[gPXE] Fwd: interrupt and serial

[Stefan Hajnoczi]

On Tue, Feb 9, 2010 at 2:22 PM, Luca <lucarx76 at gmail.com> wrote:
>> Let's discuss your setup and use case more.  Perhaps there is an
>> existing solution or we can come up with a generic solution that can
>> be merged into mainline gPXE.
>>
> I have an environment with several workstations, which I want to gPXE boot.
> I have an environment with one DHCP server.
> When gPXE starts, it gets from the DHCP server the path of the image it
> should download.
>
> I want gPXE to be sure it's talking to the right server. So I modified a
> little bit the gPXE TLS code. I assume gPXE has the certificate of the HTTPS
> server it's going to talk to. During the TLS handshake, when the server
> sends its certificate, I have gPXE validating it. If the certificate sent by
> the server is the one gPXE has then gPXE knows it is talking to right
> server.
>
> I have different HTTPS servers, and I want to be free to add a new one
> whenever I need. So I do not want to have this certificate compiled into
> gPXE.  That's why when gPXE starts I want gPXE to talk to a serial server
> and get the certificate of the HTTPS is going to talk to.

Other users have expressed interest in certificate validation and I
think this is a general problem.

I know little about how TLS works, but would it be possible to solve
this by using a certificate authority instead of comparing against
specific certificates?  When a client connects to a server, it will
check that the server's certificate has been signed by the certificate
authority?  That way you can support as many servers as you like, but
they must be signed by the certificate authority (you).

>> What are the constraints of your setup?  Do you have a DHCP server?
>> Can you set the boot filename handed out by the DHCP server?
>>
>
> The communication between the serial server and gPXE is secure, while the
> communication between gPXE and the DHCP is over an open channel. That's why
> I do not want to pass this information to gPXE as a DHCP option.

This is an interesting scenario: you do not trust the LAN.  What if
the DHCP server hands out a http, tftp, etc URL instead of a HTTPS URL
with your code in place?  I guess you have explicitly disabled all
other protocols, including plain HTTP.

I am interested in your thoughts about secure network booting.  At the
moment there are no "best practices" on the wiki.

>> When gPXE starts it could send a 'hello' message over the serial port
>> and wait for a response.  The server would respond with the
>> configuration.  I don't think there is a technical limitation
>> preventing one from implementing this config-over-serial approach.
>>
>
> That's what I'm doing right now. The problem I have is that as I use
> polling, sometimes I looses characters sent by the server (I do very few
> operations when I get a character, but still sometimes I miss some
> characters). This would not happen if I used interrupts.

Without seeing the code I can't be sure what is going on.  However,
adding interrupt handlers to the mix is not going to make the code go
any faster.  If you currently have a tight receive loop and it is
missing characters, then something is wrong.  Is the baud rate set
correctly on the serial port?

Stefan