SCP and SFTP
curl supports the SCP and SFTP protocols if built with a prerequisite 3rd party library: libssh2, libssh or wolfSSH.
SCP and SFTP are both protocols that are built on top of SSH, a secure and encrypted data protocol that is similar to TLS but differs in a few important ways. For example, SSH does not use certificates of any sort but instead it uses public and private keys. Both SSH and TLS provide strong crypto and secure transfers when used correctly.
The SCP protocol is generally considered to be the black sheep of the two since it is not portable and usually only works between Unix systems.
URLs
SFTP and SCP URLs are similar to other URLs and you download files using these protocols the same as with others:
curl sftp://example.com/file.zip -u user
and:
curl scp://example.com/file.zip -u user
SFTP (but not SCP) supports getting a file listing back when the URL ends with a trailing slash:
curl sftp://example.com/ -u user
Note that both these protocols work with "users" and you do not ask for a file anonymously or with a standard generic name. Most systems require that users authenticate, as outlined below.
When requesting a file from an SFTP or SCP URL, the file path given is
considered to be the absolute path on the remote server unless you
specifically ask for the path relative to the user's home directory. You do that by
making sure the path starts with /~/. This is quite the opposite to how FTP
URLs work and is a common cause for confusion among users.
For user daniel to transfer todo.txt from his home directory, it would
look similar to this:
curl sftp://example.com/~/todo.txt -u daniel
Authentication
Authentication with curl against an SSH server (when you specify an SCP or SFTP URL) is done like this:
- curl connects to the server and learns which authentication methods that this server offers
- curl then tries the offered methods one by one until one works or they all failed
curl attempts to use your public key as found in the .ssh subdirectory in
your home directory if the server offers public key authentication. When doing
so, you still need to tell curl which username to use on the server. For
example, the user 'john' lists the entries in his home directory on the remote
SFTP server called 'sftp.example.com':
curl -u john: sftp://sftp.example.com/
If curl cannot authenticate with the public key for any reason, it instead attempts to use the username + password if the server allows it and the credentials are passed on the command line.
For example, the same user from above has the password RHvxC6wUA on a remote
system and can download a file via SCP like this:
curl -u john:RHvxC6wUA -O scp://ssh.example.com/file.tar.gz
Known hosts
A secure network client needs to make sure that the remote host is exactly the host that it thinks it is communicating with. With TLS based protocols, it is done by the client verifying the server's certificate.
With SSH protocols there are no server certificates, but instead each server can provide its unique key. Unlike TLS, SSH has no certificate authorities or anything so the client simply has to make sure that the host's key matches what it already knows (via other means) it should look like.
The matching of keys is typically done using hashes of the key and the file
that the client stores the hashes for known servers is often called
known_hosts and is put in a dedicated SSH directory. On Linux systems that
is usually called ~/.ssh.
When curl connects to a SFTP and SCP host, it makes sure that the host's key
hash is already present in the known hosts file or it denies continued
operation because it cannot trust that the server is the right one. Once the
correct hash exists in known_hosts curl can perform transfers.
To force curl to skip checking and obeying to the known_hosts file, you can
use the -k / --insecure command-line option. You must use this option with
extreme care since it makes it possible for man-in-the-middle attacks not to
be detected.