repo/core/curl/files/CURLOPT_SSH_KEYFUNCTION.3

.\" generated by cd2nroff 0.1 from CURLOPT_SSH_KEYFUNCTION.md
.TH CURLOPT_SSH_KEYFUNCTION 3 "2024-08-02" libcurl
.SH NAME
CURLOPT_SSH_KEYFUNCTION \- callback for known host matching logic
.SH SYNOPSIS
.nf
#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);
.fi
.SH 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 \fICURLOPT_SSH_KNOWNHOSTS(3)\fP is also set.

This callback function gets passed the CURL handle, the key from the
known_hosts file \fIknownkey\fP, the key from the remote site \fIfoundkey\fP,
info from libcurl on the matching status and a custom pointer (set with
\fICURLOPT_SSH_KEYDATA(3)\fP). It MUST return one of the following return
codes to tell libcurl how to act:
.IP 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)
.IP 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.
.IP 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.
.IP CURLKHSTAT_REJECT
The host+key is rejected. libcurl denies the connection to continue and it is
closed.
.IP 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.
.SH DEFAULT
NULL
.SH PROTOCOLS
This functionality affects scp and sftp
.SH EXAMPLE
.nf
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);
}
}
.fi
.SH AVAILABILITY
Added in curl 7.19.6
.SH RETURN VALUE
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
.SH SEE ALSO
.BR CURLOPT_SSH_KEYDATA (3),
.BR CURLOPT_SSH_KNOWNHOSTS (3)