Hello @Jesús Martínez Fernández ,
Thank you for reaching out to the Microsoft Q&A platform. Happy to answer your question.
Network appliances like forinet fortigate is not certified by azure backup for a VM backup. For Azure VM Linux backups, Azure Backup supports the list of Linux distributions endorsed by Azure.
Reference: https://learn.microsoft.com/en-us/azure/backup/backup-support-matrix-iaas#operating-system-support-linux
https://learn.microsoft.com/en-us/azure/virtual-machines/linux/endorsed-distros
Other bring-your-own Linux distributions might work as long as the Azure VM agent for Linux is available on the VM, and as long as Python is supported.
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