FTP Error 501: Syntax Error in Parameters
FTP reply code 501 means the server understood the command verb but the parameters that followed are syntactically invalid. The command itself is fine — the arguments are wrong. Common triggers include invalid file paths (containing illegal characters), malformed PORT command parameters (wrong IP/port format), incorrect TYPE parameters (something other than A, I, E, or L), or invalid arguments to SITE commands. The client should fix the parameters and resend the command.
Status
501 Syntax Error in ParametersExample
220 ftp.example.com FTP server ready
USER alice
331 Password required
PASS ****
230 User alice logged in
TYPE Z
501 Syntax error in parameters or argumentsHow to Troubleshoot
Check the command parameters
Review the exact parameters being sent. For file paths, use forward slashes and avoid special characters. For PORT, ensure the format is correct: PORT h1,h2,h3,h4,p1,p2.
Quote filenames with spaces
If the file or directory name contains spaces, try quoting the path or escaping the spaces. Different servers handle quoting differently.
Test with a simple path
Try the command with a simple filename (no spaces, no special characters) to confirm the command works. Then gradually add complexity to identify which character causes the error.
Common Causes
Invalid file or directory path
highThe path argument contains characters the server does not accept, or uses the wrong path separator (backslash vs forward slash). Some servers reject paths with spaces unless quoted.
Malformed PORT command
mediumThe PORT command requires six comma-separated numbers (h1,h2,h3,h4,p1,p2) representing the IP and port. If the format is wrong or the values are out of range (0-255 for each octet), the server returns 501.
Invalid TYPE parameter
lowThe TYPE command only accepts A (ASCII), I (Image/Binary), E (EBCDIC), or L (Local byte size). Sending TYPE with any other parameter results in 501.
Related Error Codes
Syntax Error, Command Unrecognized
The server did not recognize the FTP command. It may be misspelled, malformed, or unsupported.
Requested Action Not Taken, File Unavailable
The file or directory operation failed. The path does not exist, or access is denied.
Bad Sequence of Commands
The FTP commands were sent in the wrong order. The server requires a specific command sequence.
References
| Specification | Section |
|---|---|
| RFC 959 | RFC 959 §4.2 |
Frequently Asked Questions
This reference was compiled from official RFCs, protocol specifications, and hands-on troubleshooting experience. AI tools were used primarily for formatting and organizing the content on the page.