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117 lines
5.6 KiB
Plaintext
117 lines
5.6 KiB
Plaintext
Peer SSL Certificate Verification
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=================================
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libcurl performs peer SSL certificate verification by default. This is done
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by using CA cert bundle that the SSL library can use to make sure the peer's
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server certificate is valid.
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If you communicate with HTTPS or FTPS servers using certificates that are
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signed by CAs present in the bundle, you can be sure that the remote server
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really is the one it claims to be.
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Until 7.18.0, curl bundled a severely outdated ca bundle file that was
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installed by default. These days, the curl archives include no ca certs at
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all. You need to get them elsewhere. See below for example.
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If the remote server uses a self-signed certificate, if you don't install a CA
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cert bundle, if the server uses a certificate signed by a CA that isn't
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included in the bundle you use or if the remote host is an impostor
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impersonating your favorite site, and you want to transfer files from this
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server, do one of the following:
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1. Tell libcurl to *not* verify the peer. With libcurl you disable this with
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curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, FALSE);
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With the curl command line tool, you disable this with -k/--insecure.
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2. Get a CA certificate that can verify the remote server and use the proper
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option to point out this CA cert for verification when connecting. For
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libcurl hackers: curl_easy_setopt(curl, CURLOPT_CAPATH, capath);
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With the curl command line tool: --cacert [file]
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3. Add the CA cert for your server to the existing default CA cert bundle.
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The default path of the CA bundle used can be changed by running configure
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with the --with-ca-bundle option pointing out the path of your choice.
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To do this, you need to get the CA cert for your server in PEM format and
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then append that to your CA cert bundle.
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If you use Internet Explorer, this is one way to get extract the CA cert
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for a particular server:
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o View the certificate by double-clicking the padlock
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o Find out where the CA certificate is kept (Certificate>
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Authority Information Access>URL)
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o Get a copy of the crt file using curl
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o Convert it from crt to PEM using the openssl tool:
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openssl x509 -inform DES -in yourdownloaded.crt \
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-out outcert.pem -text
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o Append the 'outcert.pem' to the CA cert bundle or use it stand-alone
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as described below.
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If you use the 'openssl' tool, this is one way to get extract the CA cert
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for a particular server:
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o openssl s_client -connect xxxxx.com:443 |tee logfile
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o type "QUIT", followed by the "ENTER" key
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o The certificate will have "BEGIN CERTIFICATE" and "END CERTIFICATE"
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markers.
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o If you want to see the data in the certificate, you can do: "openssl
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x509 -inform PEM -in certfile -text -out certdata" where certfile is
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the cert you extracted from logfile. Look in certdata.
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o If you want to trust the certificate, you can append it to your
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cert_bundle or use it stand-alone as described. Just remember that the
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security is no better than the way you obtained the certificate.
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4. If you're using the curl command line tool, you can specify your own CA
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cert path by setting the environment variable CURL_CA_BUNDLE to the path
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of your choice.
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If you're using the curl command line tool on Windows, curl will search
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for a CA cert file named "curl-ca-bundle.crt" in these directories and in
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this order:
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1. application's directory
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2. current working directory
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3. Windows System directory (e.g. C:\windows\system32)
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4. Windows Directory (e.g. C:\windows)
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5. all directories along %PATH%
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5. Get a better/different/newer CA cert bundle! One option is to extract the
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one a recent Firefox browser uses by running 'make ca-bundle' in the curl
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build tree root, or possibly download a version that was generated this
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way for you:
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http://curl.haxx.se/docs/caextract.html
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Neglecting to use one of the above methods when dealing with a server using a
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certificate that isn't signed by one of the certificates in the installed CA
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cert bundle, will cause SSL to report an error ("certificate verify failed")
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during the handshake and SSL will then refuse further communication with that
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server.
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Peer SSL Certificate Verification with NSS
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==========================================
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If libcurl is build with NSS support then depending on the OS distribution it
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is probably required to take some additional steps to use the system-wide CA
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cert db. RedHat ships with an additional module libnsspem.so which enables NSS
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to read the OpenSSL PEM CA bundle. With OpenSuSE this lib is missing, and NSS
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can only work with its own internal formats. Also NSS got a new database
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format:
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https://wiki.mozilla.org/NSS_Shared_DB
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Starting with version 7.19.7 libcurl will check for the NSS version it runs,
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and add automatically the 'sql:' prefix to the certdb directory (either the
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hardcoded default /etc/pki/nssdb or the directory configured with SSL_DIR
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environment variable) if a version 3.12.0 or later is detected.
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To check which certdb format your distribution provides examine the default
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certdb location /etc/pki/nssdb; the new certdb format can be identified by
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the filenames cert9.db, key4.db, pkcs11.txt; filenames of older versions are
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cert8.db, key3.db, modsec.db.
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Usually these cert databases are empty; but NSS also has built-in CAs which are
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provided through a shared library libnssckbi.so; if you want to use these
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built-in CAs then create a symlink to libnssckbi.so in /etc/pki/nssdb:
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ln -s /usr/lib[64]/libnssckbi.so /etc/pki/nssdb/libnssckbi.so
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