Ubuntu Manpage: CURLOPT_SSL_VERIFYHOST - verify the certificate's name against host

Provided by: libcurl4-doc_8.9.1-2ubuntu2.2_all

NAME

       CURLOPT_SSL_VERIFYHOST - verify the certificate's name against host

SYNOPSIS

       #include <curl/curl.h>

       CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSL_VERIFYHOST, long verify);

DESCRIPTION

Pass a long set to 2L to make libcurl verify the host in the server's TLS certificate.

When negotiating a TLS connection, the server sends a certificate indicating its identity.

When CURLOPT_SSL_VERIFYHOST(3) is set to 1 or 2, the server certificate must indicate that it was made
for the hostname or address curl connects to, or the connection fails. Simply put, it means it has to
have the same name in the certificate as is used in the URL you operate against.

curl considers the server the intended one when the Common Name field or a Subject Alternate Name field
in the certificate matches the hostname in the URL to which you told curl to connect.

When the verify value is 0, the connection succeeds regardless of the names in the certificate. Use that
ability with caution,

This option controls checking the server's certificate's claimed identity. The separate
CURLOPT_SSL_VERIFYPEER(3) options enables/disables verification that the certificate is signed by a
trusted Certificate Authority.

WARNING: disabling verification of the certificate allows bad guys to man-in-the-middle the communication
without you knowing it. Disabling verification makes the communication insecure. Just having encryption
on a transfer is not enough as you cannot be sure that you are communicating with the correct end-point.

When libcurl uses secure protocols it trusts responses and allows for example HSTS and Alt-Svc
information to be stored and used subsequently. Disabling certificate verification can make libcurl trust
and use such information from malicious servers.

MATCHING

       A certificate can have the name as a wildcard. The only asterisk (*) must then be the left-most character
       and  it must be followed by a period. The wildcard must further contain more than one period as it cannot
       be set for a top-level domain.

       A certificate can be set for a numerical IP address (IPv4 or IPv6), but  then  it  should  be  a  Subject
       Alternate Name kind and its type should correctly identify the field as an IP address.

LIMITATIONS

       Secure  Transport: If verify value is 0, then SNI is also disabled. SNI is a TLS extension that sends the
       hostname to the server. The server may use that information to do such things as sending back a  specific
       certificate  for  the hostname, or forwarding the request to a specific origin server. Some hostnames may
       be inaccessible if SNI is not sent.

DEFAULT

PROTOCOLS

       This functionality affects all TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.

       All TLS backends support this option.

EXAMPLE

       int main(void)
       {
         CURL *curl = curl_easy_init();
         if(curl) {
           curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");

           /* Set the default value: strict name check please */
           curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, 2L);

           curl_easy_perform(curl);
         }
       }

AVAILABILITY

       Added in curl 7.8.1

HISTORY

       In 7.28.0 and earlier: the value 1 was treated as a debug option of some sorts, not supported anymore due
       to frequently leading to programmer mistakes.

       From 7.28.1 to 7.65.3: setting it to 1 made curl_easy_setopt(3) return an  error  and  leaving  the  flag
       untouched.

       From 7.66.0: libcurl treats 1 and 2 to this option the same.

RETURN VALUE

       Returns CURLE_OK if TLS is supported, and CURLE_UNKNOWN_OPTION if not.