How do I get lftp to use SSL/TLS security mechanism from the command line?

Question

I'm trying to log into an ftps site. I've tried giving the login creds at the command line (and putting set parameters in ~/.lftprc, then opening an lftp session and typing those parameters with lftp job control statements. Regardless, I keep hitting the same roadblock:

 421 Sorry, cleartext sessions are not accepted on this server.
 Please reconnect using SSL/TLS security mechanisms.

I got furthest with the following parameters, but keep getting the error above.

How do I get lftp to use SSL/TLS security mechanism from the command line?

The objective is to script the access to this ftps site using bash (programming without using expect).

 lftp
 lftp :~> set ssl-allow false
 lftp :~> set passive-mode yes
 lftp :~> open ftp.abc.com
 lftp ftp.abc.com:~> login theuser
 Password:
 lftp [email protected]:~> cd
  `cd' at 0 [Delaying before reconnect: 26]
 CTRL-C
 lftp [email protected]:~> debug
 lftp [email protected]:~> cd
 ---- Connecting to ftp.abc.com (XX.XXX.XX.XX) port 21
 <--- 220-Welcome to the Yahoo! Web Hosting FTP server
 <--- 220-Need help? Get all details at:
 <--- 220-http://help.yahoo.com/help/us/webhosting/gftp/
 <--- 220-
 <--- 220-No anonymous logins accepted.
 <--- 220-Yahoo!
 <--- 220-Local time is now 15:30. Server port: 21.
 <--- 220-This is a private system - No anonymous login
 <--- 220 You will be disconnected after 5 minutes of inactivity.
 ---> FEAT
 <--- 211-Extensions supported:
 <---  EPRT
 <---  IDLE
 <---  MDTM
 <---  SIZE
 <---  MFMT
 <---  REST STREAM
 <---  MLST type*;size*;sizd*;modify*;UNIX.mode*;UNIX.uid*;UNIX.gid*;unique*;
 <---  MLSD
 <---  XDBG
 <---  AUTH TLS
 <---  PBSZ
 <---  PROT
 <---  TVFS
 <---  ESTA
 <---  PASV
 <---  EPSV
 <---  SPSV
 <---  ESTP
 <--- 211 End.
 ---> OPTS MLST type;size;modify;UNIX.mode;UNIX.uid;UNIX.gid;
 <--- 200  MLST OPTS type;size;sizd;modify;UNIX.mode;UNIX.uid;UNIX.gid;unique;
 ---> USER theuser
 <--- 421 Sorry, cleartext sessions are not accepted on this server.
 Please reconnect using SSL/TLS security mechanisms.

Answer 1

It seems like lftp is not configured correctly on many systems, which makes it unable to verify server certificates (producing Fatal error: Certificate verification: Not trusted).

The web (and answers in this post) is full of suggestions to fix this by disabling certificate verification or encryption altogether. This is unsecure as it allows man-in-the-middle attacks to pass unnoticed.

The better solution is to configure certificate verification correctly, which is easy, fortunately. To do so, add the following line to /etc/lftp.conf (or alternatively ~/.lftp/rc, or ~/.config/lftp/rc):

set ssl:ca-file "/etc/ssl/certs/ca-certificates.crt"

ca-certificates.crt is a file that contains all CA certificates of the system. The location used above is the one from Ubuntu and may vary on different systems. To generate or update the file, run update-ca-certificates:

sudo update-ca-certificates

If your system does not have this command, you can create one manually like this:

cat /etc/ssl/certs/*.pem | sudo tee /etc/ssl/certs/ca-certificates.crt > /dev/null

Answer 2

lftp :~> set ssl-allow false

You've explicitly set ssl-allow to false. But this must be true if lftp should attempt to use SSL.