FTP Transfer Very Slow When Windows Firewall Enabled

We have a scheduled task that transfers a large file (~130 MB) between two Windows servers using the Windows command-line FTP program. It's painfully slow (taking about 30 minutes) and occasionally terminates before the transfer has completed. The servers are 2003 (sending) and 2008 (receiving).

If I transfer the file manually, using FileZilla's GUI, it runs considerably faster and is done in a couple of minutes. I can't use this approach though, because we want this to run automatically and FileZilla can't be scripted.

I've also noticed, when I disable the Windows 2003 Firewall, command line FTP is suddenly much faster, and reaches the same speed as FileZilla.

So, it seems I have a workaround in disabling/enabling the firewall before and after the transfer. But after Googling extensively, this doesn't seem to be a common problem, and I get the feeling something is configured wrongly somewhere.

Can anyone shed any light on why disabling the firewall improves the speed so dramatically? And why FileZilla is faster than command-line FTP (when the Firewall is enabled)?

Solution 1:

ICMP is an Internet host requirement. Sometimes admins block all ICMP because they "heard somewhere it was a security risk". But ICMP is not optional and TCP relies on it. (At a minimum, 'fragmentation required' must be permitted.)

Solution 2:

1. you need to know what is blocking first. Download Process Monitor from Sysinternals (Microsoft).

2. In that program, select only "show network activities" (a button on the bar on the upper left)

3. check the packets and find the FTP ones.

   server1.domain.com:12345 -> server2.domain.com:21

4. righ click on the name of the process and select include to add the process that is running the ftp.

5. test the ftp tranfer with the firewall and without it.

6. compare both data. it should be a port blocked by the firewall. you should open that port between both PCs.