Build SSH config Hosts line from AWS subnet query

build-ssh-hosts-from-aws-subnets.sh

#!/bin/bash

# If your AWS account has a lot of subnets and you want to have an SSH config entry for all of the subnets, then
# building the Hosts line by hand can be quite cumbersome.  This script will automatically fetch the subnets
# from AWS for your account and build a Hosts line with wildcards to cover all of the IP ranges

ALL=""
for CIDR in `aws ec2 describe-subnets --query Subnets[].CidrBlock --output text`; do
  #echo $CIDR
  HOST_MIN=$(ipcalc $CIDR | ggrep -Po '(?<=HostMin:)[ ]*([^ ]*)'  | sed 's/ //g')
  HOST_MAX=$(ipcalc $CIDR | ggrep -Po '(?<=HostMax:)[ ]*([^ ]*)'  | sed 's/ //g')
  #echo "Min: \"$HOST_MIN\""
  #echo "Max: \"$HOST_MAX\""


  ENDS_WITH_MIN="\.1$"
  ENDS_WITH_MAX="\.254$"
  
  if [[ $HOST_MIN =~ $ENDS_WITH_MIN ]] && [[ $HOST_MAX =~ $ENDS_WITH_MAX ]]; then
    #echo "Easy min/max"
    CURRENT=$(echo $HOST_MIN | sed 's/[0-9]*\.[0-9]*\.\([0-9]*\)\.[0-9]*/\1/' )
    END=$(echo $HOST_MAX | sed 's/[0-9]*\.[0-9]*\.\([0-9]*\)\.[0-9]*/\1/' )

    PREFIX=$(echo $HOST_MIN | sed 's/\([0-9]*\.[0-9]*\.\)[0-9]*\.[0-9]*/\1/' )
    PREFIX_MAX=$(echo $HOST_MAX | sed 's/\([0-9]*\.[0-9]*\.\)[0-9]*\.[0-9]*/\1/' )
    if [ "$PREFIX" != "$PREFIX_MAX" ]; then
      echo "Unsupported mismatching min/max prefixes: $PREFIX $PREFIX_MAX"
      exit 1
    fi

    #echo Start $CURRENT
    #echo End $END
    while [ "$CURRENT" -le "$END" ]; do
      #echo "${PREFIX}${CURRENT}.*"
      ALL="$ALL ${PREFIX}${CURRENT}.*"
      CURRENT=$((CURRENT+1))
    done 
  fi 
done

echo "SSH host line should be: $ALL"