Commong Network Utilities

###Commong Network Utilities###

Features:

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