CURLOPT\_SSH\_HOSTKEYFUNCTION(3) Library Functions Manual CURLOPT\_SSH\_HOSTKEYFUNCTION(3)

# [NAME](#NAME)

CURLOPT\_SSH\_HOSTKEYFUNCTION - callback to check host key

# [SYNOPSIS](#SYNOPSIS)

```
#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](#DESCRIPTION)

Pass a pointer to your callback function, which should match the prototype shown above. It overrides [*CURLOPT\_SSH\_KNOWNHOSTS(3)*](/man/os/curl/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)*](/man/os/curl/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](#CURLKHMATCH_OK)

The host key is accepted, the connection should continue.

[CURLKHMATCH\_MISMATCH](#CURLKHMATCH_MISMATCH)

the host key is rejected, the connection is canceled.

# [DEFAULT](#DEFAULT)

NULL

# [PROTOCOLS](#PROTOCOLS)

This functionality affects scp and sftp

# [EXAMPLE](#EXAMPLE)

```
struct mine {


  void *custom;
};
static 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](#NOTES)

Work only with the libssh2 backend.

# [AVAILABILITY](#AVAILABILITY)

Added in curl 7.84.0

# [RETURN VALUE](#RETURN_VALUE)

[*curl\_easy\_setopt(3)*](/man/os/curl/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)*](/man/os/curl/libcurl-errors.3).

# [SEE ALSO](#SEE_ALSO)

[**CURLOPT\_SSH\_HOSTKEYDATA**(3)](/man/os/curl/CURLOPT_SSH_HOSTKEYDATA.3), [**CURLOPT\_SSH\_KNOWNHOSTS**(3)](/man/os/curl/CURLOPT_SSH_KNOWNHOSTS.3)

2026-10-14 libcurl

---

Document: CURLOPT_SSH_HOSTKEYFUNCTION(3) from curl 8.23.0-r0 (Chainguard OS)
Source: https://github.com/curl/curl.git