Securing Your Service

    Linkerd’s automatic mTLS is done in a way that’s completely transparent to the application. Of course, sometimes it’s helpful to be able to validate whether mTLS is in effect!

    Note

    Linkerd uses Kubernetes ServiceAccounts to define service identity. This requires that the feature (on by default) has not been disabled on the pods. See the Kubernetes service account documentation for more.

    To validate that mTLS is working, you can view a summary of the HTTP connections between services that are managed by Linkerd using the command. For example:

    The output will look like:

    1. SRC                  DST                  CLIENT                       SERVER                       MSG
    2. linkerd-controller   linkerd-prometheus   linkerd-controller.linkerd   linkerd-prometheus.linkerd   -
    3. linkerd-web          linkerd-controller   linkerd-web.linkerd          linkerd-controller.linkerd   -

    Instead of relying on an aggregate, it is also possible to watch the requests and responses in real time to understand what is getting mTLS’d. We can use the linkerd tap command to sample real time request data. For example:

      Looking at the control plane specifically, there will be two main types of output.

      These are calls by the . As probes are initiated from the kubelet, which is not in the mesh, there is no identity and these requests are not mTLS’d, as denoted by the message.

      Other requests to the control plane are TLS’d:

      1. req id=0:11 proxy=in  src=10.4.0.15:54740 dst=10.4.0.17:9090 tls=true :method=GET :authority=linkerd-prometheus.linkerd.svc.cluster.local:9090 :path=/api/v1/query
      2. rsp id=0:11 proxy=in  src=10.4.0.15:54740 dst=10.4.0.17:9090 tls=true :status=200 latency=194886µs
      3. end id=0:11 proxy=in  src=10.4.0.15:54740 dst=10.4.0.17:9090 tls=true duration=121µs response-length=375B

      As both linkerd-prometheus and linkerd-web are in the mesh and using HTTP to communicate, the requests are automatically mTLS’d, as denoted by the tls=true output.

      Linkerd includes a debug sidecar that comes with a selection of commands that make it easier to verify and debug the service mesh itself. For example, with our , we can add the debug sidecar by running:

      1. curl -sL https://run.linkerd.io/emojivoto.yml \
      2.   | linkerd inject --enable-debug-sidecar - \
      3.   | kubectl apply -f -

      We can then establish a remote shell directly in the debug container of a pod in the voting service with:

      Once we’re inside the debug sidecar, the built-in command can be used to inspect the raw packets on the network interface. For example:

      1. tshark -i any -d tcp.port==8080,ssl | grep -v 127.0.0.1

      This tells tshark that port 8080 might be TLS’d, and to ignore localhost (as that traffic will always be unencrypted). The output will show both unencrypted communication, such as Prometheus scraping metrics, as well as the primary application traffic being automatically mTLS’d.

      1.   132 11.391486903    10.4.0.23 → 10.4.0.17    HTTP 2039 HTTP/1.1 200 OK  (text/plain)
      2.   133 11.391540872    10.4.0.17 → 10.4.0.23    TCP 68 46766 → 4191 [ACK] Seq=557 Ack=3942 Win=1329 Len=0 TSval=3389590636 TSecr=1915605020
      3.   134 12.128190076    10.4.0.25 → 10.4.0.23    TLSv1.2 154 Application Data
      4.   140 12.129497053    10.4.0.23 → 10.4.0.25    TLSv1.2 149 Application Data
      5.   141 12.129534848    10.4.0.25 → 10.4.0.23    TCP 68 48138 → 8080 [ACK] Seq=1089 Ack=985 Win=236 Len=0 TSval=2234109459 TSecr=617799816
      6.   143 13.140288400    10.4.0.25 → 10.4.0.23    TLSv1.2 150 Application Data
      7.   148 13.141219945    10.4.0.23 → 10.4.0.25    TLSv1.2 136 Application Data

      In this guide, we’ve provided several different ways to validate whether Linkerd has been able to automatically upgrade connections to mTLS. Note that there are several reasons why Linkerd may not be able to do this upgrade—see the “Caveats and future work” section of the Linkerd automatic mTLS documentation—so if you are relying on Linkerd for security purposes, this kind of validation can be instructive.