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cni-ipvlan-vpc-k8s: IPvlan Overlay-free Kubernetes Networking in AWS

cni-ipvlan-vpc-k8s contains a set of CNI and IPAM plugins to provide a simple, host-local, low latency, high throughput, and compliant networking stack for Kubernetes within Amazon Virtual Private Cloud (VPC) environments by making use of Amazon Elastic Network Interfaces (ENI) and binding AWS-managed IPs into Pods using the Linux kernel's IPvlan driver in L2 mode.

The plugins are designed to be straightforward to configure and deploy within a VPC. Kubelets boot and then self-configure and scale their IP usage as needed, without requiring the often recommended complexities of administering overlay networks, BGP, disabling source/destination checks, or adjusting VPC route tables to provide per-instance subnets to each host (which is limited to 50-100 entries per VPC). In short, cni-ipvlan-vpc-k8s significantly reduces the network complexity required to deploy Kubernetes at scale within AWS.

The maximum number of Pods per AWS instance is determined by ENI limits. Instance types offering 8 ENIs can scale up to and beyond the default Kubernetes limit of 110 pods per instance.

Features

  • Designed and tested on Kubernetes in AWS (v1.10 with cri-o, docker, and containerd)
  • No overlay network; very low overhead with IPvlan
  • No external or local network services required outside of the AWS EC2 API; host-local scale up and scale down of network resources
  • Unnumbered point-to-point interfaces connect Pods with their Kubelet and Daemon Sets using their well-known Kubernetes IPs and optionally provide IPv4 internet connectivity via NAT by directing traffic over the primary private IP of the boot ENI making use of Amazon's Public IPv4 addressing attribute feature.
  • No asymmetric routing; no VPC routing table changes required
  • Pod IPs are directly addressable from non-Kubernetes VPC hosts, easing migration of existing pre-Kubernetes service meshes and infrastructure.
  • Automatic discovery of AWS resources, minimal plugin configuration required.

How it Works

The primary EC2 boot ENI with its primary private IP is used as the IP address for the node. Our CNI plugins manage additional ENIs and private IPs on those ENIs to assign IP addresses to Pods.

Each Pod contains two network interfaces, a primary IPvlan interface and an unnumbered point-to-point virtual ethernet interface. These interfaces are created via a chained CNI execution.

CNI Overview Diagram

  • IPvlan interface: The IPvlan interface with the Pod’s IP is used for all VPC traffic and provides minimal overhead for network packet processing within the Linux kernel. The master device is the ENI of the associated Pod IP. IPvlan is used in L2 mode with isolation provided from all other ENIs, including the boot ENI handling traffic for the Kubernetes control plane.
  • Unnumbered point-to-point interface: A pair of virtual ethernet interfaces (veth) without IP addresses is used to interconnect the Pod’s network namespace to the default network namespace. The interface is used as the default route (non-VPC traffic) from the Pod and additional routes are created on each side to direct traffic between the node IP and the Pod IP over the link. For traffic sent over the interface, the Linux kernel borrows the IP address from the IPvlan interface for the Pod side and the boot ENI interface for the Kubelet side. Kubernetes Pods and nodes communicate using the same well-known addresses regardless of which interface (IPvlan or veth) is used for communication. This particular trick of “IP unnumbered configuration” is documented in RFC5309.

Internet egress

For applications where Pods need to directly communicate with the Internet, by setting the default route to the unnumbered point-to-point interface, our stack can source NAT traffic from the Pod over the primary private IP of the boot ENI, which enables making use of Amazon’s Public IPv4 addressing attribute feature. When enabled, Pods can egress to the Internet without needing to manage Elastic IPs or NAT Gateways.

CNI Overview Diagram

Host namespace interconnect

Kubelets and Daemon Sets have high bandwidth, host-local access to all Pods running on the instance — traffic doesn’t transit ENI devices. Source and destination IPs are the well-known Kubernetes addresses on either side of the connect.

  • kube-proxy: We use kube-proxy in iptables mode and it functions as expected. The Pod's source IP is retained -- Kubernetes Services see connections from the Pod's source IP. The unnumbered point-to-point interface is used to loop traffic between kube-proxy in the default namespace for outbound connections created in the Pod namespace.
  • kube2iam: Traffic from Pods to the AWS Metadata service transits over the unnumbered point-to-point interface to reach the default namespace before being redirected via destination NAT. The Pod’s source IP is maintained as kube2iam runs as a normal Daemon Set.

VPC optimizations

Our design is heavily optimized for intra-VPC traffic where IPvlan is the only overhead between the instance’s ethernet interface and the Pod network namespace. We bias toward traffic remaining within the VPC and not transiting the IPv4 Internet where veth and NAT overhead is incurred. Unfortunately, many AWS services require transiting the Internet; however, both DynamoDB and S3 offer VPC gateway endpoints.

While we have not yet implemented IPv6 support in our CNI stack, we have plans to do so in the near future. IPv6 can make use of the IPvlan interface for both VPC traffic as well as Internet traffic, due to AWS’s use of public IPv6 addressing within VPCs and support for egress-only Internet Gateways. NAT and veth overhead will not be required for this traffic.

We’re planning to migrate to a VPC endpoint for DynamoDB and use native IPv6 support for communication to S3. Biasing toward extremely low overhead IPv6 traffic with higher overhead for IPv4 Internet traffic is the right future direction.

Using with Kubernetes

Supported container runtimes

cni-ipvlan-vpc-k8s is used in production at Lyft with cri-o for non-GPU workloads and Docker w/ nvidia-docker for GPU workloads.

Note that for cri-o, manage_network_ns_lifecycle must be set to true.

Prerequisites

  1. By default, we use a secondary (and tertiary, ...) ENI adapter for all Pod networking. This allows isolation by security groups or other constraints on the Kubelet control plane. This requires that the hosts you are running on can attach at least two ENI adapters. See: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html Most hosts support > 1 adapter, except for some of the smallest hardware types.

  2. AWS VPC with a recommended minimum number of subnets equal to the maximum number of attached ENIs. In the normal case of supporting up to the default 110 Pods per instance, you'll want five subnets (one for the control plane on the boot ENI and four subnets for the Pod ENIs). The example configuration uses adapter index 1 onward for Pods. We recommend creating a secondary IPv4 CIDR block for Kubernetes deployments within existing VPCs and subnet appropriately for the number of ENIs. In our primary region, we divide up our secondary IPv4 CIDR (/16) into 5 /20s per AZ with 3 AZs. Datadog has provided code which removes the restriction on one subnet per ENI; however, we've yet to test it thoroughly at Lyft.

  3. (Optional) AWS subnets tagged if you want to limit which ones can be used.

  4. The kubelet process must be started with the --node-ip option if you also use --cloud-provider=aws. Use the primary IP on the boot ENI adapter (eth0).

  5. AWS permissions allowing at least these actions on the Kubelet role:

     "ec2:DescribeSubnets"
     "ec2:AttachNetworkInterface"
     "ec2:AssignPrivateIpAddresses"
     "ec2:UnassignPrivateIpAddresses"
     "ec2:CreateNetworkInterface"
     "ec2:DescribeNetworkInterfaces"
     "ec2:DetachNetworkInterface"
     "ec2:DeleteNetworkInterface"
     "ec2:ModifyNetworkInterfaceAttribute"
     "ec2:DescribeVpcs"
     "ec2:DescribeVpcPeeringConnections"
    

    ec2:DescribeVpcs is required for m5 and c5 instances because the AWS metadata server does not return the secondary CIDR block on these instance types. This requirement will be removed when the issue is fixed.

    ec2:DescribeVpcPeeringConnections is only required if routeToVpcPeers is enabled on the plugin.

    See Security Considerations below for more on the implications of these permissions.

Building

cni-ipvlan-vpc-k8s requires dep for dependency management. Please see https://github.com/golang/dep#setup for build instructions. In a pinch, you may go get -u github.com/golang/dep/cmd/dep.

go get github.com/lyft/cni-ipvlan-vpc-k8s
cd $GOPATH/src/github.com/lyft/cni-ipvlan-vpc-k8s
make build

Example Configuration

This example CNI conflist creates Pod IPs on the secondary and above ENI adapters and chains with the upstream ipvlan plugin (0.7.0 or later required) and the cni-ipvlan-vpc-k8s-unnumbered-ptp plugin to create unnumbered point-to-point links back to the default namespace from each Pod. New interfaces will be attached to subnets tagged with kubernetes_kubelet = true, and created with the defined security groups.

Routes are automatically formed for the VPC on the ipvlan adapter.

ipMasq is enabled to use the host-IP for egress to the Internet as well as providing access to services such as kube2iam. kube2iam is not a dependency of this software.

{
    "cniVersion": "0.3.1",
    "name": "cni-ipvlan-vpc-k8s",
    "plugins": [
	{
	    "cniVersion": "0.3.1",
	    "type": "cni-ipvlan-vpc-k8s-ipam",
	    "interfaceIndex": 1,
	    "subnetTags": {
		"kubernetes_kubelet": "true"
	    },
	    "secGroupIds": [
		"sg-1234",
		"sg-5678"
	    ]
	},
	{
	    "cniVersion": "0.3.1",
	    "type": "cni-ipvlan-vpc-k8s-ipvlan",
	    "mode": "l2"
	},
	{
	    "cniVersion": "0.3.1",
	    "type": "cni-ipvlan-vpc-k8s-unnumbered-ptp",
	    "hostInterface": "eth0",
	    "containerInterface": "veth0",
	    "ipMasq": true
	}
    ]
}

Other configuration flags

In the above cni-ipvlan-vpc-k8s-ipam config, several options are available:

  • interfaceIndex: We also recommend never using the boot ENI adapter with this plugin (though it is possible). By setting interfaceIndex to 1, the plugin will only allocate IPs (and add new adapters) starting at eth1.
  • subnetTags: When allocating new adapters, by default the plugin will use all available subnets within the availability zone. You can restrict which subnets the plugin will use by specifying key / value tag names that must be matched in order for the plugin to be considered. These tags are set via the AWS API or in the AWS Console on the subnet object.
  • secGroupIds: When allocating a new ENI adapter, these interface groups will be assigned to the adapter. Specify the sg-xxxx interface group ID.
  • skipDeallocation: true or false - when set to true, this plugin will never remove a secondary IP address from an adapter. Useful in workloads that churn many pods to reduce the AWS ratelimits for configuring the VPC (which are low and cannot be raised above a certain threshold).
  • routeToVpcPeers: true or false - When set to true, the plugin will make a (cached) call to DescribeVpcPeeringConnections to enumerate all peered VPCs. Routes will be added so connections to these VPCs will be sourced from the IPvlan adapter in the pod and not through the host masquerade.
  • reuseIPWait: Seconds to wait before free IP addresses are made available for reuse by Pods. Defaults to 60 seconds. reuseIPWait functions as both a lock to prevent addresses from being grabbed by Pods spinning up in between the stages of chained CNI plugin execution and as a method of delaying when a new Pod can grab the same IP address of a terminating Pod.

IP address lifecycle management

As new Pods are created, if needed, secondary IP addresses are added to secondary ENI adapters until they reach capacity. A lightweight file-based registry stores hints containing free IP addresses available to the instance to prevent unnecessary churn from adding and removing IPs to and from ENI adapters, which is a fairly heavyweight AWS process. By default, free IP addresses are made available for reuse by Pods after being unused for at least 60 seconds. To handle cases where IPs are not frequently reused by Pods, and an excess of free IP addresses becomes available on an instance, a systemd timer is recommended to garbage collect these old IPs.

Sample cni-gc.service:

Description=Garbage collect IPs unused for 15 minutes

[Service]
Type=oneshot
ExecStart=/usr/local/bin/cni-ipvlan-vpc-k8s-tool registry-gc --free-after=15m

Sample cni-gc.timer:

[Unit]
Description=Run cni-gc every 5 minutes

[Timer]
OnBootSec=5min
OnUnitActiveSec=5min

[Install]
WantedBy=timers.target

The CLI Tool

This plugin ships a CLI tool which can be useful to inspect the state of the system or perform certain actions (such as provisioning an adapter at instance cloud-init time).

Run cni-ipvlan-vpc-k8s-tool --help for a complete listing of options.

NAME:
   cni-ipvlan-vpc-k8s-tool - Interface with ENI adapters and CNI bindings for those

USAGE:
   cni-ipvlan-vpc-k8s-tool [global options] command [command options] [arguments...]

VERSION:
   v-next

COMMANDS:
 new-interface             Create a new interface
 remove-interface          Remove an existing interface
 deallocate                Deallocate a private IP
 allocate-first-available  Allocate a private IP on the first available interface
 free-ips                  List all currently unassigned AWS IP addresses
 eniif                     List all ENI interfaces and their setup with addresses
 addr                      List all bound IP addresses
 subnets                   Show available subnets for this host
 limits                    Display limits for ENI for this instance type
 bugs                      Show any bugs associated with this instance
 vpccidr                   Show the VPC CIDRs associated with current interfaces
 vpcpeercidr               Show the peered VPC CIDRs associated with current interfaces
 registry-list             List all known free IPs in the internal registry
 registry-gc               Free all IPs that have remained unused for a given time interval
 help, h                   Shows a list of commands or help for one command

GLOBAL OPTIONS:
   --help, -h     show help
   --version, -v  print the version

COPYRIGHT:
   (c) 2017-2018 Lyft Inc.

Security Considerations

In Kubernetes, pods and kubelets are assumed to have static IP addresses that are assigned for the lifetime of the object. However, the EC2 IAM permissions required by cni-ipvlan-vpc-k8s enable authorized principals to manipulate network interfaces and IP addresses, which could be used to remap IP addresses and "take over" the IP address of an existing pod or kubelet. Such an IP address takeover could allow impersonation of a pod or kubelet at the network layer, and disrupt the availability of your Kubernetes cluster.

IP address takeovers are possible in the following situations:

  • Compromise of a kubelet instance configured to run cni-ipvlan-vpc-k8s with the required IAM permissions.
  • Use (or abuse) of the EC2 ENI and IP Address manipulation APIs by a user or service in your AWS account authorized to do so.

Consider taking the following actions to reduce the likelihood and impact of IP takeover attacks:

  • Limit the number of principals authorized to manipulate ENIs and IP addresses.
  • Do not rely exclusively on the Kubernetes control plane to ensure you're connected to the pod you expect. Deploy mutual TLS (mTLS) or other end-to-end authentication to authenticate clients and pods at the application layer.

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