Passive Transfers Fail When Client Has CGNAT IP

[John Hossbach]

Reference: ​my original forum post

In my server config, I'm using a hostname to provide the correct external IP for passive transfers. I also have the "Use the default host for local connections" option enabled. This works great for the ​private network address space, however, it does not recognize the ​Carrier-grade NAT (CGNAT) address space, which is also non-Internet-routable IP space. Can we please get 100.64.0.0/10 added to the exception list? I've confirmed this is still a problem with FileZilla Server 1.7.3.

Not recognizing this CGNAT space results in failed passive transfers and the following error:

Data peer IP [A.B.C.D] differs from control peer IP [100.x.x.x]: this shouldn't happen, aborting the data connection.

What's happening here is that the CGNAT address space is being treated as a non-local connection and is being sent the "public" IP (resolved from the hostname provided) in the 227 response. Instead, CGNAT address space should be treated the same as private network address space and the 227 response should have the server's IP.

Sample logs:
100.x.x.x - Client CGNAT IP (actual IP assigned to client)
9.8.7.6 - Client Public Outbound NAT IP
1.2.3.4 - Server

{{{2023-10-02T16:00:23.572Z << [FTP Session 15 100.x.x.x] 220-FileZilla Server 1.7.3
2023-10-02T16:00:23.572Z << [FTP Session 15 100.x.x.x] 220 Please visit https://filezilla-project.org/
2023-10-02T16:00:23.650Z >> [FTP Session 15 100.x.x.x] USER myuser
2023-10-02T16:00:23.650Z << [FTP Session 15 100.x.x.x] 331 Please, specify the password.
2023-10-02T16:00:23.728Z >> [FTP Session 15 100.x.x.x] PASS
2023-10-02T16:00:23.806Z << [FTP Session 15 100.x.x.x myuser] 230 Login successful.
2023-10-02T16:00:23.900Z >> [FTP Session 15 100.x.x.x myuser] PWD
2023-10-02T16:00:23.900Z << [FTP Session 15 100.x.x.x myuser] 257 "/" is current directory.
2023-10-02T16:00:23.994Z >> [FTP Session 15 100.x.x.x myuser] CWD somedir
2023-10-02T16:00:23.994Z << [FTP Session 15 100.x.x.x myuser] 250 CWD command successful
2023-10-02T16:00:24.088Z >> [FTP Session 15 100.x.x.x myuser] PASV
2023-10-02T16:00:24.088Z << [FTP Session 15 100.x.x.x myuser] 227 Entering Passive Mode (1,2,3,4,195,234)
2023-10-02T16:00:24.228Z !! [FTP Session 15 100.x.x.x myuser] Data peer IP [9.8.7.6] differs from control peer IP [100.x.x.x]: this shouldn't happen, aborting the data connection.
2023-10-02T16:00:24.244Z >> [FTP Session 15 100.x.x.x myuser] TYPE I
2023-10-02T16:00:24.244Z << [FTP Session 15 100.x.x.x myuser] 200 Type set to I
2023-10-02T16:00:24.322Z >> [FTP Session 15 100.x.x.x myuser] SIZE somefile.ext
2023-10-02T16:00:24.322Z << [FTP Session 15 100.x.x.x myuser] 213 2015
2023-10-02T16:00:24.385Z >> [FTP Session 15 100.x.x.x myuser] RETR somefile.ext
2023-10-02T16:00:24.385Z << [FTP Session 15 100.x.x.x myuser] 425 Unable to build data connection: EINVAL - Invalid argument passed
2023-10-02T16:00:24.463Z >> [FTP Session 15 100.x.x.x myuser] QUIT
2023-10-02T16:00:24.463Z << [FTP Session 15 100.x.x.x myuser] 221 Goodbye.}}}