Elastic Compute Service:Configure custom images for instance initialization

Prerequisites

Your image must satisfy the following requirements:

• The first partition, typically /dev/sda1 or /dev/vda1, must be writable.

• The file system type of the first partition must be FAT32, ext2, ext3, ext4, or UFS.

  Note

  You can use the blkid /dev/sdXn command to verify the file system type, where sdXn represents the first partition name, such as /sda1.

• The size of the virtual file of the customized Linux image must be larger than 5 GiB.

  Note

  You can check the size of the partition using the df -h /dev/sdXn command, where sdXn is the first partition name, such as /sda1.

Procedure

1. Log on to the virtual machine where the image is created as the root user.

2. Create the aliyun_custom_image directory in the root directory of the first partition of the server from which the image is created.

   mkdir /aliyun_custom_image

   When the ECS instance created from the customized Linux image is started for the first time, Alibaba Cloud writes instance configurations such as the hostname, password, and DNS server to the os.conf file in the aliyun_custom_image directory. If the os.conf file does not exist, Alibaba Cloud will create one.

   The following example shows the os.conf file content:

   hostname=<yourHostName>
   password=<yourPassword>
   eth0_ip_addr=10.0.0.2
   eth0_mac_addr=00:xx:xx:xx:xx:23
   eth0_netmask=255.255.255.0
   eth0_gateway=10.0.0.1
   eth0_route="0.0.0.0/0 10.0.0.1"
   dns_nameserver="7.7.X.X 8.8.8.8"

   The following table describes the parameters in the preceding example. You can configure the parameters based on your needs.

   Parameter	Description
   hostname	The hostname.
   password	The password of the root user.
   eth0_ip_addr	The IP address of the eth0 NIC.
   eth0_mac_addr	The MAC address of the eth0 NIC.
   eth0_netmask	The network mask of the eth0 NIC.
   eth0_gateway	The default gateway of the eth0 NIC.
   eth0_route	The eth0 internal routes. By default, routes are separated with spaces.
   dns_nameserver	The DNS address list. By default, addresses are separated with spaces.

3. Create a parsing script within the image, such as customized-config.service, to interpret and apply the system configuration from the os.conf file.

   This script automates initial configuration tasks such as setting the hostname, root password, DNS server, and network configurations when the ECS instance is started for the first time.

   Refer to the following sample script:

   #!/bin/bash
   
   ### BEGIN INIT INFO
   # Provides:          os-conf
   # Required-Start:    $local_fs $network $named $remote_fs
   # Required-Stop:
   # Should-Stop:
   # Default-Start:     2 3 4 5
   # Default-Stop:      0 1 6
   # Short-Description: Initialize system configuration
   ### END INIT INFO
   
   # Define the directory for the first partition and os.conf file
   first_partition_dir='/boot/'
   os_conf_dir=${first_partition_dir}/aliyun_custom_image
   os_conf_file=${os_conf_dir}/os.conf
   
   # Load os.conf if it exists
   load_os_conf() {
       if [[ -f $os_conf_file ]]; then
           . $os_conf_file
           return 0
       else
           return 1
       fi
   }
   
   # Cleanup function to remove os.conf and ensure directory exists
   cleanup() {
       rm $os_conf_file >& /dev/null
       mkdir -p $os_conf_dir
   }
   
   # Configure the root password
   config_password() {
       if [[ -n $password ]]; then
           password=$(echo $password | base64 -d)
           if [[ $? == 0 && -n $password ]]; then
               echo "root:$password" | chpasswd
           fi
       fi
   }
   
   # Set the hostname
   config_hostname() {
       if [[ -n $hostname ]]; then
           sed -i "s/^HOSTNAME=.*/HOSTNAME=$hostname/" /etc/sysconfig/network
           hostname $hostname
       fi
   }
   
   # Configure DNS settings
   config_dns() {
       if [[ -n $dns_nameserver ]]; then
           dns_conf=/etc/resolv.conf
           sed -i '/^nameserver.*/d' $dns_conf
           for i in $dns_nameserver; do
               echo "nameserver $i" >> $dns_conf
           done
       fi
   }
   
   # Determine if the network is classic or VPC
   is_classic_network() {
       grep -q 'eth1' $os_conf_file
   }
   
   # Configure network settings
   config_network() {
       /etc/init.d/network stop
       config_interface eth0 ${eth0_ip_addr} ${eth0_netmask} ${eth0_mac_addr}
       config_route eth0 "${eth0_route}"
       if is_classic_network ; then
           config_interface eth1 ${eth1_ip_addr} ${eth1_netmask} ${eth1_mac_addr}
           config_route eth1 "${eth1_route}"
       fi
       /etc/init.d/network start
   }
   
   # Configure network interface
   config_interface() {
       local interface=$1
       local ip=$2
       local netmask=$3
       local mac=$4
       interface_cfg="/etc/sysconfig/network-scripts/ifcfg-${interface}"
       cat << EOF > $interface_cfg
   DEVICE=$interface
   IPADDR=$ip
   NETMASK=$netmask
   HWADDR=$mac
   ONBOOT=yes
   BOOTPROTO=static
   EOF
   }
   
   # Set the default gateway
   config_default_gateway() {
       local gateway=$1
       sed -i "s/^GATEWAY=.*/GATEWAY=$gateway/" /etc/sysconfig/network
   }
   
   # Configure routing
   config_route() {
       local interface=$1
       local route="$2"
       route_conf=/etc/sysconfig/network-scripts/route-${interface}
       > $route_conf
       echo $route | sed 's/;/\n/' | \
           while read line; do
               dst=$(echo $line | awk '{print $1}')
               gw=$(echo $line | awk '{print $2}')
               if ! grep -q "$dst" $route_conf 2> /dev/null; then
                   echo "$dst via $gw dev $interface" >> $route_conf
               fi
               if [[ "$dst" == "0.0.0.0/0" ]]; then
                   config_default_gateway $gw
               fi
           done
   }
   
   ################## sysvinit service portal ####################
   
   # Start the configuration process
   start() {
       if load_os_conf ; then
           config_password
           config_network
           config_hostname
           config_dns
           cleanup
           return 0
       else
           echo "not load $os_conf_file"
           return 0
       fi
   }
   
   RETVAL=0
   
   # Service action handling
   case "$1" in
       start)
           start
           RETVAL=$?
       ;;
       *)
           echo "Usage: $0 {start}"
           RETVAL=3
       ;;
   esac
   
   exit $RETVAL

4. Set the parsing script to run automatically on system startup.

   The following sample commands apply to Ubuntu systems. Modify the startup commands based on your operating system.

   systemctl daemon-reload
   systemctl enable customized-config.service

   Note

   When a custom image is created based on a Customized Linux image, the automatic startup script is also included. Alibaba Cloud will execute the configurations of the os.conf file when the instance is started for the first time.

References

After you configure the image, you can select Customized Linux as the operating system version when importing the custom image. For more information, see Import custom images.