Prepare a CentOS-based virtual machine for Azure

Note

Azure has two different deployment models for creating and working with resources: Resource Manager and classic. This article covers using both models, but Microsoft recommends that most new deployments use the Resource Manager model.

Prerequisites

This article assumes that you have already installed a CentOS (or similar derivative) Linux operating system to a virtual hard disk. Multiple tools exist to create .vhd files, for example a virtualization solution such as Hyper-V. For instructions, see Install the Hyper-V Role and Configure a Virtual Machine.

CentOS installation notes

  • Please see also General Linux Installation Notes for more tips on preparing Linux for Azure.
  • The VHDX format is not supported in Azure, only fixed VHD. You can convert the disk to VHD format using Hyper-V Manager or the convert-vhd cmdlet. If you are using VirtualBox this means selecting Fixed size as opposed to the default dynamically allocated when creating the disk.
  • When installing the Linux system it is recommended that you use standard partitions rather than LVM (often the default for many installations). This will avoid LVM name conflicts with cloned VMs, particularly if an OS disk ever needs to be attached to another identical VM for troubleshooting. LVM or RAID may be used on data disks.
  • Kernel support for mounting UDF file systems is required. At first boot on Azure the provisioning configuration is passed to the Linux VM via UDF-formatted media that is attached to the guest. The Azure Linux agent must be able to mount the UDF file system to read its configuration and provision the VM.
  • Linux kernel versions below 2.6.37 do not support NUMA on Hyper-V with larger VM sizes. This issue primarily impacts older distributions using the upstream Red Hat 2.6.32 kernel, and was fixed in RHEL 6.6 (kernel-2.6.32-504). Systems running custom kernels older than 2.6.37, or RHEL-based kernels older than 2.6.32-504 must set the boot parameter numa=off on the kernel command-line in grub.conf. For more information see Red Hat KB 436883.
  • Do not configure a swap partition on the OS disk. The Linux agent can be configured to create a swap file on the temporary resource disk. More information about this can be found in the steps below.
  • All of the VHDs must have sizes that are multiples of 1 MB.

CentOS 6.x

  1. In Hyper-V Manager, select the virtual machine.

  2. Click Connect to open a console window for the virtual machine.

  3. In CentOS 6, NetworkManager can interfere with the Azure Linux agent. Uninstall this package by running the following command:

     # sudo rpm -e --nodeps NetworkManager
    
  4. Create or edit the file /etc/sysconfig/network and add the following text:

     NETWORKING=yes
     HOSTNAME=localhost.localdomain
    
  5. Create or edit the file /etc/sysconfig/network-scripts/ifcfg-eth0 and add the following text:

     DEVICE=eth0
     ONBOOT=yes
     BOOTPROTO=dhcp
     TYPE=Ethernet
     USERCTL=no
     PEERDNS=yes
     IPV6INIT=no
    
  6. Modify udev rules to avoid generating static rules for the Ethernet interface(s). These rules can cause problems when cloning a virtual machine in Microsoft Azure or Hyper-V:

     # sudo ln -s /dev/null /etc/udev/rules.d/75-persistent-net-generator.rules
     # sudo rm -f /etc/udev/rules.d/70-persistent-net.rules
    
  7. Ensure the network service will start at boot time by running the following command:

     # sudo chkconfig network on
    
  8. If you would like to use the OpenLogic mirrors that are hosted within the Azure datacenters, then replace the /etc/yum.repos.d/CentOS-Base.repo file with the following repositories. This will also add the [openlogic] repository that includes additional packages such as the Azure Linux agent:

     [openlogic]
     name=CentOS-$releasever - openlogic packages for $basearch
     baseurl=http://olcentgbl.trafficmanager.net/openlogic/$releasever/openlogic/$basearch/
     enabled=1
     gpgcheck=0
    
     [base]
     name=CentOS-$releasever - Base
     #mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
     baseurl=http://olcentgbl.trafficmanager.net/centos/$releasever/os/$basearch/
     gpgcheck=1
     gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
    
     #released updates
     [updates]
     name=CentOS-$releasever - Updates
     #mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra
     baseurl=http://olcentgbl.trafficmanager.net/centos/$releasever/updates/$basearch/
     gpgcheck=1
     gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
    
     #additional packages that may be useful
     [extras]
     name=CentOS-$releasever - Extras
     #mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra
     baseurl=http://olcentgbl.trafficmanager.net/centos/$releasever/extras/$basearch/
     gpgcheck=1
     gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
    
     #additional packages that extend functionality of existing packages
     [centosplus]
     name=CentOS-$releasever - Plus
     #mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra
     baseurl=http://olcentgbl.trafficmanager.net/centos/$releasever/centosplus/$basearch/
     gpgcheck=1
     enabled=0
     gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
    
     #contrib - packages by Centos Users
     [contrib]
     name=CentOS-$releasever - Contrib
     #mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=contrib&infra=$infra
     baseurl=http://olcentgbl.trafficmanager.net/centos/$releasever/contrib/$basearch/
     gpgcheck=1
     enabled=0
     gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
    
    Note

    The rest of this guide will assume you are using at least the [openlogic] repo, which will be used to install the Azure Linux agent below.

  9. Add the following line to /etc/yum.conf:

     http_caching=packages
    
  10. Run the following command to clear the current yum metadata and update the system with the latest packages:

    # yum clean all
    

    Unless you are creating an image for an older version of CentOS, it is recommended to update all the packages to the latest:

    # sudo yum -y update
    

    A reboot may be required after running this command.

  11. (Optional) Install the drivers for the Linux Integration Services (LIS).

    Important

    The step is required for CentOS 6.3 and earlier, and optional for later releases.

    # sudo rpm -e hypervkvpd  ## (may return error if not installed, that's OK)
    # sudo yum install microsoft-hyper-v
    

    Alternatively, you can follow the manual installation instructions on the LIS download page to install the RPM onto your VM.

  12. Install the Azure Linux Agent and dependencies:

    # sudo yum install python-pyasn1 WALinuxAgent
    

    The WALinuxAgent package will remove the NetworkManager and NetworkManager-gnome packages if they were not already removed as described in step 3.

  13. Modify the kernel boot line in your grub configuration to include additional kernel parameters for Azure. To do this, open /boot/grub/menu.lst in a text editor and ensure that the default kernel includes the following parameters:

    console=ttyS0 earlyprintk=ttyS0 rootdelay=300
    

    This will also ensure all console messages are sent to the first serial port, which can assist Azure support with debugging issues.

    In addition to the above, it is recommended to remove the following parameters:

    rhgb quiet crashkernel=auto
    

    Graphical and quiet boot are not useful in a cloud environment where we want all the logs to be sent to the serial port. The crashkernel option may be left configured if desired, but note that this parameter will reduce the amount of available memory in the VM by 128MB or more, which may be problematic on the smaller VM sizes.

    Important

    CentOS 6.5 and earlier must also set the kernel parameter numa=off. See Red Hat KB 436883.

  14. Ensure that the SSH server is installed and configured to start at boot time. This is usually the default.

  15. Do not create swap space on the OS disk.

    The Azure Linux Agent can automatically configure swap space using the local resource disk that is attached to the VM after provisioning on Azure. Note that the local resource disk is a temporary disk, and might be emptied when the VM is deprovisioned. After installing the Azure Linux Agent (see previous step), modify the following parameters in /etc/waagent.conf appropriately:

    ResourceDisk.Format=y
    ResourceDisk.Filesystem=ext4
    ResourceDisk.MountPoint=/mnt/resource
    ResourceDisk.EnableSwap=y
    ResourceDisk.SwapSizeMB=2048    ## NOTE: set this to whatever you need it to be.
    
  16. Run the following commands to deprovision the virtual machine and prepare it for provisioning on Azure:

    # sudo waagent -force -deprovision
    # export HISTSIZE=0
    # logout
    
  17. Click Action -> Shut Down in Hyper-V Manager. Your Linux VHD is now ready to be uploaded to Azure.


CentOS 7.0+

Changes in CentOS 7 (and similar derivatives)

Preparing a CentOS 7 virtual machine for Azure is very similar to CentOS 6, however there are several important differences worth noting:

  • The NetworkManager package no longer conflicts with the Azure Linux agent. This package is installed by default and we recommend that it is not removed.
  • GRUB2 is now used as the default bootloader, so the procedure for editing kernel parameters has changed (see below).
  • XFS is now the default file system. The ext4 file system can still be used if desired.

Configuration Steps

  1. In Hyper-V Manager, select the virtual machine.

  2. Click Connect to open a console window for the virtual machine.

  3. Create or edit the file /etc/sysconfig/network and add the following text:

     NETWORKING=yes
     HOSTNAME=localhost.localdomain
    
  4. Create or edit the file /etc/sysconfig/network-scripts/ifcfg-eth0 and add the following text:

     DEVICE=eth0
     ONBOOT=yes
     BOOTPROTO=dhcp
     TYPE=Ethernet
     USERCTL=no
     PEERDNS=yes
     IPV6INIT=no
     NM_CONTROLLED=no
    
  5. Modify udev rules to avoid generating static rules for the Ethernet interface(s). These rules can cause problems when cloning a virtual machine in Microsoft Azure or Hyper-V:

     # sudo ln -s /dev/null /etc/udev/rules.d/75-persistent-net-generator.rules
    
  6. If you would like to use the OpenLogic mirrors that are hosted within the Azure datacenters, then replace the /etc/yum.repos.d/CentOS-Base.repo file with the following repositories. This will also add the [openlogic] repository that includes packages for the Azure Linux agent:

     [openlogic]
     name=CentOS-$releasever - openlogic packages for $basearch
     baseurl=http://olcentgbl.trafficmanager.net/openlogic/$releasever/openlogic/$basearch/
     enabled=1
     gpgcheck=0
    
     [base]
     name=CentOS-$releasever - Base
     #mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
     baseurl=http://olcentgbl.trafficmanager.net/centos/$releasever/os/$basearch/
     gpgcheck=1
     gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
    
     #released updates
     [updates]
     name=CentOS-$releasever - Updates
     #mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra
     baseurl=http://olcentgbl.trafficmanager.net/centos/$releasever/updates/$basearch/
     gpgcheck=1
     gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
    
     #additional packages that may be useful
     [extras]
     name=CentOS-$releasever - Extras
     #mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra
     baseurl=http://olcentgbl.trafficmanager.net/centos/$releasever/extras/$basearch/
     gpgcheck=1
     gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
    
     #additional packages that extend functionality of existing packages
     [centosplus]
     name=CentOS-$releasever - Plus
     #mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra
     baseurl=http://olcentgbl.trafficmanager.net/centos/$releasever/centosplus/$basearch/
     gpgcheck=1
     enabled=0
     gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
    
    Note

    The rest of this guide will assume you are using at least the [openlogic] repo, which will be used to install the Azure Linux agent below.

  7. Run the following command to clear the current yum metadata and install any updates:

     # sudo yum clean all
    

    Unless you are creating an image for an older version of CentOS, it is recommended to update all the packages to the latest:

     # sudo yum -y update
    

    A reboot maybe required after running this command.

  8. Modify the kernel boot line in your grub configuration to include additional kernel parameters for Azure. To do this, open /etc/default/grub in a text editor and edit the GRUB_CMDLINE_LINUX parameter, for example:

     GRUB_CMDLINE_LINUX="rootdelay=300 console=ttyS0 earlyprintk=ttyS0 net.ifnames=0"
    

    This will also ensure all console messages are sent to the first serial port, which can assist Azure support with debugging issues. It also turns off the new CentOS 7 naming conventions for NICs. In addition to the above, it is recommended to remove the following parameters:

     rhgb quiet crashkernel=auto
    

    Graphical and quiet boot are not useful in a cloud environment where we want all the logs to be sent to the serial port. The crashkernel option may be left configured if desired, but note that this parameter will reduce the amount of available memory in the VM by 128MB or more, which may be problematic on the smaller VM sizes.

  9. Once you are done editing /etc/default/grub per above, run the following command to rebuild the grub configuration:

     # sudo grub2-mkconfig -o /boot/grub2/grub.cfg
    
  10. If building the image from VMWare, VirtualBox or KVM: Ensure the Hyper-V drivers are included in the initramfs:

    Edit /etc/dracut.conf, add content:

    add_drivers+=”hv_vmbus hv_netvsc hv_storvsc”
    

    Rebuild the initramfs:

    # sudo dracut –f -v
    
  11. Install the Azure Linux Agent and dependencies:

    # sudo yum install python-pyasn1 WALinuxAgent
    # sudo systemctl enable waagent
    
  12. Do not create swap space on the OS disk.

    The Azure Linux Agent can automatically configure swap space using the local resource disk that is attached to the VM after provisioning on Azure. Note that the local resource disk is a temporary disk, and might be emptied when the VM is deprovisioned. After installing the Azure Linux Agent (see previous step), modify the following parameters in /etc/waagent.conf appropriately:

    ResourceDisk.Format=y
    ResourceDisk.Filesystem=ext4
    ResourceDisk.MountPoint=/mnt/resource
    ResourceDisk.EnableSwap=y
    ResourceDisk.SwapSizeMB=2048    ## NOTE: set this to whatever you need it to be.
    
  13. Run the following commands to deprovision the virtual machine and prepare it for provisioning on Azure:

    # sudo waagent -force -deprovision
    # export HISTSIZE=0
    # logout
    
  14. Click Action -> Shut Down in Hyper-V Manager. Your Linux VHD is now ready to be uploaded to Azure.

Next steps

You're now ready to use your CentOS Linux virtual hard disk to create new virtual machines in Azure. If this is the first time that you're uploading the .vhd file to Azure, see steps 2 and 3 in Creating and uploading a virtual hard disk that contains the Linux operating system.