Q

Clients getting 'connection reset' and authentication failures during file transfers

I'm about at my wits end with this problem, and I suspect it's something simple, but I need another pair of eyes to look at it. Clients connecting to our FTP server can download files just fine. However, when they upload files, they get random "connection reset" and authentication failures during multiple-file transfers.

Here's an example, edited for security:

22:49:14 123.456.78.90 0207 [295]USER 0207 - 331 0 0 0 0 
22:49:14 123.456.78.90 0207 [295]PASS - - 230 0 0 0 0 
22:49:22 123.456.78.90 0207 [295]DELE E011.dwg - 250 0 0 0 0 
22:49:22 123.456.78.90 0207 [295]DELE E012.dwg - 250 0 0 0 0 
22:49:22 123.456.78.90 0207 [295]DELE E101.dwg - 250 0 0 0 16 
22:49:22 123.456.78.90 0207 [295]DELE E102.dwg - 250 0 0 0 0 
22:49:22 123.456.78.90 0207 [295]DELE E103.dwg - 250 0 0 0 0 
22:49:53 123.456.78.90 0207 [295]created Copy+of+E011.dwg - 226 0 0 790403 18156 
22:50:10 123.456.78.90 0207 [295]created Copy+of+E012.dwg - 226 0 0 636455 16891 
22:50:46 123.456.78.90 0207 [295]created Copy+of+E101.dwg - 226 0 0 1549612 35313 
22:51:26 123.456.78.90 0207 [295]created Copy+of+E102.dwg - 226 0 0 1545573 39812 
22:51:59 123.456.78.90 0207 [295]created Copy+of+E103.dwg - 226 0 0 1393001 33062 
22:52:25 123.456.78.90 0207 [295]created Copy+of+E181.dwg - 226 0 0 908052 24922 
22:52:45 123.456.78.90 0207 [295]created E011.dwg - 226 0 0 790403 19672 
22:53:01 123.456.78.90 0207 [295]created E012.dwg - 226 0 0 636455 16015 
22:53:38 123.456.78.90 0207 [295]created E101.dwg - 226 0 0 1549612 36704 
22:54:19 123.456.78.90 0207 [295]created E102.dwg - 226 0 0 1545573 39531 
22:54:58 123.456.78.90 0207 [295]created E103.dwg - 226 0 0 1393001 38969 
22:55:58 123.456.78.90 0207 [296]USER 0207 - 331 0 0 0 0 
22:55:58 123.456.78.90 0207 [296]PASS - - 230 0 0 0 0 
22:55:58 123.456.78.90 0207 [296]DELE E181.dwg - 550 32 0 0 0 
22:58:11 123.456.78.90 0207 [295]created E181.dwg - 426 10038 0 598016 192781 

Notice that the client attempts to log in again (from 295 to 296) after the 2nd to last file is completely downloaded. In this case, the client is IE 6.1, which (to my knowledge) doesn't multi-thread. For whatever reason, that login fails, which in time botches the file transfer (426 being the connection reset error code, I believe).

I've done quite a bit to troubleshoot this, but I can't really find the solution. I thought since the ISA services are running on the same box, there might be port blocking with PASV, but there's no blocking reported in the ISA logs. Packet sniffing shows that right about the time of the 426 error, there's some invalid checksums on the packets, so I did send something to my ISP, but I honestly don't think it's related. I'm just following up on every lead I can. Thanks for your help.

That's quite a puzzle, and there's nothing obviously wrong. I was going to recommend taking a sniffer to it and seeing if you saw some non-standard communication so that you could identify whether it was the client or server, but it sounds like you've already done that. By the way, invalid checksums on the packets shouldn't stop the session. FTP uses TCP, and TCP will automatically retransmit any packets with bad checksums.

Most likely, the problem is caused by incompatibilities between the FTP server (IIS?), the FTP client (IE in this case) or the firewall (ISA). FTP is not the most commonly used protocol nowadays, so it's entirely possible that an intermittent compatibility problem slipped past the Microsoft QA team. If you are using IIS, then MS is responsible for all the components, and you should call your Microsoft support person.

Short of that, the best I can suggest is to try replacing each of the components and attempting to reproduce the problem. I bet you'll discover that replacing either of two of the three components will eliminate the problem, which points to an incompatibility between those components. Then, you can be fairly sure that it's a software but and not a configuration problem, and you can take it to the vendor for resolution.

This was first published in May 2004

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