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.
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 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.
SEE ALSO¶
netstat(1),
ifconfig(8),
routed(8)
HISTORY¶
The
ping command appeared in
4.3BSD.