icmp.7: Document icmp_ratelimit and icmp_ratemask (new in Linux 2.4.10)

Text taken from Documentation/networking/ip-sysctl.txt

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/icmp.7

@@ -83,6 +83,46 @@ Maximum rate for sending
 packets.
 These packets are sent when a packet arrives with an invalid IP header.
 .TP
+.IR icmp_ratelimit " (integer; default: 1000; since Linux 2.4.10)"
+.\" The following taken from 2.6.28-rc4 Documentation/networking/ip-sysctl.txt
+Limit the maximum rates for sending ICMP packets whose type matches
+.IR icmp_ratemask
+(see below) to specific targets.
+0 to disable any limiting,
+otherwise the minimum space between responses in milliseconds.
+.TP
+.IR icmp_ratemask " (integer; default: see below; since Linux 2.4.10)"
+.\" The following taken from 2.6.28-rc4 Documentation/networking/ip-sysctl.txt
+Mask made of ICMP types for which rates are being limited.
+
+Significant bits: IHGFEDCBA9876543210
+.br
+Default mask:     0000001100000011000 (0x1818)
+
+Bit definitions (see the kernel source file
+.IR include/linux/icmp.h ):
+
+.in +4n
+.nf
+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
+.fi
+.in
+
+The bits marked with an asterisk are rate limited by default
+(see the default mask above).
+.TP
 .IR icmp_timeexceed_rate " (Linux 2.2 to 2.4.10)"
 Maximum rate for sending
 .B ICMP_TIME_EXCEEDED