PING

What is ping ?

Why to use PING ?

Syntax of PING ?

Ping is a computer network administration utility used to test the reachability of a host on an Internet Protocol (IP) network and to measure the round-trip time for messages sent from the originating host to a destination computer. The name comes from active sonar terminology which sends a pulse of sound and listens for the echo to detect objects underwater.

Ping operates by sending Internet Control Message Protocol (ICMP) echo request packets to the target host and waiting for an ICMP response. In the process it measures the time from transmission to reception (round-trip time) and records any packet loss. The results of the test are printed in the form of a statistical summary of the response packets received, including the minimum, maximum, and the mean round-trip times, and sometimes the standard deviation of the mean.

Depending on actual implementation, the ping utility may be executed with various command-line switches to enable special operational modes. For example, options include specifying the packet size of the probe, automatic repeated operation for sending a specified count of probes, and time stamping.

Many operating systems provide a companion utility, ping6, for probing Internet Protocol version 6 (IPv6) hosts, but some systems may include this capability in ping.

Ping may be abused as a simple form of denial-of-service attack in the form of a ping flood, in which the attacker overwhelms the victim with ICMP echo request packets.

History

The ping utility was authored by Mike Muuss in December 1983 as a tool to troubleshoot problems in an IP network. He was inspired by a remark by David Mills on using ICMP echo packets for IP network diagnosis and measurements. Mike named it after the sound that sonar makes, since its methodology is similar to sonar's echo location.

Host discovery or ping scanning or ping sweep is a feature of network scanning tools, such as nmap.

RFC 1122 prescribes that any host must process an echo-request and issue an echo-reply in return This has been characterized as a security risk.

Sample ping test

The following is the output of running ping with the target www.example.com for five probes.

Syntax :Type ping <hostname> or ping <IP address>.

The utility summarizes its results after completing the ping probes. The shortest round trip time was 9.674 ms, the average was 10.968 ms, and the maximum value was 11.726 ms. The measurement had a standard deviation of 0.748 ms.

Message format

ICMP packet

IP Datagram
	Bits 0–7	Bits 8–15	Bits 16–23	Bits 24–31
IP Header (20 bytes)	Version/IHL	Type of service	Length
	Identification		flags and offset
	Time To Live (TTL)	Protocol	Checksum
	Source IP address
	Destination IP address
ICMP Header (8 bytes)	Type of message	Code	Checksum
	Header Data
ICMP Payload (optional)	Payload Data

Generic composition of an ICMP 32-byte packet:[6]

·         IP Header (in blue):  Protocol set to 1 (ICMP) and Type of Service set to 0.

·         ICMP Header (in red): Type of ICMP message (8 bits)

Code (8 bits)

Checksum (16 bits), calculated with the ICMP part of the packet (the IP header is not used). It is the 16-bit one's complement of the one's complement sum of the ICMP message starting with the Type field[7]

Header Data (32 bits) field, which in this case (ICMP echo request and replies), will be composed of identifier (16 bits) and sequence number (16 bits).

ICMP Payload

Payload for the different kind of answers; can be an arbitrary length, left to implementation detail. However, the packet including IP and ICMP headers must be less than the maximum transmission unit of the network or risk being fragmented.

Data Transportation

Echo request

The echo request ("ping") is an ICMP message whose data is expected to be received back in an echo reply ("pong"). The host must respond to all echo requests with an echo reply containing the exact data received in the request message.

00	01	02	03	04	05	06	07	08	09	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
Type = 8								Code = 0								Header Checksum
Identifier																Sequence Number
Data

The Identifier and Sequence Number can be used by the client to match the reply with the request that caused the reply. In practice, most Linux systems use a unique identifier for every ping process, and sequence number is an increasing number within that process. Windows uses a fixed identifier, which varies between Windows versions, and a sequence number that is only reset at boot time.

The data received in the Echo Request must be entirely included in the Echo Reply.

Echo reply

The echo reply ("pong") is an ICMP message generated in response to an echo request, and is mandatory for all hosts and routers.

00	01	02	03	04	05	06	07	08	09	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
Type = 0								Code = 0								Header Checksum
Identifier																Sequence Number
Data

Type and code must be set to 0.

The identifier and sequence number can be used by the client to determine which echo requests are associated with the echo replies.

The data received in the echo request must be entirely included in the echo reply.

Possible reply messages include the following:

H, !N, or !P – host, network or protocol unreachable

S – source route failed

F – fragmentation needed

U or !W – destination network/host unknown

I – source host is isolated

A – communication with destination network administratively prohibited

Z – communication with destination host administratively prohibited

Q – for this ToS the destination network is unreachable

T – for this ToS the destination host is unreachable

X – communication administratively prohibited

V – host precedence violation

C – precedence cutoff in effect.