FTP issue and how to resolve it

Jonathan 400 Reputation points
2026-10-06T07:46:43.29+00:00

Hi,

I got the issue like

200 Type set to I.Command: PORT 10,0,2,15,195,12 Response: 501 Server cannot accept argument.Error: Failed to retrieve directory listing

on FTP Any advice? Must I use passive for FTP?

Windows for business | Windows Server | Networking | Network connectivity and file sharing
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  1. AI answer

    2026-10-06T07:46:58.4733333+00:00

    Yes—passive FTP is the recommended mode in most network and Internet scenarios.

    The log shows the client is sending a PORT command, which means it is using active FTP. In active mode, the server must connect back to the client on the IP and port provided in that command. That commonly fails behind NAT or firewalls, and active mode is no longer recommended for Internet scenarios.

    What the error indicates:

    • PORT 10,0,2,15,195,12 contains a private client IP address (10.0.2.15).
    • A server on the Internet generally cannot connect back to that private address.
    • In IIS FTP, 501 means syntax error in parameters or arguments.
    • A failed directory listing is consistent with a data connection problem, not a login problem.

    What to do:

    1. Switch the FTP client to passive mode.
      • Passive mode lets the client open both the control and data connections.
      • This works better through NAT and firewalls.
    2. Make sure the FTP server allows passive data ports through the firewall.
      • FTP needs port 21 for the control channel.
      • Data transfers and directory listings need a secondary data channel.
      • If passive mode is used, the server must have its passive port range opened through all firewall layers.
    3. If using IIS FTP, configure a fixed passive port range and allow it consistently.
      • A Microsoft Q&A resolution for similar “login works but directory listing times out” behavior recommends defining a passive range such as 49152-65535 in IIS and allowing port 21 plus that passive range in the firewall layers.
    4. Check FTP server logs for the exact data-channel failure.
      • On Windows FTP servers, logs can help confirm whether the data connection is being rejected or blocked.

    Why passive mode is preferred:

    • Active mode requires the server to initiate the data connection back to the client.
    • Passive mode requires the client to initiate both connections.
    • Microsoft documentation states active connections are no longer recommended because they do not work well in Internet scenarios.

    References:

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  2. Tracy Le 13,210 Reputation points Independent Advisor
    2026-10-06T15:17:37.0533333+00:00

    Hello Jonathan,

    Based on the error shown, I would recommend trying Passive Mode first. The FTP client is sending a PORT command that contains the private address 10.0.2.15, and the server is rejecting it with "501 Server cannot accept argument". This typically happens when the client is behind NAT and the server cannot establish the return data connection required by Active Mode.

    If Passive Mode works, the issue is likely related to the network translation between the client and the FTP server. If you must use Active Mode, verify that the client has a directly reachable address or consider using a bridged network configuration instead of NAT.

    If the FTP server is IIS FTP, also confirm that the passive port range and any firewall rules are configured correctly. If this helps, please accept the answer.

    Tracy Le.

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