#include <curl/curl.h>

enum curl_khstat {
 CURLKHSTAT_FINE_ADD_TO_FILE,
 CURLKHSTAT_FINE,
 CURLKHSTAT_REJECT, /* reject the connection, return an error */
 CURLKHSTAT_DEFER, /* do not accept it, but we can't answer right
 now so this causes a CURLE_DEFER error but
 otherwise the connection will be left intact
 etc */
 CURLKHSTAT_FINE_REPLACE
};

enum curl_khmatch {
 CURLKHMATCH_OK, /* match */
 CURLKHMATCH_MISMATCH, /* host found, key mismatch! */
 CURLKHMATCH_MISSING, /* no matching host/key found */
};

struct curl_khkey {
 const char *key; /* points to a null-terminated string encoded with
 base64 if len is zero, otherwise to the "raw"
 data */
 size_t len;
 enum curl_khtype keytype;
};

int ssh_keycallback(CURL *easy,
 const struct curl_khkey *knownkey,
 const struct curl_khkey *foundkey,
 enum curl_khmatch,
 void *clientp);

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSH_KEYFUNCTION,
 ssh_keycallback);
 DESCRIPTION
Pass a pointer to your callback function, which should match the prototype
shown above.

It gets called when the known_host matching has been done, to allow the
application to act and decide for libcurl how to proceed. The callback will
only be called if CURLOPT_SSH_KNOWNHOSTS(3) is also set.

This callback function gets passed the CURL handle, the key from the
known_hosts file knownkey, the key from the remote site foundkey,
info from libcurl on the matching status and a custom pointer (set with
CURLOPT_SSH_KEYDATA(3)). It MUST return one of the following return
codes to tell libcurl how to act:
 CURLKHSTAT_FINE_REPLACE
The new host+key is accepted and libcurl will replace the old host+key into
the known_hosts file before continuing with the connection. This will also
add the new host+key combo to the known_host pool kept in memory if it wasn't
already present there. The adding of data to the file is done by completely
replacing the file with a new copy, so the permissions of the file must allow
this. (Added in 7.73.0)
 CURLKHSTAT_FINE_ADD_TO_FILE
The host+key is accepted and libcurl will append it to the known_hosts file
before continuing with the connection. This will also add the host+key combo
to the known_host pool kept in memory if it wasn't already present there. The
adding of data to the file is done by completely replacing the file with a new
copy, so the permissions of the file must allow this.
 CURLKHSTAT_FINE
The host+key is accepted libcurl will continue with the connection. This will
also add the host+key combo to the known_host pool kept in memory if it wasn't
already present there.
 CURLKHSTAT_REJECT
The host+key is rejected. libcurl will deny the connection to continue and it
will be closed.
 CURLKHSTAT_DEFER
The host+key is rejected, but the SSH connection is asked to be kept alive.
This feature could be used when the app wants to somehow return back and act
on the host+key situation and then retry without needing the overhead of
setting it up from scratch again.
 DEFAULT
NULL
 PROTOCOLS
SFTP and SCP
 EXAMPLE
static int keycb(CURL *easy,
 const struct curl_khkey *knownkey,
 const struct curl_khkey *foundkey,
 enum curl_khmatch,
 void *clientp)
{
 /* 'clientp' points to the callback_data struct */
 /* investigate the situation and return the correct value */
 return CURLKHSTAT_FINE_ADD_TO_FILE;
}
{
 curl_easy_setopt(curl, CURLOPT_URL, "sftp://example.com/thisfile.txt");
 curl_easy_setopt(curl, CURLOPT_SSH_KEYFUNCTION, keycb);
 curl_easy_setopt(curl, CURLOPT_SSH_KEYDATA, &callback_data);
 curl_easy_setopt(curl, CURLOPT_SSH_KNOWNHOSTS, "/home/user/known_hosts");

 curl_easy_perform(curl);
}

 AVAILABILITY
Added in 7.19.6
 RETURN VALUE
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
 "SEE ALSO"
 CURLOPT_SSH_KEYDATA "(3), " CURLOPT_SSH_KNOWNHOSTS "(3), "