openssh unix dev - Mar 2015 - One SSH_FXP_DATA record split over SSH_MSG_CHANNEL_DATA packets

Hi,

I'm developing my own sftp client.
When I try to read a file larger than 16371 bytes from a OpenSSH server my
program fails.
And that is because it doesn't expect a SSH_FXP_DATA record to be send
through two SSH_MSG_CHANNEL_DATA packets.

For instance:
  First a SSH_MSG_CHANNEL_DATA packet containing the start of a SSH_FXP_DATA
record
    read_packet: done 16393
    read_packet: done
5E0000007B0000400000003FFF67000000FC00003FF60101010101010101....

  And then a SSH_MSG_CHANNEL_DATA packet containing the last part the
SSH_FXP_DATA record
    read_packet: done 12
    read_packet: done 5E0000007B00000003AA1133

Is this normal, to be expected, behavior? I can't find anything regarding
this in the RFC's.

I see this on OpenSSH_6.4 and OpenSSH_4.3 using these settings
aes128-cbc
aes128-cbc
hmac-sha1
hmac-sha1
none
none

Anton

On Thu, 19 Mar 2015, Anton Scheffer wrote:
> Hi,
> 
> I'm developing my own sftp client.
>
> When I try to read a file larger than 16371 bytes from a OpenSSH
> server my program fails. And that is because it doesn't expect a
> SSH_FXP_DATA record to be send through two SSH_MSG_CHANNEL_DATA
> packets.
...
> Is this normal, to be expected, behavior? I can't find anything
> regarding this in the RFC's.
Yes, the filexfer runs atop the channel protocol and is completely
ignorant of the latter's framing of its messages. In OpenSSH, they
are totally separate programs.

-d