503 error ftp

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.

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I have an FTP server running on an AWS virtual server. We have about 100 users connecting to it over the course of a day, uploading images and other files. All but one are working perfectly. Files come in, not a problem.

We have 1 single user that causes the following log lines to be generated — the username and IP has been removed intentionally and the «***» has been added to highlight the error line:

> 227 Entering Passive Mode (54,79,122,6,195,96)
> STOR media/UV1358A_3.jpg
> 150 Opening data channel for file upload to server of "/media/UV1358A_3.jpg"
> 226 Successfully transferred "/media/UV1358A_3.jpg"
> PASV
> 227 Entering Passive Mode (54,79,122,6,195,141)
> STOR media/UV1358A_4.jpg
> 150 Opening data channel for file upload to server of "/media/UV1358A_4.jpg"
> PASV
> 227 Entering Passive Mode (54,79,122,6,195,136)
> 226 Successfully transferred ""
> STOR media/UV1358A_5.jpg
***********************************************
> 503 Bad sequence of commands.
***********************************************
> PASV
> 227 Entering Passive Mode (54,79,122,6,195,80)
> PORT 122,99,115,5,212,227
> 200 Port command successful
> PORT 122,99,115,5,226,227
> 200 Port command successful
> PORT 122,99,115,5,130,124
> 200 Port command successful
> STOR media/UV1358A_9.jpg
> 150 Opening data channel for file upload to server of "/media/UV1358A_9.jpg"
> PORT 122,99,115,5,152,62
> 200 Port command successful
> STOR media/UV1358A_10.jpg
> 150 Opening data channel for file upload to server of "/media/UV1358A_10.jpg"
> PORT 122,99,115,5,161,49
> 200 Port command successful

We’re using FileZilla Server 0.9.55 on a Windows 2012 box.

My question, as stated in the title is essentially.. Is this our issue on the server end, or is this theirs?
Is this 503 error always caused by the FTP client screwing something up or is there the possibility that the FTP server is interpreting something wrongly?

I’m happy to go back to the customer and say «It is our issue», but I suspect it’s not at our end.

Thanks

Moderator: Project members

I installed FTP Server Services on my IIS 8.5/Windows 7 workstation, and tried to FTP using FileZilla from my laptop (client — Windows 8). I tried to configure the Network Settings using Network Wizard on FileZilla, and here is the output. [I have masked the IP with ‘x’ on the output for privacy]

Connecting to probe.filezilla-project.org
Response: 220 FZ router and firewall tester ready
USER FileZilla
Response: 331 Give any password.
PASS 3.9.0.5
Response: 230 logged on.
Checking for correct external IP address
IP xxx.xxx.xxx.xxx bdj-bac-bgd-ic
Response: 200 OK
PREP 50307
Response: 200 Using port 50307, data token 860523756
PORT 139,102,163,82,196,131
Response: 200 PORT command successful
LIST
Response: 150 opening data connection
Response: 503 Failure of data connection.
Server sent unexpected reply.
Connection closed

I also tried to FTP from the laptop using Windows PowerShell, I was able to connect but when I try to get file, I get the following error.

ftp> get file.jpg
200 PORT command successful.
125 Data connection already open; Transfer starting.
> .:No CSI structure available
550 Access is denied.
ftp: 131072 bytes received in 0.15Seconds 891.65Kbytes/sec.

Then I started an SSH session using PuTTY, and tried FTP to the same server & everything worked fine.

I want FTP to work from FileZilla on my laptop. What configuration setting may be missing?