The kubelet is the primary “node agent” that runs on each node. The kubelet works in terms of a PodSpec. A PodSpec is a YAML or JSON object that describes a pod. The kubelet takes a set of PodSpecs that are provided through various mechanisms (primarily through the apiserver) and ensures that the containers described in those PodSpecs are running and healthy.
Other than from an PodSpec from the apiserver, there are three ways that a container manifest can be provided to the Kubelet.
File: Path passed as a flag on the command line. This file is rechecked every 20 seconds (configurable with a flag).
HTTP endpoint: HTTP endpoint passed as a parameter on the command line. This endpoint is checked every 20 seconds (also configurable with a flag).
HTTP server: The kubelet can also listen for HTTP and respond to a simple API (underspec’d currently) to submit a new manifest.
--address=0.0.0.0: The IP address for the Kubelet to serve on (set to 0.0.0.0 for all interfaces) --allow-privileged[=false]: If true, allow containers to request privileged mode. [default=false] --api-servers=: List of Kubernetes API servers for publishing events, and reading pods and services. (ip:port), comma separated. --cadvisor-port=4194: The port of the localhost cAdvisor endpoint --cert-dir="/var/run/kubernetes": The directory where the TLS certs are located (by default /var/run/kubernetes). If --tls-cert-file and --tls-private-key-file are provided, this flag will be ignored. --cgroup-root="": Optional root cgroup to use for pods. This is handled by the container runtime on a best effort basis. Default: '', which means use the container runtime default. --chaos-chance=0: If > 0.0, introduce random client errors and latency. Intended for testing. [default=0.0] --cloud-config="": The path to the cloud provider configuration file. Empty string for no configuration file. --cloud-provider="": The provider for cloud services. Empty string for no provider. --cluster-dns="": IP address for a cluster DNS server. If set, kubelet will configure all containers to use this for DNS resolution in addition to the host's DNS servers --cluster-domain="": Domain for this cluster. If set, kubelet will configure all containers to search this domain in addition to the host's search domains --config="": Path to the config file or directory of files --configure-cbr0[=false]: If true, kubelet will configure cbr0 based on Node.Spec.PodCIDR. --container-runtime="docker": The container runtime to use. Possible values: 'docker', 'rkt'. Default: 'docker'. --containerized[=false]: Experimental support for running kubelet in a container. Intended for testing. [default=false] --cpu-cfs-quota[=true]: Enable CPU CFS quota enforcement for containers that specify CPU limits --docker-endpoint="": If non-empty, use this for the docker endpoint to communicate with --docker-exec-handler="native": Handler to use when executing a command in a container. Valid values are 'native' and 'nsenter'. Defaults to 'native'. --enable-custom-metrics[=false]: Support for gathering custom metrics. --enable-debugging-handlers[=true]: Enables server endpoints for log collection and local running of containers and commands --enable-server[=true]: Enable the Kubelet's server --event-burst=10: Maximum size of a bursty event records, temporarily allows event records to burst to this number, while still not exceeding event-qps. Only used if --event-qps > 0 --event-qps=5: If > 0, limit event creations per second to this value. If 0, unlimited. --experimental-flannel-overlay[=false]: Experimental support for starting the kubelet with the default overlay network (flannel). Assumes flanneld is already running in client mode. [default=false] --file-check-frequency=20s: Duration between checking config files for new data --google-json-key="": The Google Cloud Platform Service Account JSON Key to use for authentication. --hairpin-mode="promiscuous-bridge": How should the kubelet setup hairpin NAT. This allows endpoints of a Service to loadbalance back to themselves if they should try to access their own Service. Valid values are "promiscuous-bridge", "hairpin-veth" and "none". --healthz-bind-address=127.0.0.1: The IP address for the healthz server to serve on, defaulting to 127.0.0.1 (set to 0.0.0.0 for all interfaces) --healthz-port=10248: The port of the localhost healthz endpoint --host-ipc-sources="*": Comma-separated list of sources from which the Kubelet allows pods to use the host ipc namespace. [default="*"] --host-network-sources="*": Comma-separated list of sources from which the Kubelet allows pods to use of host network. [default="*"] --host-pid-sources="*": Comma-separated list of sources from which the Kubelet allows pods to use the host pid namespace. [default="*"] --hostname-override="": If non-empty, will use this string as identification instead of the actual hostname. --http-check-frequency=20s: Duration between checking http for new data --image-gc-high-threshold=90: The percent of disk usage after which image garbage collection is always run. Default: 90% --image-gc-low-threshold=80: The percent of disk usage before which image garbage collection is never run. Lowest disk usage to garbage collect to. Default: 80% --kube-api-burst=10: Burst to use while talking with kubernetes apiserver --kube-api-qps=5: QPS to use while talking with kubernetes apiserver --kube-reserved=: A set of ResourceName=ResourceQuantity (e.g. cpu=200m,memory=150G) pairs that describe resources reserved for kubernetes system components. Currently only cpu and memory are supported. See http://releases.k8s.io/HEAD/docs/user-guide/compute-resources.md for more detail. [default=none] --kubeconfig="/var/lib/kubelet/kubeconfig": Path to a kubeconfig file, specifying how to authenticate to API server (the master location is set by the api-servers flag). --kubelet-cgroups="": Optional absolute name of cgroups to create and run the Kubelet in. --lock-file="": <Warning: Alpha feature> The path to file for kubelet to use as a lock file. --log-flush-frequency=5s: Maximum number of seconds between log flushes --low-diskspace-threshold-mb=256: The absolute free disk space, in MB, to maintain. When disk space falls below this threshold, new pods would be rejected. Default: 256 --manifest-url="": URL for accessing the container manifest --manifest-url-header="": HTTP header to use when accessing the manifest URL, with the key separated from the value with a ':', as in 'key:value' --master-service-namespace="default": The namespace from which the kubernetes master services should be injected into pods --max-open-files=1000000: Number of files that can be opened by Kubelet process. [default=1000000] --max-pods=110: Number of Pods that can run on this Kubelet. --maximum-dead-containers=240: Maximum number of old instances of containers to retain globally. Each container takes up some disk space. Default: 100. --maximum-dead-containers-per-container=2: Maximum number of old instances to retain per container. Each container takes up some disk space. Default: 2. --minimum-container-ttl-duration=1m0s: Minimum age for a finished container before it is garbage collected. Examples: '300ms', '10s' or '2h45m' --minimum-image-ttl-duration=2m0s: Minimum age for a unused image before it is garbage collected. Examples: '300ms', '10s' or '2h45m'. Default: '2m' --network-plugin="": <Warning: Alpha feature> The name of the network plugin to be invoked for various events in kubelet/pod lifecycle --network-plugin-dir="/usr/libexec/kubernetes/kubelet-plugins/net/exec/": <Warning: Alpha feature> The full path of the directory in which to search for network plugins --node-ip="": IP address of the node. If set, kubelet will use this IP address for the node --node-labels=: <Warning: Alpha feature> Labels to add when registering the node in the cluster. Labels must be key=value pairs separated by ','. --node-status-update-frequency=10s: Specifies how often kubelet posts node status to master. Note: be cautious when changing the constant, it must work with nodeMonitorGracePeriod in nodecontroller. Default: 10s --non-masquerade-cidr="10.0.0.0/8": Traffic to IPs outside this range will use IP masquerade. --oom-score-adj=-999: The oom-score-adj value for kubelet process. Values must be within the range [-1000, 1000] --outofdisk-transition-frequency=5m0s: Duration for which the kubelet has to wait before transitioning out of out-of-disk node condition status. Default: 5m0s --pod-cidr="": The CIDR to use for pod IP addresses, only used in standalone mode. In cluster mode, this is obtained from the master. --pod-infra-container-image="gcr.io/google_containers/pause:2.0": The image whose network/ipc namespaces containers in each pod will use. --port=10250: The port for the Kubelet to serve on. --read-only-port=10255: The read-only port for the Kubelet to serve on with no authentication/authorization (set to 0 to disable) --really-crash-for-testing[=false]: If true, when panics occur crash. Intended for testing. --reconcile-cidr[=true]: Reconcile node CIDR with the CIDR specified by the API server. No-op if register-node or configure-cbr0 is false. [default=true] --register-node[=true]: Register the node with the apiserver (defaults to true if --api-servers is set) --register-schedulable[=true]: Register the node as schedulable. No-op if register-node is false. [default=true] --registry-burst=10: Maximum size of a bursty pulls, temporarily allows pulls to burst to this number, while still not exceeding registry-qps. Only used if --registry-qps > 0 --registry-qps=5: If > 0, limit registry pull QPS to this value. If 0, unlimited. [default=5.0] --resolv-conf="/etc/resolv.conf": Resolver configuration file used as the basis for the container DNS resolution configuration. --rkt-path="": Path of rkt binary. Leave empty to use the first rkt in $PATH. Only used if --container-runtime='rkt' --rkt-stage1-image="": image to use as stage1. Local paths and http/https URLs are supported. If empty, the 'stage1.aci' in the same directory as '--rkt-path' will be used --root-dir="/var/lib/kubelet": Directory path for managing kubelet files (volume mounts,etc). --runonce[=false]: If true, exit after spawning pods from local manifests or remote urls. Exclusive with --api-servers, and --enable-server --runtime-cgroups="": Optional absolute name of cgroups to create and run the runtime in. --serialize-image-pulls[=true]: Pull images one at a time. We recommend *not* changing the default value on nodes that run docker daemon with version < 1.9 or an Aufs storage backend. Issue #10959 has more details. [default=true] --streaming-connection-idle-timeout=4h0m0s: Maximum time a streaming connection can be idle before the connection is automatically closed. 0 indicates no timeout. Example: '5m' --sync-frequency=1m0s: Max period between synchronizing running containers and config --system-cgroups="": Optional absolute name of cgroups in which to place all non-kernel processes that are not already inside a cgroup under `/`. Empty for no container. Rolling back the flag requires a reboot. (Default: ""). --system-reserved=: A set of ResourceName=ResourceQuantity (e.g. cpu=200m,memory=150G) pairs that describe resources reserved for non-kubernetes components. Currently only cpu and memory are supported. See http://releases.k8s.io/HEAD/docs/user-guide/compute-resources.md for more detail. [default=none] --tls-cert-file="": File containing x509 Certificate for HTTPS. (CA cert, if any, concatenated after server cert). If --tls-cert-file and --tls-private-key-file are not provided, a self-signed certificate and key are generated for the public address and saved to the directory passed to --cert-dir. --tls-private-key-file="": File containing x509 private key matching --tls-cert-file. --volume-plugin-dir="/usr/libexec/kubernetes/kubelet-plugins/volume/exec/": <Warning: Alpha feature> The full path of the directory in which to search for additional third party volume plugins --volume-stats-agg-period=1m0s: Specifies interval for kubelet to calculate and cache the volume disk usage for all pods and volumes. To disable volume calculations, set to 0. Default: '1m'