Setting the root (or other) passwords in a Linux guest

I wrote this example for the CentOS Dojo on Friday (which by the way will be recorded and available on YouTube afterwards).

You can use or modify this script to change the password in /etc/shadow for root or any other user of a guest.

The $5$ causes it to use a SHA-256-encrypted password, but you can change this to $6$ to use SHA-512 (both assume you are using glibc on the host).

#!/usr/bin/perl -w

use strict;
#use Sys::Virt;
use Sys::Guestfs;

my $vm = "dojo";
my $user = "root";
my $newpw = "1234567";

my $salt;
my @chars = ("A".."Z", "a".."z", "0".."9", ".", "/");
$salt .= $chars[rand @chars] for 1..16;
my $crypted = crypt ($newpw, '$5$' . $salt . '$');

my $g = Sys::Guestfs->new ();
$g->set_trace (1);
$g->add_domain ($vm, libvirturi => "qemu:///session");
$g->launch ();
$g->mount ("/dev/fedora/root", "/");

my @shadow = $g->read_lines ("/etc/shadow");
s/^root:.*?:/root:$crypted:/ foreach @shadow;

$g->write ("/etc/shadow", join ("\n", @shadow) . "\n");
$g->chmod (0, "/etc/shadow");

$g->touch ("/.autorelabel");