Although TCP/IP checks that all packets are received, malformed packets or other mishaps can occur, leading the FTP client to believe that a transfer was successful when it was not.
The Server's file integrity command is defined as XCRC. Once an XCRC enabled client performs a transfer, it can request the server to do a checksum calculation on the file. If it matches the checksum on the client, then the transfer is deemed successful. Performing XCRC checksum calculations is processor intensive; enable or disable the feature accordingly.
XCRC is a proprietary command and is not defined nor endorsed by any FTP-related RFC. Competing servers who want to implement this command may do so using the syntax described below.
XCRC <File Name>
XCRC <File Name>, <EP>
XCRC <File Name>, <SP>, <EP>
SP = Starting Point in bytes (from where to start CRC calculating)
EP = Ending Point in bytes (where to stop CRC calculating)
FTP Client Log Example
COMMAND:> XCRC "/Program Files/MSN Gaming Zone/Windows/chkrzm.exe" 0 42575
SP and EP are optional parameters. If not specified then it calculates the CRC for the whole file. If only EP is specified, then the CRC calculation starts from the beginning of the file to the EP.
This command can be used for a single file at a time. It does not allow file lists as parameters.
The standard CRC32 algorithm is used (for speed and efficiency).
A client can invoke this command for uploads, downloads, and single and Multi-Part Transfers.
|
Server Reply |
Indicates |
|
250 <XCRC> |
calculated CRC value |
|
450 Requested file action not taken |
file is busy |
|
550 Requested action not taken |
file is not found or has no read permission; or the SP or EP are not correct |
To enable file integrity (XCRC) checking
In the Administrator, connect to the server, then click the Server tab.
In the left pane, click the user or User Setting Level that you want to configure.
In the right pane, click the Security tab.
Select the Allow XCRC command check box to enable XCRC file integrity checking.
Click Apply.