Fixing a Broken DNS in Linux Mint 13

Clearly I just have to start remembering more IP addresses

So last week I needed to go abroad (to CERN as it happens) and when I got there, my Mint 13 running laptop decided to throw a paddy and stop talking to the DNS. I could ping usual Google IPs (8.8.8.8 for example) but DNS just hung despite being reported to correctly (and pingable) in the Network Settings. Having a look at my resolv.conf, I could see that my DNS settings were set to my Uni one. I couldn't remember if these were put in automatically by the ethernet connection in my office or I'd just dumped that there randomly for some reason, but tellingly, these DNSs were NOT pingable (I guess they had fallen over or something).

So, I thought, not a problem - just change the DNSs to Google's and all should be well. Except it wasn't. The laptop was still refusing to talk to anyone by name. Luckily, my phone was not having these troubles and some frantic searching led me to the following Blog. It appears someone in Ubuntu land was trying to be clever and started using dnsmasq instead of resolv.conf. I don't know why this was done and really can't be bothered to find out, but in this occasion it meant I could not find whatever black magic was needed to force the Google DNS to be used.

By *mostly* following the blog, I discovered the following worked like a charm to get control of the DNS back to resolv.conf where (in my humble - and probably naive - opinion) it should still reside. First up, (and be aware, this should all be done under su/root), stop Mint from using dnsmasq by going to:

/etc/NetworkManager/NetworkManager.conf

And commenting out the dns line:

#dns=dnsmasq

Next, get resolvconf to sort out it's links and bring back resolv.conf:

dpkg-reconfigure resolvconf

Just to be sure, do a few more housekeeping bits and pieces and follow up with a restart:

resolvconf --create-runtime-directories
resolvconf --enable-updates
reboot

And now you should be able to edit /etc/resolv.conf as usual and have Mint listen to you. Note that you may want to also change:

/etc/resolvconf/resolv.conf.d/original

for a more permanent setting as I believe on restart resolvconf will recreate the resolv.conf with this. 

Last thing to note: While messing around, I tried to select 'Automatic (DHCP) addresses only' under IPv4 Settings for the wireless I was using. THIS WAS A BAD IDEA! It stopped the above working for other reasons I didn't understand. I plan to revisit all this at some point to try to better understand how DNS is handled in Mint/Ubuntu as I'm sure there was probably a better way around this...

Editing Bootable DVDs as ISO images

 When someone says ISO, I reach for my Q*Bert

A friend recently asked how to go about changing the contents of an install DVD with freely available tools that didn't have a size limit. This was something I hadn't explicitly done before and sounded like quite a fun thing to try (you may not entirely agree with the use of the word 'fun' there) so I thought I'd look into it and try to hack my Win7 install DVD a bit. For this I used Ubuntu (though I'm guessing almost any Linux install would work), dd, mount, mkisofs and a script called geteltorito.pl (available here). I was going to use ISO Master but hit problems which I will get to later. If you're of the Windows persuasion, I'm not sure how to do this using free tools, however there's nothing to stop you creating a live boot CD/USB stick of a Linux distro, going into that and just going into some area on your existing HD.

Anyway, the first step was to create an ISO image of the DVD. This was fairly easy using:

 
markwslater@markwslater-System-Product-Name:~$ dd if=/dev/cdrom of=~/win7_image.iso
6298216+0 records in
6298216+0 records out
3224686592 bytes (3.2 GB) copied, 229.392 s, 14.1 MB/s
markwslater@markwslater-System-Product-Name:~$ ls -ltrh ~/win7_image.iso 
-rw-rw-r-- 1 markwslater markwslater 3.1G Mar  5 23:26 /home/markwslater/win7_image.iso

So we now have the ISO image which we could (in theory) start messing with through various tools. Apparently (though I didn't confirm this) most of the free ones or trials for Windows have an upper limit on the size of ISO you can mess with (around CD size by all accounts). Looking around I happened across ISO Master and thought this would do the job. Unfortunately, plugging the above ISO image in only ended up with a README that said:
 

This disc contains a "UDF" file system and requires an operating system
that supports the ISO-13346 "UDF" file system specification.

Which was not the most helpful. So, after a bit of searching I found the following thread that explained how to do things with basic Linux tools. The key thing is to preserve the master boot record of the ISO and recreate it with this intact. The aforementioned geteltorito.pl perl script did a grand job of this:

 
markwslater@markwslater-System-Product-Name:~$ ./geteltorito.pl  win7_image.iso > ~/boot.bin
Booting catalog starts at sector: 22 
Manufacturer of CD: Microsoft Corporation
Image architecture: x86
Boot media type is: no emulation
El Torito image starts at sector 734 and has 8 sector(s) of 512 Bytes
Image has been written to stdout ....
markwslater@markwslater-System-Product-Name:~$

It's then quite easy to mount the existing ISO image (read only), copy this elsewhere, change the permissions and do what's necessary:

 
mkdir windvd
sudo mount -t auto -o loop win7_image.iso /home/markwslater/windvdcp -r windvd windvd_rw
chmod u+w windvd_rw -R
echo stuff > windvd_rw/my_file.txt

So all that remains is to recreate the ISO with the boot.bin file created above:

 
mkisofs -udf -b boot.bin -no-emul-boot -hide boot.bin -relaxed-filenames -joliet-long -D -o ~/new_win7.iso  ~/windvd_rw

Burn this to a DVD however you wish and bob's-your-uncle, you have recreated the win7 install DVD after messing with it :)