CURLOPT_SSH_HOSTKEYFUNCTION(3) Library Functions Manual
NAME
CURLOPT_SSH_HOSTKEYFUNCTION - callback to check host key
SYNOPSIS
#include <curl/curl.h>
int keycallback(void *clientp,
int keytype,
const char *key,
size_t keylen);
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSH_HOSTKEYFUNCTION,
keycallback);
DESCRIPTION
Pass a pointer to your callback function, which should match the
prototype shown above. It overrides CURLOPT_SSH_KNOWNHOSTS(3).
This callback gets called when the verification of the SSH host key is
needed.
key is keylen bytes long and is the key to check. keytype says what
type it is, from the CURLKHTYPE_* series in the curl_khtype enum.
clientp is a custom pointer set with CURLOPT_SSH_HOSTKEYDATA(3).
This option is used to verify new SSH connections only. Once the
connection has been vetted by this callback it is deemed vetted and may
be reused again without invoking this callback again.
The callback must return one of the following return codes to tell
libcurl how to act:
CURLKHMATCH_OK
The host key is accepted, the connection should continue.
CURLKHMATCH_MISMATCH
the host key is rejected, the connection is canceled.
DEFAULT
NULL
PROTOCOLS
This functionality affects scp and sftp
EXAMPLE
struct mine {
void *custom;
};
static int hostkeycb(void *clientp, /* passed with CURLOPT_SSH_HOSTKEYDATA */
int keytype, /* CURLKHTYPE */
const char *key, /* host key to check */
size_t keylen) /* length of the key */
{
/* 'clientp' points to the callback_data struct */
/* investigate the situation and return the correct value */
return CURLKHMATCH_OK;
}
int main(void)
{
struct mine callback_data;
CURL *curl = curl_easy_init();
if(curl) {
CURLcode result;
curl_easy_setopt(curl, CURLOPT_URL, "sftp://example.com/thisfile.txt");
curl_easy_setopt(curl, CURLOPT_SSH_HOSTKEYFUNCTION, hostkeycb);
curl_easy_setopt(curl, CURLOPT_SSH_HOSTKEYDATA, &callback_data);
result = curl_easy_perform(curl);
curl_easy_cleanup(curl);
}
}
NOTES
Work only with the libssh2 backend.
AVAILABILITY
Added in curl 7.84.0
RETURN VALUE
curl_easy_setopt(3) returns a CURLcode indicating success or error.
CURLE_OK (0) means everything was OK, non-zero means an error occurred,
see libcurl-errors(3).
SEE ALSO
CURLOPT_SSH_HOSTKEYDATA(3), CURLOPT_SSH_KNOWNHOSTS(3)
libcurl 2026-09-03 CURLOPT_SSH_HOSTKEYFUNCTION(3)
curl 8.22.0 - Generated Sat Sep 5 14:28:51 CDT 2026