@JGStew

My random thoughts.


Downloading Files With Bigfix: Prefetches

The majority of BigFix content downloads file(s) and then does something with them.

The downloads could be many different things:

The “Best” way to download files with BigFix is to use something called a prefetch.

Why Prefetches? Speed and Security

A prefetch allows BigFix to securely download files while validating that the downloaded file is not corrupted or maliciously altered.

A prefetch allows the BigFix client to check it’s own caches before downloading a file it may already have, which speeds things up. It also allows the client to request the file from a Relay server which has it’s own cache. This allows many BigFix clients to all download the same file at the same time without having to reach far outside of the local network. This can be a huge advantage for slow WAN links, or even local links between floors and buildings.

What is a Prefetch?

A prefetch has a few components, the most important being the size of the file and it’s hash(es).

Prefetch Statements

The most common form of a prefetch is called a Prefetch Statement, and it takes this form:

prefetch theFileName sha1:theSha1 size:theSize http://theURL sha256:theSha256

Example:

Prefetch Blocks

Prefetch blocks are a less common form of prefetch. They are really only needed for rare extra functionality or very complex situations. It is useful to be able to understand a prefetch block if you come across one, but it is not likely that you will need to create one.

Prefetch blocks have all the same general parts as prefetch statements, but in a block structure:

begin prefetch block
    add prefetch item name=theFileName sha1=theSha1 sha256=theSha256 size=theSize url=http://theURL
end prefetch block

It is possible to convert a prefetch statement into a prefetch block and vis versa:

Example:

begin prefetch block
	add prefetch item name=unzip.exe sha1=e1652b058195db3f5f754b7ab430652ae04a50b8 sha256=8d9b5190aace52a1db1ac73a65ee9999c329157c8e88f61a772433323d6b7a4a size=167936 url=http://software.bigfix.com/download/redist/unzip-5.52.exe
end prefetch block

Generating Prefetches

You can figure out the needed info for a prefetch many different ways.

Once all the parts are determined, you can just combine them by hand into a prefetch. This isn’t ideal and it is much nicer to use BigFix specific tools to more completely automate the process.


Work In Progress

I plan to document many of these options in detail.