PING(8) NetBSD System Manager's Manual PING(8)
NAME
ping -- send ICMP ECHO_REQUEST packets to network hosts
SYNOPSIS
ping [-aCDdfLnoPQqRrv] [-c count] [-E policy] [-g gateway] [-h host] [-I srcaddr] [-i interval] [-l preload] [-p pattern] [-s packetsize] [-T ttl] [-t tos] [-w deadline] 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 timespec'' and then an arbitrary number of ``pad'' bytes used to fill out the packet. The options are as follows: -a Emit an audible beep (by sending an ascii BEL character to the standard error output) after each non-duplicate response is received. This is disabled for flood pings as it would probably cause temporary insanity. -C Send timestamps in compat format; two 32 bit words in little endian format, the first one representing seconds, and the second one representing microseconds. -c count Stop after sending (and waiting the specified delay to receive) count ECHO_RESPONSE packets. -D Set the Don't Fragment bit in the IP header. This can be used to determine the path MTU. -d Set the SO_DEBUG option on the socket being used. -E policy Use IPsec policy specification string policy for packets. For the format of specification string, please refer ipsec_set_policy(3). Please note that this option is same as -P in KAME/FreeBSD and KAME/BSDI (as -P was already occupied in NetBSD). -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 net- work and should be used with caution. -g gateway Use Loose Source Routing to send the ECHO_REQUEST packets via gateway. -h host is an alternate way of specifying the target host instead of as the last argument. -I srcaddr Set the source IP address to srcaddr which can be a hostname or an IP number. For multicast datagrams, it also specifies the outgoing interface. -i interval Wait interval seconds between sending each packet. The default is to wait for one second between each packet, except when the -f option is used the wait interval is 0.01 seconds. -L Disable loopback when sending to multicast destinations, so the transmitting host doesn't see the ICMP requests. -l preload If preload is specified, ping sends that many packets as fast as possible before falling into its normal mode of behavior. Only the super-user may use this option. -n Numeric output only. No attempt will be made to look up symbolic names for host addresses. -o Exit successfully after receiving one reply packet. -P Use a pseudo-random sequence for the data instead of the default, fixed sequence of incrementing 8-bit integers. This is useful to foil compression on PPP and other links. -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 Do not display responses such as Network Unreachable ICMP mes- sages concerning the ECHO_REQUESTs sent. -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. This should show the path to the target host and back, which is especially useful in the case of asymmetric routing. Note that the IP header is only large enough for nine such addresses, and only seven when using the -g option. This is why it was necessary to invent traceroute(8). 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. The maximum allowed value is 65467 bytes. -T ttl Use the specified time-to-live. -t tos Use the specified hexadecimal type of service. -v Verbose output. ICMP packets other than ECHO_RESPONSE that are received are listed. -w deadline Specifies a timeout, in seconds, before ping exits regardless of how many packets have been sent or received. 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 calcula- tion, although the round trip time of these packets is used in calculat- ing 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. The summary information can be displayed while ping is running by sending it a SIGINFO signal (see the ``status'' argument for stty(1) for more information). ping continually sends one datagram per second, and prints one line of output for every ECHO_RESPONSE returned. On a trusted system with IP Security Options enabled, if the network idiom is not MONO, ping also prints a second line containing the hexadecimal representation of the IP security option in the ECHO_RESPONSE. If the -c count option is given, only that number of requests is sent. No output is produced if there is no response. 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 an interrupt (SIGINT), a brief summary is dis- played. When not using the -f (flood) option, the first interrupt, usu- ally generated by control-C or DEL, causes ping to wait for its outstand- ing requests to return. It will wait no longer than the longest round trip time encountered by previous, successful pings. The second inter- rupt stops ping immediately. This program is intended for use in network testing, measurement and man- agement. 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 arbi- trary 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 sizeof(struct timespec) (16) large, ping uses the first sizeof(struct timespec) bytes to include a timestamp to compute round trip times. Otherwise if the data space is at least eight bytes large (or the -C flag is specified), ping uses the first eight bytes of this space to include a timestamp to compute round trip times. If there are not enough bytes of pad 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 retrans- missions. 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 returns 0 on success (the host is alive), and non-zero if the argu- ments are incorrect or the host is not responding.
SEE ALSO
netstat(1), icmp(4), inet(4), ip(4), ifconfig(8), routed(8), spray(8), traceroute(8)
HISTORY
The ping command appeared in 4.3BSD. IPsec support was added by WIDE/KAME project.
BUGS
Flood pinging is not recommended in general, and flood pinging a broad- cast or multicast address should only be done under very controlled con- ditions. The ping program has evolved differently under different operating sys- tems, and in some cases the same flag performs a different function under different operating systems. The -t flag conflicts with FreeBSD. The -a, -c, -I, -i, -l, -P, -p, -s, and -t flags conflict with Solaris. Some 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. NetBSD 6.1.3 September 10, 2011 NetBSD 6.1.3
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