Today I was faced with the following problem: I was trying to configure one of the Ethernet interfaces of an OpenBSD 4.5 box with both a dynamic address leased via DHCP, but also a static IP address. Initially, I tried this:
# cat /etc/hostname.vr2 dhcp inet 1.1.1.11 255.255.255.0 NONE up # sh /etc/netstart vr2
The problem with this approach is that dhclient never gets daemonized because netstart gets it annoyed: dhclient notices that something else reconfigured the interface and commits suicide. So, then I thought about reversing the order of the first two lines:
# cat /etc/hostname.vr2 inet 1.1.1.11 255.255.255.0 NONE dhcp up # sh /etc/netstart vr2
Now dhclient daemonizes but also removes all previously configured IP addresses, so the statically configured address configured via the first line is wiped by dhclient. Not very nice.
Turns out the solution lies in /etc/dhclient.conf:
# cat /etc/dhclient.conf
interface "vr2" {
supersede domain-name "example.com";
supersede domain-name-servers 1.1.1.1;
}
alias {
interface "vr2";
fixed-address 1.1.1.11;
option subnet-mask 255.255.255.0;
}
The alias stanza allows one to define an additional, aliased IP address for an interface. Which allows the machine to be always reachable on a fixed IP address.
Neat.
Configuring a diskless Ubuntu
December 9th, 2007
This post is not about doing a PXE-based network installation of Ubuntu. There are already many posts describing how to do this. This post is about setting up an Ubuntu workstation in diskless mode, such as the workstation boots via PXE and the root filesystem is mounted over NFS.
The process consists on the following main steps:
- Setting up the DHCP server
- Setting up the TFTP server and configuring PXE boot
- Setting up the NFS server
- Bootstrapping a Ubuntu installation into the client’s root filesystem
- Booting up the diskless workstation
1. Setting up the DHCP server
PXE-enabled workstations need to get an additional option during the DHCP negotiation that will point them to a TFTP server where the PXE-compatible boot loader code can be downloaded. Configuring dnsmasq to hand this option to clients is just as easy as adding the following line to /etc/dnsmasq.conf:
dhcp-boot=pxelinux.0,tftp.lan,10.42.242.13
and restarting the dnsmasq service.
2. Setting up the TFTP server and configuring PXE boot
Now that DHCP has been configured, the next step is setting up the TFTP server. The TFTP server will be used by PXE-compatible clients to download the PXE boot loader code, and also the Linux kernel and Linux initial RAM disk.
I will use H. P. Anvin’s TFTP server as it’s widely used and works fairly well:
root@tftp.lan:# apt-get install tftpd-hpa
tftpd-hpa does not integrate automatically with xinetd so if you want to run the TFTP server under xinetd, you will have to create the following file, which was ported from the inetd description that is created automatically in /etc/inetd.conf when tftpd-hpa is installed:
service tftp
{
disable = no
id = chargen-dgram
socket_type = dgram
protocol = udp
user = root
wait = yes
server = /usr/sbin/in.tftpd
server_args = -s /var/lib/tftpboot/
}
then restarting the xinetd service.
Configuring PXE boot is just a matter of copying the PXE boot loader code, a configuration file, the Linux kernel and initial RAM disks under the TFTP root. First, install syslinux and copy the PXE boot loader code to the TFTP server root:
root@tftp.lan:# apt-get install syslinux root@tftp.lan:# cp /usr/lib/syslinux/pxelinux.0 /var/lib/tftpboot/ root@tftp.lan:# mkdir /var/lib/tftpboot/pxelinux.cfg
The Linux PXE boot loader code, syslinux, expects that a configuration file describing what kernel, its boot parameters, and the initial RAM disk to use to be stored within a directory named pxelinux.cfg just under the TFTP server root.
Client-specific config files can be created (based on the client MAC address, for example). We will create a config file that is suitable for any client. This configuration file is called default and can be created by running the following commands:
root@tftp.lan:# KERNEL_VERSION=2.6.22-14-generic
root@tftp.lan:# NFS_IPADDR=$(host nfs.lan | cut -d' ' -f4)
root@tftp.lan:# cat > /var/lib/tftpboot/pxelinux.cfg/default << EOF
> LABEL linux
> KERNEL vmlinuz-${KERNEL_VERSION}
> APPEND root=/dev/nfs initrd=initrd.img-${KERNEL_VERSION} nfsroot=${NFS_IPADDR}:/home/nfsroot ip=dhcp rw
> EOF
3. Setting up the NFS server
In this step, we will configure the NFS server and export the directory where the client’s root filesystem will be stored.
Let’s start by installing the NFS server packages:
root@nfs.lan:# apt-get install nfs-kernel-server nfs-common
I will use /home/nfsroot as the root for the client’s root filesystem
root@nfs.lan:# mkdir /home/nfsroot
Next, add the following line to /etc/exports in order to export the the client’s root filesystem:
/home/nfsroot *,gss/krb5(rw,no_subtree_check,async,no_root_squash)
Then re-export all the filesystems:
root@nfs.lan:# exportfs -avr
4. Bootstrapping a Ubuntu installation into the client’s root filesystem
The following steps will bootstrap the installation of a minimal Ubuntu Hardy Heron GNU/Linux system into the client’s root:
root@nfs.lan:# debootstrap --arch i386 hardy \ /home/nfsroot http://ch.archive.ubuntu.com/ubuntu/
Only the minimum required packages will be downloaded from the Internet and installed into /home/nfsroot. The output of the previous command should look like this:
I: Retrieving Release I: Retrieving Packages I: Validating Packages I: Resolving dependencies of required packages... I: Resolving dependencies of base packages... I: Found additional required dependencies: libdb4.6 I: Checking component main on http://ch.archive.ubuntu.com/ubuntu... I: Retrieving adduser I: Validating adduser ... I: Base system installed successfully.
Once the system has been bootstrapped, we need to populate fstab. At least, /proc and / must get mounted, but other filesystems might be referenced in this file too, like additional NFS exports, swap files, and so on. The difference with respect a traditional, disk-based Ubuntu installation, is that the root filesystem gets mounted via NFS by the intial RAM disk, and is referenced by the kernel’s /dev/nfs block device.
The contents of /home/nfsroot/etc/fstab should look like this:
# /etc/fstab: static file system information. # # <file system> <mount point> <type> <options> <dump> <pass> proc /proc proc defaults 0 0 /dev/nfs / nfs defaults 0 0
Another difference with a traditional Ubuntu system is that we don’t want the network interfaces be managed by Network Manager. In fact, the main (probably Ethernet) network interface was already configured by the kernel based on PXE’s supplied configuration. Letting Nework Manager reconfigure or manage this network interface might mean losing the connection to the NFS server and, thus, rendering the system unusable.
To stop Network Manager from managing the main network interface, the contents of /home/nfsroot/etc/network/interfaces must look like this:
# Used by ifup(8) and ifdown(8). See the interfaces(5) manpage or # /usr/share/doc/ifupdown/examples for more information. # The loopback network interface auto lo iface lo inet loopback # The primary network interface, commented out for NFS root iface eth0 inet manual
Finally, we will give the client a generic name. This can be done by storing that generic name into /etc/hostname:
root@nfs.lan:# echo client.lan > /home/nfsroot/etc/hostname
Booting up the diskless workstation
Once everything is set up, the last thing to do is testing that the diskless workstation is able to boot via PXE. The mechanics of booting via PXE depend on the machine. On some machines, it’s just a matter of pressing ESC to get into a menu that allows to override the boot device. On others, the same can be done by pressing F12 during the POST. On most modern systems it’s possible to reconfigure the system to always boot from the network.
If the client is able to boot from PXE successfully, this is what you will briefly see on your screen:
CLIENT MAC ADDR: 00 11 22 33 44 55 66 GUID: 00000000 0000 0000 0000 000000000000 CLIENT IP: 192.168.0.2 MASK: 255.255.255.0 DHCP IP: 192.168.0.1 GATEWAY IP: 192.168.0.1 PXELNUX 3.36 Debian-2007-08-30 Copyright (C) 1994-2007 H. Peter Anvin UNDI data segment at: 00094140 UNDI data segment size: 94B0 UNDI code segment at: 0009D5F0 UNDI code segment size: 20B0 PXE entry point found (we hope) at 9D5F:0106 My IP address seems to be C0A80001 192.168.0.2 ip=192.168.0.2:192.168.0.1:192.168.0.1:255.255.255.0 TFTP prefix: Tring to load: pxelinux.cfg/00-01-02-03-04-05 Tring to load: pxelinux.cfg/C0A80001 Tring to load: pxelinux.cfg/C0A8000 Tring to load: pxelinux.cfg/C0A800 Tring to load: pxelinux.cfg/C0A80 Tring to load: pxelinux.cfg/C0A8 Tring to load: pxelinux.cfg/C0A Tring to load: pxelinux.cfg/C0 Tring to load: pxelinux.cfg/C Tring to load: pxelinux.cfg/default boot: Loading vmlinuz...
If the system boots up, we will be dropped into a text-mode console where we can log in as root with no password. Of course, the first thing you should do is creating a password for the root user, but if you have been able to get to this point, you probably know how to do it 
Once you are logged in as your in your new diskless workstation, you will probably want to add more functionality. For example, installing the OpenSSH server, or the packages for the typical Ubuntu desktop system. Before you do that, we will need to add more Ubuntu repositories to /etc/apt/sources.list. This is how mine looks like:
deb http://ch.archive.ubuntu.com/ubuntu/ feisty main restricted deb-src http://ch.archive.ubuntu.com/ubuntu/ feisty main restricted deb http://ch.archive.ubuntu.com/ubuntu/ feisty-updates main restricted deb-src http://ch.archive.ubuntu.com/ubuntu/ feisty-updates main restricted deb http://ch.archive.ubuntu.com/ubuntu/ feisty universe deb-src http://ch.archive.ubuntu.com/ubuntu/ feisty universe deb http://ch.archive.ubuntu.com/ubuntu/ feisty multiverse deb-src http://ch.archive.ubuntu.com/ubuntu/ feisty multiverse deb http://ch.archive.ubuntu.com/ubuntu/ feisty-backports main restricted universe multiverse deb-src http://ch.archive.ubuntu.com/ubuntu/ feisty-backports main restricted universe multiverse deb http://security.ubuntu.com/ubuntu feisty-security main restricted deb-src http://security.ubuntu.com/ubuntu feisty-security main restricted deb http://security.ubuntu.com/ubuntu feisty-security universe deb-src http://security.ubuntu.com/ubuntu feisty-security universe deb http://security.ubuntu.com/ubuntu feisty-security multiverse deb-src http://security.ubuntu.com/ubuntu feisty-security multiverse
Finally, I decided to install OpenSSH, OpenNTPD and the typical Ubuntu desktop:
root@client.lan:# apt-get update root@client.lan:# apt-get install openssh-server openntpd ubuntu-desktop
Integrated DHCP and DNS services using OpenWRT
May 15th, 2006
dnsmasq offers a lightweight, functional and integrated DHCP and DNS service. Using it on OpenWRT brings up and embedded, flexible DNS service, with a very small footprint, for small or home offices.
dnsmasq acts as a caching DNS server and DHCP server. It reserves a DNS domain, called the local DNS domain and usually being .lan, for local name resolution. When queried for an A RR inside the local DNS domain, dnsmasq looks at file /etc/hosts for a match. If one is found, its corresponding IP is returned as the query result. When queried for a PTR RR, it looks into file /etc/hosts for a match by IP and, if one is found, its correspoding hostname, qualified with the local DNS domain, is returned. Thus, /etc/hosts behaves much like a DNS master zone file.
Also, if the DHCP server funcionality of dnsmasq is enabled, when a query under the local DNS domain fails (no record is found in /etc/hosts), it will try to resolve the query from the DHCP lease database.
The DHCP lease database is usually stored at /tmp/dhcp.leases. Its format is pretty simple: it’s a text file, where each line represents an active DHCP lease. Each line is made up of five fields:
- Time of lease expiration
In epoch time (seconds since 1970). States when the lease will expire. Most DHCP clients will try to renew the lease when it reaches 80% of its valid lifetime.
- Client MAC address
The MAC address corresponding to the client to which the lease belongs.
- Leased IP address
A valid IP address, taken from the DHCP pool, which is actually and currently leased to the client whose MAC address is listed in the previous field.
- Client hostname
If known, holds the unqualified host name of the client machine. Else, an asterisk is stored here.
- Client ID
Simon Kelley defines it as:
The client-ID is used as the computer’s unique-ID in preference to the MAC address, if it’s available. Some DHCP clients provide it, and some don’t. The ones that do normally derive it from the MAC address unless explicity configured, but it could be something like a serial number, which would protect a computer from losing its identify if the network interface were replaced.
If not know, an asterisk is stored here.
A sample DHCP database lease:
# cat /tmp/dhcp.leases 1147729862 00:16:3e:3b:56:f1 192.168.0.11 rhel * 1147725355 00:0c:29:09:3d:58 192.168.0.10 rhel-devel *
In this case, there are two active DHCP leases, one for client rhel, another one for rhel-devel.
OpenWRT uses a rc.d script stored at /etc/init.d/S50dnsmasq which, for a squashfs firmware is a symbolic link to /rom/etc/init.d/S50dnsmasq. This rc.d script tries to configure the dnsmasq daemon using NVRAM variables, which helps a lot when reflashing. However, I have found more convenient to the use the traditional /etc/dnsmasq.conf file instead.
Replacing the OpenWRT rc.d script with a custom one, in order to leverage dnsmasq.conf, is as simple as removing /etc/init.d/S50dnsmasq and invoking the dnsmasq daemon directly:
rm -f /etc/init.d/S50dnsmasq cat > /etc/init.d/S50dnsmasq < < EOF #/bin/sh /usr/sbin/dnsmasq EOF
Here is a sample of a /etc/dnsmasq.conf file I use on my Linksys WRT54G router running OpenWRT White Russian RC5:
# filter what we send upstream domain-needed bogus-priv filterwin2k localise-queries # allow /etc/hosts and dhcp lookups via *.lan local=/lan/ domain=lan expand-hosts # enable dhcp (start,end,netmask,leasetime) dhcp-authoritative dhcp-range=10.0.0.10,10.0.0.100,255.255.255.128,12h dhcp-leasefile=/tmp/dhcp.leases # use /etc/ethers for static hosts; same format as --dhcp-host # [hwaddr] [ipaddr] read-ethers # other useful options: # default route(s): dhcp-option=3,192.168.1.1,192.168.1.2 # dns server(s): dhcp-option=6,192.168.1.1,192.168.1.2 dhcp-option=3,10.0.0.126 dhcp-option=6,10.0.0.122