Edit and repack .iso bootable image

Edit_Repack_ISO_tutorial.md

On Linux

Installing mkisofs

apt-get install mkisofs

Editing ISO image

mkdir /tmp/custom_iso

cd /tmp/custom_iso/

mount -t iso9660 -o loop ~/original.iso /mnt/

cd /mnt

mnt is read-only, thus we need to copy the files in it in order to modify the files.

tar cf - . | (cd /tmp/custom_iso; tar xfp -)

You can now modify the files for making a preseed for example!

Back in .iso

If not, go back to your /tmp/custom_iso folder

cd /tmp/custom_iso

Copy this line, modify it and paste it in order to create the .iso

mkisofs -o output.iso -b isolinux.bin -c boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -J -R -V "Ubuntu Custom ISO Preseed" .

Make it bootable with isohybrid

Install isohybrid:

sudo apt install syslinux-utils

Then simply type the following to make it bootable:

sudo isohybrid output.iso

On macOS

Install brew

Follow https://brew.sh/

Install requirements

In order to install mkisofs, you need to get the cdrtools package for macOS:

brew install cdrtools

Prepare folders

Make two folders: iso and custom.

ro_iso is going to be where the files of the iso will be mounted, read only.

custom will be where you will be able to modify the files

mkdir ro_iso && mkdir custom

Attaching, mounting and editing

hdiutil attach -nomount ubuntu.iso

List the disks and get the disk image name usually /dev/disk2:

diskutil list

We will use /dev/disk2.

Now you can mount the disk into the read-only folder:

mount -t cd9660 /dev/disk2 ro_iso

We now need to copy the files in the custom folder in order to modify the files:

tar cf - . | (cd $HOME/yourfolder/custom; tar xfp -)

You can now begin editing the files.

Back in .iso

If not yet, go back to your custom folder.

Copy this line, modify it and paste it in order to create the .iso

sudo mkisofs -o output.iso -b isolinux.bin -c boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -J -R -V "Ubuntu Custom ISO Preseed" .

Make it bootable with isohybrid

Use the isohybrid.pl file of this gist like the following:

sudo perl isohybrid.pl output.iso

umount and detach

Don't forget to do it, or you will can't be able to delete the ro_iso folder

umount /dev/disk2

hdiutil detach /dev/disk2

You now have a bootable USB iso file :)

isohybrid.pl

#!/usr/bin/perl
## -----------------------------------------------------------------------
##
##   Copyright 2002-2008 H. Peter Anvin - All Rights Reserved
##   Copyright 2009 Intel Corporation; author: H. Peter Anvin
##
##   This program is free software; you can redistribute it and/or modify
##   it under the terms of the GNU General Public License as published by
##   the Free Software Foundation, Inc., 53 Temple Place Ste 330,
##   Boston MA 02111-1307, USA; either version 2 of the License, or
##   (at your option) any later version; incorporated herein by reference.
##
## -----------------------------------------------------------------------
##   From: https://gist.github.com/jsarenik/e184b4061263dbd7d3a3
## 

#
# Post-process an ISO 9660 image generated with mkisofs/genisoimage
# to allow "hybrid booting" as a CD-ROM or as a hard disk.
#

use bytes;
use Fcntl;

# User-specifyable options
%opt = (
    # Fake geometry (zipdrive-style...)
    'h'      => 64,
    's'      => 32,
    # Partition number
    'entry'  => 1,
    # Partition offset
    'offset' => 0,
    # Partition type
    'type'   => 0x17,		# "Windows hidden IFS"
    # MBR ID
    'id'     => undef,
);

%valid_range = (
    'h'      => [1, 256],
    's'      => [1, 63],
    'entry'  => [1, 4],
    'offset' => [0, 64],
    'type'   => [0, 255],
    'id'     => [0, 0xffffffff],
    'hd0'    => [0, 2],
    'partok' => [0, 1],
);

# Boolean options just set other options
%bool_opt = (
    'nohd0'    => ['hd0', 0],
    'forcehd0' => ['hd0', 1],
    'ctrlhd0'  => ['hd0', 2],
    'nopartok' => ['partok', 0],
    'partok'   => ['partok', 1],
);

sub usage() {
    print STDERR "Usage: $0 [options] filename.iso\n",
    "Options:\n",
    "  -h          Number of default geometry heads\n",
    "  -s          Number of default geometry sectors\n",
    "  -entry      Specify partition entry number (1-4)\n",
    "  -offset     Specify partition offset (default 0)\n",
    "  -type       Specify partition type (default 0x17)\n",
    "  -id         Specify MBR ID (default random)\n",
    "  -forcehd0   Always assume we are loaded as disk ID 0\n",
    "  -ctrlhd0    Assume disk ID 0 if the Ctrl key is pressed\n",
    "  -partok     Allow booting from within a partition\n";
    exit 1;
}

# Parse a C-style integer (decimal/octal/hex)
sub doh($) {
    my($n) = @_;
    return ($n =~ /^0/) ? oct $n : $n+0;
}

sub get_random() {
    # Get a 32-bit random number
    my $rfd, $rnd;
    my $rid;

    if (open($rfd, "< /dev/urandom\0") && read($rfd, $rnd, 4) == 4) {
	$rid = unpack("V", $rnd);
    }

    close($rfd) if (defined($rfd));
    return $rid if (defined($rid));

    # This sucks but is better than nothing...
    return ($$+time()) & 0xffffffff;
}

sub get_hex_data() {
    my $mbr = '';
    my $line, $byte;
    while ( $line = <DATA> ) {
	chomp $line;
	last if ($line eq '*');
	foreach $byte ( split(/\s+/, $line) ) {
	    $mbr .= chr(hex($byte));
	}
    }
    return $mbr;
}

while ($ARGV[0] =~ /^\-(.*)$/) {
    $o = $1;
    shift @ARGV;
    if (defined($bool_opt{$o})) {
	($o, $v) = @{$bool_opt{$o}};
	$opt{$o} = $v;
    } elsif (exists($opt{$o})) {
	$opt{$o} = doh(shift @ARGV);
	if (defined($valid_range{$o})) {
	    ($l, $h) = @{$valid_range{$o}};
	    if ($opt{$o} < $l || $opt{$o} > $h) {
		die "$0: valid values for the -$o parameter are $l to $h\n";
	    }
	}
    } else {
	usage();
    }
}

($file) = @ARGV;

if (!defined($file)) {
    usage();
}

open(FILE, "+< $file\0") or die "$0: cannot open $file: $!\n";
binmode FILE;

#
# First, actually figure out where mkisofs hid isolinux.bin
#
seek(FILE, 17*2048, SEEK_SET) or die "$0: $file: $!\n";
read(FILE, $boot_record, 2048) == 2048 or die "$0: $file: read error\n";
($br_sign, $br_cat_offset) = unpack("a71V", $boot_record);
if ($br_sign ne ("\0CD001\1EL TORITO SPECIFICATION" . ("\0" x 41))) {
    die "$0: $file: no boot record found\n";
}
seek(FILE, $br_cat_offset*2048, SEEK_SET) or die "$0: $file: $!\n";
read(FILE, $boot_cat, 2048) == 2048 or die "$0: $file: read error\n";

# We must have a Validation Entry followed by a Default Entry...
# no fanciness allowed for the Hybrid mode [XXX: might relax this later]
@ve = unpack("v16", $boot_cat);
$cs = 0;
for ($i = 0; $i < 16; $i++) {
    $cs += $ve[$i];
}
if ($ve[0] != 0x0001 || $ve[15] != 0xaa55 || $cs & 0xffff) {
    die "$0: $file: invalid boot catalog\n";
}
($de_boot, $de_media, $de_seg, $de_sys, $de_mbz1, $de_count, 
 $de_lba, $de_mbz2) = unpack("CCvCCvVv", substr($boot_cat, 32, 32));
if ($de_boot != 0x88 || $de_media != 0 ||
    ($de_segment != 0 && $de_segment != 0x7c0) || $de_count != 4) {
    die "$0: $file: unexpected boot catalog parameters\n";
}

# Now $de_lba should contain the CD sector number for isolinux.bin
seek(FILE, $de_lba*2048+0x40, SEEK_SET) or die "$0: $file: $!\n";
read(FILE, $ibsig, 4);
if ($ibsig ne "\xfb\xc0\x78\x70") {
    die "$0: $file: bootloader does not have a isolinux.bin hybrid signature.".
        "Note that isolinux-debug.bin does not support hybrid booting.\n";
}

# Get the total size of the image
(@imgstat = stat(FILE)) or die "$0: $file: $!\n";
$imgsize = $imgstat[7];
if (!$imgsize) {
    die "$0: $file: cannot determine length of file\n";
}
# Target image size: round up to a multiple of $h*$s*512
$h = $opt{'h'};
$s = $opt{'s'};
$cylsize = $h*$s*512;
$frac = $imgsize % $cylsize;
$padding = ($frac > 0) ? $cylsize - $frac : 0;
$imgsize += $padding;
$c = int($imgsize/$cylsize);
if ($c > 1024) {
    print STDERR "Warning: more than 1024 cylinders ($c).\n";
    print STDERR "Not all BIOSes will be able to boot this device.\n";
    $cc = 1024;
} else {
    $cc = $c;
}

# Preserve id when run again
if (defined($opt{'id'})) {
    $id = pack("V", doh($opt{'id'}));
} else {
    seek(FILE, 440, SEEK_SET) or die "$0: $file: $!\n";
    read(FILE, $id, 4);
    if ($id eq "\x00\x00\x00\x00") {
	$id = pack("V", get_random());
    }
}

# Print the MBR and partition table
seek(FILE, 0, SEEK_SET) or die "$0: $file: $!\n";

for ($i = 0; $i <= $opt{'hd0'}+3*$opt{'partok'}; $i++) {
    $mbr = get_hex_data();
}
if ( length($mbr) > 432 ) {
    die "$0: Bad MBR code\n";
}

$mbr .= "\0" x (432 - length($mbr));

$mbr .= pack("VV", $de_lba*4, 0); 	# Offset 432: LBA of isolinux.bin
$mbr .= $id;				# Offset 440: MBR ID
$mbr .= "\0\0";				# Offset 446: actual partition table

# Print partition table
$offset  = $opt{'offset'};
$psize   = $c*$h*$s - $offset;
$bhead   = int($offset/$s) % $h;
$bsect   = ($offset % $s) + 1;
$bcyl    = int($offset/($h*$s));
$bsect  += ($bcyl & 0x300) >> 2;
$bcyl   &= 0xff;
$ehead   = $h-1;
$esect   = $s + ((($cc-1) & 0x300) >> 2);
$ecyl    = ($cc-1) & 0xff;
$fstype  = $opt{'type'};	# Partition type
$pentry  = $opt{'entry'};	# Partition slot

for ( $i = 1 ; $i <= 4 ; $i++ ) {
    if ( $i == $pentry ) {
	$mbr .= pack("CCCCCCCCVV", 0x80, $bhead, $bsect, $bcyl, $fstype,
		     $ehead, $esect, $ecyl, $offset, $psize);
    } else {
	$mbr .= "\0" x 16;
    }
}
$mbr .= "\x55\xaa";

print FILE $mbr;

# Pad the image to a fake cylinder boundary
seek(FILE, $imgstat[7], SEEK_SET) or die "$0: $file: $!\n";
if ($padding) {
    print FILE "\0" x $padding;
}

# Done...
close(FILE);

exit 0;
__END__
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Awesome stuff! This works outside of docker, but unfortunately I didn't go through the trouble of making this work on the CI machine, on gitlab, in docker. It didn't work because I couldn't use loopback devices, so I couldn't use mount -o loop.

However I found this article, that shows how unpacking an iso can be done with a command line executable 7z:
https://www.tecmint.com/extract-files-from-iso-files-linux/

Thank you @AkdM!

I confirm that this is the only way to repack a CentOS ISO on Mac and make it bootable from USB.
I tried everything I could find in internet during last 3 days and none of the resources mention that it's mandatory to run isohybrid to make it bootable from USB, and of course no one explains how to get isohybrid on Mac.

Thanks brother.

Very nicely formatted Perl!

What about Windows ISOs?