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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 will try 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 will attempt to upgrade the connection to TLS but if that fails, it will still continue 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 will make 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 will return an error if TLS is not negotiated.
Get a file over HTTPS:
curl https://www.example.com/
Ciphers
TLS versions
Last modified 7mo ago
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