---
c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
SPDX-License-Identifier: curl
Title: CURLOPT_SSH_KEYFUNCTION
Section: 3
Source: libcurl
See-also:
  - CURLOPT_SSH_KEYDATA (3)
  - CURLOPT_SSH_KNOWNHOSTS (3)
Protocol:
  - SFTP
  - SCP
Added-in: 7.19.6
---

# NAME

CURLOPT_SSH_KEYFUNCTION - callback for known host matching logic

# SYNOPSIS

~~~c
#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(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 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%

# EXAMPLE

~~~c
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%

# RETURN VALUE

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