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The CS1010 Programming Environment

C is a common programming language and you can find different implementations of C compiler on many platforms. Wikipedia lists more than 40 different C compilers. 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.

Our platform of choice is Ubuntu 18.04 (or later) using the clang compiler (version 6.0 or later).

The school has provided a list of computing servers for you to use. You can access them remotely via ssh, or secure shell. The hosts are named pe111, pe112, ... , pe120. (pe stands for "programming environment"). We will refer to these servers generally as the PE hosts.

pe115 and pe116

For this semester, the two servers pe115 and pe116 are not available.

You can choose which of the 10 8 hosts to use. You share the same home directory across all the hosts (this home directory, however, is different from that of sunfire).

For simplicity, the following guide uses pe111 in all examples. Do keep in mind that you can use other hosts for CS1010 to spread out the load.

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 ssh and edit your program with either vim.

Basic Requirements

  1. You need to have an SoC UNIX account. If you do not have one, you can apply for one online.

  2. Once you have an account, you need to activate your access to the PE hosts, which is part of the SoC computer clusters.

  3. You need an ssh client. macOS and Linux users should already have it installed by default. Windows 10 users should have it, too, through an update back in 2018. Please type ssh in your command prompt to confirm. If it is not available, you might want to update your Windows 10 or install the Windows Subsystem for Linux and use Linux.

For older versions of Windows, such as those used in the SoC's programming labs, you can check out XShell 6 (free for home/school use), or PuTTY. These are GUI-based programs so the command lines instructions below do not apply.

The Command to SSH

Run: 

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ssh <username>@pe111.comp.nus.edu.sg

Replace <username> with your SoC UNIX username, for instance, I would do: 

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ssh ooiwt@pe111.comp.nus.edu.sg

After the command above, following the instructions on the screen. The first time you ever connect to pe111.comp.nus.edu.sg, you will be warned that you are connecting to a previously unknown host. Say yes, and you will be prompted with your SoC UNIX password.

Accessing The PE Hosts from Outside SoC

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.

Option 1: Using SoC VPN

One way is to set up a Virtual Private Network (VPN) (See instruction here). The staff at helpdesk@comp.nus.edu.sg or the IT helpdesk in COM1, Level 1, will be able to help with you setting up if needed.

SoC VPN vs NUS VPN

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.

Windows 10 Users: FortiClient from Windows Store

Students have reported that running FortiClient downloaded from the Windows Store does not allow one to ssh from WSL to sunfire as expected. Therefore, you should download and install For Windows 10 users, please download and install FortiClient VPN directly from FortiClient's website.

Option 2: Tunneling through Sunfire

The alternative is to use ssh tunnels.

SoC's Sunfire (sunfire.comp.nus.edu.sg) is configured to allow your connection if it's originating from a local telco (See more details here).

There are two ways to achieve this, and in both ways, it appears to the PE hosts that Sunfire is the client.

SSH through sunfire

Connect to sunfire at sunfire.comp.nus.edu.sg via command line ssh with the option -t. 

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ssh -t <username>@sunfire.comp.nus.edu.sg ssh pe111.comp.nus.edu.sg

Alternatively: 

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ssh pe111.comp.nus.edu.sg -J <username>@sunfire.comp.nus.edu.sg

SSH Port Forwarding

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 (sunfire), which opens a connection to a preset destination server (e.g., pe111). This method causes the host to seem as if it is hosted on a local port, e.g. localhost:7000, allowing you to use your favorite SCP program (e.g. FileZilla) to access the PE host.

To use local port forwarding (from local port 7000), connect to sunfire using 

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ssh -L 7000:pe111.comp.nus.edu.sg:22 <username>@sunfire.comp.nus.edu.sg
This command opens an SSH tunnel from port 7000 of your machine to port 22 (the default SSH port) of pe111.comp.nus.edu.sg via sunfire. After successful login, open a separate SSH (or SCP) connection from your machine to localhost at port 7000 to access the PE host:

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ssh <username>@localhost -p 7000

PuTTY supports SSH port forwarding, so this setup can also be used on Windows.

Setting up SSH Keys

Once you are comfortable with UNIX, you can set up a pair of public/private keys for authentication.

You can use 

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ssh-keygen -t rsa

to generate a pair of keys on your local computer. Keep the private key id_rsa on your local machine in the hidden ~/.ssh directory, and copy the public key id_rsa.pub to your home directory on PE pe111. On pe111, run 

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cat id_rsa.pub >> ~/.ssh/authorized_keys

Make sure that the permission for .ssh both on the local machine and on PE are set to 700 and the files id_rsa on the local machine and authorized_keys on the remote machine are set to 600. Once set up, you need not enter your password every time you run ssh or scp.

Troubleshooting

Suppose you try to connect to pe111 using either: 

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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
and you get the following error:

  1. ssh: Could not resolve hostname pe111.comp.nus.edu.sg

    ssh cannot recognize the name pe111, it is likely that you are not connected to the SoC VPN.

  2. Connection closed by 192.168.48.xxx port 22

    You have connected to the PE host, but you are kicked out because you have no permission to use the host.

    Make sure you have activated your access to "SoC computer clusters" here: https://mysoc.nus.edu.sg/~myacct/services.cgi

  3. Permission denied, please try again

    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: https://mysoc.nus.edu.sg/~newacct/.

    Check that you have entered your username correctly. It is case sensitive.

    If you have lost your password, go here: https://mysoc.nus.edu.sg/~myacct/iforgot.cgi

  4. ssh: connect to host sunfire.comp.nus.edu.sg port 22: Operation timed out

    It means that you failed to connect to sunfire via ssh. There could be two reasons for this: (i) sunfire or its ssh service is down; (ii) you are connecting via a network where sunfire is not accessible (such as outside Singapore).

    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.

  5. Could not chdir to home directory /home/o/ooiwt: Permission denied

    This error means that you have successfully connect to the PE hosts, but you have no access to your own home directory.

    This should not happen. Please send an email with the above error message to helpdesk@comp.nus.edu.sg, 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.

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