Category Archives: Linux

How to Configure and Use Passwordless SSH for Infrastructure Automation on Ubuntu

In the previous post, we revisited SSH public key authentication protocol that brought passwordless SSH to life. We perused and summarized the RFCs. To visualize the authentication process, we drew the arrows that highlight the sequences of messages exchanged between the client and the server.

Not to get overwhelmed with theories, we set up a mini lab to demonstrate how a client should authenticate itself against a remote server with passwordless SSH. We ran the experiment in a cloud environment provided by Digital Ocean.

And probably you may ask this question, “Why running the experiment in the cloud?”

The very simple answer to this is “to emulate the situations when passwordless SSH shines”.

Let’s go into more details by reviewing the use cases.
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Passwordless SSH Concept and How to Setup on Ubuntu

Secure Shell (SSH) as defined in RFC4251 is a protocol for secure remote login and other secure network services over an insecure network. SSH consists of three main components: transport layer protocol, user authentication protocol and the connection protocol.

The transport protocol for SSH 2.0 as elaborated in RFC4253 provides a secured channel over an insecure network by performing host authentication, key exchange, encryption and integrity protection, and also deriving unique session ID that can be used by higher-level protocols.

The authentication protocol as elaborated in RFC4252 provides a suite of mechanisms that can be used to authenticate the client user to the server. There are three authentication mechanisms for SSH: public key authentication, password authentication, and host-based authentication.

The connection protocol as elaborated in RFC4254 specifies a mechanism to multiplex multiple data channels into a single encrypted tunnel over the secured and authenticated transport. The channels can be used for various purposes such as interactive shell sessions, remote command executions, forwarding arbitrary TCP/IP ports/connections over the secure transport, forwarding X11 connections, and accessing the secure subsystems on the server host.
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CUDA Compatibility of NVIDIA Display / GPU Drivers

The last post about CUDA installation guide was for CUDA 9.2. We went through several types of CUDA installation methods, including the multiple-version CUDA installs. While the guide is still valid for CUDA 9.2, NVIDIA keeps releasing newer versions of CUDA. As a concrete example, when this article was written, the latest CUDA version has been CUDA 10, taking the spotlight from CUDA 9.2. If we are about to upgrade to CUDA 10, how can we achieve so? Can we simply upgrade the CUDA toolkit without upgrading the display driver?

Handling CUDA Version Upgrade

CUDA version upgrade itself can be a misleading term because since CUDA 8.0, multiple versions of CUDA can be installed on the same machine. But let’s have a simple scenario where we already have CUDA 9.1 installed and only want to upgrade to CUDA 10. NVIDIA states that each version of CUDA toolkit requires certain minimum NVIDIA display version that should be satisfied. This means that when upgrading to newer version of CUDA toolkit, we need to make sure that the currently installed display driver version is newer/bigger than the minimum compatible display driver version. In other words, standard CUDA upgrade involves two upgrade processes: CUDA (toolkit) upgrade and driver upgrade. The following picture visualizes the standard upgrade process from CUDA 9.1 to CUDA 10: the toolkit is upgraded from 9.1 to 10 and the driver is upgraded from 390 to 410.

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How to Resolve The Error “Illegal instruction (core dumped)” when Running “import tensorflow” in a Python Program

There are several ways to install TensorFlow on Ubuntu. The easiest way is to install via pip. Unfortunately, this easy installation may result in a bumpy first time experience of running TensorFlow. Consider the following one line Python script:

$ python -c 'import tensorflow as tf;'

This should be where the excitement begins, the moment where conviction about the new era of AI-powered banalities starts to bloom. Yet, the reality can be unexpectedly different. Executing the command may immediately raise this very infamous error:

Illegal instruction (core dumped)

This means that TensorFlow has crashed even before it does anything. What a surprise!

The good thing is that we can run gdb to debug Python and start analyzing the call stack. But what’s even better is that we can save the brilliance for later. This error has been repeatedly reported and has conveniently sit on its fame for a while, as reflected on the issue page. Continue reading

How to Build and Install The Latest TensorFlow without CUDA GPU and with Optimized CPU Performance on Ubuntu

In this post, we are about to accomplish something less common: building and installing TensorFlow with CPU support-only on Ubuntu server / desktop / laptop. We are targeting machines with older CPU, as for example those without Advanced Vector Extensions (AVX) support. This kind of setup can be a choice when we are not using TensorFlow to build a new AI model but instead only for obtaining the prediction (inference) served by a trained AI model. Compared with model training, the model inference is less computational intensive. Hence, instead of performing the computation using GPU acceleration, the task can be simply handled by CPU.

tl;dr The WHL file from TensorFlow CPU build is available for download from this Github repository.

Since we will build TensorFlow with CPU support only, the physical server will not need to be equipped with additional graphics card(s) to be mounted on the PCI slot(s). This is different with the case when we build TensorFlow with GPU support. For such case, we need to have at least one external (non built-in) graphics card that supports CUDA. Naturally, running TensorFlow with CPU pertains to be an economical approach to deep learning. Then how about the performance? Some benchmark results have shown that GPU performs better than CPU when performing deep learning tasks, especially for model training. However, this does not mean that TensorFlow CPU cannot be a feasible option. With proper CPU optimization, TensorFlow can exhibit improved performance that is comparable to its GPU counterpart. When cost is a more serious issue, let’s say we can only do the model training and inference in the cloud, leaning towards TensorFlow CPU can be a decision that also makes more sense from financial standpoint. Continue reading