VbAsyncSocket

Simple and thin WinSock API wrappers for VB6 loosly based on the original CAsyncSocket wrapper in MFC.

Description

Base class cAsyncSocket wraps OS non-blocking sockets that can be used to implement various network components in VB6 -- clients and servers -- and supports both async and blocking network communications.

Additionally in contrib folder there are helper classes that build-up on cAsyncSocket base to provide familiar interfaces for completing common tasks.

For instance class cWinSockRequest exposes same-named methods, properties and events as the OS provided WinHttpRequest object but can be used to easily access generic TCP/UDP servers instead.

Then class cTlsClient provides streaming support (familiar Read/Write methods) for accessing both plain-text TCP servers (e.g. http) and SSL/TLS secured ones (e.g. https) using OS provided SSPI/Schannel implementation w/o any additional dependency (e.g. OpenSSL library).

For simple demo TCP/UDP client/server implementation take a look at test\Chat folder. Same client and server sides are capable of communicating over reliable TCP channels and fast UDP datagrams.

For sample clients to various network services (https, SMTP over SSL, etc.) open sample project in test\Basic folder.

Usage

Start by including src\cAsyncSocket.cls with companion src\cAsyncSocketHelperWindow.cls in your project to have a convenient wrapper to most WinSock API functions. Note that cAsyncSocketHelperWindow uses self-subclassing implementation that can be unstable to the End button/statement in VBIDE.

Optionally you can add contrib\cTlsClient.cls to your project for SSL/TLS secured channels and/or contrib\cWinSockRequest.cls for familiar class methods when accessing TCP/UDP network services.

Sample SMTP with STARTTLS

A working sample with error checking omitted for brevity for accessing smtp.gmail.com over port 587.

At first the communication goes over unencrypted plain-text socket, then later its switched to TLS secured one before issuing last QUIT command.

With New cTlsClient
    .SetTimeouts 0, 5000, 5000, 5000
    .Connect "smtp.gmail.com", 587
    Debug.Print .ReadText();
    .WriteText "HELO 127.0.0.1" & vbCrLf
    Debug.Print .ReadText();
    .WriteText "STARTTLS" & vbCrLf
    Debug.Print .ReadText();
    .StartTls "smtp.gmail.com"
    Debug.Print "TLS handshake complete: " & .TlsHostAddress
    .WriteText "QUIT" & vbCrLf
    Debug.Print .ReadText();
End With

Produces debug output in Immediate Window similar to this:

220 smtp.gmail.com ESMTP l8sm10315047wmf.39 - gsmtp
250 smtp.gmail.com at your service
220 2.0.0 Ready to start TLS
TLS handshake complete: smtp.gmail.com
221 2.0.0 closing connection l8sm10315047wmf.39 - gsmtp

ToDo

• Allow client to perform TLS server certificate check
• Allow client to assign client certificate for connection
• Provide UI for end-user to choose suitable certificates from Personal certificate store
• Add wrappers for http and ftp protocols
• Add WinSock control replacement
• Add more samples (incl. vbcurl.exe utility)
• Refactor subclassing thunk to use msg queue not to re-enter IDE in debug mode