我有一个嵌入式设备（运行mbedTLS），它只能包含数量非常有限的服务器证书，我想使用curl验证我放在设备上的PEM文件。我使用icanhazip.com（IPv6）来验证。要创建PEM文件，我使用openSSL：

openssl s_client -connect icanhazip.com:443 -showcerts

并将证书部分存储在icanhazip_com.pem中，并尝试使用curl进行验证：

curl --verbose --cacert icanhazip_com.pem https://icanhazip.com

但我得到一个错误：

$ curl --verbose --cacert icanhazip_com.pem https://icanhazip.com
* Rebuilt URL to: https://icanhazip.com/
* timeout on name lookup is not supported
*   Trying 2607:ff68:116:f33d::13...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0* Connected to icanhazip.com (2607:ff68:116:f33d::13) port 443 (#0)
* ALPN, offering http/1.1
* Cipher selection: ALL:!EXPORT:!EXPORT40:!EXPORT56:!aNULL:!LOW:!RC4:@STRENGTH
* successfully set certificate verify locations:
*   CAfile: icanhazip_com.pem
  CApath: none
* TLSv1.2 (OUT), TLS header, Certificate Status (22):
} [5 bytes data]
* TLSv1.2 (OUT), TLS handshake, Client hello (1):
} [512 bytes data]
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0* TLSv1.2 (IN), TLS handshake, Server hello (2):
{ [108 bytes data]
* TLSv1.2 (IN), TLS handshake, Certificate (11):
{ [2792 bytes data]
* TLSv1.2 (OUT), TLS alert, Server hello (2):
} [2 bytes data]
* SSL certificate problem: unable to get issuer certificate
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
* Closing connection 0
} [5 bytes data]
* TLSv1.2 (OUT), TLS alert, Client hello (1):
} [2 bytes data]
curl: (60) SSL certificate problem: unable to get issuer certificate
More details here: http://curl.haxx.se/docs/sslcerts.html

curl performs SSL certificate verification by default, using a "bundle"
 of Certificate Authority (CA) public keys (CA certs). If the default
 bundle file isn't adequate, you can specify an alternate file
 using the --cacert option.
If this HTTPS server uses a certificate signed by a CA represented in
 the bundle, the certificate verification probably failed due to a
 problem with the certificate (it might be expired, or the name might
 not match the domain name in the URL).
If you'd like to turn off curl's verification of the certificate, use
 the -k (or --insecure) option.