curl / libcurl / API / curl_easy_setopt / CURLOPT_SSH_KEYFUNCTION

CURLOPT_SSH_KEYFUNCTION explained

Name

CURLOPT_SSH_KEYFUNCTION - callback for known host matching logic

Synopsis

#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 cannot answer right
                        now. Causes a CURLE_PEER_FAILED_VERIFICATION error but
                        the connection is left intact */
  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 match,
                    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 is only called if CURLOPT_SSH_KNOWNHOSTS 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). 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 replaces the old host+key into the known_hosts file before continuing with the connection. This also adds the new host+key combo to the known_host pool kept in memory if it was not 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 appends it to the known_hosts file before continuing with the connection. This also adds the host+key combo to the known_host pool kept in memory if it was not 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 continues with the connection. This also adds the host+key combo to the known_host pool kept in memory if it was not already present there.

CURLKHSTAT_REJECT

The host+key is rejected. libcurl denies the connection to continue and it is 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 return and act on the host+key situation and then retry without needing the overhead of setting it up from scratch again.

Default

NULL

Protocols

This functionality affects scp and sftp

Example

struct mine {
  void *custom;
};
 
static int keycb(CURL *easy,
                 const struct curl_khkey *knownkey,
                 const struct curl_khkey *foundkey,
                 enum curl_khmatch match,
                 void *clientp)
{
  /* 'clientp' points to the callback_data struct */
  /* investigate the situation and return the correct value */
  return CURLKHSTAT_FINE_ADD_TO_FILE;
}
 
int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    struct mine callback_data;
    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 curl 7.19.6

Return value

Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.