This guide provides educational material that is not specific to Calico.
In this guide you will learn:
- What is Kubernetes egress?
- Why should you restrict egress traffic and how can you do it?
- What is "NAT outgoing" and when is it used?
- What is an egress gateway, and why might you want to use one?
What is Kubernetes egress?
In this guide we are using the term Kubernetes egress to describe connections being made from pods to anything outside of the cluster.
In contrast to ingress traffic, where Kubernetes has the Ingress resource type to help manage the traffic, there is no Kubernetes Egress resource. Instead, how the egress traffic is handled at a networking level is determined by the Kubernetes network implementation / CNI plugin being used by the cluster. In addition, if a service mesh is being used, this can add egress behaviors on top of those the network implementation provides.
There are three areas of behavior worth understanding for egress traffic, so you can choose a networking and/or service mesh setup that best suits your needs:
- Restricting egress traffic
- Outgoing NAT behavior
- Egress gateways
Restricting egress traffic
It's a common security requirement and best practice to restrict outgoing connections from the cluster. This is normally achieved using Network Policy to define egress rules for each microservice, often in conjunction with a default deny policy that ensures outgoing connections are denied by default, until a policy is defined to explicitly allow specific traffic.
One limitation when using Kubernetes Network Policy to restrict access to specific external resources, is that the external resources need to be specified as IP addresses (or IP address ranges) within the policy rules. If the IP addresses associated with an external resource change, then every policy that referenced those IP addresses needs to be updated with the new IP addresses. This limitation can be circumvented using Calico Use external IPs or networks rules in policy, or Calico Enterprise's support for domain names in policy rules..
Note in addition to everything mentioned so far, perimeter firewalls can also be used to restrict outgoing connections, for example to allow connections only to particular external IP address ranges, or external services. However, since perimeter firewalls typically cannot distinguish individual pods, the rules apply equally to all pods in the cluster. This provides some defense in depth, but cannot replace the requirement for network policy.
Network Address Translation (NAT) is the process of mapping an IP address in a packet to a different IP address as the packet passes through the device performing the NAT. Depending on the use case, NAT can apply to the source or destination IP address, or to both addresses.
In the context of Kubernetes egress, NAT is used to allow pods to connect to services outside of the cluster if the pods have IP addresses that are not routable outside of the cluster (for example, if the pod network is an overlay).
For example, if a pod in an overlay network attempts to connect to an IP address outside of the cluster, then the node hosting the pod uses SNAT (Source Network Address Translation) to map the non-routable source IP address of the packet to the node's IP address before forwarding on the packet. The node then maps response packets coming in the opposite direction back to the original pod IP address, so packets flow end-to-end in both directions, with neither pod or external service being aware the mapping is happening.
In most clusters this NAT behavior is configured statically across the whole of the cluster. When using Calico, the NAT behavior can be configured at a more granular level for particular address ranges using IP pools. This effectively allows the scope of "non-routable" to be more tightly defined than just "inside the cluster vs outside the cluster", which can be useful in some enterprise deployment scenarios.
Another approach to Kubernetes egress is to route all outbound connections via one or more egress gateways. The gateways SNAT (Source Network Address Translation) the connections so the external service being connected to sees the connection as coming from the egress gateway. The main use case is to improve security, either with the egress gateway performing a direct security role in terms of what connections it allows, or in conjunction with perimeter firewalls (or other external entities). For example, so that perimeter firewalls see the connections coming from well known IP addresses (the egress gateways) rather than from dynamic pod IP addresses they don't understand.
Egress gateways are not a native concept in Kubernetes itself, but are implemented by some Kubernetes network implementations and some service meshes. For example, Calico provides egress gateway functionality, plus the ability to map namespaces (or even individual pods) to specific egress gateways. Perimeter firewalls (or other external security entities) can then effectively provide per namespace security controls, even though they do not have visibility to dynamic pod IP addresses.
As an alternative approach to egress gateways, Calico allows you to control pod IP address ranges based on namespace, or node, or even at the individual pod level. Assuming no outgoing NAT is required, this provides a very simple way for perimeter firewalls (or other external security entities) to integrate with Kubernetes for both ingress and egress traffic. (Note that this approach relies on having enough address space available to sensibly assign IP address ranges, for example to each namespace, so it can lead to IP address range exhaustion challenges for large scale deployments. In these scenarios, using egress gateways is likely to be a better option.)