ping - Online in the Cloud

PROGRAM:

NAME

ping — send ICMP ECHO_REQUEST packets to network hosts

SYNOPSIS

ping [-Rdfnqrv] [-c count] [-i wait] [-l preload] [-p pattern] [-s packetsize] host

DESCRIPTION

Ping uses the ICMP protocol's mandatory ECHO_REQUEST datagram to elicit an ICMP
ECHO_RESPONSE from a host or gateway. ECHO_REQUEST datagrams (``pings'') have an IP and
ICMP header, followed by a “struct timeval” and then an arbitrary number of ``pad'' bytes
used to fill out the packet. The options are as follows:

-c count
Stop after sending (and receiving) count ECHO_RESPONSE packets.

-d Set the SO_DEBUG option on the socket being used.

-f Flood ping. Outputs packets as fast as they come back or one hundred times per
second, whichever is more. For every ECHO_REQUEST sent a period ``.'' is printed,
while for every ECHO_REPLY received a backspace is printed. This provides a rapid
display of how many packets are being dropped. Only the super-user may use this
option. This can be very hard on a network and should be used with caution.

-i wait
Wait wait seconds between sending each packet. The default is to wait for one
second between each packet. This option is incompatible with the -f option.

-l preload
If preload is specified, ping sends that many packets as fast as possible before
falling into its normal mode of behavior.

-n Numeric output only. No attempt will be made to lookup symbolic names for host
addresses.

-p pattern
You may specify up to 16 ``pad'' bytes to fill out the packet you send. This is
useful for diagnosing data-dependent problems in a network. For example, “-p ff”
will cause the sent packet to be filled with all ones.

-q Quiet output. Nothing is displayed except the summary lines at startup time and
when finished.

-R Record route. Includes the RECORD_ROUTE option in the ECHO_REQUEST packet and
displays the route buffer on returned packets. Note that the IP header is only
large enough for nine such routes. Many hosts ignore or discard this option.

-r Bypass the normal routing tables and send directly to a host on an attached network.
If the host is not on a directly-attached network, an error is returned. This
option can be used to ping a local host through an interface that has no route
through it (e.g., after the interface was dropped by routed(8)).

-s packetsize
Specifies the number of data bytes to be sent. The default is 56, which translates
into 64 ICMP data bytes when combined with the 8 bytes of ICMP header data.

-v Verbose output. ICMP packets other than ECHO_RESPONSE that are received are listed.

When using ping for fault isolation, it should first be run on the local host, to verify
that the local network interface is up and running. Then, hosts and gateways further and
further away should be ``pinged''. Round-trip times and packet loss statistics are
computed. If duplicate packets are received, they are not included in the packet loss
calculation, although the round trip time of these packets is used in calculating the
minimum/average/maximum round-trip time numbers. When the specified number of packets have
been sent (and received) or if the program is terminated with a SIGINT, a brief summary is
displayed.

This program is intended for use in network testing, measurement and management. Because of
the load it can impose on the network, it is unwise to use ping during normal operations or
from automated scripts.

ICMP PACKET DETAILS

An IP header without options is 20 bytes. An ICMP ECHO_REQUEST packet contains an
additional 8 bytes worth of ICMP header followed by an arbitrary amount of data. When a
packetsize is given, this indicated the size of this extra piece of data (the default is
56). Thus the amount of data received inside of an IP packet of type ICMP ECHO_REPLY will
always be 8 bytes more than the requested data space (the ICMP header).

If the data space is at least eight bytes large, ping uses the first eight bytes of this
space to include a timestamp which it uses in the computation of round trip times. If less
than eight bytes of pad are specified, no round trip times are given.

DUPLICATE AND DAMAGED PACKETS

Ping will report duplicate and damaged packets. Duplicate packets should never occur, and
seem to be caused by inappropriate link-level retransmissions. Duplicates may occur in many
situations and are rarely (if ever) a good sign, although the presence of low levels of
duplicates may not always be cause for alarm.

Damaged packets are obviously serious cause for alarm and often indicate broken hardware
somewhere in the ping packet's path (in the network or in the hosts).

TRYING DIFFERENT DATA PATTERNS

The (inter)network layer should never treat packets differently depending on the data
contained in the data portion. Unfortunately, data-dependent problems have been known to
sneak into networks and remain undetected for long periods of time. In many cases the
particular pattern that will have problems is something that doesn't have sufficient
``transitions'', such as all ones or all zeros, or a pattern right at the edge, such as
almost all zeros. It isn't necessarily enough to specify a data pattern of all zeros (for
example) on the command line because the pattern that is of interest is at the data link
level, and the relationship between what you type and what the controllers transmit can be
complicated.

This means that if you have a data-dependent problem you will probably have to do a lot of
testing to find it. If you are lucky, you may manage to find a file that either can't be
sent across your network or that takes much longer to transfer than other similar length
files. You can then examine this file for repeated patterns that you can test using the -p
option of ping.

TTL DETAILS

The TTL value of an IP packet represents the maximum number of IP routers that the packet
can go through before being thrown away. In current practice you can expect each router in
the Internet to decrement the TTL field by exactly one.

The TCP/IP specification states that the TTL field for TCP packets should be set to 60, but
many systems use smaller values (4.3 BSD uses 30, 4.2 used 15).

The maximum possible value of this field is 255, and most Unix systems set the TTL field of
ICMP ECHO_REQUEST packets to 255. This is why you will find you can ``ping'' some hosts,
but not reach them with telnet(1) or ftp(1).

In normal operation ping prints the ttl value from the packet it receives. When a remote
system receives a ping packet, it can do one of three things with the TTL field in its
response:

· Not change it; this is what Berkeley Unix systems did before the 4.3BSD-Tahoe release.
In this case the TTL value in the received packet will be 255 minus the number of
routers in the round-trip path.

· Set it to 255; this is what current Berkeley Unix systems do. In this case the TTL
value in the received packet will be 255 minus the number of routers in the path from
the remote system to the pinging host.

· Set it to some other value. Some machines use the same value for ICMP packets that they
use for TCP packets, for example either 30 or 60. Others may use completely wild
values.

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