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5 ping: Packets to network hosts

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. Synopsis:

     ping [option...] host...
--address
Send ICMP_ADDRESS packets.
--echo
Send ICMP_ECHO requests (default).
--timestamp
Send ICMP_TIMESTAMP packets.
-t type
--type=type
Send TYPE packets.
-c n
--count=n
Stop after sending (and receiving) n ECHO_RESPONSE packets.
-d
-debug
Set the SO_DEBUG option on the socket being used.
-i n
--interval=n
Wait n seconds between sending each packet. The default is to wait for one second between each packet. This option is incompatible with the -f option.
-n
--numeric
Numeric output only. No attempt will be made to lookup symbolic names for host addresses.
-r
--ignore-routing
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).
-f
--flood
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.
-l n
--preload=n
If n is specified, ping sends that many packets as fast as possible before falling into its normal mode of behavior.
-p pat
--pattern=pat
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.
-R
--route
Record route. Includes the RECORD_ROUTE field 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.
-s n
--size=n
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.
-w n
--timeout=n
Stop after n seconds.
-W n
--linger=n
Maximum number of seconds n to wait for a response.

5.1 Using ping for network fault isolation

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.

5.2 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).

5.3 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.

5.4 TTL Details

The TTL, Time To Live 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 or ftp.

In normal operation ping prints the TTL value from the packet it receives. When a remote system receives a ICMP 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.

Many hosts and gateways ignore the RECORD_ROUTE field, since the maximum IP header length is far to small to hold all the routes. There's not much that can be done about this.

Flood pinging is not recommended in general, and flood pinging the broadcast address should only be done under very controlled conditions.