k8s

Kubernetes Short Notes(4)

2020 年 1 月 6 日2021 年 6 月 3 日
Ops

tags: `k8s`

Networking

Networking for Linux Basics

Network Switch

A switch is a device in a computer network that connects other devices together, can only enable a communication within a network

Host A(192.168.1.10)[eth0] &harr Switch(192.168.1.0) &harr [eth0]Host B(192.168.1.11)

# For Network A

$ ip link

$ ip addr add 192.168.1.10/24 dev eth0 # set a ip addr for interface eth0

# For Network B

$ ip link

$ ip addr add 192.168.1.11/24 dev eth0

# Test

$ ping 192.168.1.11

Router

A router is a device/service that provides the function of routing IP packets between networks

Switch(192.168.1.0) <–> [192.168.1.1]Router[192.168.2.1] <–> Switch(192.168.2.0)

Route/Gateway

A gateway (in network terms) is a router that describes the function for connectivity

# For Network A

$ ip route add 192.168.2.0/24 via 192.168.1.1

# For Network B

$ ip route add 192.168.1.0/24 via 192.168.2.1

Default Gateway

If none of these forwarding rules in the routing table is appropriate for a given destination address, the default gateway is chosen as the default router of last resort

1 2	$ ip route show default

Forwording packets between interfaces

By default in linux, packets are not forwarded from one interface to the next, for security reasons

Explicity allow it

1 2	echo 1 > /proc/sys/net/ipv4/ip_forward

Persists the settings

1 2	net.ipv4.ip_forward = 1

DNS

Translate host name to IP address by configure the /etc/hosts

When a environment has too many entries and IP address are not persistent, we need a DNS server

$ cat /etc/resolv.conf

nameserver 192.168.1.100

The host will lookup an entry in /etc/hosts first, then lookup in the DNS. This order can be changed by configure file /etc/nsswitch.conf

$ cat /etc/nsswitch.conf

passwd: files

group: files

shadow: files

gshadow: files

hosts: files dns

networks: files

protocols: db files

services: db files

ethers: db files

rpc: db files

netgroup: nis

You can configure the DNS server to forward unknown host name to the public name server in the Internet, for example reach www.google.com

private DNS → Root DNS → .com DNS → google DNS → cache the result

When looking for a host in the same domain, we want to simple use the host name not the full name, such as using web not web.mycompany.com, therefore we specify the domain name you want to append in /etc/resolv.conf

$ cat /etc/resolv.conf

search mycompany.com

There are records stores in DNS with specific types:

A: ipv4
AAAA: ipv6
CNAME: name to name mapping

You can use tools like nslookup, dig to debug, note that nslookup only query from dns, not files

There are plenty DNS solutions, such as CoreDNS, except configure from files, CoreDNS supports other ways of configuring DNS entries through plugins like kubernetes

Network Namespace

A namespace is a way of scoping a particular set of identifiers

Linux provides namespaces for networking and processes, if a process is running within a process namespace, it can only see and communicate with other processes in the same namespace

Linux starts up with a default network namespace

Each network namespace has its own routing table and has its own set of iptables

# Create namespace

ip netns add red

# List namespace

ip netns list

# List interface

ip link

# List interface in namespace

ip netns exec red ip link

# or

ip -n red link

Connect namespaces together using a virtual Ethernet pair (or virtual cable, pipe)

# Create veth pair

$ ip link add veth-red type veth peer name veth-blue

# Attach each interface to the appropriate namespace

$ ip link set veth-red netns red

$ ip link set veth-blue netns blue

# Assign IP to each namespaces

$ ip -n red addr add 192.168.15.1 dev veth-red

$ ip -n blue addr add 192.168.15.2 dev veth-blue

# Bring up the interface for each device within the respective namespace

$ ip -n red link set veth-red up

$ ip -n blue link set veth-blue up

# List ARP table to see neighbor

$ ip netns exec red arp

# Ping across namespace

$ ip netns exec red ping 192.168.15.2

When there more of namespaces need connected, use a virtial switch to create a virtial network. There few solutions:

Linux Bridge
Open vSwitch

# Create a virtial switch interface

$ ip link add v-net-0 type bridge

# Bring the interface up

$ ip link set dev v-net-0 up

# Create cables for each namespace to connect to the bridge

$ ip link add veth-red type veth peer name veth-red-br

$ ip link add veth-blue type veth peer name veth-blue-br

# Attach one end to the appropriate namespace

$ ip link set veth-red netns red

$ ip link set veth-blue netns blue

# Attach the other end to the bridge

$ ip link set veth-red-br master v-net-0

$ ip link set veth-blue-br master v-net-0

# Assign IP to each namespaces

$ ip -n red addr add 192.168.15.1 dev veth-red

$ ip -n blue addr add 192.168.15.2 dev veth-blue

# Bring up the interface for each device within the respective namespace

$ ip -n red link set veth-red up

$ ip -n blue link set veth-blue up

# Assign IP address to the bridge (since it’s just another interface on the host)

$ ip addr add 12.168.15.3/24 dev v-net-0

# Ping accross namespaces

$ ip netns exec red ping 192.168.15.2

When a private virtual network need to reach the outer network, it need a gateway, the host is the gateway

1 2	$ ip netns exec red ip route add 192.168.1.0/24 via 192.168.15.3

For destination network to response, enable NAT on host acting as a gateway.

Add a new rule in the NAT IP table in the POSTROUTING chain to masquerade or replace the from address on all packets coming from the source network 192.168.15.0 with its own IP address.

Thus anyone receiving these packets outside the network will think that they are coming from the host and not from within the namespaces

1 2	$ iptables -t nat -A POSTROUTING -s 192.168.15.0/24 -j MAS

Add a route using default gateway to outside world

1 2	$ ip netns exec red ip route add default via 192.168.15.3

For outside world to reach the namespace in a private network, add a port forwarding rule using IP tables to say any traffic coming to port 80 on the localhost is to be forwarded to port 80 on the IP assigned to the namespace

$ iptables \

-t nat \

-A PREROUTING \

--dport 80 \

--to-destination 192.168.15.1:80 \

-j DNAT

Kubernetes Short Notes(3)

2019 年 12 月 17 日2019 年 12 月 18 日
Ops

Cluster Maintainance

OS Upgrade

Pod Eviction Timeout

When the nodes was down for more than 5 minute(default) then the pods are terminated; pod will recreate if has replicaset

Drain, Cordon, Uncordon

We’re not sure the node will come back online in 5 minutes, therefore we can drain the node.

After the drained node upgraded and come back, it still unschedulable, uncordon the node to make it schedulable.

Note that the previouse pods won’t be automatically reschedule back to the nodes.

Cluster Upgrade

The core control plane components’s version can be different, but should follow certain rules:

the kube-api is the primary component, none of the other components’s version must not be higher than the kube-api
the components can be lower in 1-2 versions
- kube-api: x
- Controlloer-manager, kube-scheduler: x, x-1
- kubelet, kube-proxy: x, x-1, x-2
the kubectl can be one version higher than kube-api: x+1, x, x-1

The kubernetes support only up to the recent 3 minor versions. The recommanded approch is to update one minor version at a time.

Update the cluster depend on how you deploy them:

cloud provider: few clicks at the UI
kubeadm: using upgrade argument (you should upgrade the kubeadm first!)
the hard way from scratch: manually upgrade components by yourself

Two major steps:

upgrade master node, the control plane componets goes down, all management function are down, only the applications deploy on worker nodes keeps serving
update worker nodes, with strategies:
- upgrade all at once with downtimes
- upgrade one at a time
- create new nodes and remove the workloads, then finally remove old nodes

When you run command like kubectl get nodes, the VERSION is indicat the version of the kubelet

Backup and Restore

Master / Node DR

Cordon & drain
Provision replacement master / node

ETCD DR

Option: Backup resources

Saving objects as a copy by query the kube-api

1 2	kubectl get all --namespace=default -o yaml > default-deplayment-services.yaml

Option: Backup ETCD

Making copies of the ETCD data directory

# etcd.service

ExecStart=/user/local/bin/etcd \

--name= ${ETCD_NAME}

...

--data-dir=/var/lib/etcd

Or use the etcd command line tool

Make a snap shot

ETCDTL_API=3 etcdctl backup save etcd.db \

--endpoint=http://127.0.0.1:2379 \

--cacert=/etc/etcd/ca.crt \

--cert=/etc/etcd/etcd-server.crt \

--key=/etc/etcd/etcd

Remember to specify the certification files for authentication

Stop kube-api

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service kube-apiserver stop

Restore snapshot

ETCDCTL_API=3 etcdctl snapshot restore etcd.db \

-- data-dir=/var/lib/etcd-backup-dir \

--initial-cluster master-1=https://192.169.5.11:2380,master-2=https://192.168.5.12:2380 \

--initial-cluster-token etck-cluster-1 \

--initial-advertise-peer-urls https://${INTERNAL_IP}:2380

When ETCD restore from a backup, it initialize a new cluster configuration and configures the members of ETCD as new members to a new cluster. This is to prevent a new member from accidentally joining an existing cluster.
For example, using a snapshot to provision a new etcd-cluster from testing purpose. You don’t want the members in the new test cluster to accidentally join the production cluster.

Configure the etcd.service with new data directory and new cluster token

During a restore, you must provide a new cluster token and the same initial cluster configuration
Restart ETCD service

1
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3

systemctl daemon-reload
service etcd restart
Start kube-api

1
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service kube-apiserver start

Persistant Volume DR

You can’t relay on kubernetes to for backing up and restore persistant volumes.

If you’re using cloud provider specific persistant volumes like EBS volumes, Azure managed disks or GCE persistent disks, you should use cloud provider snapshot APIs

Kubernetes Short Notes (2)

2019 年 10 月 6 日2019 年 10 月 6 日
Ops

Scheduling

Manual Scheduling

Bind the pod to node by nodeName property, before that, the pod stays in the Pending state

Manutal ways to bind:

specify the spec.nodeName, not updatable

create the Binding object

curl --header "Content-Type:application/json" \

--request POST --data '{"apiVersion": "v1", "kind": "Binding" ...}' \

http://$SERVER/api/v1/namespaces/default/pods/$PODNAME/binding/

Labeling

Use to group and select the objects, for example a ReplicaSet object configs:

metadata.labels sets the ReplicaSet itself
spec.template.metadata.lables sets the Pod
spec.selector.matchLabels defines how ReplicaSet to discover the Pod

Annotations

Use to record other details for intergration purpose e.g. build info, contact details

Restriction

Taint/Toleration

Limit pods without tolerations cannot get scheduled to a tainted node

Taint the nodes

1
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3

kubectl taint nodes master node-role.kubernetes.io/master:NoSchedule # Taint
ubectl taint nodes master node-role.kubernetes.io/master:NoSchedule- # UnTaint
Set the pods’ tolerance, three behavior are avaliable if not tolerant:
- NoSchedule
PreferNoSchedule: not guaranteed

NoExecute: new pods=NoSchedule, existed pods=evicted

Note the value in tolerations keys must use double quotes

Node Selector

Limit the pod to get scheduled to one kind of node only

Lable the node
Set the nodeSelector

Note there is no OR or NOT conditions, use node affinity instead

Node Affinity

Limit the pod to get scheduled to one or more particular nodes

Lable the node
Set the nodeAffinity
operators: In, NotIn, Exists, DoesNotExist, Gt, Lt
3 types

Combines the Taint/Toleration with NodeSelector or NodeAffinity to cover the scenarios

Resources

Request

The scheduling base on the resource requests
By default, k8s assumes a pod requires 0.5 cpu and 256Mi memory

Limit

By default, k8s limit a pod to 1 cpu and 512Mi memory
When a pod try to exceed resources beyond the limit
- cpu: k8s throttles the cpu won’t kill
- memory: k8s kill the pod with OOM

Static Pods

Use in creating control plane components (kube admin tools)

Without the intervention from the kube-api server, the kubelet can manage a node independently by monitor config files in the file system, and be able to create, recreate, update and delete the POD only object

–pod-manifest-path=/etc/Kubernetes/manifest
–config=kubeconfig.yaml (staticPodPath)

While the static pod created, the kube-api only get a readable mirror and not have the ability to update/delete it

Multiple Scheduler

copy the kube-scheduler configs from /etc/kubernetes/manifests
rename the scheduler --scheduler-name
if one master nodes with multiple scheduler:
- set the --leader-elect=false
if multiple masters with multiple scheduler, only one scheduler can active at a time
- set the --leader-elect=true
- set the --lock-object-name to differentiate the custom scheduler from default if multiple master
specify the scheduler for pod by schedulerName

2019 年 9 月 27 日2020 年 1 月 6 日
Ops

Cluster Architecture

Master Node

ETCD cluster
kube-scheduler
kube-controller-manager

These components communicate via kube-api server

Worker Node

container runtime engine, e.g. Docker, Rocket, ContainerD
kubelet: agent that runs and listen for instructions from kube-api
containers

The services deploy within worker nodes communicate with each other via kube-proxy

Objectives

ETCD

a distributed reliable key-value store
client commuications on port 2379
server to server on port 2380

kube-api

primary management component
setup:
1. using kube-admin tools
  - deploy kube-api as a pod in kube-system namespace
  - the manifests is at /etc/kubernetes/manifests/kube-apiserver.yaml
  - the options is at /etc/systemd/system/kube-apiserver.service
  - search for kube-apiserver process on master node
    
    1
    2
    
    ps -aux | grep kube-apiserver
example: apply deployment using kubectl
1. authenticates user
validate the HTTP requests

the kube-scheduler monitored the changes from the kube-api, then:

retrieve the node information from kube-api

schedule the pod to some node through kube-api to kubelet

update the pod info to ETCD

kube-controller-manager

continuously monitors the state of components
the controllers packages into a single process called Kube-Controller-Manager, which includes:
1. deployment-controller, cronjob, service-account-controller …
2. namespace-controller, job-contorller, node-controller …
3. endpoint-controller, replicaset, replication-controller(replica set) …
remediate situation

kube-scheduler

decide which pod goes to which node
1. filter nodes
2. rank nodes

kubelet

follow the instruction from kube-scheduler to controll the container runtime engine (e.g. docker) that run or remove a container
using kube-admin tools to deploy cluster, the kubelet are not installed by default in worker nodes, need intstall manually

kube-proxy

runs on each nodes in the cluster
create iptables rules on each nodes to forward traffic heading to the IP of the services to the IP of the actual pods
kube-admin tool deploy kube-proxy as daemonset in each nodes

pod

the container are encapsulated into a pod
is a single instance of an application, the smallest object in k8s
containers in same pod shares storages and network namespaces, created and removed in the same time
multi-container pod is rare use case

ReplicationController

apiVersion support in v1
the process to monitor the pods
maintain the HA and specified number of pods that running on all nodes
only care about the pod which RestartPolicy is set to Always
scalable and replacable application should be managed by the controller
use cases: rolling updates, multiple release tracks (multiple replication controller replica the same pod but using different labels)

ReplicaSets

next generation of ReplicationController
api version support in apps/v1
enhance the filtering in .spec.selector (the major difference)
be aware of the non-template pod that has same lables
using Deployment as a replacement is recommended, it own and manage its ReplicaSets

Deployment

provide replication vis replicaset and other:
- rolling update
- rollout
- pause and resume

Namespace

namespaces created at cluster creation
1. kube-system
kube-public

default

each namespace can be assigned quota of resources
a DNS entry with SERVICE_NAME.NAMESPACE.svc.cluster.local format is automatically created when at service creation
1. the cluster.local is the default domain name of the cluster
permanently config the namespace

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kubectl config set-context $(kubectl config current-context) --namespace=$NAMESPACE

k8s