Test a network connection. 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'.
Syntax
ping [options] destination_host Options -c count Stop after sending (and receiving) count ECHO_RESPONSE packets. -d Debug, Set the SO_DEBUG option on the socket being used. -f Flood ping, output packets as fast as they come back or 100 times per second. -i wait Set an interval of wait seconds between sending each packet. default=one second. (incompatible with -f) -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 Specify up to 16 `pad' bytes to fill out the packet sent. This is useful for diagnosing data-dependent problems in a network. eg, `-p ff' will fill the packet sent with all ones. -q Quiet output. Only display the summary lines at startup time and when finished. -R Record route. Includes the RECORD_ROUTE option in the ECHO_REQUEST packet and display 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 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.
Ping 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.
Flood Ping
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.
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.
Flood pinging is not recommended in general, and flood pinging the broadcast address should only be done under very controlled conditions.
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 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 a serious cause for alarm and often indicate broken hardware somewhere in the ping packet's path (in the network or in the hosts).
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. 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 Time To Live, (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.
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:
"There's a Nong Nang Ning, Where the trees go Ping! " ~ Spike Milligan
Related:
ping man page - Apple.com
netstat - show network status
ifconfig - configure network interface parameters
routed - network RIP and router discovery routing daemon
