---
c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
SPDX-License-Identifier: curl
Title: CURLOPT_SSH_HOSTKEYFUNCTION
Section: 3
Source: libcurl
See-also:
  - CURLOPT_SSH_HOSTKEYDATA (3)
  - CURLOPT_SSH_KNOWNHOSTS (3)
Protocol:
  - SFTP
  - SCP
Added-in: 7.84.0
---

# NAME

CURLOPT_SSH_HOSTKEYFUNCTION - callback to check host key

# SYNOPSIS

~~~c
#include <curl/curl.h>

int keycallback(void *clientp,
                int keytype,
                const char *key,
                size_t keylen);

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSH_HOSTKEYFUNCTION,
                          keycallback);
~~~

# DESCRIPTION

Pass a pointer to your callback function, which should match the prototype
shown above. It overrides CURLOPT_SSH_KNOWNHOSTS(3).

This callback gets called when the verification of the SSH host key is needed.

**key** is **keylen** bytes long and is the key to check. **keytype** says
what type it is, from the **CURLKHTYPE_*** series in the **curl_khtype** enum.

**clientp** is a custom pointer set with CURLOPT_SSH_HOSTKEYDATA(3).

This option is used to verify new SSH connections only. Once the connection
has been vetted by this callback it is deemed vetted and may be reused again
without invoking this callback again.

The callback must return one of the following return codes to tell libcurl how
to act:

## CURLKHMATCH_OK

The host key is accepted, the connection should continue.

## CURLKHMATCH_MISMATCH

the host key is rejected, the connection is canceled.

# DEFAULT

NULL

# %PROTOCOLS%

# EXAMPLE

~~~c
struct mine {
  void *custom;
};

int hostkeycb(void *clientp,    /* passed with CURLOPT_SSH_HOSTKEYDATA */
              int keytype,      /* CURLKHTYPE */
              const char *key,  /* host key to check */
              size_t keylen)    /* length of the key */
{
  /* 'clientp' points to the callback_data struct */
  /* investigate the situation and return the correct value */
  return CURLKHMATCH_OK;
}
int main(void)
{
  struct mine callback_data;
  CURL *curl = curl_easy_init();
  if(curl) {
    CURLcode result;
    curl_easy_setopt(curl, CURLOPT_URL, "sftp://example.com/thisfile.txt");
    curl_easy_setopt(curl, CURLOPT_SSH_HOSTKEYFUNCTION, hostkeycb);
    curl_easy_setopt(curl, CURLOPT_SSH_HOSTKEYDATA, &callback_data);

    result = curl_easy_perform(curl);
    curl_easy_cleanup(curl);
  }
}
~~~

# NOTES

Work only with the libssh2 backend.

# %AVAILABILITY%

# RETURN VALUE

curl_easy_setopt(3) returns a CURLcode indicating success or error.

CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
libcurl-errors(3).