CURLOPT_SSH_HOSTKEYFUNCTION(3)

✨ NAME ✨

okay so this is CURLOPT_SSH_HOSTKEYFUNCTION(3) — the libcurl manpage for the callback that checks the SSH host key. yes bestie, we are literally doing host-key verification today 💅🔐

🌸 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 🦋

okay so you pass a pointer to your callback function, and it should match the prototype shown above. This overrides CURLOPT_SSH_KNOWNHOSTS(3) — like, the callback is taking over, periodt 👑.

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).

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. slay ✅
• CURLKHMATCH_MISMATCH — the host key is rejected, the connection is canceled. bye 👋💀

🎀 DEFAULT 🎀

NULL

🌈 PROTOCOLS 🌈

This functionality affects scp and sftp — no cap 🫶

💖 EXAMPLE 💖

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) {
    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);

    curl_easy_perform(curl);
  }
}

🧚 NOTES 🧚

Work only with the libssh2 backend. that's the rule, bestie 😌

⭐ AVAILABILITY ⭐

Added in curl 7.84.0 — iconic behavior since then 💜

🔥 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).

👑 SEE ALSO 👑

CURLOPT_SSH_HOSTKEYDATA(3), CURLOPT_SSH_KNOWNHOSTS(3)

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that's the whole vibe from libcurl, dated 2026-10-12 — catch you in the next manpage, bestie 💖✨🦋