DNS Proxying
In addition to capturing application traffic, Istio can also capture DNS requests to improve the performance and usability of your mesh.
When proxying DNS, all DNS requests from an application will be redirected to the sidecar, which stores a local mapping of domain names to IP addresses. If the request can be handled by the sidecar, it will directly return a response to the application, avoiding a roundtrip to the upstream DNS server. Otherwise, the request is forwarded upstream following the standard /etc/resolv.conf
DNS configuration.
While Kubernetes provides DNS resolution for Kubernetes Service
s out of the box, any custom ServiceEntry
s will not be recognized. With this feature, ServiceEntry
addresses can be resolved without requiring custom configuration of a DNS server. For Kubernetes Service
s, the DNS response will be the same, but with reduced load on kube-dns
and increased performance.
This functionality is also available for services running outside of Kubernetes. This means that all internal services can be resolved without clunky workarounds to expose Kubernetes DNS entries outside of the cluster.
Getting started
This feature is not currently enabled by default. To enable it, install Istio with the following settings:
$ cat <<EOF | istioctl install -y -f -
apiVersion: install.istio.io/v1alpha1
kind: IstioOperator
spec:
meshConfig:
defaultConfig:
proxyMetadata:
# Enable basic DNS proxying
ISTIO_META_DNS_CAPTURE: "true"
# Enable automatic address allocation, optional
ISTIO_META_DNS_AUTO_ALLOCATE: "true"
EOF
This can also be enabled on a per-pod basis with the proxy.istio.io/config
annotation:
kind: Deployment
metadata:
name: curl
spec:
...
template:
metadata:
annotations:
proxy.istio.io/config: |
proxyMetadata:
ISTIO_META_DNS_CAPTURE: "true"
ISTIO_META_DNS_AUTO_ALLOCATE: "true"
...
DNS capture In action
To try out the DNS capture, first setup a ServiceEntry
for some external service:
$ kubectl apply -f - <<EOF
apiVersion: networking.istio.io/v1
kind: ServiceEntry
metadata:
name: external-address
spec:
addresses:
- 198.51.100.1
hosts:
- address.internal
ports:
- name: http
number: 80
protocol: HTTP
EOF
Bring up a client application to initiate the DNS request:
$ kubectl label namespace default istio-injection=enabled --overwrite
$ kubectl apply -f @samples/curl/curl.yaml@
Without the DNS capture, a request to address.internal
would likely fail to resolve. Once this is enabled, you should instead get a response back based on the configured address
:
$ kubectl exec deploy/curl -- curl -sS -v address.internal
* Trying 198.51.100.1:80...
Address auto allocation
In the above example, you had a predefined IP address for the service to which you sent the request. However, it’s common to access external services that do not have stable addresses, and instead rely on DNS. In this case, the DNS proxy will not have enough information to return a response, and will need to forward DNS requests upstream.
This is especially problematic with TCP traffic. Unlike HTTP requests, which are routed based on Host
headers, TCP carries much less information; you can only route on the destination IP and port number. Because you don’t have a stable IP for the backend, you cannot route based on that either, leaving only port number, which leads to conflicts when multiple ServiceEntry
s for TCP services share the same port. Refer
to the following section for more details.
To work around these issues, the DNS proxy additionally supports automatically allocating addresses for ServiceEntry
s that do not explicitly define one. This is configured by the ISTIO_META_DNS_AUTO_ALLOCATE
option.
When this feature is enabled, the DNS response will include a distinct and automatically assigned address for each ServiceEntry
. The proxy is then configured to match requests to this IP address, and forward the request to the corresponding ServiceEntry
. When using ISTIO_META_DNS_AUTO_ALLOCATE
, Istio will automatically allocate non-routable VIPs (from the Class E subnet) to such services as long as they do not use a wildcard host. The Istio agent on the sidecar will use the VIPs as responses to the DNS lookup queries from the application. Envoy can now clearly distinguish traffic bound for each external TCP service and forward it to the right target. For more information check respective Istio blog about smart DNS proxying.
To try this out, configure another ServiceEntry
:
$ kubectl apply -f - <<EOF
apiVersion: networking.istio.io/v1
kind: ServiceEntry
metadata:
name: external-auto
spec:
hosts:
- auto.internal
ports:
- name: http
number: 80
protocol: HTTP
resolution: DNS
EOF
Now, send a request:
$ kubectl exec deploy/curl -- curl -sS -v auto.internal
* Trying 240.240.0.1:80...
As you can see, the request is sent to an automatically allocated address, 240.240.0.1
. These addresses will be picked from the 240.240.0.0/16
reserved IP address range to avoid conflicting with real services.
External TCP services without VIPs
By default, Istio has a limitation when routing external TCP traffic because it is unable to distinguish between multiple TCP services on the same port. This limitation is particularly apparent when using third party databases such as AWS Relational Database Service or any database setup with geographical redundancy. Similar, but different external TCP services, cannot be handled separately by default. For the sidecar to distinguish traffic between two different TCP services that are outside of the mesh, the services must be on different ports or they need to have globally unique VIPs.
For example, if you have two external database services, mysql-instance1
and mysql-instance2
, and you create service entries for both, client sidecars will still have a single listener on 0.0.0.0:{port}
that looks up the IP address of only mysql-instance1
, from public DNS servers, and forwards traffic to it. It cannot route traffic to mysql-instance2
because it has no way of distinguishing whether traffic arriving at 0.0.0.0:{port}
is bound for mysql-instance1
or mysql-instance2
.
The following example shows how DNS proxying can be used to solve this problem. A virtual IP address will be assigned to every service entry so that client sidecars can clearly distinguish traffic bound for each external TCP service.
Update the Istio configuration specified in the Getting Started section to also configure
discoverySelectors
that restrict the mesh to namespaces withistio-injection
enabled. This will let us use any other namespaces in the cluster to run TCP services outside of the mesh.$ cat <<EOF | istioctl install -y -f - apiVersion: install.istio.io/v1alpha1 kind: IstioOperator spec: meshConfig: defaultConfig: proxyMetadata: # Enable basic DNS proxying ISTIO_META_DNS_CAPTURE: "true" # Enable automatic address allocation, optional ISTIO_META_DNS_AUTO_ALLOCATE: "true" # discoverySelectors configuration below is just used for simulating the external service TCP scenario, # so that we do not have to use an external site for testing. discoverySelectors: - matchLabels: istio-injection: enabled EOF
Deploy the first external sample TCP application:
$ kubectl create ns external-1 $ kubectl -n external-1 apply -f samples/tcp-echo/tcp-echo.yaml
Deploy the second external sample TCP application:
$ kubectl create ns external-2 $ kubectl -n external-2 apply -f samples/tcp-echo/tcp-echo.yaml
Configure
ServiceEntry
to reach external services:$ kubectl apply -f - <<EOF apiVersion: networking.istio.io/v1 kind: ServiceEntry metadata: name: external-svc-1 spec: hosts: - tcp-echo.external-1.svc.cluster.local ports: - name: external-svc-1 number: 9000 protocol: TCP resolution: DNS --- apiVersion: networking.istio.io/v1 kind: ServiceEntry metadata: name: external-svc-2 spec: hosts: - tcp-echo.external-2.svc.cluster.local ports: - name: external-svc-2 number: 9000 protocol: TCP resolution: DNS EOF
Verify listeners are configured separately for each service at the client side:
$ istioctl pc listener deploy/curl | grep tcp-echo | awk '{printf "ADDRESS=%s, DESTINATION=%s %s\n", $1, $4, $5}' ADDRESS=240.240.105.94, DESTINATION=Cluster: outbound|9000||tcp-echo.external-2.svc.cluster.local ADDRESS=240.240.69.138, DESTINATION=Cluster: outbound|9000||tcp-echo.external-1.svc.cluster.local
Cleanup
$ kubectl -n external-1 delete -f @samples/tcp-echo/tcp-echo.yaml@
$ kubectl -n external-2 delete -f @samples/tcp-echo/tcp-echo.yaml@
$ kubectl delete -f @samples/curl/curl.yaml@
$ istioctl uninstall --purge -y
$ kubectl delete ns istio-system external-1 external-2
$ kubectl label namespace default istio-injection-