### Commong Network Utilities ###

  1. Useful for basic troubleshooting


PING:
 Features:
  1. ability to communicate with hosts using ICMP
   a. PING sends ICMP echo-requests
   b. PING expects to receive ICMP echo-replies

Task: PING some hosts and evaluate the output
  1. ping localhost (127.0.0.1)
  2. ping -c 3 localhost - sends 3 ICMP echo-requests
Note: 'ping localhost' performs name resolution using /etc/hosts
/etc/hosts stores static name-to-IP mappings

Note: 127.0.0.0/8 is fully-reserved to the loopback adapter of ALL IPv4 hosts

  3. ping -c 3 192.168.75.199
  4. ping -c 3 -i 3 192.168.75.199 - delays PINGs to 3 seconds apart

Note: PING defaults to a standard 1-second interval
Note: Firewall(s) may block ICMP traffic, causing PING to fail

TELNET:
 Features:
  1. Great for basic TCP port diagnosis

Task:
  1. Connect to TCP ports on various hosts
   a. telnet 192.168.75.100 22
   b. telnet www.linuxcbt.com 80

NETSTAT:
 Features:
  1. Provides network connection information from /proc/net/*

Task:
  1. Return useful information for various protocols
   a. netstat
   b. netstat -a - returns all protocols/sockets
   c. netstat -ntlp - returns all TCP LISTENERS without name resolution
   d. netstat -nulp - returns all UDP lISTENERS without name resolution

Note: netstat uses /etc/services to translate ports to names
Note: 0.0.0.0:514 - this means that Syslog will accept traffic to any of the defined IP addresses/interfaces on the system

   e. netstat -ntp - returns established connections (sockets)
   f. netstat -rn - returns the routing table


ARP:
 Features:
  1. Resolves layer-2 (OSI model) MAC addresses to layer-3 IP addresses


Task:
 1. Examine MAC addresses using: ifconfig and arp
  a. ifconfig - returns our local MAC addresses
   Link encap:Ethernet  HWaddr 00:02:B3:98:41:08
  b. arp -a - returns MAC to IP mappings

Note: When 2 TCP/IP hosts communicate, ARP is performed to translate the IP address (v6/v4) to a MAC address.

Note: If a one or more routers separate the communicating hosts, then the MAC address of the default router's (gateway's) interface is stored by each client