curl supports the TLS version of many protocols. HTTP has HTTPS, FTP has FTPS, LDAP has LDAPS, POP3 has POP3S, IMAP has IMAPS and SMTP has SMTPS.
If the server side supports it, you can use the TLS version of these protocols with curl.
There are two general approaches to do TLS with protocols. One of them is to speak TLS already from the first connection handshake while the other is to upgrade the connection from plain-text to TLS using protocol specific instructions.
With curl, if you explicitly specify the TLS version of the protocol (the one
that has a name that ends with an 'S' character) in the URL, curl tries to
connect with TLS from start, while if you specify the non-TLS version in the
URL you can usually upgrade the connection to TLS-based with the --ssl
option.
The support table looks like this:
| Clear | TLS version | --ssl |
|---|---|---|
| HTTP | HTTPS | no |
| LDAP | LDAPS | no |
| FTP | FTPS | yes |
| POP3 | POP3S | yes |
| IMAP | IMAPS | yes |
| SMTP | SMTPS | yes |
The protocols that can do --ssl all favor that method. Using --ssl means
that curl attempts to upgrade the connection to TLS but if that fails, it
still continues with the transfer using the plain-text version of the
protocol. To make the --ssl option require TLS to continue, there is
instead the --ssl-reqd option which makes the transfer fail if curl cannot
successfully negotiate TLS.
Require TLS security for your FTP transfer:
curl --ssl-reqd ftp://ftp.example.com/file.txt
Suggest TLS to be used for your FTP transfer:
curl --ssl ftp://ftp.example.com/file.txt
Connecting directly with TLS (to HTTPS://, LDAPS://, FTPS:// etc) means that
TLS is mandatory and curl returns an error if TLS is not negotiated.
Get a file over HTTPS:
curl https://www.example.com/