Remote Linux Explained
Remote Linux refers to Linux workstations or nodes that do not boot the Linux kernel from local media, but instead receive their startup instructions over a network, typically Ethernet. Due to the ease of configuring both the Linux kernel and the operating system itself, Linux is being customized to work in many disparate environments, from web serving, to cluster computing and X servers.
The first question you might be asking yourself is why start Linux remotely? After all, installing Linux locally is a matter of sticking in the CD, answering a few questions and going out for a double latte while the workstation installs. This is true for the typical single-workstation installation; however, once you start to manage and install a lot of workstations, particularly in a cluster or server-farm setting, local media becomes less practical. With the advent of dense servers from companies like RLX Technologies, Inc., booting remotely becomes a necessity, as dense servers do not provide diskette or CD-ROM drives on the nodes. Dense servers expect to be brought up over the local fast Ethernet connection and administered remotely. The main advantages of remote Linux are:
Centralized, hands-off administration: while many installations do have someone on site that can jockey CDs and diskettes 24/7, there are also many hands-off sites (sites where no one is physically present at the site for long periods of time). At these sites, when a programmer is working remotely and needs to boot a node using a special image that's on local media, he or she is out of luck until someone is there to load the correct media.
Dense server solution: since the trend is toward centralized remote administration for clusters and server farms, CD and diskette drives become rather anachronistic. The very presence of local media on the nodes means that the nodes must take on a certain form factor, thereby increasing the minimum size of the nodes. For higher density node packaging, CD-ROM and diskette drives are being phased out in certain segments of the industry.
Versatility: sometimes one can fix a problem with a filesystem that prevents the node from coming up from local media. For example, you can run fsck on a local hard drive on a remotely booted machine in order to fix a filesystem problem.
Cost and security: why pay for media you don't need? Some companies are selling workstations without hard drives and other local media that are intended for use as secure terminal servers. Secure, in this sense, means that if employees do not have access to local media on their workstations, it is more difficult to capture sensitive data.
Helps eliminate version skew: in the case where all workstations are booted remotely from a single kernel image, it eliminates the problem of updating local media for kernel patches or enhancements. You can update the single remote kernel image once, instead of propagating the change to a set of workstation hard drives or, worse, to their local boot diskettes.
The remote boot process mimics the local boot process but with a few important distinctions. From a logical perspective, without talking about the services that perform these tasks, this is basically what happens during the network boot process:
The node is powered up or reset and conditioned to boot from the network.
The node broadcasts its unique Ethernet MAC address, looking for a server.
The server, previously conditioned to listen for specific MAC addresses, responds with the correct IP address for the node. Alternately, the server responds to any broadcast on its physical network with IP information from a designated range of IP addresses.
The node receives its IP information and configures its Ethernet adaptor for TCP/IP communications.
The node requests a kernel over its newly configured adaptor.
The server responds by sending a network loader to the client, which will load the network boot kernel.
The network boot loader mounts the root filesystem as read-only.
The network loader reads the network boot kernel sent from the server into local memory and transfers control to it.
The kernel mounts root as read/write, mounts other filesystems and starts the init process.
Init brings up the customized Linux services for the node, and it comes up completely.
From this description, we see that the booting node has several dependencies on the server: a network boot kernel, a root filesystem and a method of transporting the kernel and IP information from the server to the node.
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