[QUESTION] - `list()` returns `BadResponse` via 'fake VPN'

[somidad]

Before start, 'fake VPN' I mentioned here is an app called 'Unicorn HTTPs'. The purpose of this app is bypassing Korea's HTTPS SNI inspection policy (to prevent from accessing sites regulated by the gov). Detailed mechanisms are unknown but they explain it as 'fake VPN'. (I also asked them about technical details.)

When I launch this app and enable it, list() method returns BadResponse. When it is disabled it returns an expected result (Vec<String>). Also, macOS builtin FTP (via Finder) works fine.

Questions are:

1. What is a condition that list() method returns BadResponse? In BadResponse documentation, it only says The response syntax is invalid.
2. How can I further dig the issue? BadResponse variant has no more information unlike other variants. Does inspecting FTP packet, for example, via Wireshark work?

[somidad]

I quickly captured packets response to FTP LIST command and the following is its Hexdump and ASCII code (-: fake VPN off, +: fake VPN on)

 0000   5c e9 1e c0 a3 1f 64 e5 99 a5 c5 18 08 00 45 00   \.....d.......E.
-0010   05 dc d1 1b 40 00 6a 06 d3 2d c3 ee e2 23 c0 a8   ....@.j..-...#..
-0020   00 18 2a fa dd 97 19 f1 64 bb f6 eb c7 05 50 10   ..*.....d.....P.
-0030   04 02 0a 5b 00 00 30 34 2d 31 36 2d 31 37 20 20   ...[..04-16-17  
+0010   05 dc d4 09 40 00 6a 06 d0 3f c3 ee e2 23 c0 a8   ....@.j..?...#..
+0020   00 18 2b 06 eb 1f 87 97 7e 12 0f 60 c6 99 50 10   ..+.....~..`..P.
+0030   04 02 5d c1 00 00 30 34 2d 31 36 2d 31 37 20 20   ..]...04-16-17  
 0040   30 38 3a 35 35 50 4d 20 20 20 20 20 20 20 20 20   08:55PM         
 0050   20 20 20 20 20 20 37 35 31 39 39 37 20 33 38 33         751997 383
 0060   33 31 2d 30 30 31 2e 7a 69 70 0d 0a 30 34 2d 31   31-001.zip..04-1
 0070   36 2d 31 37 20 20 30 38 3a 35 35 50 4d 20 20 20   6-17  08:55PM   
 0080   20 20 20 20 20 20 20 20 20 20 20 20 37 35 31 36               7516
 0090   34 38 20 33 38 33 33 31 2d 30 30 32 2e 7a 69 70   48 38331-002.zip
 00a0   0d 0a 30 35 2d 31 30 2d 31 37 20 20 30 38 3a 35   ..05-10-17  08:5
 # Unchanged parts are omitted

[somidad]

Here are fields in TCP packet showing differences:

Header Checksum: 0xd32d [validation disabled] Header Checksum: 0xd03f [validation disabled]

Source Port: 11002 Source Port: 11014 (I believe this difference is due to FTP passive mode)

Sequence Number (raw): 435250363 Sequence Number (raw): 2274852370

Acknowledgment number (raw): 4142647045 Acknowledgment number (raw): 258000537

Checksum: 0x0a5b [unverified] Checksum: 0x5dc1 [unverified]

I believe such differences seem irrelevant to on/off of fake VPN... Am I looking into incorrect one?