Linux Core as Network Host

 Core Linux is a small modular system with community build extensions. Based on downloaded extensions the Core can perfectly meets your needs.  For instance if you want to build a network host, you install the extensions that somehow interact with network traffic such as tcpdump, iperf, iptables, d-itg, mtr, etc. Moreover you can convert your Linux Core installation to a modern Linux router or multi-layer switch by installing extensions such as Quagga and Openvswitch.

In this tutorial I will show how to enable IPv6 support on Linux Core, explain SSH server installation and configuration and suggest installation of several extensions which extend the functionality of Core Linux as a network host. Below are the examples such extensions.

Bash - is an sh-compatible shell that incorporates useful features from the Korn shell (ksh) and C shell (csh).

Bash Completion - completion feature in Bash permits typing a partial command, then pressing the [Tab] key to auto-complete

Openssh - encrypts all traffic to effectively eliminate eavesdropping, connection hijacking, and other attacks.

Iptables - the userspace tool to control the linux kernel firewall.

Tcpdump - a tool for network monitoring and data acquisition.

Iperf3 - a tool for measuring Internet bandwidth performance.

Iproute2 - a collection of utilities for controlling TCP/IP networking and traffic control.

Mtr - combines the functionality of the traceroute and ping programs in a single network diagnostic tool.

D-ITG -  a distributed traffic generator and it is capable to generate traffic at network, transport, and application layer.

Nmap - an utility for network discovery and security auditing.

Ncat - a feature-packed networking utility which reads and writes data across networks from the command line. It's part of nmap installation.

Hping3 - send (almost) arbitrary TCP/IP packets to network hosts.

Note: Core Linux installation on Qemu image and tweaking Core for redirecting output to a serial port is explained here.

The Core Linux virtual disks are available for download here.

1. IPv6 Support in Core Linux

a)  Download and install ipv6 extension

$ tce-load -wi ipv6-$(uname -r | cut -d "-" -f1)-tinycore64.tcz

b)  Load IPv6 module to kernel and make it persistent

$ echo "modprobe ipv6" >> /opt/bootlocal.sh
$ sudo modprobe ipv6
$ /usr/bin/filetool.sh -b

2. SSH Server Installation and Configuration

First install openssh.tcz extension.

$ tce-load -wi openssh

a) Set password for user 'tc' and 'root'

In order to login to Core using SSH you have to assign password for the accounts you use to login. Then add the shadow file that stores password hashes to the list of files that will remain permanent after Core reboot.

$ sudo su
# passwd tc
# passwd root
# exit

$ echo "/etc/shadow" >> /opt/.filetool.lst

b) Generate host keys for SSH

$ sudo su
# ssh-keygen -f /usr/local/etc/ssh/ssh_host_rsa_key -N '' -t rsa -b 2048
# ssh-keygen -f /usr/local/etc/ssh/ssh_host_dsa_key -N '' -t dsa -b 1024
# ssh-keygen -f /usr/local/etc/ssh/ssh_host_ecdsa_key -N '' -t ecdsa -b 521
# ssh-keygen -f /usr/local/etc/ssh/ssh_host_ed25519_key -N '' -t ed25519 -b 2048

Explanation:
-N null pasphrase
-b length of key

c) Create configuration file for SSH daemon

# mv  /usr/local/etc/ssh/sshd_config_example /usr/local/etc/ssh/sshd_config

Add /usr/local/etc/ssh/ directory to the list of files and directories which content is saved.

# echo "/usr/local/etc/ssh/" >> /opt/.filetool.lst

d) Start sshd service

# /usr/local/sbin/sshd

Add SSH to the list of services started after boot.

# echo "/usr/local/sbin/sshd" >> /opt/bootlocal.sh

e) Save content of file /opt/.filetool.lst

# /usr/bin/filetool.sh -b

3. Bash Shell

$ tce-load -wi bash.tcz

Set BASH as default shell for users in /etc/passwd.

$ sudo su
# sed -i 's/\/bin\/sh/\/bin\/bash/g' /etc/passwd

# echo "/etc/passwd" >> /opt/.filetool.lst
# /usr/bin/filetool.sh -b

4. Bash Completion

$ tce-load -wi bash-completion.tcz

Create the file .bash_profile with the following content.

echo '[[ $PS1 && -f /usr/local/share/bash-completion/bash_completion ]] && . /usr/local/share/bash-completion/bash_completion' >> /home/tc/.bash_profile

# /usr/bin/filetool.sh -b

5. Iptables 

$ tce-load -wi iptables.tcz

Load basic IPv4 and Ipv6 iptables modules to kernel.

$ sudo modprobe iptable_nat
$ sudo modprobe iptable_mangle

$ sudo modprobe ip6table_filter
$ sudo modprobe ip6_tables

$ echo "modprobe iptable_nat" >> /opt/bootlocal.sh
$ echo "modprobe iptable_mangle" >> /opt/bootlocal.sh
$ echo "modprobe ip6table_filter" >> /opt/bootlocal.sh
$ echo "modprobe ip6_tables" >> /opt/bootlocal.sh

$ /usr/bin/filetool.sh -b

6. Script for Dhclient

There is no ISC dhclient extension available in Linux Core repository. Instead of that Core uses udhcpc to get IP address from DHCP server. The script below calls udhcpc when command dhclient is invoked.

$ sudo su

# echo "udhcpc" > /sbin/dhclient
# chmod 744 /sbin/dhclient

# echo "/sbin/dhclient" >> /opt/.filetool.lst
# /usr/bin/filetool.sh -b

7. Basic Network Extensions Installation

$ tce-load -wi  tcpdump iperf3  iproute2 mtr d-itg nmap hping3

8. Delete History Artifacts

Delete SSH known hosts.

$ rm /home/tc/.ssh/known_hosts

Delete Almquist and BASH history.

$ sudo su
# echo > /home/tc/.ash_history
# echo > /root/.ash_history

# history -c && history -w
# exit
$ history -c && history -w

$ /usr/bin/filetool.sh -b

End.

References
http://wiki.tinycorelinux.net/wiki:persistence_for_dummies
http://wiki.tinycorelinux.net/wiki:boot_codes_explained
http://wiki.tinycorelinux.net/wiki:boot_options