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It’s also possible to use DHCP to specify a preconfiguration file to download from the network. DHCP allows specifying a filename. Normally this is a file to netboot, but if it appears to be an URL then installation media that support network preseeding will download the file from the URL and use it as a preconfiguration file. Here is an example of how to set it up in the dhcpd.conf for version 3 of the
ISC DHCP server (the isc-dhcp-server Ubuntu package).
if substring (option vendor-class-identifier, 0, 3) = "d-i" { filename "http://host/preseed.cfg";
}
Note: The above example limits this filename to DHCP clients that identify themselves as "d-i", so it will not affect regular DHCP clients, but only the installer. You can also put the text in a stanza for only one particular host to avoid preseeding all installs on your network.
A good way to use the DHCP preseeding is to only preseed values specific to your network, such as the Ubuntu mirror to use. This way installs on your network will automatically get a good mirror selected, but the rest of the installation can be performed interactively. Using DHCP preseeding to fully automate Ubuntu installs should only be done with care.