Add support for client certificate validation to x509_cert input plugin

[cruscio]

Feature Request (possibly Bug Report)

Add support for client certificate validation to x509_cert input plugin

Current behavior:

For a certificate with only TLS Web Client Authentication key usage and no TLS Web Server Authentication, the plugin outputs verification=invalid and verification_error="x509: certificate specifies an incompatible key usage.

For the same certificate, with the following extensions, openssl verify outputs OK

        X509v3 extensions:
            X509v3 Key Usage: critical
                Digital Signature, Key Encipherment
            X509v3 Extended Key Usage:
                TLS Web Client Authentication
            X509v3 Basic Constraints: critical
                CA:FALSE
            X509v3 Subject Key Identifier:
                4F:97:3B:EE:D4:1A:F5:34:3E:3D:9B:E0:8E:D0:22:61:6B:CC:5E:E0
            X509v3 Authority Key Identifier:
                keyid:3C:88:FB:67:50:81:F4:6A:FD:71:31:0A:98:42:B2:68:90:EE:B3:04

            X509v3 Subject Alternative Name:
                DNS:some.client.server.name.com

Desired behavior:

Valid TLS Web Client Authentication certificates without TLS Web Server Authentication should be identified as valid.

Use case:

My Telegraf agents output to Kafka using TLS client certificates for authentication. I'd like to monitor the client certificates and alert when they near expiration. It works in the current version of Telegraf (1.14), but includes a message that they're invalid, when they are not.

For reference, these certificates were generated by CFSSL with the following usage

      "client": {
        "usages": [
          "signing",
          "key encipherment",
          "client auth"
        ],

[cruscio]

The more I think about it, the more it seems a bug, not a feature request. If it's a valid x509 certificate (and can be validated by openssl) then it shouldn't be reported as invalid by Telegraf.

[cruscio]

Here are steps_to_reproduce.txt using cfssl (to generate certificates) and telegraf docker containers in Linux. Note that the issue also occurs in Telegraf for windows.

[danielnelson]

I think the change in #7760 will take care of the issue, would you be able to test with any of these development builds:

• telegraf-1.15.0~07f4cbf2-0.x86_64.rpm
• telegraf_1.15.0~07f4cbf2-0_amd64.deb
• telegraf-1.15.0~07f4cbf2_windows_amd64.zip

[cruscio]

Hey @danielnelson - I just tested the 1.14.5 release and it's working great. Thanks for turning this around so quickly!