|PING(8)||System Manager's Manual||PING(8)|
ping — send ICMP
ECHO_REQUEST packets to network hosts
ping uses the ICMP protocol's mandatory
ECHO_REQUEST datagram to elicit an ICMP
ECHO_REPLY 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:
ECHO_REQUESTpackets have been sent. If count is 0, send an unlimited number of packets.
SO_DEBUGoption on the socket being used. This option has no effect on OpenBSD.
ECHO_REQUESTsent, a period ‘.’ is printed, while for every
ECHO_REPLYreceived a backspace is printed. This provides a rapid display of how many packets are being dropped. Only the superuser may use this option.
pingsends that many packets as fast as possible before falling into its normal mode of behavior. Only root may set a preload value.
RECORD_ROUTEoption in the
ECHO_REQUESTpacket 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,
pingwill print the route list and then truncate it at the correct spot. Many hosts ignore or discard this option.
ECHO_REPLYthat are received are listed.
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
running by sending it a
SIGINFO signal (see the
status argument of
stty(1) for more
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
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
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
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
packet's path (in the network or in the hosts).
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
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
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
ping exits 0 if at least one reply is
received, and >0 if no reply is received or an error occurred.
ping command appeared in
Many hosts and gateways ignore the
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.
|March 24, 2014||OpenBSD-5.6|