CURLOPT_SSH_KEYFUNCTION explained
Name
CURLOPT_SSH_KEYFUNCTION - callback for known host matching logic
Synopsis
#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 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). It MUST return one of the following return codes to tell libcurl how to act:
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)
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.
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.
The host+key is rejected. libcurl denies the connection to continue and it is closed.
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
This functionality affects scp and sftp
Example
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
Added in curl 7.19.6
Return value
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
See also
CURLOPT_SSH_KEYDATA(3), CURLOPT_SSH_KNOWNHOSTS(3)
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