Tribblix: manual page: CURLOPT_SSL

CURLOPT_SSL_VERIFYHOST(3) Introduction to Library Functions

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

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).