Implement a new IP option (not compiled/enabled by default) to allow

applications to specify a non-local IP address when bind()'ing a socket
to a local endpoint.

This allows applications to spoof the client IP address of connections
if (obviously!) they somehow are able to receive the traffic normally
destined to said clients.

This patch doesn't include any changes to ipfw or the bridging code to
redirect the client traffic through the PCB checks so TCP gets a shot
at it. The normal behaviour is that packets with a non-local destination
IP address are not handled locally. This can be dealth with some IPFW hackery;
modifications to IPFW to make this less hacky will occur in subsequent
commmits.

Thanks to Julian Elischer and others at Ironport. This work was approved
and donated before Cisco acquired them.

Obtained from:	Julian Elischer and others
MFC after:	2 weeks

sys/conf/NOTES

@@ -633,6 +633,14 @@ options 	ALTQ_PRIQ	# Priority Queueing
 options 	ALTQ_NOPCC	# Required if the TSC is unusable
 options 	ALTQ_DEBUG
 
+# IP optional behaviour.
+# IP_NONLOCALBIND disables the check that bind() usually makes that the
+# Address is one that is assigned to an interface on this machine.
+# It allows transparent proxies to pretend to be other machines.
+# How the packet GET to that machine is a problem solved elsewhere,
+# smart routers, ipfw fwd, etc.
+options        IP_NONLOCALBIND		#Allow impersonation for proxies.
+
 # netgraph(4). Enable the base netgraph code with the NETGRAPH option.
 # Individual node types can be enabled with the corresponding option
 # listed below; however, this is not strictly necessary as netgraph

sys/conf/options

@@ -392,6 +392,7 @@ IPFIREWALL_VERBOSE	opt_ipfw.h
 IPFIREWALL_VERBOSE_LIMIT	opt_ipfw.h
 IPSEC			opt_ipsec.h
 IPSEC_DEBUG		opt_ipsec.h
+IP_NONLOCALBIND		opt_inet.h
 IPSEC_FILTERTUNNEL	opt_ipsec.h
 IPSTEALTH
 IPX

sys/netinet/in.h

@@ -441,6 +441,7 @@ __END_DECLS
 #define	IP_FAITH		22   /* bool; accept FAITH'ed connections */
 
 #define	IP_ONESBCAST		23   /* bool: send all-ones broadcast */
+#define	IP_NONLOCALOK		24   /* allow bind to spoof other machines */
 
 #define	IP_FW_TABLE_ADD		40   /* add entry */
 #define	IP_FW_TABLE_DEL		41   /* delete entry */

sys/netinet/in_pcb.c

@@ -35,6 +35,7 @@
 __FBSDID("$FreeBSD$");
 
 #include "opt_ddb.h"
+#include "opt_inet.h"
 #include "opt_ipsec.h"
 #include "opt_inet6.h"
 #include "opt_mac.h"
@@ -346,7 +347,11 @@ in_pcbbind_setup(struct inpcb *inp, struct sockaddr *nam, in_addr_t *laddrp,
 		} else if (sin->sin_addr.s_addr != INADDR_ANY) {
 			sin->sin_port = 0;		/* yech... */
 			bzero(&sin->sin_zero, sizeof(sin->sin_zero));
-			if (ifa_ifwithaddr((struct sockaddr *)sin) == 0)
+			if (
+#if defined(IP_NONLOCALBIND)
+			    ((inp->inp_flags & INP_NONLOCALOK) == 0) &&
+#endif
+			    (ifa_ifwithaddr((struct sockaddr *)sin) == 0))
 				return (EADDRNOTAVAIL);
 		}
 		laddr = sin->sin_addr;

sys/netinet/in_pcb.h

@@ -411,6 +411,8 @@ void 	inp_4tuple_get(struct inpcb *inp, uint32_t *laddr, uint16_t *lp,
 #define	INP_FAITH		0x200	/* accept FAITH'ed connections */
 #define	INP_RECVTTL		0x400	/* receive incoming IP TTL */
 #define	INP_DONTFRAG		0x800	/* don't fragment packet */
+#define	INP_NONLOCALOK		0x1000	/* Allow bind to spoof any address */
+					/* - requires options IP_NONLOCALBIND */
 
 #define IN6P_IPV6_V6ONLY	0x008000 /* restrict AF_INET6 socket for v6 */
 

sys/netinet/ip_output.c

@@ -33,6 +33,7 @@
 __FBSDID("$FreeBSD$");
 
 #include "opt_ipfw.h"
+#include "opt_inet.h"
 #include "opt_ipsec.h"
 #include "opt_mac.h"
 #include "opt_mbuf_stress_test.h"
@@ -95,6 +96,12 @@ SYSCTL_INT(_net_inet_ip, OID_AUTO, mbuf_frag_size, CTLFLAG_RW,
 	&mbuf_frag_size, 0, "Fragment outgoing mbufs to this size");
 #endif
 
+#if defined(IP_NONLOCALBIND)
+static int ip_nonlocalok = 0;
+SYSCTL_INT(_net_inet_ip, OID_AUTO, nonlocalok,
+	CTLFLAG_RW|CTLFLAG_SECURE, &ip_nonlocalok, 0, "");
+#endif
+
 static void	ip_mloopback
 	(struct ifnet *, struct mbuf *, struct sockaddr_in *, int);
 
@@ -866,6 +873,13 @@ ip_ctloutput(struct socket *so, struct sockopt *sopt)
 			return (error);
 		}
 
+#if defined(IP_NONLOCALBIND)
+		case IP_NONLOCALOK:
+			if (! ip_nonlocalok) {
+			error = ENOPROTOOPT;
+			break;
+		}
+#endif
 		case IP_TOS:
 		case IP_TTL:
 		case IP_MINTTL:
@@ -937,6 +951,11 @@ ip_ctloutput(struct socket *so, struct sockopt *sopt)
 			case IP_DONTFRAG:
 				OPTSET(INP_DONTFRAG);
 				break;
+#if defined(IP_NONLOCALBIND)
+			case IP_NONLOCALOK:
+				OPTSET(INP_NONLOCALOK);
+				break;
+#endif
 			}
 			break;
 #undef OPTSET