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.
OPTIONS
-a
Audible ping.
-A
Adaptive ping. Interpacket interval adapts to round-trip time, so that
effectively not more than one (or more, if preload is set) unanswered probes
present in the network. Minimal interval is 200msec for not super-user.
On networks with low rtt this mode is essentially equivalent to flood mode.
-b
Allow pinging a broadcast address.
-B
Do not allow ping to change source address of probes.
The address is bound to one selected when ping starts.
-c count
Stop after sending count ECHO_REQUEST
packets. With
deadline
option, ping waits for
count ECHO_REPLY packets, until the timeout expires.
-d
Set the SO_DEBUG option on the socket being used.
Essentially, this socket option is not used by Linux kernel.
-F flow label
Allocate and set 20 bit flow label on echo request packets.
(Only ping6). If value is zero, kernel allocates random flow label.
-f
Flood ping. For every ECHO_REQUEST sent a period ``.'' is printed,
while for ever ECHO_REPLY received a backspace is printed.
This provides a rapid display of how many packets are being dropped.
If interval is not given, it sets interval to zero and
outputs packets as fast as they come back or one hundred times per second,
whichever is more.
Only the super-user may use this option with zero interval.
-i interval
Wait interval seconds between sending each packet.
The default is to wait for one second between each packet normally,
or not to wait in flood mode. Only super-user may set interval
to values less 0.2 seconds.
-I interface address
Set source address to specified interface address. Argument
may be numeric IP address or name of device. When pinging IPv6
link-local address this option is required.
-l preload
If preload is specified,
ping sends that many packets not waiting for reply.
Only the super-user may select preload more than 3.
-L
Suppress loopback of multicast packets. This flag only applies if the ping
destination is a multicast address.
-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 tos
Set Quality of Service -related bits in ICMP datagrams.
tos can be either decimal or hex number.
Traditionally (RFC1349), these have been interpreted as: 0 for reserved
(currently being redefined as congestion control), 1-4 for Type of Service
and 5-7 for Precedence.
Possible settings for Type of Service are: minimal cost: 0x02,
reliability: 0x04, throughput: 0x08, low delay: 0x10. Multiple TOS bits
should not be set simultaneously. Possible settings for
special Precedence range from priority (0x20) to net control (0xe0). You
must be root (CAP_NET_ADMIN capability) to use Critical or
higher precedence value. You cannot set
bit 0x01 (reserved) unless ECN has been enabled in the kernel.
In RFC2474, these fields has been redefined as 8-bit Differentiated
Services (DS), consisting of: bits 0-1 of separate data (ECN will be used,
here), and bits 2-7 of Differentiated Services Codepoint (DSCP).
-q
Quiet output.
Nothing is displayed except the summary lines at startup time and
when finished.
-R
Record route. (IPv4 only)
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
interface.
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 provided the option -I is also
used.
-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.
-S sndbuf
Set socket sndbuf. If not specified, it is selected to buffer
not more than one packet.
-t ttl
Set the IP Time to Live.
-T timestamp option
Set special IP timestamp options.
timestamp option may be either
tsonly (only timestamps),
tsandaddr (timestamps and addresses) or
tsprespec host1 [host2 [host3 [host4]]]
(timestamp prespecified hops).
-M hint
Select Path MTU Discovery strategy.
hint may be either do
(prohibit fragmentation, even local one),
want (do PMTU discovery, fragment locally when packet size
is large), or dont (do not set DF flag).
-U
Print full user-to-user latency (the old behaviour). Normally
ping
prints network round trip time, which can be different
f.e. due to DNS failures.
-v
Verbose output.
-V
Show version and exit.
-w deadline
Specify a timeout, in seconds, before
ping
exits regardless of how many
packets have been sent or received. In this case
ping
does not stop after
count
packet are sent, it waits either for
deadline
expire or until
count
probes are answered or for some error notification from network.
-W timeout
Time to wait for a response, in seconds. The option affects only timeout
in absense of any responses, otherwise ping waits for two RTTs.
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. Shorter current statistics
can be obtained without termination of process with signal
SIGQUIT.
If ping does not receive any reply packets at all it will
exit with code 1. If a packet
count
and
deadline
are both specified, and fewer than
count
packets are received by the time the
deadline
has arrived, it will also exit with code 1.
On other error it exits with code 2. Otherwise it exits with code 0. This
makes it possible to use the exit code to see if a host is alive or
not.
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 of size of struct timeval
ping uses the beginning bytes of this space to include
a timestamp which it uses in the computation of round trip times.
If the data space is shorter, 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.
