FTP Server Problem, clients getting disconnected

Hello,

I'm running a FTP server on my Windows Server 2008 R2, using Filezilla Server to manage accounts.  I've had zero problems using it as a client, and most of our clients have no issues either.  But it seems like our more "corporate" clients tend to have connectivity problems, where they can't access it at all or can start downloads that get interrupted.  I'm guessing this is because of the IT infrastructure of the offices that they are working in, and some sort of firewall issues.

I'm a developer not a sys admin, so I'm at a loss here.  I don't know what to tell this client who insists it's been very problematic.  Do you have any advise as to what some general issues may be.  Thanks,

Justin
JT_SIROAsked:
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cavp76Connect With a Mentor Commented:
First: make sure their clients are configured to use passive mode. Second: if possible, that they open outbound port 20 in their firewall.
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Wayne MichaelSenior Software DeveloperCommented:
we switched to a web based product from IPSWITCH for the same reasons.

you just have to get their IT staff involved to help track down what part of the network is blocking it.
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AlexPaceConnect With a Mentor Commented:
Your server software may allow you to specify a range of ports to be used for passive mode file transfers.  Choose a range (# > 1024) that contains at least as many ports as the maximum number of simultaneous client connections then make the range 50% bigger to support future growth.

Now when you are telling people how to connect to your server make sure to tell  them your passive data channel range.  This will allow them to poke holes in their firewall that matches your range.  Their IT staff can get all difficult if you don't restrain the port range because having a lot of open ports keeps them up at night.
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Aaron TomoskyConnect With a Mentor SD-WAN SimplifiedCommented:
I also use filezilla for the same stuff. I setup both active and passive connections to allow for anyone to connect. The only people that have issues now are ones that try to use the FTP built into their browsers. Is this what you are seeing?
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JT_SIROAuthor Commented:
Thanks for the all the advise so far.  I don't really understand the whole passive ports thing, but I think I should be fine.  I checked my settings, they're set to "Use custom port range:" 3850 - 3859.  And I never have more than 2-3 clients on simultaneously.

Another thing to mention is that this server is within my companies network and firewall.  Their admin gave me port 2121 to access it from an FTP client.  So I require all clients to enter port 2121 when they log in.  Does this restrict what I can do?  Does that mean that only one client can access it?  
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AlexPaceCommented:
Lots of clients access a web server on port 80.  It sounds like you are doing the right things but at some point if you want to help the clients you'll need them to provide you with logs that show protocol-level tracing so you can see what they were actually trying to do when they disconnected.  You could look in your logs too but unless they only connect once per day you'll need to know the exact time of the connection and failure or risk wasting a bunch of time looking for a needle in a haystack.
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JT_SIROAuthor Commented:
Another thing I just noticed is that "Block Server to Server Transfers" WAS checked.  I unchecked it.

I'm guessing that could be why some of my more corporate clients have experienced problems from their offices, but were fine at home.  Thoughts?
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Aaron TomoskyConnect With a Mentor SD-WAN SimplifiedCommented:
Widen your port range. By default filezilla clients and other clients use up to 10 connections at a time. This will require a change in your router as well. I use a 100 wide range personally
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