cluster witout internet connection

Note: CentOS 7 has reached end-of-life (EOL). These instructions are intended for maintaining legacy systems.

This document records procedures for:

• Accessing the internet from an offline server via SSH tunnel
• Fixing the CentOS 7 repository issue
• Installing Environment Modules from source

1. Internet Connection via SSH Tunnel

If server A does not have direct internet access, but another server does, we can tunnel traffic through SSH using dynamic port forwarding. Although we use CentOS 7 here, the approach in this section also works for Rocky Linux 8.

References:

• https://unix.stackexchange.com/questions/157743/tunnel-yum-traffic-through-another-server-via-ssh
• https://unix.stackexchange.com/questions/111972/connect-to-the-internet-on-a-vpn-connected-server-without-internet

1.1 Configure YUM to Use Proxy

Edit:

/etc/yum.conf

Add:

proxy=socks5h://localhost:1080

1.2 Establish SSH Tunnel

ssh -D 1080 YOUR_USER@YOUR_SERVER_WITH_FULL_WEB_ACCESS

This creates a local SOCKS5 proxy on port 1080. Keep this terminal open.

1.3 Test the Connection

curl --socks5 127.0.0.1:1080 http://www.baidu.com

If successful, internet traffic is being routed through the SSH tunnel.

1.4 Configure Proxy for Conda and Git

Edit ~/.condarc:

proxy_servers:
  http: socks5://127.0.0.1:1080
  https: socks5://127.0.0.1:1080

Edit ~/.gitconfig:

[http]
    proxy = socks5://127.0.0.1:1080
[https]
    proxy = socks5://127.0.0.1:1080

2. Fix: mirrorlist.centos.org No Longer Available

Since CentOS 7 is EOL, the default mirrorlist no longer resolves.

Reference:

• https://serverfault.com/questions/1161816/mirrorlist-centos-org-no-longer-resolve

2.1 Switch to Vault Repository

sed -i 's/mirror.centos.org/vault.centos.org/g' /etc/yum.repos.d/*.repo
sed -i 's/^#.*baseurl=http/baseurl=http/g' /etc/yum.repos.d/*.repo
sed -i 's/^mirrorlist=http/#mirrorlist=http/g' /etc/yum.repos.d/*.repo

These lines essentially replace mirror.centos.org to vault.centos.org in /etc/yum.repos.d/CentOS-Base.repo.

2.2 Update System

yum update

3. Install Environment Modules

Environment Modules allow dynamic modification of shell environments, which is useful for managing compilers, MPI, and scientific software.

Documentation:

• https://modules.readthedocs.io/en/stable/INSTALL.html

3.1 Download Source (with SSH tunnel)

curl --socks5 127.0.0.1:1080 -LJO \
https://github.com/cea-hpc/modules/releases/download/v5.4.0/modules-5.4.0.tar.gz

3.2 Extract

tar xfz modules-5.4.0.tar.gz
cd modules-5.4.0

3.3 Compile and Install

./configure --prefix=/usr/share/Modules \
            --modulefilesdir=/usr/share/Modules/modulefiles
make
make install

3.4 Enable Modules at Shell Startup

ln -s /usr/share/Modules/init/profile.sh /etc/profile.d/modules.sh
ln -s /usr/share/Modules/init/profile.csh /etc/profile.d/modules.csh

Log out and back in, or run:

source /etc/profile.d/modules.sh

3.5 Test Modules

module sh-to-mod bash example/source-script-in-modulefile/foo-1.2/foo-setup.sh arg1 > modulefiles/foo/1.2

module avail
module load foo/1.2

Final Remarks

These procedures are primarily for maintaining legacy CentOS 7 cluster nodes.

For new deployments, consider:

• Rocky Linux or AlmaLinux
• A modern workload manager (e.g., SLURM)
• Lmod instead of classic Environment Modules

Maintaining clear documentation is essential for long-term cluster stability.