Modifying Wininet,dll to implement Pause & Resume at FTP Protocol leve

Programmatically starting download pausing and then restarting again

Let say for example you are downloading a file: MyHugeFooFile.zip.

1. Typically send a RETRIEVE (RETR) command for file MyHugeFooFile.zip to the FTP server to start download. Store data received from server to a temp file for example Temp_MyHugeFooFile.zip. So any time download is aborted, size of temp file is the actual useful data transferred so far of MyHugeFooFile.zip.
2. Let say all of a sudden ISP connection is broken or FTP server is down or your FTP client crashed or you manually stopped the download.
3. Now Next time when you want to restart download, read the size of temp file (i.e. useful data downloaded so far), and seek to the end of the temp file so further download will append the data from end.
4. Now send a REST command to FTP server and pass the file size of temp file as parameter. (a typical syntax would be REST 250 if size of temp file was 250). FTP server will send a response stating something like "Restarting from byte 250..".
5. Now you can send RETRIEVE (RETR) again for file MyHugeFooFile.zip and FTP server will start sending data from byte offset 251 onwards. 
6. Now it is your responsibility to append the data coming from server to the incomplete file Temp_MyHugeFooFile.zip and you are all set.
7. Once the transfer is complete rename the temp file to the actual file name on WINDOWS using C++ it should typically look like RenameFile(Temp_MyHugeFooFile.zip, MyHugeFooFile.zip); Done.

Programmatically determining if a particular FTP server supports Pause & Resume

Three simple steps

1. Once connected to FTP server try sending a REST command with 0 ( zero ) as argument.
2. If commands fail or server return something like command is not supported. That means this server doesn't support Pause and Resume.
3. If FTP server return a response stating something like "Restarting from byte 0" that means server supports Pause & Resume

Now if you just started writing code you may have question, how to send these commands the FTP  server. Answer is so many ways. Raw socket is one, WinInet is another. In my sample code I have utilized WinInet's API FtpCommand to send direct commands to FTP server.

How Wininet API work

BOOL WINAPI FtpOpenFile(
HINTERNET hConnect,
LPCTSTR lpszFileName,
DWORD dwAccess,
DWORD dwFlags,
DWORD dwContext);

Three simple steps

1,Type to I PASV
2,SIZE filename
3, some nouse action

If we change SIZE filename command to REST pos ,it would resume from pos.

so I modify the function like this:

BOOL WINAPI FtpOpenFile(
HINTERNET hConnect,
LPCTSTR lpszFileName,
DWORD dwAccess,
DWORD dwFlags,
LPCTSTR *plpszSeekPos );

the argument  plpszSeekPos  is used as follow:

CString sSeekPos;
SSeekPos.Format("%ld",dwSeekPos);
lpSeekPose=sSeekPos.LockBuffer();
plpszSeekPos=&lpSeekPose;

Modify Wininet.dll

Debug into FTPOpenFile function ,F11 F11 ....

then you would find three function entry to format string:

761BB256   and         ebx,80000000h
761BB25C   mov         eax,761DE8F0h
………………………………………………………
761BB261   jne         761BB268
761BB263   mov         eax,761DE8E8h
761BB276   call        761BC0A2 
                          format("RETR %s");

761BC213   push        edi
761BC214   mov         edi,dword ptr [ebp+70h]
761BC217   push        eax
761BC218   push        761DE9ECh
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
761BC21D   push        edi
761BC21E   push        0
761BC225   call        761BC1C3
                format("TYPE %s");

761BC7F9   test        eax,eax  //没有用
761BC7FB   jne         761BC89D //没有用
761BC801   push        dword ptr [ebp+78h]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lpstrFileAtServer
761BC804   push        761DE93Ch
761BC809   push        dword ptr [ebp+58h]
761BC80C   call        761BC003
                format("SIZE %s",lpstrFileAtServer)

Replace string resource first. change SIZE %s to REST %s.

At address 761BC7F9:

761BC7F9  85 C0 0F 85 9C 00 00  吚.厹..
761BC800  00 FF 75 78

761BC7F9   test        eax,eax 
761BC7FB   jne         761BC89D
761BC801   push        dword ptr [ebp+78h]

Modify the 11 bytes to:

761BC7F9  90 8B C5 05 50 01 00  悑..P..
761BC800  00 50 90 90

      nop;
      mov eax, ebp    ;
      add eax, 0x150  ;
      push eax ;
      nop;
      nop;

that's DONE!.

Usage

        char lpPos[64];//note: lpPos must in stack not in heap
        memset(lpPos,0,64);
        sprintf(lpPos,"%ld",dOffsetToSeek);

        m_hOpenFile = ::FtpOpenFile(m_hOpenUrlHandle,sFileNameAtServer
, GENERIC_R
EAD
        , FTP_TRANSFER_TYPE_BINARY, (DWORD)&lpPos);

Note

You should put this modified dll in current directory.