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Peer SSL Certificate Verification
Since version 7.10, libcurl performs peer SSL certificate verification by
default. This is done by installing a default CA cert bundle on 'make install'
(or similar), that CA bundle package is used by default on operations against
SSL servers.
Alas, if you communicate with HTTPS servers using certificates that are signed
by CAs present in the bundle, you will not notice any changed behavior and you
will seamlessly get a higher security level on your SSL connections since you
can be sure that the remote server really is the one it claims to be.
If the remote server uses a self-signed certificate, if you don't install
curl's CA cert bundle, if the server uses a certificate signed by a CA that
isn't included in the bundle or if the remoste host is an imposter
impersonating your favourite site, and you want to transfer files from this
server, do one of the following:
1. Tell libcurl to *not* verify the peer. With libcurl you disable with with
curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, FALSE);
With the curl command tool, you disable this with -k/--insecure.
2. Get a CA certificate that can verify the remote server and use the proper
option to point out this CA cert for verification when connecting. For
libcurl hackers: curl_easy_setopt(curl, CURLOPT_CAPATH, capath);
With the curl command tool: --cacert [file]
3. Add the CA cert for your server to the existing default CA cert bundle.
The default path of the CA bundle installed with the curl package is:
/usr/local/share/curl/curl-ca-bundle.crt, which can be changed by running
configure with the --with-ca-bundle option pointing out the path of your
Neglecting to use one of the above menthods when dealing with a server using a
certficate that isn't signed by one of the certficates in the installed CA
cert bundle, will cause SSL to report an error ("certificate verify failed")
during the handshake and SSL will then refuse further communication with that
This procedure has been deemed The Right Thing even though it adds this extra
trouble for some users, since it adds security to a majority of the SSL
connections that previously weren't really secure. It turned out many people
were using previous versions of curl/libcurl without realizing the need for
the CA cert options to get truly secure SSL connections.