PuTTY wish dll-frontend

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A lot of people ask for a DLL that implements an SSH client, so that they can include it in their own programs and automatically get securely in touch with other systems and issue commands.

I'm not entirely sure when (or whether) we'll have the energy to get round to this, but I've thought about the design aspects a fair bit, and this is what I've come up with.

When I hear "DLL for PuTTY" I tend to think of something very much like Plink, but using a very simple sort of function-call interface rather than reading from stdin and writing to stdout. Something like this:

  SSH_Session mysess;
  SSH_Channel mychan;
  
  mysess = puttydll_open("myhost.example.com", 22);
  mychan = puttydll_start_command(mysess, "cvs server");
  puttydll_send_data(mychan, cvs_auth_data, len);
  len = puttydll_read_data(mychan, buffer, sizeof(buffer));

... and so on. So you'd end up with `SSH_Channel' handles which worked a little like file handles - you could do a blocking read on a particular one of them, for example, or wait until one of a selection of them became ready. (If you wanted to wait for data from either an SSH channel or a Windows-side object, interfacing between this sort of select() and the Windows equivalents might be fun, but I'm sure it could be worked out. Perhaps an API function which would hand you a Windows event object that would get set when one of the channels was active.)

The biggest immediate problems I see with it are:

• PuTTY needs its own event loop, so that it can respond to server- side requests (such as repeat key exchange and server-side pings) in the SSH connection it's managing. This might not be easy to integrate with whatever kind of event loop the calling program is using - if indeed it's using an event loop at all; the kind of code I quote above would probably much rather stay sequential than become event-driven with explicit state. Hence I think the simplest way to arrange this is to have the DLL spawn a thread to manage PuTTY's own network event loop, which can then run without interfering with what the rest of the program is doing. That way, the calling program could call the send-data function, go off and do its own thing for a while, and then come back and see if data had come back for it to read yet.
• (On the other hand, if the caller did want to integrate PuTTY into their own event loop so that they received immediate notification when data came in, that would be another thing we'd want to allow them to do. Perhaps allowing them to provide an event object which we could set when particular things happened might be a useful approach.)
• The DLL interface would need careful design. In particular, what should happen about interactive prompts - passwords, passphrases, host key verification, and so on? Should all of those things be fatal errors (limiting the DLL to being used in a context where unattended login is possible), or what? I think it would be a mistake to have this sort of prompt go directly to the user; it would probably be better to notify the calling program in some way, and leave it to decide how to handle it. (Although perhaps providing helper routines that gave standard dialog boxes for these prompts might also be an interesting option.)

However, none of those issues is insurmountable, and once they're taken care of I don't see why a DLL providing an API like the one above shouldn't be perfectly possible to write just by implementing a single front-end module to replace (say) plink.c.

There's a third-party DLL encapsulation, W-PuTTY-CD, on our Links page.

Audit trail for this wish.