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MAN page from Trustix iputils-0.0.20001011-1tr.i586.rpm

PING

Section: Maintenance Commands (8)
Index
BSD mandoc
iputils-ss990107 

NAME

ping - sendICMP ECHO_REQUESTpackets to network hosts 

SYNOPSIS

ping[-LRUbdfnqrvV][-c count][-i interval][-l preload][-p pattern][-s packetsize][-t ttl][-w deadline][-I interface address][-T timestamp option][-M hint]host 

DESCRIPTION

Pinguses theICMPprotocol's mandatoryECHO_REQUESTdatagram to elicit anICMP ECHO_RESPONSEfrom a host or gateway.ECHO_REQUESTdatagrams (``pings'') have an IP andICMPheader,followed by a``struct timeval''and then an arbitrary number of ``pad'' bytes used to fill out thepacket.The options are as follows:Other options are:

-b
Allow pinging a broadcast address.
-c count
Stop after sendingcountECHO_REQUESTpackets. Withdeadlineoption, ping waits forcountECHO_REPLYpackets, until the timeout expires.
-d
Set theSO_DEBUGoption 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 everyECHO_REQUESTsent a period ``.'' is printed, while for everECHO_REPLYreceived a backspace is printed.This provides a rapid display of how many packets are being dropped.Only the super-user may use this option.Bf -emphasisThis can be very hard on a network and should be used with caution.Ef
-i wait
Waitwaitsecondsbetween sending each packet The default is to wait for one second between each packet.This option is incompatible with the-foption.
-I interface address
Set source address to specified interface address.
-l preload
Ifpreloadis specified,pingsends that many packets as fast as possible before falling into its normalmode of behavior.Only the super-user may use this option.
-L
Suppress loopback of multicast packets. This flag only applies if the pingdestination 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 allones.
-q
Quiet output.Nothing is displayed except the summary lines at startup time andwhen finished.
-R
Record route.Includes theRECORD_ROUTEoption in theECHO_REQUESTpacket and displaysthe 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 attachednetwork.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 interfacethat has no route through it (e.g., after the interface was dropped byrouted(8)).
-s packetsize
Specifies the number of data bytes to be sent. The default is 56, which translates into 64ICMPdata bytes when combinedwith the 8 bytes ofICMPheader data.
-t ttl
Set the IP Time to Live for multicasted packets. This flag only applies ifthe ping destination is a multicast address.
-T timestamp option
Set special IP timestamp options.Timestamp optionmay be either tsonly(only timestamps), tsandaddr(timestamps and addresses), or tsprespec host1 [host2 [host3 [host 4]]](timestamp preprecified hops).
-M hint
Select Path MTU Discovery strategy.hintmay 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 true user-to-user latency (the old behaviour).
-v
Verbose output.
-V
Show version.
-w deadline
Specify a timeout, in seconds, before ping exits regardless of how manypackets have been sent or received.

When usingpingfor fault isolation, it should first be run on the local host, to verifythat 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 packetloss calculation, although the round trip time of these packets is usedin calculating the minimum/average/maximum round-trip time numbers.When the specified number of packets have been sent (and received) orif the program is terminated with aSIGINT a brief summary is displayed.

If pingdoes not receive any reply packets at all it will exit with code 1.If a packetcountanddeadlineare both specified, and fewer thancountpackets are received by the time thedeadlinehas arrived,it will also exit with code 1. On other error it exits with code 2. Otherwise it exits with code 0. Thismakes it possible to use the exit code to see if a host is alive ornot.

This program is intended for use in network testing, measurement andmanagement.Because of the load it can impose on the network, it is unwise to usepingduring normal operations or from automated scripts. 

ICMP PACKET DETAILS

An IP header without options is 20 bytes.AnICMPECHO_REQUESTpacket contains an additional 8 bytes worthofICMPheader followed by an arbitrary amount of data.When apacketsizeis given, this indicated the size of this extra piece of data (thedefault is 56).Thus the amount of data received inside of an IP packet of typeICMPECHO_REPLYwill always be 8 bytes more than the requested data space(theICMPheader).

If the data space is at least eight bytes large,pinguses the first eight bytes of this space to include a timestamp whichit uses in the computation of round trip times.If less than eight bytes of pad are specified, no round trip times aregiven. 

DUPLICATE AND DAMAGED PACKETS

Pingwill report duplicate and damaged packets.Duplicate packets should never occur, and seem to be caused byinappropriate link-level retransmissions.Duplicates may occur in many situations and are rarely (if ever) agood sign, although the presence of low levels of duplicates may notalways be cause for alarm.

Damaged packets are obviously serious cause for alarm and oftenindicate broken hardware somewhere in thepingpacket's path (in the network or in the hosts). 

TRYING DIFFERENT DATA PATTERNS

The (inter)network layer should never treat packets differently dependingon the data contained in the data portion.Unfortunately, data-dependent problems have been known to sneak intonetworks and remain undetected for long periods of time.In many cases the particular pattern that will have problems is somethingthat doesn't have sufficient ``transitions'', such as all ones or allzeros, 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 (forexample) on the command line because the pattern that is of interest isat the data link level, and the relationship between what you type andwhat the controllers transmit can be complicated.

This means that if you have a data-dependent problem you will probablyhave 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 sentacross your network or that takes much longer to transfer than othersimilar length files.You can then examine this file for repeated patterns that you can testusing the-poption ofping  

TTL DETAILS

TheTTLvalue of an IP packet represents the maximum number of IP routersthat the packet can go through before being thrown away.In current practice you can expect each router in the Internet to decrementtheTTLfield by exactly one.

TheTCP/IPspecification states that theTTLfield forTCPpackets shouldbe set to 60, but many systems use smaller values (4.3BSDuses 30, 4.2 used15).

The maximum possible value of this field is 255, and most Unix systems settheTTLfield ofICMP ECHO_REQUESTpackets to 255.This is why you will find you can ``ping'' some hosts, but not reach themwithtelnet(1)orftp(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 thingswith theTTLfield in its response:

  • Not change it; this is what Berkeley Unix systems did before theBSD 4.3 tahoerelease.In this case theTTLvalue in the received packet will be 255 minus thenumber of routers in the round-trip path.
  • Set it to 255; this is what current Berkeley Unix systems do.In this case theTTLvalue in the received packet will be 255 minus thenumber of routers in the pathfromthe remote systemtotheping inghost.
  • Set it to some other value.Some machines use the same value forICMPpackets that they use forTCPpackets, for example either 30 or 60.Others may use completely wild values.

 

BUGS

Many Hosts and Gateways ignore theRECORD_ROUTEoption.

The maximum IP header length is too small for options likeRECORD_ROUTEtobe completely useful.There's not much that that can be done about this, however.

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

SEE ALSO

netstat(1),ifconfig(8) 

HISTORY

Thepingcommand appeared inBSD 4.3


 

Index

NAME
SYNOPSIS
DESCRIPTION
ICMP PACKET DETAILS
DUPLICATE AND DAMAGED PACKETS
TRYING DIFFERENT DATA PATTERNS
TTL DETAILS
BUGS
SEE ALSO
HISTORY

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