That's correct...

If you take a look at ftp.c at line 1990 which is where it performs that parsing just before the error message you saw is output:

      if(6 == sscanf(str, "%d,%d,%d,%d,%d,%d",
                      &ip[0], &ip[1], &ip[2], &ip[3],
                      &port[0], &port[1]))

It needs to look for an IPv6 address if that fails, and then use the IPv6 address instead of the parsed IPv4 address afterwards.

Do you fancy having a go at implementing this for us as you have an ideal test bed for it?

---
** [bugs:#1468] Cannot connect to FTP on link-local IPv6**
**Status:** open
**Created:** Sat Jan 03, 2015 10:27 PM UTC by Vojtech Kral
**Last Updated:** Sat Jan 03, 2015 10:27 PM UTC
**Owner:** nobody
    $ curl -u user:password -l ftp://name-that-resolves-to-link-local-ipv6/
    curl: (14) Couldn't interpret the 227-response
    
    Response wireshark log:
    0000   32 32 37 20 45 6e 74 65 72 69 6e 67 20 50 61 73  227 Entering Pas
    0010   73 69 76 65 20 4d 6f 64 65 20 28 66 65 38 30 3a  sive Mode (fe80:
    0020   3a __ __ __ __ 3a __ __ __ __ 3a __ __ __ __ 3a  :____:____:____:
    0030   32 63 36 66 25 77 6c 61 6e 30 2c 32 32 33 2c 31  2c6f%wlan0,223,1
    0040   36 31 29 0d 0a                                   61)..
    
    I censored a part of my server's IPv6
    My guess is the zone ID / scope ID is not parsed
    
    
    $ curl -V
    curl 7.39.0 (x86_64-unknown-linux-gnu) libcurl/7.39.0 OpenSSL/1.0.1j zlib/1.2.8 libidn/1.29 libssh2/1.4.3
    Protocols: dict file ftp ftps gopher http https imap imaps pop3 pop3s rtsp scp sftp smtp smtps telnet tftp
    Features: AsynchDNS IDN IPv6 Largefile GSS-API SPNEGO NTLM NTLM_WB SSL libz TLS-SRP
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