Linux flavours vary slightly when netbooting, here are the two most common versions I use.

default syslinux/menu.c32
prompt 0

MENU TITLE Linux Mint Installer Menu
menu separator

Label Linux - Mint 19.0 amd64
KERNEL mint/19.0/vmlinuz
APPEND initrd=mint/19.0/initrd.lz boot=casper locale=en_GB netboot=nfs nfsroot=192.168.1.111:/export/tftp/mint/19.0/iso toram systemd.mask=dev-hugepages.mount systemd.mask=dev-mqueue.mount systemd.mask=sys-fs-fuse-connections.mount systemd.mask=sys-kernel-config.mount systemd.mask=sys-kernel-debug.mount systemd.mask=tmp.mount

Label Linux - Mint 18.3 amd64
KERNEL mint/18.3/vmlinuz
APPEND initrd=mint/18.3/initrd.lz root=/dev/nfs boot=casper netboot=nfs nfsroot=192.168.1.111:/export/tftp/mint/18.3/iso nosplash --

LABEL Return
MENU LABEL ^Return to Main Menu
KERNEL menu.c32
APPEND pxelinux.cfg/default

There are differences between Mint 18.x and 19.x - here is a summary:

• KERNEL - Path to vmlinuz kernel image, as found at [root_of_mint_iso]/casper/vmlinuz
• initrd - Path to initrd.lz root filesystem image, as found at [root_of_mint_iso]/casper/initrd.lz
• nfsroot - NFS path to the extracted Mint 18.x iso image
• root - Instruct the installer to mount the root filesystem over NFS
• boot - Use the 'casper' installer (default on Mint)

• KERNEL - Path to vmlinuz kernel image, as found at [root_of_mint_iso]/casper/vmlinuz
• initrd - Path to initrd.lz root filesystem image, as found at [root_of_mint_iso]/casper/initrd.lz
• nfsroot - NFS path to the extracted Mint 19.x iso image
• boot - Use the 'casper' installer (default on Mint)
• netboot - Instruct the installer to mount the root filesystem over NFS (this is a change from 18.x)
• toram - Unpack the root filesystem to RAM; do not try to mount NFS read/write
• systemd.* - A heap of directives that stop systemd from trying to bring up several filesystems in read/write mode

There is a bug in the Mint 19.x installer if the installer filesystem is not mounted in RAM it will error out; hence the additional flags added to the kernel compared to Mint 18.x.

Most Ubuntu versions netboot cleanly without any specific options. Here is a summary of my current Ubuntu 16/18 LTS netboot menu:

default syslinux/menu.c32
prompt 0

MENU TITLE Linux Ubuntu Installer Menu
menu separator

Label Linux - Ubuntu 18.04 LTS amd64
KERNEL ubuntu/18.04/linux
APPEND initrd=ubuntu/18.04/initrd.gz vga=788 locale=en_GB.UTF-8 keyboard-configuration/layoutcode=gb

Label Linux - Ubuntu 16.04 LTS amd64
KERNEL ubuntu/16.04/linux
APPEND initrd=ubuntu/16.04/initrd.gz vga=788 locale=en_GB.UTF-8 keyboard-configuration/layoutcode=gb

LABEL Return
MENU LABEL ^Return to Main Menu
KERNEL menu.c32
APPEND pxelinux.cfg/default