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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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>How Jigdo Works (optional)</TITLE
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><H1
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><A
NAME="HOWJIGDOWORKS"
></A
>3. How Jigdo Works (optional)</H1
><P
>You don't need to know this material to download Debian ISOs, but it may help demystify how
jigdo works. If you're not interested in the details, simply fast forward to <A
HREF="downloadingyourfirstimage.html"
>Section 4</A
>, "How Do I Use Jigdo".</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PREPARINGTHEISOFORDOWNLOAD"
></A
>3.1. Preparing The ISO For Download</H2
><P
>A CD image is a filesystem called iso9660, but for this discussion, we can safely talk
about a CD image as being a big file called an "ISO image" (about 650MB) that contains files
at various offsets. For instance, if a CD contains a 567 byte file named README, the ISO
image might contain the README file's contents between offsets 20480000 and 20480567. You can
visualize a CD image as:</P
><TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> --------------------------------------------------------
ISO Image: |xxxx| file-0 |xx| file-1 |xxx| file-2 |x| file-3 |xxxx|
--------------------------------------------------------
</PRE
></FONT
></TD
></TR
></TABLE
><P
>The "x" areas of the image contain things like directory information, zero padding, disk
name, boot block, etc.</P
><P
><SPAN
CLASS="APPLICATION"
>jigdo-file</SPAN
> takes two things as input: the complete CD image
(so the ISO already needs to have been made) and a set of files which may or may not be in the
image. Here's a visualization of jigdo-file's input:</P
><TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> --------------------------------------------------------
ISO Image: |xxxx| file-0 |xx| file-1 |xxx| file-2 |x| file-3 |xxxx|
--------------------------------------------------------
---------- ---------- ---------- ----------
Loose Files: | file-0 | | file-1 | | file-3 | | file-4 |
---------- ---------- ---------- ----------
</PRE
></FONT
></TD
></TR
></TABLE
><P
>Through magic, jigdo-file finds out which of the loose files are contained in the ISO
image and their offsets within the ISO file. It outputs two files: a ".template" file and a
".jigdo" file.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="THE.TEMPLATEFILE"
></A
>3.2. The .template File</H2
><P
>Given an input of an ISO image and a set of files which may or may not be in the ISO
image, jigdo-file outputs a .template file for that ISO image. Here's what the
<TT
CLASS="FILENAME"
>.template</TT
> file looks like:</P
><TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> --------------------------------------------------------
.template: |xxxx| md5-0 |xx| md5-1 |xxx|cccccccc|x| md5-3 |xxxx|
--------------------------------------------------------
</PRE
></FONT
></TD
></TR
></TABLE
><P
>jigdo-file found that the files <TT
CLASS="FILENAME"
>file-0</TT
>, <TT
CLASS="FILENAME"
>file-1</TT
>
and <TT
CLASS="FILENAME"
>file-3</TT
> were contained in the ISO image. It removed the contents of
the these files and replaced them with each file's md5 checksum (the
<TT
CLASS="FILENAME"
>md5-0</TT
>, <TT
CLASS="FILENAME"
>md5-1</TT
>, etc).</P
><P
>The "<TT
CLASS="LITERAL"
>x</TT
>" data (directory information, zero padding, etc)
within the ISO image is compressed and written to the .template file. Finally, any files
within the ISO image that weren't supplied as loose files (like <TT
CLASS="FILENAME"
>file-2</TT
>)
are also compressed and written to the .template file. This is shown as "<TT
CLASS="LITERAL"
>c</TT
>" data in the .template file visualization.</P
><P
>Loose files which were supplied to <SPAN
CLASS="APPLICATION"
>jigdo-file</SPAN
> that aren't
found in the ISO image (like <TT
CLASS="FILENAME"
>file-4</TT
>) are ignored.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="THE.JIGDOFILE"
></A
>3.3. The .jigdo File</H2
><P
>Given an input of an ISO image and a set of loose files which may or may not be in the
ISO image, jigdo-file outputs a .jigdo file for that ISO image. The Debian .jigdo files are
gzipped, so you need to use zcat or zless to view them. Here's what a .jigdo file looks like
when you gunzip it:</P
><TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> md5-0=http://somemirror.org/file-0
md5-1=http://somemirror.org/file-1
md5-2=http://somemirror.org/file-2
md5-3=http://somemirror.org/file-3
</PRE
></FONT
></TD
></TR
></TABLE
><P
>The .jigdo file simply provides a mapping between the md5sum of a file within the ISO
image and the download URL of that file. There are some other things within the
<TT
CLASS="FILENAME"
>.jigdo</TT
> file,
and if you look through it, you'll see the <TT
CLASS="FILENAME"
>.jigdo</TT
> file has the same
format as a ".ini" file. It should be self explanatory, but if you want the nitty-gritty
details, see the jigdo documentation.</P
><P
>The format shown above is not quite what you'd see in a typical .jigdo file, but it's
very similar. If you look at the [Servers] section at the bottom of the .jigdo file, you'll
see exactly what the difference is between what I showed above and an actual
<TT
CLASS="FILENAME"
>.jigdo</TT
> file.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="DOWNLOADINGTHEIMAGE"
></A
>3.4. Downloading The Image</H2
><P
>Once you use <SPAN
CLASS="APPLICATION"
>jigdo-file</SPAN
> to generate a
<TT
CLASS="FILENAME"
>.jigdo</TT
> and .<TT
CLASS="FILENAME"
>template</TT
> file for an ISO image, anyone
can use <SPAN
CLASS="APPLICATION"
>jigdo-lite</SPAN
> to download that image. jigdo-lite downloads
all the files of a Debian ISO using <SPAN
CLASS="APPLICATION"
>wget</SPAN
>, assembles them and forms
a copy of the original ISO image on the fly.</P
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>Downloading Your First Image (In 5 Easy Steps)</TD
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