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        !            23: .TH CURLOPT_SSH_KEYFUNCTION 3 "May 31, 2017" "libcurl 7.70.0" "curl_easy_setopt options"
        !            24: 
        !            25: .SH NAME
        !            26: CURLOPT_SSH_KEYFUNCTION \- callback for known host matching logic
        !            27: .SH SYNOPSIS
        !            28: .nf
        !            29: #include <curl/curl.h>
        !            30: 
        !            31: enum curl_khstat {
        !            32:   CURLKHSTAT_FINE_ADD_TO_FILE,
        !            33:   CURLKHSTAT_FINE,
        !            34:   CURLKHSTAT_REJECT, /* reject the connection, return an error */
        !            35:   CURLKHSTAT_DEFER,  /* do not accept it, but we can't answer right
        !            36:                         now so this causes a CURLE_DEFER error but
        !            37:                         otherwise the connection will be left intact
        !            38:                         etc */
        !            39: };
        !            40: 
        !            41: enum curl_khmatch {
        !            42:   CURLKHMATCH_OK,       /* match */
        !            43:   CURLKHMATCH_MISMATCH, /* host found, key mismatch! */
        !            44:   CURLKHMATCH_MISSING,  /* no matching host/key found */
        !            45: };
        !            46: 
        !            47: struct curl_khkey {
        !            48:   const char *key; /* points to a zero-terminated string encoded with
        !            49:                       base64 if len is zero, otherwise to the "raw"
        !            50:                       data */
        !            51:   size_t len;
        !            52:   enum curl_khtype keytype;
        !            53: };
        !            54: 
        !            55: int ssh_keycallback(CURL *easy,
        !            56:                     const struct curl_khkey *knownkey,
        !            57:                     const struct curl_khkey *foundkey,
        !            58:                     enum curl_khmatch,
        !            59:                     void *clientp);
        !            60: 
        !            61: CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSH_KEYFUNCTION,
        !            62:                           ssh_keycallback);
        !            63: .SH DESCRIPTION
        !            64: Pass a pointer to your callback function, which should match the prototype
        !            65: shown above.
        !            66: 
        !            67: It gets called when the known_host matching has been done, to allow the
        !            68: application to act and decide for libcurl how to proceed. The callback will
        !            69: only be called if \fICURLOPT_SSH_KNOWNHOSTS(3)\fP is also set.
        !            70: 
        !            71: This callback function gets passed the CURL handle, the key from the
        !            72: known_hosts file \fIknownkey\fP, the key from the remote site \fIfoundkey\fP,
        !            73: info from libcurl on the matching status and a custom pointer (set with
        !            74: \fICURLOPT_SSH_KEYDATA(3)\fP). It MUST return one of the following return
        !            75: codes to tell libcurl how to act:
        !            76: 
        !            77: .IP CURLKHSTAT_FINE_ADD_TO_FILE
        !            78: The host+key is accepted and libcurl will append it to the known_hosts file
        !            79: before continuing with the connection. This will also add the host+key combo
        !            80: to the known_host pool kept in memory if it wasn't already present there. The
        !            81: adding of data to the file is done by completely replacing the file with a new
        !            82: copy, so the permissions of the file must allow this.
        !            83: .IP CURLKHSTAT_FINE
        !            84: The host+key is accepted libcurl will continue with the connection. This will
        !            85: also add the host+key combo to the known_host pool kept in memory if it wasn't
        !            86: already present there.
        !            87: .IP CURLKHSTAT_REJECT
        !            88: The host+key is rejected. libcurl will deny the connection to continue and it
        !            89: will be closed.
        !            90: .IP CURLKHSTAT_DEFER
        !            91: The host+key is rejected, but the SSH connection is asked to be kept alive.
        !            92: This feature could be used when the app wants to somehow return back and act
        !            93: on the host+key situation and then retry without needing the overhead of
        !            94: setting it up from scratch again.
        !            95: .SH DEFAULT
        !            96: NULL
        !            97: .SH PROTOCOLS
        !            98: SFTP and SCP
        !            99: .SH EXAMPLE
        !           100: .nf
        !           101: static int keycb(CURL *easy,
        !           102:                  const struct curl_khkey *knownkey,
        !           103:                  const struct curl_khkey *foundkey,
        !           104:                  enum curl_khmatch,
        !           105:                  void *clientp)
        !           106: {
        !           107:   /* 'clientp' points to the callback_data struct */
        !           108:   /* investigate the situation and return the correct value */
        !           109:   return CURLKHSTAT_FINE_ADD_TO_FILE;
        !           110: }
        !           111: {
        !           112:   curl_easy_setopt(curl, CURLOPT_URL, "sftp://example.com/thisfile.txt");
        !           113:   curl_easy_setopt(curl, CURLOPT_SSH_KEYFUNCTION, keycb);
        !           114:   curl_easy_setopt(curl, CURLOPT_SSH_KEYDATA, &callback_data);
        !           115:   curl_easy_setopt(curl, CURLOPT_SSH_KNOWNHOSTS, "/home/user/known_hosts");
        !           116: 
        !           117:   curl_easy_perform(curl);
        !           118: }
        !           119: .fi
        !           120: .SH AVAILABILITY
        !           121: Added in 7.19.6
        !           122: .SH RETURN VALUE
        !           123: Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
        !           124: .SH "SEE ALSO"
        !           125: .BR CURLOPT_SSH_KEYDATA "(3), " CURLOPT_SSH_KNOWNHOSTS "(3), "