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How to determine if FTP clients are using FTPS

How to determine if FTP clients are using FTPS

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One of my colleagues here at Microsoft, Emmanuel Boersma, just reminded me of an email thread that we had several weeks ago, where a customer had asked him how they could tell if FTPS was being used on their FTP server. He had pointed out that when he looks at his FTP log files, the port number was always 21, so it wasn't as easy as looking at a website's log files and looking for port 80 for HTTP versus port 443 for HTTPS. I had sent him the following notes, and I thought that they might make a good blog. ;-)

As I mentioned earlier, we had discussed the control channel is typically over port 21 for both FTP and FTPS, so you can't rely on the port. But having said that, I mentioned that you will see certain verbs in your FTP logs that will let you know when FTPS is being used, and that’s a reliable way to check.

With that in mind, I suggested the following two methods that you can use to determine if FTPS is being used:

  • If the port number is something other than 990, and you see the following verbs being used (and succeeding), then Explicit FTPSis being used:
    • AUTH
    • PBSZ
    • PROT
  • If the port is 990, then Implicit FTPS is being used. (This means the FTPS is always on.)

For example, see the highlighted data in following FTP log file excerpts:

Explicit FTPS over port 21:

#Fields: date time c-ip cs-username cs-host s-ip s-port cs-method cs-uri-stem sc-status sc-win32-status sc-substatus sc-bytes cs-bytes time-taken
2011-06-30 22:11:24 ::1 - - ::1 21 ControlChannelOpened - - 0 0 0 0 0
2011-06-30 22:11:24 ::1 - - ::1 21 AUTH TLS 234 0 0 31 10 16
2011-06-30 22:11:27 ::1 - - ::1 21 PBSZ 0 200 0 0 69 8 0
2011-06-30 22:11:27 ::1 - - ::1 21 PROT P 200 0 0 69 8 0
2011-06-30 22:11:36 ::1 - - ::1 21 USER robert 331 0 0 69 13 0
2011-06-30 22:11:42 ::1 robert - ::1 21 PASS *** 230 0 0 53 15 2808

Implicit FTPS over port 990:

#Fields: date time c-ip cs-username cs-host s-ip s-port cs-method cs-uri-stem sc-status sc-win32-status sc-substatus sc-bytes cs-bytes time-taken
2011-06-30 22:16:55 ::1 - - ::1 990 ControlChannelOpened - - 0 0 0 0 0
2011-06-30 22:16:58 ::1 - - ::1 990 USER robert 331 0 0 69 13 0
2011-06-30 22:16:58 ::1 robert - ::1 990 PASS *** 230 0 0 53 15 78
2011-06-30 22:16:58 ::1 robert - ::1 990 SYST - 500 5 51 1005 6 0
2011-06-30 22:16:58 ::1 robert - ::1 990 FEAT - 211 0 0 313 6 0
2011-06-30 22:16:58 ::1 robert - ::1 990 OPTS UTF8+ON 200 0 0 85 14 0
2011-06-30 22:16:58 ::1 robert - ::1 990 PBSZ 0 200 0 0 69 8 0
2011-06-30 22:16:58 ::1 robert - ::1 990 PROT P 200 0 0 69 8 0

FWIW – An explanation about Implicit FTPS and Explicit FTPS can be found in the following articles:

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