Errors can be frustrating, especially if they are cryptic, like this:

Response: 150 opening data connection
Response: 503 Failure of data connection.

You know that the connection has failed (obviously!), but where?

Here at Bobcares.com, we help Web hosts, VPS users and website users troubleshoot such errors as part of our Outsourced Hosting Support services.

503 failure of data connection is a common error seen in FileZilla FTP client.

What is “503 failure of data connection”?

This error says that the FTP client was unable to contact the server’s FTP port, which means that the connection was blocked somewhere along the way.

It could be in the user’s PC, the modem, within the office network, ISP, or the server.

Here are the top 5 causes we’ve seen for this error, and how we fix them.

1. FTP client set to Active mode

FTP transfers can happen in Active or Passive mode.

To use Active FTP, users must configure their PCs to allow incoming connections FROM the server to a non-standard port (Port 21 is the standard FTP port. Port 30231 is a non-standard one).

Such an incoming connection closely resembles a cyber attack, and most firewalls block such non-standard connections.

We’ve seen cases where FTP users accidentally set their FTP mode to Active, causing the connection to fail.

To solve this, we help the users navigate the settings and choose Passive mode. We recommend this to be set by default for all FTP accounts.

2. Unsupported FTP features (eg. SFTP, SSL, etc.)

Some hosts offer SFTP or FTP over SSH. It uses port 22 instead of the traditional FTP port 21.

Similarly, some hosts offer FTP over SSL that uses port 990.

We’ve seen users mixing up these two, or even trying to establish a SSL/TLS connection where no such facility exists.

When users attempt to connect to an unsupported port, the server firewall will block that connection, and it’ll result in the error 503 failure of data connection.

To fix this we recommend customers to use the basic FTP protocol. To set it in FileZilla, go to  File > Site Manager > Encryption > Only use plain FTP(insecure).

3. Blocked by firewall or antivirus software

The FileZilla error 503 failure of data connection goes on to state:

“This means that some router and/or firewall is still interfering with Filezilla.”

So, many users try to turn off their PC firewall, but still face issues.

We’ve seen that this is sometimes caused by Antivirus and Antispyware tools that acts as a second layer of firewall.

We resolve this by asking the users to turn of all security applications one by one. This helps us figure out exactly which app is causing the error, and fix its settings.

4. Blocked by intermediate routers or gateway devices

FTP users are sometimes unaware of firewalls that exist outside their PC.

This is especially true in large offices where people sit behind a local network (LAN).

We’ve seen routers, gateways or proxies that block outbound connections through port 22 (SFTP) or 990 (FTP over TLS).

To resolve this, we ask customers to turn off their gateways or routers, and establish a direct connection to the net. This helps us isolate where the block is happening.

Once we’ve confirmed that the issue is indeed with the intermediate devices, the users can work with their IT team to fix the device.

5. Port range not open at the server

The proper functioning of FTP requires that the server accept connections in all standard ports (eg. 21, 20, 22 and 990).

In addition, it also should accept connections on passive FTP ports, which varies from one server to another.

The trouble is, server firewalls allow only connections to the standard ports by default. If it’s not specifically set to allow connections to Passive FTP ports, the connections will fail.

We’ve seen newly setup VPS servers blocking Passive FTP ports.

Similarly, we’ve seen FTP ports of newly setup FTP servers in Windows IIS blocked as well in firewall.

To fix this, we add firewall exclusion rules:

• Linux IPtables – Add the rule iptables -A INPUT -p tcp --match multiport --dports XXXXX:ZZZZZ -j ACCEPT where XXXXX is the starting port, and ZZZZZ is the ending port in the Passive port range.
• Windows IIS –  Go to Control Panel > System and Security > Windows Firewall > Advanced Settings > Inbound Rules > FTP server rules > Actions > Enable Rule.

Conclusion

503 failure of data connection is a common FTP error caused when the FTP connection to the server is blocked by some device or software between the PC and the server. Today we’ve seen the top 5 causes for this error, and how we fix them.