshake before data is handed off to the program
>> means
>> this approach won't work out of the box. I'm looking for any insight folks
>> may
>> have regarding this.
>>
>> My original approach to the handshake included setting one of the reserved
>> bits in the TCP header to indicate the first 32 bits of the payload were
>> the
>> real source address. However this would be reliant on SYN packets
>> containing
>> a payload. Does the Linux kernel allow this?
For 3.7+ you can use TCP Fast Open.

For a quick trial experiment, you can just set
sysctl net.ipv4.tcp_fastopen=0x603 on both end hosts and use
sendmsg(..., MSG_FASTOPEN) instead of connect() then send(). the
sendmsg() will behave as a combo call of connect() and send() and
return similar errno. accept() will return after data in the SYN is
received instead of after handshake is completed.

>>
>> -
>>
>> [1] Barring any non store-and-forward network behavior like dropping
>> packets
>>      with questionable source addresses. Considering recent NTP-related
>> news
>>      this seems to be a not-entirely common activity :)
>> [2] This is of course reliant on both programs knowing the proper key for
>> the
>>      other.
>
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