Quick boot server

Time for a little tutorial!

When fixing computers I’d often like to boot a live-CD so I can freely inspect and play around, instead of working with the shitty Windows they usually have.

But what if there isn’t a pen (or writable CD) at hand? Today we’re gonna setup a quick boot server so that computers can boot from us! Wow.

Advantages of this method:

No need to find a pen with enough space, you just need an Ethernet cable.
Quick to use. Set it up, connect the cable, boot via network and you’re done.
Also acts as a router, so when the computer boots it can access the Internet through your connection.
Portable. Start and stop serving whenever you want.

Disadvantages:

Can be slower than a pen sometimes.
Getting the BIOS to cooperate and boot from network is, in my experience, tricky.

Preparing

sudo apt-get install dnsmasq syslinux-common nfs-kernel-server

Now we have everything we need. But before continuing, edit /etc/default/dnsmasq and set ENABLED to 0 to prevent dnsmasq to start automatically. Then, stop it:

sudo service dnsmasq stop

And while we’re at, you should follow these steps to make sure NetworkManager won’t mess with the wired interface!

Basic setup

mkdir myserver && cd myserver

Okay, the first thing is to setup a simple DHCP server with dnsmasq. Create dnsmasq.conf with this content:

listen-address=192.168.99.1
bind-interfaces
dhcp-range=192.168.99.100,192.168.99.199,12h
dhcp-authoritative

Let’s now start dnsmasq to test the config works:

sudo ifconfig eth0 up 192.168.99.1/24
sudo dnsmasq -C dnsmasq.conf

Now connect an Ethernet cable to the other computer and verify it gets an IP as expected. Bonus! Dnsmasq already forwards DNS requests and sets the gateway so you just need to:

sudo iptables -t nat -A POSTROUTING -s 192.168.99.1/24 -j MASQUERADE

and now the computer can also access the Internet through the NAT we just created!

Adding TFTP

Now on to the interesting thing: network booting.

TFTP is frequently used by bootloaders to download the image when booting via network. Let’s create the directory we’ll serve over TFTP:

mkdir tftp

Now take the ISO of your favorite distribution (I recommend LUbuntu) and extract its contents at the directory we just created:

mkdir /tmp/iso
sudo mount -t iso9660 /path/to/lubuntu.iso /tmp/iso
cp -r /tmp/iso/* tftp
sudo umount /tmp/iso

Now let’s put in pxelinux.0, which will be the invoked by the bootloader:

cp /usr/lib/syslinux/pxelinux.0 tftp

And create the configuration files to tell pxelinux where to find the Linux kernel, and the options:

mkdir tftp/pxelinux.cfg
echo "DEFAULT casper/vmlinuz.efi initrd=casper/initrd.lz boot=casper root=/dev/nfs netboot=nfs nfsroot=192.168.99.1:/absolute/path/to/tftp quiet splash --" > tftp/pxelinux.cfg/default

Make sure casper/vmlinux.efi exists within tftp and if not, change it as appropiate. Same with casper/initrd.lz. Also change /absolute/path/to/tftp to, well, you know.

Make sure the paths have no spaces or strange characters in them.

Start it!

Let’s export the tftp folder through NFS (so that the booted Linux can access it):

sudo tee -a /etc/exports <<< "/absolute/path/to/tftp   192.168.99.0/24(ro,no_subtree_check,fsid=DDD)"
sudo service nfs-kernel-server restart

(where DDD is a random number)

And finally, let’s tell dnsmasq to serve TFTP! Add this to dnsmasq.conf:

enable-tftp
dhcp-boot=pxelinux.0
tftp-root=/absolute/path/to/tftp

We’re done! Restart dnsmasq:

sudo killall dnsmasq
sudo dnsmasq -C dnsmasq.conf

Now it’s time to reboot the other computer, make it boot via network, and pray it’ll work. Note: it takes some time to boot. Be patient.

Automating everything

Keep in mind this is designed to be temporal: we don’t want to turn our laptop into a permanent boot server, instead we start and stop the server when needed.

Starting and stopping the server is three or two commands; we can create two scripts to handle that for us.

Create startserver next to dnsmasq.conf:

#!/bin/bash
# Start a DHCP, DNS and TFTP boot server, and a NAT.
cd $(dirname $0)

sudo ifconfig eth0 up 192.168.99.1/24
sudo iptables -t nat -A POSTROUTING -s 192.168.99.0/24 -j MASQUERADE
sudo dnsmasq -C dnsmasq.conf

Create stopserver too:

#!/bin/bash
# Start a DHCP, DNS and TFTP boot server, and a NAT.
cd $(dirname $0)

sudo killall dnsmasq
sudo iptables -t nat -F

Make them executable:

chmod +x startserver stopserver

And that’s it! Next time you need to boot Linux on a computer (or need the NAT) just do ./startserver and connect the cable, and ./stopserver when done.

Conclusion

I find myself using scripts like these much of the time I need to fix problems on foreign computers, especially these two, and I hope they’ll be useful to someone else.

Keep in mind we’re not setting any firewalling, i.e. hosts from the “outside” can access our NAT-ed hosts. Adding firewalling is left as an excercise to the reader.