# icmp

## NAME



           icmp - Linux IPv4 ICMP kernel module.
    


## DESCRIPTION



           This kernel protocol module implements the Internet Control
           Message Protocol defined in RFC 792.  It is used to signal error
           conditions and for diagnosis.  The user doesn't interact directly
           with this module; instead it communicates with the other protocols
           in the kernel and these pass the ICMP errors to the application
           layers.  The kernel ICMP module also answers ICMP requests.
    
           A user protocol may receive ICMP packets for all local sockets by
           opening a raw socket with the protocol **IPPROTO_ICMP**.  See raw(7)
           for more information.  The types of ICMP packets passed to the
           socket can be filtered using the **ICMP_FILTER** socket option.  ICMP
           packets are always processed by the kernel too, even when passed
           to a user socket.
    
           Linux limits the rate of ICMP error packets to each destination.
           **ICMP_REDIRECT** and **ICMP_DEST_UNREACH** are also limited by the
           destination route of the incoming packets.
    
       **/proc interfaces**
           ICMP supports a set of _/proc_ interfaces to configure some global
           IP parameters.  The parameters can be accessed by reading or
           writing files in the directory _/proc/sys/net/ipv4/_.  Most of these
           parameters are rate limitations for specific ICMP types.  Linux
           2.2 uses a token bucket filter to limit ICMPs.  The value is the
           timeout in jiffies until the token bucket filter is cleared after
           a burst.  A jiffy is a system dependent unit, usually 10ms on i386
           and about 1ms on alpha and ia64.
    
           _icmp_destunreach_rate_ (Linux 2.2 to Linux 2.4.9)
                  Maximum rate to send ICMP Destination Unreachable packets.
                  This limits the rate at which packets are sent to any
                  individual route or destination.  The limit does not affect
                  sending of **ICMP_FRAG_NEEDED** packets needed for path MTU
                  discovery.
    
           _icmp_echo_ignore_all_ (since Linux 2.2)
                  If this value is nonzero, Linux will ignore all **ICMP_ECHO**
                  requests.
    
           _icmp_echo_ignore_broadcasts_ (since Linux 2.2)
                  If this value is nonzero, Linux will ignore all **ICMP_ECHO**
                  packets sent to broadcast addresses.
    
           _icmp_echoreply_rate_ (Linux 2.2 to Linux 2.4.9)
                  Maximum rate for sending **ICMP_ECHOREPLY** packets in response
                  to **ICMP_ECHOREQUEST** packets.
    
           _icmp_errors_use_inbound_ifaddr_ (Boolean; default: disabled; since
           Linux 2.6.12)
                  If disabled, ICMP error messages are sent with the primary
                  address of the exiting interface.
    
                  If enabled, the message will be sent with the primary
                  address of the interface that received the packet that
                  caused the ICMP error.  This is the behavior that many
                  network administrators will expect from a router.  And it
                  can make debugging complicated network layouts much easier.
    
                  Note that if no primary address exists for the interface
                  selected, then the primary address of the first non-
                  loopback interface that has one will be used regardless of
                  this setting.
    
           _icmp_ignore_bogus_error_responses_ (Boolean; default: disabled;
           since Linux 2.2)
                  Some routers violate RFC1122 by sending bogus responses to
                  broadcast frames.  Such violations are normally logged via
                  a kernel warning.  If this parameter is enabled, the kernel
                  will not give such warnings, which will avoid log file
                  clutter.
    
           _icmp_paramprob_rate_ (Linux 2.2 to Linux 2.4.9)
                  Maximum rate for sending **ICMP_PARAMETERPROB** packets.  These
                  packets are sent when a packet arrives with an invalid IP
                  header.
    
           _icmp_ratelimit_ (integer; default: 1000; since Linux 2.4.10)
                  Limit the maximum rates for sending ICMP packets whose type
                  matches _icmp_ratemask_ (see below) to specific targets.  0
                  to disable any limiting, otherwise the minimum space
                  between responses in milliseconds.
    
           _icmp_ratemask_ (integer; default: see below; since Linux 2.4.10)
                  Mask made of ICMP types for which rates are being limited.
    
                  Significant bits: IHGFEDCBA9876543210
                  Default mask:     0000001100000011000 (0x1818)
    
                  Bit definitions (see the Linux kernel source file
                  _include/linux/icmp.h_):
                       0 Echo Reply
                       3 Destination Unreachable *
                       4 Source Quench *
                       5 Redirect
                       8 Echo Request
                       B Time Exceeded *
                       C Parameter Problem *
                       D Timestamp Request
                       E Timestamp Reply
                       F Info Request
                       G Info Reply
                       H Address Mask Request
                       I Address Mask Reply
    
           The bits marked with an asterisk are rate limited by default (see
           the default mask above).
    
           _icmp_timeexceed_rate_ (Linux 2.2 to Linux 2.4.9)
                  Maximum rate for sending **ICMP_TIME_EXCEEDED** packets.  These
                  packets are sent to prevent loops when a packet has crossed
                  too many hops.
    
           _ping_group_range_ (two integers; default: see below; since Linux
           2.6.39)
                  Range of the group IDs (minimum and maximum group IDs,
                  inclusive) that are allowed to create ICMP Echo sockets.
                  The default is "1 0", which means no group is allowed to
                  create ICMP Echo sockets.
    


## VERSIONS



           Support for the **ICMP_ADDRESS** request was removed in Linux 2.2.
    
           Support for **ICMP_SOURCE_QUENCH** was removed in Linux 2.2.
    


## NOTES



           As many other implementations don't support **IPPROTO_ICMP** raw
           sockets, this feature should not be relied on in portable
           programs.
    
           **ICMP_REDIRECT** packets are not sent when Linux is not acting as a
           router.  They are also accepted only from the old gateway defined
           in the routing table and the redirect routes are expired after
           some time.
    
           The 64-bit timestamp returned by **ICMP_TIMESTAMP** is in milliseconds
           since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
    
           Linux ICMP internally uses a raw socket to send ICMPs.  This raw
           socket may appear in netstat(8) output with a zero inode.
    


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