Network Configuration (Files and Commands)

Files

/etc/hostname – holds the hostname
/etc/hosts – file mapping of IP addresses – hostnames
/etc/networks – maps network addresses -names
/etc/host.conf – replaced by nsswitch
/etc/resolv.conf – client dns configuration
/etc/services – maps port numbers to names
/etc/nsswich.conf – name service switch configuration file, determines how system looks up name and various other things.

Commands

ifconfig – show and configure network interfaces
route – show and configure network routes
dhcpcd, dhcpclient, pmp – DHCP clients
dig – tests DNS servers
host – query and debug DNS servers
hostname – display hostname, and NIS domaind setting
netstat – shows what service is listening on what port
ping – sends ICMP echo request to hosts
traceroute – show the path (display routes) that a network connection takes.
mtr – works like traceroute in real time
tcpdump – packet sniffer command that displays the contents of packets received on a network interface.
route -n – view the routing table
whois – queries information about the owner of the domain

Netstat
netstat -a -u -t – Prints ports in use
netstat -r – Prints routing table
netstat -i – Prints interfaces
netstat -g – Prints multicast groups
netstat -M – Prints masqueraded connections
netstat -s – Prints statistics

NTP Server and Client Configuration

NTP stands for Network Time Protocol, and it is an Internet protocol used to synchronize the clocks of computers to some time reference. NTP is an Internet standard protocol that is implemented via UDP over port 123. It is the best way to set your system clock correctly.

According to a Survey of the NTP Network, there were at least 175,000 hosts running NTP in the Internet. Source: http://alumni.media.mit.edu/~nelson/research/ntp-survey99/

Here is a step by step guide on how to configure NTP on your Ubuntu machine. Please read my previous post to better understand this guide for NTP Server and Client Configuration.

NTP Server Configuration

Step 1: Install NTP server on your machine.

sudo apt-get install ntp

Step 2: Configure NTP server. NTP’s default main configuration file is located at /etc/ntp.conf.
a: add a number of servers to the server list

server 0.asia.pool.ntp.org
server 1.asia.pool.ntp.org

b: restrict the server’s type of access

restrict otherntp.server.org mask 255.255.255.255 nomodify notrap noquery
restrict ntp.research.gov mask 255.255.255.255 nomodify notrap noquery

Servers are not allowed to modify the run-time configuration or query your own NTP server.

c: set the localhost to have full access without any restricting keywords

restrict 127.0.0.1

d: define the network where NTP synchronization requests are accecpted by the NTP server

restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap

Step 3: Stop NTP.

/etc/init.d/ntp stop

Step 4: Run ntpdate command.

ntpdate 192.168.1.1

Step 5: Start NTP for the settings to take effect.

/etc/init.d/ntp restart

Step 6: Check if NTP is synchronized.

ntpq -p

Configured time servers are listed. For correct synchronization, the delay and offset values should be non-zero and the jitter value should be under 100.

NTP Client Configuration

Follow the same steps provided above and set your server configuration setting to your own server’s current ip address.
server configuration settings on ntp.conf

#server 192.168.1.1