PING(8) | System Manager's Manual | PING(8) |
ping
— send ICMP
ECHO_REQUEST packets to network hosts
ping |
[-DdEefLnqRv ]
[-c count]
[-I ifaddr]
[-i wait]
[-l preload]
[-p pattern]
[-s packetsize]
[-T toskeyword]
[-t ttl]
[-V rtable]
[-w maxwait] host |
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:
-c
countECHO_REQUEST
packets have been sent. If
count is 0, send an unlimited number of
packets.-D
Don't Fragment
bit.-d
SO_DEBUG
option on the socket being used.
This option has no effect on OpenBSD.-E
-e
-f
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.
-I
ifaddr-i
wait-f
option.-L
-l
preloadping
sends that many packets as fast as possible
before falling into its normal mode of behavior. Only root may set a
preload value.-n
-p
pattern-q
-R
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-T
toskeyword-t
ttl-V
rtable-v
ECHO_REPLY
that are received are listed.-w
maxwaitWhen 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.
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 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.
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).
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
.
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:
ping
exits 0 if at least one reply is
received, and >0 if no reply is received or an error occurred.
The ping
command appeared in
4.3BSD.
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.
March 24, 2014 | OpenBSD-6.0 |