Prerequisite - Networking

Basic IP and Route Commands

# To see interfaces on a host
ip link

# To add an IP address to a link temporarily (will not survive reboot)
# where dev means device and eth0 is the device to add the address to
ip addr add 192.168.0.1/24 dev etho0
# Permanently
sudo ifconfig eth0 192.168.0.1 netmask 255.255.255.0”

# To see the routes on the system (route is part of net-tools)
route
# or
ip route 

# To add a new route (gateway) from our current network of 192.168.1.0/24 to 192.168.2.0/24.
# What network are we routing to and via what router (gateway)
ip route add 192.168.2.0/24 via 192.168.1.1
# Now add the reciprocal route to the machine with an IP in the 192.168.2.0/24 network
ip route add 192.168.1.0/24 via 192.168.2.1
# To add a default route for any network the machine can't already talk to
# default can also be 0.0.0.0
ip route add default via 192.168.2.1
# Often a default route is all you need unless your network has different gateways for different network, then you need an entry for each.

# To see what ports are open and how many connections they have do 
netstat -plnt

Setting up a Linux Host as a Router

Assume we have three systems, A, B and C where A is on network 192.168.1.x and C is on 2.x and B has two interfaces, eth0 and eth1 where eth0 has an ip on 1.x and eth1 has an ip on 2.x.
If we add routes from A to B and C to B then B can act as a router except that Linux does not by default route traffic between two different physical interfaces.

We can change this by editing /proc/sys/net/ipv4/ip_forward which defaults to 0. To route the traffic, change this to a 1 echo 1 > /proc/sys/net/ipv4/ip_forward.

Editing /proc/sys/* does not persist changes across reboot, to make this change permanent we must edit /etc/sysctl.conf and change net.ipv4.ip_forward = 1

DNS

Hosts File

The hosts file is located at /etc/hosts
Add an entry to give another system a name.

echo "192.168.1.11      db" > /etc/hosts

Setting a DNS Server

The /etc/resolv.conf file should contain an entry to the DNS server that looks like this nameserver 192.168.1.100
You can have as many nameserver lines as needed, private DNS server first then public after that with backups after that, etc.

You can also configure your private DNS server with forward any unknown requests to a public DNS server.

Hosts / DNS Search Order

By default the system looks at /etc/hosts first and if it finds an entry there it uses it, if not then it checks the nameserver specified in /etc/resolv.conf, however, the order can be changed by editing the hosts line in /etc/nsswitch.conf. Default is hosts: files dns, switching the order to hosts: dns files will switch the order they are checked in.

Adding a Default Domain

Add a search entry in /etc/resolv.conf with your companies domain to enable short name resolution.

Example: If you had a web server at web.mycompany.com but wanted to be able to get to it internally with just web then add search mycompany.com to the resolv.conf file. This entry is a space separated list of of domains to search: search mycompany.com pro.ycompany.com

Record Types

Tools

# Ping
ping 192.168.0.1

# nslookup, does not look in hosts file, is designed to talk to DNS servers
nslookup www.google.com

# dig, returns more details
dig www.google.com