Backport of Add support for Nomad transparent proxy into release/1.4.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #3795 to be assessed for backporting due to the inclusion of the label backport/1.4.x.

The below text is copied from the body of the original PR.

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Nomad will implement support for Connect transparent proxy. Unlike in K8s, the
CNI plugin can't contact the Nomad API to read allocation metadata (pod labels)
to get the iptables configuration, and doesn't use the rest of the Consul-K8s
control plane to inject that metadata. Instead, Nomad will pass the iptables
configuration JSON-serialized in the CNI arguments.

This changeset implements the behavior switch by detecting the IPTABLES_CONFIG
argument in the CNI arguments. This hypothetically allows for non-Nomad
workflows to use the same code path, if desired.

Ref: hashicorp/nomad#10628

How I've tested this PR

In addition to new unit tests, I ran a build of Nomad that includes hashicorp/nomad#20183, deployed as in production via systemd, along with a Consul agent configured to support DNS. Run the following Nomad job specification:

countdash with tproxy

job "countdash" {

  group "api" {
    network {
      mode = "bridge"
    }

    service {
      name = "count-api"
      port = "9001"

      check {
        type     = "http"
        path     = "/health"
        expose   = true
        interval = "3s"
        timeout  = "1s"

        check_restart {
          limit = 0
        }
      }

      connect {
        sidecar_service {
          proxy {
            transparent_proxy {}
          }
        }
      }
    }

    task "web" {
      driver = "docker"

      config {
        image          = "hashicorpdev/counter-api:v3"
        auth_soft_fail = true
      }
    }
  }

  group "dashboard" {
    network {
      mode = "bridge"

      port "http" {
        static = 9010
        to     = 9002
      }
    }

    service {
      name = "count-dashboard"
      port = "9002"

      check {
        type     = "http"
        path     = "/health"
        expose   = true
        interval = "3s"
        timeout  = "1s"

        check_restart {
          limit = 0
        }
      }

      connect {
        sidecar_service {
          proxy {
            transparent_proxy {}
          }
        }
      }
    }

    task "dashboard" {
      driver = "docker"

      env {
        COUNTING_SERVICE_URL = "http://count-api.virtual.consul:9001"
      }

      config {
        image          = "hashicorpdev/counter-dashboard:v3"
        auth_soft_fail = true
      }
    }
  }
}

Configure the following service intentions:

Kind = "service-intentions"
Name = "count-api"
Sources = [
  {
    Name   = "count-dashboard"
    Action = "allow"
  }
]

Once the job ran, I visited http://<nomad_ip_address>:9010 in my browser, and saw the countdash page with the green "connected" badge. All services appear healthy in the Consul UI.

How I expect reviewers to test this PR

• Verify there's no interference with the existing k8s code paths
• As I've done above, if necessary (although the only changes here are to argument parsing). You should be able to run nomad agent -dev -dev-connect on Linux along with a Consul agent to get a suitable reproduction.

Checklist

• Tests added
• CHANGELOG entry added

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Overview of commits

• f3810b1 - 0a2bc2c - 69687c3

[tgross]

LGTM