NAME

ping, ping6 — send ICMP ECHO_REQUEST packets to network hosts

SYNOPSIS

DESCRIPTION

ping uses the ICMP protocol's mandatory ECHO_REQUEST datagram to elicit an ICMP ECHO_REPLY from a host or gateway. These 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 sending after count ECHO_REQUEST packets have been sent. If count is 0, send an unlimited number of packets.

-D

Don't fragment IP packets.

-d

Set the SO_DEBUG option on the socket being used. This option has no effect on OpenBSD.

-E

Emit an audible beep (by sending an ASCII BEL character to the standard error output) when no packet is received before the next packet is transmitted. To cater for round-trip times that are longer than the interval between transmissions, further missing packets cause a bell only if the maximum number of unreceived packets has increased. This option is disabled for flood pings.

-e

Emit an audible beep (by sending an ASCII BEL character to the standard error output) after each non-duplicate response is received. This option is disabled for flood pings.

-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 superuser may use this option.

This can be very hard on a network and should be used with caution.

-H

Try reverse lookups for addresses.

-h hoplimit

(IPv6 only) Set the hoplimit.

-I sourceaddr

Set the source address to transmit from, which is useful on machines with multiple interfaces. For unicast and multicast pings.

-i interval

Send one packet every interval seconds. The default is one second. The interval may contain a fractional portion. Only the superuser may specify a value less than one second. This option is incompatible with the -f option.

-L

Disable the loopback, so the transmitting host doesn't see the ICMP requests. For multicast pings.

-l preload

Send preload packets as fast as possible before reverting to normal behavior. Only root may set a preload value.

-m

(IPv6 only) Do not fragment unicast packets to fit the minimum IPv6 MTU. If specified twice, do this for multicast packets as well.

-n

Numeric output only. No attempt will be made to look up symbolic names from addresses in the reply.

-p pattern

Specify up to 16 pad bytes to fill out the packet sent. This is useful for diagnosing data-dependent problems in a network. For example, "-p ff" causes 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

(IPv4 only) 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. If more routes come back than should, such as due to an illegal spoofed packet, ping will print the route list and then truncate it at the correct spot. Many hosts ignore or discard this option.

-s packetsize

Specify 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. The maximum packet size is 65467 for IPv4 and 65527 for IPv6.

-T toskeyword

Change the IPv4 TOS or IPv6 Traffic Class value. toskeyword may be one of critical, inetcontrol, lowdelay, netcontrol, throughput, reliability, or one of the DiffServ Code Points: ef, af11 ... af43, cs0 ... cs7; or a number in either hex or decimal.

-t ttl

(IPv4 only) Use the specified time-to-live.

-V rtable

Set the routing table to be used for outgoing packets.

-v

Verbose output. ICMP packets other than ECHO_REPLY that are received are listed.

-w maxwait

Specify the maximum number of seconds to wait for responses after the last request has been sent. The default is 10.

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 and the standard deviation.

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. The summary information can also be displayed while ping is running by sending it a SIGINFO signal (see the status argument of stty(1) for more information).

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 24 bytes, ping uses the first sixteen bytes of this space to include a timestamp which it uses in the computation of round trip times. The following 8 bytes store a message authentication code. If less than 24 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 (4.3BSD uses 30, 4.2BSD 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.

EXIT STATUS

ping exits 0 if at least one reply is received, and >0 if no reply is received or an error occurred.

SEE ALSO

netstat(1), ifconfig(8)

HISTORY

The ping command appeared in 4.3BSD. The ping6 command was originally a separate program and first appeared in the WIDE Hydrangea IPv6 protocol stack kit.

BUGS

Many hosts and gateways ignore the RECORD_ROUTE option.

The maximum IP header length is too small for options like RECORD_ROUTE to be completely useful. There's not much that can be done about this, however.

Flood pinging is not recommended in general, and flood pinging the broadcast address should only be done under very controlled conditions.