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IPsec Cross-cluster container network solution

Using IPsec Tunnels for Cross-Cluster Container Network Communication over Public IP​

Introduction​

Kosmos is a multi-cluster solution, and networking is an essential part of it. Sometimes, there is a need for communication between Kubernetes clusters in different networks. In some cases, two or more clusters can only communicate with each other over the public internet. To address this, Kosmos has implemented a cross-cluster container network communication solution based on IPsec tunnels.

Motivation​

For the sake of disaster recovery, application deployments may require communication across different clouds or across regions within a single cloud (across VPCs). In such scenarios, container communication becomes challenging as the internal IP addresses of the machines are usually not directly accessible without a dedicated network connection. Common CNI tunnel technologies like VxLAN or IPIP may not work effectively in public internet environments. To solve this problem, Kosmos has implemented a container network communication solution based on IPsec tunnels for cross-cloud communication over the public internet. This solution addresses the need for communication across public networks while also considering data transmission security.

Goals​

The goal is to enable communication between pods in two clusters using elastic public IP addresses. The flow of traffic is illustrated in the diagram below:

IPsec_Tunnel

NOTE

This solution does not address container network communication in host network mode within a cluster. Only focuses on IPv4 container network communication and does not cover IPv6 container networks.

Design Details​

API Changes​

Cluster API Changes​

This solution adds three fields to the .spec.ClusterLinkOptions: NodeElasticIPMap, ClusterPodCIDRs, and UseExternalApiserver.

type ClusterLinkOptions struct {
    ...
    // NodeElasticIPMap presents mapping between nodename in kubernetes and elasticIP
    // +optional
    NodeElasticIPMap map[string]string `json:"nodeElasticIPMap,omitempty"`
    // +optional
    ClusterPodCIDRs []string `json:"clusterpodCIDRs,omitempty"`
    // +optional
    UseExternalApiserver bool `json:"useexternalapiserver,omitempty"`
}
  • NodeElasticIPMap field represents the mapping relationship between the NodeName in Kubernetes and the elastic public IP mounted on the node.
  • ClusterPodCIDRs field is added to input the Pod CIDR, as it is not always easy to obtain the Pod CIDR for some CNI plugins.

Typically, Kosmos retrieves the service CIDR through the kube-apiserver parameters. However, in some cases, the kube-apiserver is not a pod within the cluster. Therefore, the UseExternalApiserver field is added to handle this scenario.

Clusternode API Changes​

This solution adds a new field, ElasticIP, to the .spec, and a new field, NodeStatus, to the .status.

type ClusterNodeSpec struct {
    ...
    // +optional
    ElasticIP string `json:"elasticip,omitempty"`
}

type ClusterNodeStatus struct {
    // +optional
    NodeStatus string `json:"nodeStatus,omitempty"`
}
  • ElasticIP field describes the elastic public IP mounted on the node.
  • NodeStatus field describes the status of the node, either "Ready" or "NotReady".

Nodeconfig API Changes​

This solution adds two new fields, XfrmPoliciesXfrmStates and IPsetsAvoidMasqs, to the .spec.

type NodeConfigSpec struct {
    XfrmPolicies     []XfrmPolicy `json:"xfrmpolicies,omitempty"`
    XfrmStates       []XfrmState  `json:"xfrmstates,omitempty"`
    IPsetsAvoidMasqs []IPset      `json:"ipsetsavoidmasq,omitempty"`
}

type XfrmPolicy struct {
    LeftIP   string `json:"leftip"`
    LeftNet  string `json:"leftnet"`
    RightIP  string `json:"rightip"`
    RightNet string `json:"rightnet"`
    ReqID    int    `json:"reqid"`
    Dir      int    `json:"dir"`
}

type XfrmState struct {
    LeftIP  string `json:"leftip"`
    RightIP string `json:"rightip"`
    ReqID   int    `json:"reqid"`
    SPI     uint32 `json:"spi"`
    PSK     string `json:"PSK"`
}

type IPset struct {
    CIDR string `json:"cidr"`
    Name string `json:"name"`
}

The new XfrmPolicies and XfrmStates fields define the IPsec-related rules created by Kosmos.

IPsetsAvoidMasqs field describes the network segments that need to avoid masquerading, allowing outbound traffic from containers to retain their container IP address.

Component Modifications​

Handling scenarios where kube-apiserver pods are not within the cluster:

  • The cluster-controller optimizes the retrieval of service CIDR through the GetSvcByCreateInvalidSvc function.

Node status synchronization:

  • The node-controller synchronizes the ElasticIP field of the clusternode object based on the values in the NodeElasticIPMap field of the cluster object.
  • The node-controller now updates the .Status.NodeStatus field of the clusternode object based on the node's status, "Ready" or "NotReady".

The elector module is used for Gateway selection in Gateway mode. It can now select a Gateway from the nodes in "Ready" state.

  1. Adding support for some CNI plugins For some CNI plugins, iptables rules are added to avoid masquerading, allowing outbound traffic from containers to retain their container IP address.

  2. Building IPsec rules Typically, Kosmos creates routes to achieve container communication. In IPsec tunnel mode, Kosmos creates ip xfrm state and ip xfrm policy rules if the ElasticIP field of the clusternode is not empty.

Functions have been added to execute specific operations, equivalent to executing ip xfrm state add/del and ip xfrm policy add/del commands on the operating system.

To avoid masquerading, functions have been added to execute ipset commands and create iptables rules.

kosmosctl​

NodeElasticIP, UseExternalApiserver, and ClusterPodCIDRs input parameters have been added to populate the new fields NodeElasticIPMap, UseExternalApiserver, and ClusterPodCIDRs in the Cluster CRD.