Configure a custom Linux image

Prerequisites

Ensure that your image meets the following requirements:

• The first partition, typically /dev/sda1 or /dev/vda1, must have write permissions.

• The file system of the first partition must be one of the following: FAT32, EXT2, EXT3, EXT4, or UFS.

  Note

  You can run the blkid /dev/sdXn command to check the file system type, where /dev/sdXn represents the path to the first partition, such as /dev/sda1.

• The virtual size of the image file must be greater than 5 GiB.

  Note

  You can run the df -h /dev/sdXn command to check the space of a specific partition, where /dev/sdXn represents the path to the first partition, such as /dev/sda1.

Procedure

1. Log on to the virtual machine that you use to create the image as the root user.

2. In the root directory of the first partition of the image, create a directory named aliyun_custom_image.

   mkdir /aliyun_custom_image

   When an ECS instance created from a Customized Linux image starts for the first time, Alibaba Cloud writes instance information, such as the hostname, root user password, and DNS server addresses, to the os.conf file in the aliyun_custom_image directory. If the os.conf file does not exist, the system automatically creates it.

   The following sample shows the content of the os.conf file:

   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 sample. Configure them based on your actual environment.

   Parameter	Description
   hostname	The hostname of the instance.
   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 subnet mask of the eth0 NIC.
   eth0_gateway	The IP address of the default gateway for the eth0 NIC.
   eth0_route	A list of internal routes for eth0. By default, routes are separated by spaces.
   dns_nameserver	A list of DNS server addresses. By default, addresses are separated by spaces.

3. Create a parsing script in the image, for example, a service file named customized-config.service, to read and parse the system configurations from the os.conf file.

   This parsing script runs when the ECS instance starts for the first time to automate configurations, such as setting the hostname, root user password, DNS server addresses, and network settings.

   The following sample shows the content of the 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: The initial os-conf job, config the system.
   ### END INIT INFO
   
   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 [[ -f $os_conf_file ]]; then
           . $os_conf_file
           return 0
       else
           return 1
       fi
   }
   
   cleanup() {
       # ensure $os_conf_file is deleted, to avoid repeating config system
       rm $os_conf_file >& /dev/null
       # ensure $os_conf_dir exists
       mkdir -p $os_conf_dir
   }
   
   config_password() {
       if [[ -n $password ]]; then
           password=$(echo $password | base64 -d)
           if [[ $? == 0 && -n $password ]]; then
               echo "root:$password" | chpasswd
           fi
       fi
   }
   
   config_hostname() {
       if [[ -n $hostname ]]; then
           sed -i "s/^HOSTNAME=.*/HOSTNAME=$hostname/" /etc/sysconfig/network
           hostname $hostname
       fi
   }
   
   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
   }
   
   is_classic_network() {
       # vpc: eth0
       # classic: eth0 eth1
       grep -q 'eth1' $os_conf_file
   }
   
   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
   }
   
   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
   }
   
   config_default_gateway() {
       local gateway=$1
       sed -i "s/^GATEWAY=.*/GATEWAY=$gateway/" /etc/sysconfig/network
   }
   
   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() {
       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
   
   case "$1" in
       start)
           start
           RETVAL=$?
       ;;
       *)
           echo "Usage: $0 {start}"
           RETVAL=3
       ;;
   esac
   
   exit $RETVAL

4. Set the script to run on system startup.

   The following commands are for Ubuntu. Use the appropriate commands for your operating system.

   sudo systemctl daemon-reload
   sudo systemctl enable customized-config.service

   Note

   If you create a new custom image from an instance launched from a Customized Linux image, the new image also contains this startup script. Alibaba Cloud executes the script to apply the os.conf settings only during the instance's first boot.

Related documents

After you configure the image, you can select Customized Linux as the OS Version when you import the custom image. For more information, see Import a custom image.