Securing Your Application with mTLS
By default, Linkerd automatically enables mutual Transport Layer Security (mTLS) for TCP traffic between meshed pods, by establishing and authenticating secure, private TLS connections between Linkerd proxies. Simply add your services to Linkerd, and Linkerd will take care of the rest.
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!
Validating mTLS with
To validate that mTLS is working, you can view a summary of the TCP
connections between services that are managed by Linkerd using the
linkerd edges command. For example:
linkerd -n linkerd edges deployment
The output will look like:
SRC DST CLIENT SERVER MSG linkerd-controller linkerd-prometheus linkerd-controller.linkerd linkerd-prometheus.linkerd - linkerd-web linkerd-controller linkerd-web.linkerd linkerd-controller.linkerd -
In this example, everything is successfully mTLS'd, and the
SERVER columns denote the identities used, in the form
service-account-name.namespace. (See Linkerd's automatic mTLS
documentation for more on what these identities
mean.) If there were a problem automatically upgrading the connection with
MSG field would contain the reason why.
Validating mTLS with
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.
linkerd -n linkerd tap deploy
Looking at the control plane specifically, there will be two main types of output.
req id=0:7 proxy=in src=10.138.15.206:51558 dst=10.4.0.18:9998 tls=not_provided_by_remote :method=GET :authority=10.4.0.18:9998 :path=/ready rsp id=0:7 proxy=in src=10.138.15.206:51558 dst=10.4.0.18:9998 tls=not_provided_by_remote :status=200 latency=482µs end id=0:7 proxy=in src=10.138.15.206:51558 dst=10.4.0.18:9998 tls=not_provided_by_remote duration=32µs response-length=3B
These are calls by the Kubernetes readiness
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
Other requests to the control plane are TLS'd:
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 rsp id=0:11 proxy=in src=10.4.0.15:54740 dst=10.4.0.17:9090 tls=true :status=200 latency=194886µs 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
linkerd-web are in the mesh and using HTTP
to communicate, the requests are automatically mTLS'd, as denoted by the
Validating mTLS with tshark
The final way to validate mTLS is to look at raw network traffic within the cluster.
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 emojivoto demo application, we can add the debug sidecar by running:
curl -sL https://run.linkerd.io/emojivoto.yml \ | linkerd inject --enable-debug-sidecar - \ | kubectl apply -f -
We can then establish a remote shell directly in the debug container of a pod in
voting service with:
kubectl -n emojivoto exec -it \ $(kubectl -n emojivoto get po -o name | grep voting) \ -c linkerd-debug -- /bin/bash
Once we're inside the debug sidecar, the built-in
tshark command can be used
to inspect the raw packets on the network interface. For example:
tshark -i any -d tcp.port==8080,ssl | grep -v 127.0.0.1
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.
131 11.390338699 10.4.0.17 → 10.4.0.23 HTTP 346 GET /metrics HTTP/1.1 132 11.391486903 10.4.0.23 → 10.4.0.17 HTTP 2039 HTTP/1.1 200 OK (text/plain) 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 134 12.128190076 10.4.0.25 → 10.4.0.23 TLSv1.2 154 Application Data 140 12.129497053 10.4.0.23 → 10.4.0.25 TLSv1.2 149 Application Data 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 143 13.140288400 10.4.0.25 → 10.4.0.23 TLSv1.2 150 Application Data 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.