Allow raw sockets to process icmp echo requests.

[Christiano F. Haesbaert]

Hi, this diff adds a sysctl to disable kernel icmp echo processing and pass it
to userland via raw sockets. I'm terrible with names but I chose userecho, so
net.inet.icmp.userecho.

I did some basic tests and it seems to work ok.

I kinda need this to tunnel ip over icmp echo.

Index: netinet/icmp_var.h
===================================================================
RCS file: /cvs/src/sys/netinet/icmp_var.h,v
retrieving revision 1.13
diff -d -u -p -w -r1.13 icmp_var.h
--- netinet/icmp_var.h  13 Dec 2007 20:00:53 -0000      1.13
+++ netinet/icmp_var.h  15 Jul 2011 22:47:17 -0000
@@ -65,7 +65,8 @@ struct        icmpstat {
 #define        ICMPCTL_REDIRTIMEOUT    5       /* Remove routes added via 
redirects */
 #define        ICMPCTL_TSTAMPREPL      6       /* allow replies to timestamp 
requests */
 #define ICMPCTL_STATS          7       /* ICMP statistics */
-#define ICMPCTL_MAXID          8
+#define ICMPCTL_USERECHO       8       /* ICMP echo requests in raw sockets */
+#define ICMPCTL_MAXID          9
 
 #define ICMPCTL_NAMES { \
        { 0, 0 }, \
@@ -75,7 +76,8 @@ struct        icmpstat {
        { "rediraccept", CTLTYPE_INT }, \
        { "redirtimeout", CTLTYPE_INT }, \
        { "tstamprepl", CTLTYPE_INT }, \
-       { "stats", CTLTYPE_STRUCT } \
+       { "stats", CTLTYPE_STRUCT }, \
+       { "userecho", CTLTYPE_INT } \
 }
 
 #define ICMPCTL_VARS { \
@@ -86,7 +88,8 @@ struct        icmpstat {
        &icmp_rediraccept, \
        NULL, \
        &icmptstamprepl, \
-       NULL \
+       NULL, \
+       &icmpuserecho, \
 }
 
 #ifdef _KERNEL
Index: netinet/ip_icmp.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_icmp.c,v
retrieving revision 1.94
diff -d -u -p -w -r1.94 ip_icmp.c
--- netinet/ip_icmp.c   6 Jul 2011 01:57:37 -0000       1.94
+++ netinet/ip_icmp.c   15 Jul 2011 22:47:17 -0000
@@ -106,6 +106,7 @@
  */
 
 int    icmpmaskrepl = 0;
+int    icmpuserecho = 0;
 int    icmpbmcastecho = 0;
 int    icmptstamprepl = 1;
 #ifdef ICMPPRINTFS
@@ -482,6 +483,8 @@ icmp_input(struct mbuf *m, ...)
                break;
 
        case ICMP_ECHO:
+               if (icmpuserecho)
+                       goto raw;
                if (!icmpbmcastecho &&
                    (m->m_flags & (M_MCAST | M_BCAST)) != 0) {
                        icmpstat.icps_bmcastecho++;