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diff --git a/share/man/man4/inet.4 b/share/man/man4/inet.4
--- a/share/man/man4/inet.4
+++ b/share/man/man4/inet.4
@@ -284,6 +284,17 @@
in the reassembly queue for a packet.
0 means that the host will not accept any fragmented packets for the VNET.
This is a per-VNET limit.
+.It Va ip.allow_net0
+Boolean: allow experimental use of addresses in 0.0.0.0/8 as endpoints,
+and allow forwarding of packets with these addresses.
+.It Va ip.allow_net240
+Boolean: allow experimental use of addresses in 240.0.0.0/4 as endpoints,
+and allow forwarding of packets with these addresses.
+.It Va ip.loopback_prefixlen
+Integer: prefix length of the address space reserved for loopback purposes.
+The default is 8, meaning that 127.0.0.0/8 is reserved for loopback,
+and cannot be sent, received, or forwarded on a non-loopback interface.
+Use of other values is experimental.
.El
.Sh SEE ALSO
.Xr ioctl 2 ,
diff --git a/sys/net/vnet.h b/sys/net/vnet.h
--- a/sys/net/vnet.h
+++ b/sys/net/vnet.h
@@ -65,6 +65,7 @@
* as required for libkvm.
*/
#if defined(_KERNEL) || defined(_WANT_VNET)
+#include <machine/param.h> /* for CACHE_LINE_SIZE */
#include <sys/queue.h>
struct vnet {
diff --git a/sys/netinet/in.h b/sys/netinet/in.h
--- a/sys/netinet/in.h
+++ b/sys/netinet/in.h
@@ -383,7 +383,13 @@
#define IN_BADCLASS(i) (((in_addr_t)(i) & 0xf0000000) == 0xf0000000)
#define IN_LINKLOCAL(i) (((in_addr_t)(i) & 0xffff0000) == 0xa9fe0000)
+#ifdef _KERNEL
+#define IN_LOOPBACK(i) \
+ (((in_addr_t)(i) & V_in_loopback_mask) == 0x7f000000)
+#define IN_LOOPBACK_MASK_DFLT 0xff000000
+#else
#define IN_LOOPBACK(i) (((in_addr_t)(i) & 0xff000000) == 0x7f000000)
+#endif
#define IN_ZERONET(i) (((in_addr_t)(i) & 0xff000000) == 0)
#define IN_PRIVATE(i) ((((in_addr_t)(i) & 0xff000000) == 0x0a000000) || \
@@ -414,6 +420,18 @@
#define IN_RFC3021_MASK ((in_addr_t)0xfffffffe)
+#ifdef _KERNEL
+#include <net/vnet.h>
+
+VNET_DECLARE(bool, ip_allow_net0);
+VNET_DECLARE(bool, ip_allow_net240);
+/* Address space reserved for loopback */
+VNET_DECLARE(uint32_t, in_loopback_mask);
+#define V_ip_allow_net0 VNET(ip_allow_net0)
+#define V_ip_allow_net240 VNET(ip_allow_net240)
+#define V_in_loopback_mask VNET(in_loopback_mask)
+#endif
+
/*
* Options for use with [gs]etsockopt at the IP level.
* First word of comment is data type; bool is stored in int.
diff --git a/sys/netinet/in.c b/sys/netinet/in.c
--- a/sys/netinet/in.c
+++ b/sys/netinet/in.c
@@ -97,6 +97,28 @@
&VNET_NAME(broadcast_lowest), 0,
"Treat lowest address on a subnet (host 0) as broadcast");
+VNET_DEFINE(bool, ip_allow_net240) = false;
+#define V_ip_allow_net240 VNET(ip_allow_net240)
+SYSCTL_BOOL(_net_inet_ip, OID_AUTO, allow_net240,
+ CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(ip_allow_net240), 0,
+ "Allow use of Experimental addresses, aka Class E (240/4)");
+/* see https://datatracker.ietf.org/doc/draft-schoen-intarea-unicast-240 */
+
+VNET_DEFINE(bool, ip_allow_net0) = false;
+SYSCTL_BOOL(_net_inet_ip, OID_AUTO, allow_net0,
+ CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(ip_allow_net0), 0,
+ "Allow use of addresses in network 0/8");
+/* see https://datatracker.ietf.org/doc/draft-schoen-intarea-unicast-0 */
+
+VNET_DEFINE(uint32_t, in_loopback_mask) = IN_LOOPBACK_MASK_DFLT;
+#define V_in_loopback_mask VNET(in_loopback_mask)
+static int sysctl_loopback_prefixlen(SYSCTL_HANDLER_ARGS);
+SYSCTL_PROC(_net_inet_ip, OID_AUTO, loopback_prefixlen,
+ CTLFLAG_VNET | CTLTYPE_INT | CTLFLAG_RW,
+ NULL, 0, sysctl_loopback_prefixlen, "I",
+ "Prefix length of address space reserved for loopback");
+/* see https://datatracker.ietf.org/doc/draft-schoen-intarea-unicast-127 */
+
VNET_DECLARE(struct inpcbinfo, ripcbinfo);
#define V_ripcbinfo VNET(ripcbinfo)
@@ -251,12 +273,36 @@
{
u_long i = ntohl(in.s_addr);
- if (IN_EXPERIMENTAL(i) || IN_MULTICAST(i) || IN_LINKLOCAL(i) ||
- IN_ZERONET(i) || IN_LOOPBACK(i))
+ if (IN_MULTICAST(i) || IN_LINKLOCAL(i) || IN_LOOPBACK(i))
+ return (0);
+ if (IN_EXPERIMENTAL(i) && !V_ip_allow_net240)
+ return (0);
+ if (IN_ZERONET(i) && !V_ip_allow_net0)
return (0);
return (1);
}
+/*
+ * Sysctl to manage prefix of reserved loopback network; translate
+ * to/from mask. The mask is always contiguous high-order 1 bits
+ * followed by all 0 bits.
+ */
+static int
+sysctl_loopback_prefixlen(SYSCTL_HANDLER_ARGS)
+{
+ int error, preflen;
+
+ /* ffs is 1-based; compensate. */
+ preflen = 33 - ffs(V_in_loopback_mask);
+ error = sysctl_handle_int(oidp, &preflen, 0, req);
+ if (error || !req->newptr)
+ return (error);
+ if (preflen < 8 || preflen > 32)
+ return (EINVAL);
+ V_in_loopback_mask = 0xffffffff << (32 - preflen);
+ return (0);
+}
+
/*
* Trim a mask in a sockaddr
*/
diff --git a/sys/netinet/ip_icmp.c b/sys/netinet/ip_icmp.c
--- a/sys/netinet/ip_icmp.c
+++ b/sys/netinet/ip_icmp.c
@@ -775,8 +775,8 @@
NET_EPOCH_ASSERT();
if (IN_MULTICAST(ntohl(ip->ip_src.s_addr)) ||
- IN_EXPERIMENTAL(ntohl(ip->ip_src.s_addr)) ||
- IN_ZERONET(ntohl(ip->ip_src.s_addr)) ) {
+ (IN_EXPERIMENTAL(ntohl(ip->ip_src.s_addr)) && !V_ip_allow_net240) ||
+ (IN_ZERONET(ntohl(ip->ip_src.s_addr)) && !V_ip_allow_net0) ) {
m_freem(m); /* Bad return address */
ICMPSTAT_INC(icps_badaddr);
goto done; /* Ip_output() will check for broadcast */

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