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CURLOPT_SSH_KEYFUNCTION(3)          libcurl         CURLOPT_SSH_KEYFUNCTION(3)

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 will be 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,
                           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 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 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 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 will continue with the
              connection. This will also add 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 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),

ibcurl 8.1.2                    April 26, 2023      CURLOPT_SSH_KEYFUNCTION(3)