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        Programming Environments
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      Programming Environments
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    Basic Requirements
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  <a href="#the-command-to-ssh" class="md-nav__link">
    The Command to SSH
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  <a href="#accessing-the-pe-hosts-from-outside-soc" class="md-nav__link">
    Accessing The PE Hosts from Outside SoC
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    Option 1: Using SoC VPN
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    Option 2: Tunneling through Sunfire
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    SSH through sunfire
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    SSH Port Forwarding
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    Setting up SSH Keys
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    Troubleshooting
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      UNIX
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                <h1 id="the-cs1010-programming-environment">The CS1010 Programming Environment</h1>
<p>C is a common programming language and you can find different implementations of C compiler on many platforms.  Wikipedia <a href="https://en.wikipedia.org/wiki/List_of_compilers#C_compilers">lists more than 40 different C compilers</a>.  These different compilers support different processor architecture, operating systems, may behave slightly differently, and may support different features of C standards.  It is therefore important for CS1010 to stick to a single platform and single compiler.</p>
<p>Our platform of choice is <strong>Ubuntu 18.04 (or later)</strong> using the <code>clang</code> compiler (<strong>version 6.0 or later</strong>).</p>
<p>The school has provided a list of computing servers for you to use.  You can access them remotely via <code>ssh</code>, or secure shell.  The hosts are named <code>pe111</code>, <code>pe112</code>, ... , <code>pe120</code>.  (<code>pe</code> stands for "programming environment").  We will refer to these servers generally as the <em>PE hosts.</em></p>
<div class="admonition note">
<p class="admonition-title">pe115 and pe116</p>
<p>For this semester, the two servers <code>pe115</code> and <code>pe116</code> are not available.</p>
</div>
<p>You can choose which of the <del class="critic">10</del> 8 hosts to use.  You share the same home directory across all the hosts (this home directory, however, is different from that of <code>sunfire</code>).</p>
<p>For simplicity, the following guide uses <code>pe111</code> in all examples.  Do keep in mind that you can use other hosts for CS1010 to spread out the load.</p>
<p>While it is possible for you to complete the programming assignments on your own computers, the practical exams are done in a controlled environment using servers similar to the PE hosts.  It is therefore advisable for you to familiarize yourself with accessing the PE servers via <code>ssh</code> and edit your program with either <code>vim</code>.</p>
<h2 id="basic-requirements">Basic Requirements</h2>
<ol>
<li>
<p>You need to have an SoC UNIX account.  If you do not have one, you can <a href="https://mysoc.nus.edu.sg/~newacct/">apply for one online</a>.</p>
</li>
<li>
<p>Once you have an account, you need to <a href="https://mysoc.nus.edu.sg/~myacct/services.cgi">activate your access to the PE hosts</a>, which is part of the SoC computer clusters.</p>
</li>
<li>
<p>You need an <code>ssh</code> client.  macOS and Linux users should already have it installed by default.  Windows 10 users should install <a href="https://docs.microsoft.com/en-us/windows/wsl/install-win10">the Windows Subsystem for Linux</a> and use Linux.</p>
</li>
</ol>
<p>For older versions of Windows, such as those used in the SoC's programming labs, you can check out <a href="https://www.netsarang.com/products/xsh_overview.html">XShell 6</a> (free for home/school use), or <a href="https://www.putty.org">PuTTY</a>.  These are GUI-based programs so the command lines instructions below do not apply.</p>
<h2 id="the-command-to-ssh">The Command to SSH</h2>
<p>Run:
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span></span>1</pre></div></td><td class="code"><div class="highlight"><pre><span></span><code>ssh &lt;username&gt;@pe111.comp.nus.edu.sg
</code></pre></div>
</td></tr></table></p>
<p>Replace <code>&lt;username&gt;</code> with your SoC UNIX username, for instance, I would do:
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span></span>1</pre></div></td><td class="code"><div class="highlight"><pre><span></span><code>ssh ooiwt@pe111.comp.nus.edu.sg
</code></pre></div>
</td></tr></table></p>
<p>After the command above, following the instructions on the screen.  The first time you ever connect to <code>pe111.comp.nus.edu.sg</code>, you will be warned that you are connecting to a previously unknown host.  Say <code>yes</code>, and you will be prompted with your SoC UNIX password.</p>
<h2 id="accessing-the-pe-hosts-from-outside-soc">Accessing The PE Hosts from Outside SoC</h2>
<p>The PE hosts can only be accessed from within the School of Computing networks.  If you want to access it from outside, there are several ways.</p>
<h3 id="option-1-using-soc-vpn">Option 1: Using SoC VPN</h3>
<p>One way is to set up a Virtual Private Network (VPN) (See <a href="https://dochub.comp.nus.edu.sg/cf/guides/network/vpn">instruction here</a>).  The staff at <code>helpdesk@comp.nus.edu.sg</code> or the IT helpdesk in COM1, Level 1, will be able to help with you setting up if needed.</p>
<div class="admonition note">
<p class="admonition-title">SoC VPN vs NUS VPN</p>
<p>Note that SoC VPN is different from NUS VPN.  Connecting to NUS VPN only allows you access to the NUS internal network, but not the SoC internal network.</p>
</div>
<div class="admonition warning">
<p class="admonition-title">Windows 10 Users: FortiClient from Windows Store</p>
<p>Students have reported that running FortiClient downloaded from the Windows Store does not allow one to <code>ssh</code> from WSL to <code>sunfire</code> as expected.  Therefore, you should download and install For Windows 10 users, please download and install FortiClient VPN directly from <a href="https://forticlient.com/downloads">FortiClient's website</a>.</p>
</div>
<h3 id="option-2-tunneling-through-sunfire">Option 2: Tunneling through Sunfire</h3>
<p>The alternative is to use ssh tunnels.</p>
<p>SoC's Sunfire (<code>sunfire.comp.nus.edu.sg</code>) is configured to allow your connection if it's originating from a local telco (See <a href="https://dochub.comp.nus.edu.sg/cf/guides/unix/soc_unix_pass_your_direct_access_to_soc_unix_servers">more details here</a>).</p>
<p>There are two ways to achieve this, and in both ways, it appears to the PE hosts that Sunfire is the client.</p>
<h4 id="ssh-through-sunfire">SSH through sunfire</h4>
<p>Connect to sunfire at <code>sunfire.comp.nus.edu.sg</code> via command line <code>ssh</code> with the option <code>-t</code>.
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span></span>1</pre></div></td><td class="code"><div class="highlight"><pre><span></span><code>ssh -t &lt;username&gt;@sunfire.comp.nus.edu.sg ssh pe111.comp.nus.edu.sg
</code></pre></div>
</td></tr></table></p>
<p>Alternatively:
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span></span>1</pre></div></td><td class="code"><div class="highlight"><pre><span></span><code>ssh pe111.comp.nus.edu.sg -J &lt;username&gt;@sunfire.comp.nus.edu.sg
</code></pre></div>
</td></tr></table></p>
<h4 id="ssh-port-forwarding">SSH Port Forwarding</h4>
<p>SSH has built-in support for local and remote port forwarding, and local port forwarding can be used to connect to the programming environments.  Local port forwarding means that a port of the SSH client (your machine) is forwarded to the SSH server (<code>sunfire</code>), which opens a connection to a preset destination server (e.g., <code>pe111</code>).  This method causes the host to seem as if it is hosted on a local port, e.g. <code>localhost:7000</code>, allowing you to use your favorite SCP program (e.g. <a href="https://filezilla-project.org/">FileZilla</a>) to access the PE host.</p>
<p>To use local port forwarding (from local port <code>7000</code>), connect to <code>sunfire</code> using
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span></span>1</pre></div></td><td class="code"><div class="highlight"><pre><span></span><code>ssh -L 7000:pe111.comp.nus.edu.sg:22 &lt;username&gt;@sunfire.comp.nus.edu.sg
</code></pre></div>
</td></tr></table>
This command opens an SSH tunnel from port <code>7000</code> of your machine to port <code>22</code> (the default SSH port) of <code>pe111.comp.nus.edu.sg</code> via <code>sunfire</code>.  After successful login, open a separate SSH (or SCP) connection from your machine to <code>localhost</code> at port <code>7000</code> to access the PE host:</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span></span>1</pre></div></td><td class="code"><div class="highlight"><pre><span></span><code>ssh &lt;username&gt;@localhost -p 7000
</code></pre></div>
</td></tr></table>
<p><a href="https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html"><code>PuTTY</code></a> supports SSH port forwarding, so this setup can also be used on Windows.</p>
<h2 id="setting-up-ssh-keys">Setting up SSH Keys</h2>
<p>Once you are comfortable with UNIX, you can set up a pair of public/private keys for authentication.  </p>
<p>You can use
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span></span>1</pre></div></td><td class="code"><div class="highlight"><pre><span></span><code>ssh-keygen -t rsa
</code></pre></div>
</td></tr></table></p>
<p>to generate a pair of keys on your local computer.  Keep the private key <code>id_rsa</code> on your local machine in the hidden <code>~/.ssh</code> directory, and copy the public key <code>id_rsa.pub</code> to your home directory on PE <code>pe111</code>.  On <code>pe111</code>, run
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span></span>1</pre></div></td><td class="code"><div class="highlight"><pre><span></span><code>cat id_rsa.pub &gt;&gt; ~/.ssh/authorized_keys
</code></pre></div>
</td></tr></table></p>
<p>Make sure that the permission for <code>.ssh</code> both on the local machine and on PE are set to <code>700</code> and the files <code>id_rsa</code> on the local machine and <code>authorized_keys</code> on the remote machine are set to <code>600</code>.  Once set up, you need not enter your password every time you run <code>ssh</code> or <code>scp</code>.  </p>
<h2 id="troubleshooting">Troubleshooting</h2>
<p>Suppose you try to connect to <code>pe111</code> using either:
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span></span>1
2
3</pre></div></td><td class="code"><div class="highlight"><pre><span></span><code>ssh pe111.comp.nus.edu.sg
ssh -t sunfire.comp.nus.edu.sg ssh pe111.comp.nus.edu.sg
ssh pe111.comp.nus.edu.sg -J sunfire.comp.nus.edu.sg
</code></pre></div>
</td></tr></table>
and you get the following error:</p>
<ol>
<li>
<p><code>ssh: Could not resolve hostname pe111.comp.nus.edu.sg</code></p>
<p><code>ssh</code> cannot recognize the name <code>pe111</code>, it is likely that you are not connected to the SoC VPN.</p>
</li>
<li>
<p><code>Connection closed by 192.168.48.xxx port 22</code></p>
<p>You have connected to the PE host, but you are kicked out because you have no permission to use the host.</p>
<p>Make sure you have activated your access to "SoC computer clusters" here: <a href="https://mysoc.nus.edu.sg/~myacct/services.cgi">https://mysoc.nus.edu.sg/~myacct/services.cgi</a></p>
</li>
<li>
<p><code>Permission denied, please try again</code></p>
<p>You did not enter the correct password or username.  Please use the username and password 
of your SoC UNIX account which you have created here: <a href="https://mysoc.nus.edu.sg/~newacct/">https://mysoc.nus.edu.sg/~newacct/</a>.   </p>
<p>Check that you have entered your username correctly.  It is <em>case sensitive</em>.</p>
<p>If you have lost your password, go here: <a href="https://mysoc.nus.edu.sg/~myacct/iforgot.cgi">https://mysoc.nus.edu.sg/~myacct/iforgot.cgi</a></p>
</li>
<li>
<p><code>ssh: connect to host sunfire.comp.nus.edu.sg port 22: Operation timed out</code></p>
<p>It means that you failed to connect to <code>sunfire</code> via <code>ssh</code>.  There could be two reasons for this: (i) <code>sunfire</code> or its ssh service is down; (ii) you are connecting via a network where <code>sunfire</code> is not accessible (such as outside Singapore).  </p>
<p>The likelihood of (i) is small.  The more likely scenario is (ii), in which case, you should be able to solve it by connecting to NUS or SoC VPN.</p>
</li>
<li>
<p><code>Could not chdir to home directory /home/o/ooiwt: Permission denied</code></p>
<p>This error means that you have successfully connect to the PE hosts, but you have no access to your own home directory. </p>
<p>This should not happen.  Please send an email with the above error message to <a href="&#109;&#97;&#105;&#108;&#116;&#111;&#58;&#104;&#101;&#108;&#112;&#100;&#101;&#115;&#107;&#64;&#99;&#111;&#109;&#112;&#46;&#110;&#117;&#115;&#46;&#101;&#100;&#117;&#46;&#115;&#103;">&#104;&#101;&#108;&#112;&#100;&#101;&#115;&#107;&#64;&#99;&#111;&#109;&#112;&#46;&#110;&#117;&#115;&#46;&#101;&#100;&#117;&#46;&#115;&#103;</a>, include the PE hosts that you
connected to with this error and your username.  The system administrator can reset the permission of your home directory for you.</p>
</li>
</ol>
                
              
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