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ping(8)
NAME
ping - Sends ICMP ECHO_REQUEST packets to network hosts
SYNOPSIS
/usr/sbin/ping [-dfnqruvLR] [-c count] [-i wait] [-l preload] [-p pattern]
[-s packetsize] [-t number] host
FLAGS
-c count Stops after sending (and receiving) count ECHO_RESPONSE packets.
-d Set the SO_DEBUG option on the socket being used.
-f Floods ping. Outputs packets as fast as they come back or 100
times per second, whichever is more. For every ECHO_REQUEST
sent, a . (dot) is printed, while for every ECHO_REPLY received a
backspace is used. This provides a rapid display of how many
packets are being dropped. Only the superuser may use this
option. This can be very hard on a network and should be used
with caution (see Cautions).
-i wait Waits wait seconds between sending each packet. The default is
to wait for 1 second between each packet. This option is
incompatible with the -f option.
-L Displays every 100th reply only and displays a message indicating
lost packets.
A packet loss rate of just 1% is enough to seriously impact UDP-
based protocols like NFS. This option enables you to measure
loss rates and to determine how bursty they are. All lost
messages are reported with the following message:
Lost sequence number(s) number
The number value is a single number or a range or numbers.
-l preload
If preload is specified, ping sends that many packets as fast as
possible before falling into its normal mode of behavior. Only
the superuser may use this option. This can be very hard on a
network and should be used with caution (see Cautions).
-n Numeric output only. No attempt will be made to look up symbolic
names for host addresses. This occurs only when displaying ICMP
packets other than ECHO_RESPONSE.
-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 1s (ones).
-q Quiets output. Nothing is displayed except the summary lines at
start-up time and when finished.
-R Records 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 Bypasses the normal routing tables and directly sends 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 send ping to a local host
through an interface that has no route through it (for example,
after the interface was dropped by routed).
-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.
-t number Sets the timeout period (in seconds) for ping to wait before
terminating. The default timeout period is 10 seconds. This
option is only useful with -c 1 option.
-u Displays the time in microseconds (three decimal places). In
order to ensure this microsecond precision, the NTP_TIME and
MICRO_TIME kernel options must be on. By default NTP_TIME and
MICRO_TIME kernel options are off. If these kernel options are
off and this flag is used, the time is displayed to three decimal
places, but in milliseconds.
-v Specifies verbose output. ICMP packets other than ECHO_RESPONSE
that are received are listed.
DESCRIPTION
The ping command uses the ICMP (Internet Control Message Protocol)
protocol's mandatory ECHO_REQUEST datagram to elicit an ICMP ECHO_RESPONSE
from the specified host or gateway host, where host is a network name or IP
address. ECHO_REQUEST datagrams (pings) have an IP (Internet Protocol) and
ICMP header, followed by a struct timeval and then an arbitrary number of
pad bytes used to fill out the packet.
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 sent the ping
command. Round-trip times and packet loss statistics are computed. If
duplicate packets are received, they are not included in the packet loss
calculations, although the round-trip time of these packets is used in
calculating the minimum, average, and 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 indicates 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 8 bytes large, ping uses the first 8 bytes of
this space to include a timestamp, which it uses in the computation of
round-trip times. If less than 8 bytes of pad are specified, no round-trip
times are given.
Duplicate and Damaged Packets
The ping command 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 does not have sufficient transitions, such as
all 1s (ones) or all 0s (zeros), or a pattern right at the edge, such as
almost all 0s (zeros). It is not necessarily enough to specify a data
pattern of all 0s (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 enter 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 cannot 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 1 (one).
The TCP/IP specification states that the TTL field for TCP packets should
be set to 60, but many systems use smaller values (4.3BSD uses 30, 4.2BSD
used 15).
The maximum possible value of this field is 255, and most UNIX compatible
systems set the TTL field of ICMP ECHO_REQUEST packets to 255. This is why
you will find you can use the ping command on some hosts, but not reach
them with telnet or ftp.
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:
1. Not change it; this is what Berkeley UNIX compatible systems did
before the 4.3BSD release. In this case, the TTL value in the
received packet will be 255 minus the number of routers in the round-
trip path.
2. Set it to 255; this is what current Berkeley UNIX compatible 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
host that received the ping command.
3. 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.
CAUTIONS
Many hosts and gateways ignore the RECORD_ROUTE option.
Flooding and preloading the ping command is not recommended in general, and
flooding ping on the broadcast address should only be done under very
controlled conditions.
FILES
/usr/sbin/ping
Specifies the command path
RELATED INFORMATION
Commands: netstat(1), ifconfig(8)
Daemons: gated(8), routed(8)
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Alphabetical
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