add dns items for coredns

coredns org

hosts plguin and reverse， we can set reload durations.  

add a dns item as follow:

# $(kubectl get cm coredns -n kube-system  -o json | jq .data.Corefile |grep -o host）

function add_coredns_host(){
CM=${CM:-coredns}
 
kubectl get cm $CM -n kube-system  -o json | jq .data.Corefile |grep -o hosts
ret=$?

if [[ $ret != 0 ]]; then
  # str=`kubectl get cm $CM -n kube-system -o json | jq .data.Corefile | 
  #      sed -e '0,/prometheus/s/prometheus/hosts {\
 '"$item"'\
 }\
 /'`
  str=$(kubectl get cm $CM -n kube-system -o json | jq .data.Corefile |
           sed -e '0,/prometheus/s/prometheus/hosts {\n       '"$1"'\n    }\n    prometheus/')
else
  str=$(kubectl get cm $CM -n kube-system -o json | jq .data.Corefile |
           sed -e 's/(hosts {)/1\n       '"$1"'/')
fi

NATPATH="/data/Corefile"
kubectl patch cm $CM -n kube-system --type='json' -p '[
{"op":"replace","path":"'"$NATPATH"'","value": '"$str"'},
]'
}


item="172.16.30.4 gateway.sdwam.opennes"
add_coredns_host  "$item"

Add host item with fallthrough append 

# $(kubectl get cm coredns -n kube-system  -o json | jq .data.Corefile |grep -o host）

function add_coredns_host(){
  CM=${CM:-coredns}
  item=fallthrough

  kubectl get cm $CM -n kube-system  -o json | jq .data.Corefile |grep -o hosts
  ret=$?
  if [[ $ret != 0 ]]; then
    # str=`kubectl get cm $CM -n kube-system -o json | jq .data.Corefile | 
    #      sed -e '0,/prometheus/s/prometheus/hosts {\
 '"$item"'\
 }\
 /'`
    # str=$(kubectl get cm $CM -n kube-system -o json | jq .data.Corefile |
    #          sed -e '0,/prometheus/s/prometheus/hosts {\n       '"$1"'\n    }\n    prometheus/')
    str=$(kubectl get cm $CM -n kube-system -o json | jq .data.Corefile |
          sed -e '0,/prometheus/s/prometheus/hosts {\n       '"$item"'\n    }\n    prometheus/')
    echo "Create an empity hosts list"
    NATPATH="/data/Corefile"
    kubectl patch cm $CM -n kube-system --type='json' -p '[
      {"op":"replace","path":"'"$NATPATH"'","value": '"$str"'},
    ]'
  fi


  str=$(kubectl get cm $CM -n kube-system -o json | jq .data.Corefile |
           sed -e 's/(hosts {)/1\n       '"$1"'/')


  NATPATH="/data/Corefile"
  kubectl patch cm $CM -n kube-system --type='json' -p '[
    {"op":"replace","path":"'"$NATPATH"'","value": '"$str"'},
  ]'
}

example:

item="172.16.30.4 gateway.sdwam.opennes"
# make sure the $item is in double quotes
add_coredns_host  "$item"

Get Coredns config

function get_dns_corefile(){
  CM=${CM:-coredns}
  NS=${NS:-kube-system}
  kubectl get cm $CM -n $NS -o json | jq .data.Corefile | xargs printf
}

Get host dns item

function list_dns_host(){
  str=$(kubectl get cm $CM -n kube-system -o json | jq .data.Corefile |grep -o "[ ]*hosts[^}]*}")
  if  [[ -z "$str" ]] ;then 
    echo "no hosts list in coredns data file, exit."
    exit 1
  fi
  echo "The current hosts list:"
  printf "$str
"
}

Remove host dns item

function rm_dns_host(){
 
  str=$(kubectl get cm $CM -n kube-system -o json | jq .data.Corefile |grep -o "[ ]*hosts[^}]*}")
  if  [[ -z "$str" ]] ;then 
    echo "no hosts list in coredns data file, exit."
    exit 1
  fi
  echo "The current hosts list:"
  printf "$str
"
  echo 

  echo "----------- New date file -----------"
  CM=${CM:-coredns}
  NS=${NS:-kube-system}
  str=$(kubectl get cm $CM -n kube-system -o json | jq .data.Corefile | sed -e 's/\n[^]*'"$1"'[^]*//g')
  printf "$str"
  
  NATPATH="/data/Corefile"
  kubectl patch cm $CM -n kube-system --type='json' -p '[
  {"op":"replace","path":"'"$NATPATH"'","value": '"$str"'},
  ]'
}

Restart pod

可以不用重新启动，2分钟后会生效

kubectl scale deployment coredns -n kube-system --replicas=0
watch -n 1 "kubectl -n kube-system get pod |grep core"


kubectl scale deployment coredns -n kube-system --replicas=2  
watch -n 1 "kubectl -n kube-system get pod |grep core"

or

# https://coredns.io/2017/05/08/custom-dns-entries-for-kubernetes/

kubectl exec -n kube-system coredns-980047985-g2748 -- kill -SIGUSR1 1

validation

# https://www.cyberciti.biz/faq/centos-7-dig-command-not-found-how-to-install-dig-on-centos/
yum whatprovides '*bin/dig'

yum install bind-utils -y

# https://serverfault.com/questions/731238/contacting-dns-server-via-command-line-interface/731244
dnssv=$(kubectl get svc -A |grep dns |awk '{print $4}')

dns=demo4.xx.com
dig +add +trace @$dnssv -p 53 $dns
dig @$dnssv -p 53 $dns

REF:

CoreDNS Manual  

CoreDNS for Kubernetes Service Discovery, Take 2 

Add a Custom Host to Kubernetes  

Linux: Using sed to insert lines before or after a match