The CS2030S Programming Environment

Java version

Java is a language that continues to evolve. A new version is released every six months, and by the time we are done with CS2030S this semester, we will have Java 16. For CS2030S, we will only use Java 11, the most recent version with long-term support.

Programming Servers

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.

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

You can choose which of the eight hosts to use. You share the same home directory across all the hosts (this home directory, however, is different from that of sunfire). If you notice that one host is crowded, you can use another host to spread out the load.

While you can complete the programming assignments on your 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 or emacs (vim is recommended and supported).

Basic Requirements

You need to have an SoC Unix account. If you do not have one, you can apply for one online.
Once you have an account, you need to activate your access to the PE hosts, which is part of the SoC computer clusters.
You need a command-line ssh client. Windows 10, macOS, and Linux users should already have it installed by default.

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 line instructions below do not apply.

Basic Command to SSH

Run:

1	`ssh <username>@<hostname>`

Replace <hostname> with the host you want to log into and <username> with your SoC Unix username. Note that both are case sensitive.

For instance, I would do:

1	`ssh ooiwt@sunfire.comp.nus.edu.sg`

to log into sunfire.

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

Accessing The PE Hosts from Outside SoC

The PE hosts can only be accessed from within the School of Computing networks. To complete the lab at home and to complete the two practical assessments from home, you need to access the PE hosts from outside the SoC networks.

There are several ways to do this, the simplest way is to tunnel through sunfire, and this is the recommended method, as no extra software is required.

Option 1: Tunneling through Sunfire (Recommended for those in Singapore)

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).

Since sunfire is within the SoC network, you can log into sunfire first, then from sunfire, log into one of the PE nodes from sunfire. These two steps can be done with one command:

1	`ssh -t <username1>@sunfire.comp.nus.edu.sg ssh <username2>@pe1xx.comp.nus.edu.sg`

Note that in non-exam scenarios, username1 and username2 are both your SoC Unix username. You will be prompted for password twice. The first prompt will be for your password to sunfire, and the second, to pe1xx.

For practical exams, you will be issued a special exam account to log into the PE hosts. In this case, username1 will be your SoC Unix username and username2 will be your special exam account.

Students not in Singapore will need to access sunfire via SoC VPN. In which case, Option 2 would be better.

Option 2: Using SoC VPN (Recommended only for not in Singapore)

To set up the SOC 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, Windows 10 users should download and install FortiClient VPN directly from FortiClient's website.

Once you are connected to SoC VPN, you can run

1	`ssh <username>@pe1xx.comp.nus.edu.sg`

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

1	`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 the remote host you want to log into. On this remote host, run

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

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

Troubleshooting

If you get the following error:

ssh: Could not resolve hostname pe1xx.comp.nus.edu.sg

ssh cannot recognize the name pe1xx, likely, you are not tunneling connected to the SoC VPN.
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
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
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 SoC VPN.
Could not chdir to home directory /home/o/ooiwt: Permission denied

This error means that you have successfully connected to the PE hosts, but you have no access to your 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.