conf: Add command line switch to enable IP_FREEBIND socket option

In a couple of recent reports, we've seen that it can be useful for pasta
to forward ports from addresses which are not currently configured on the
host, but might be in future.  That can be done with the sysctl
net.ipv4.ip_nonlocal_bind, but that does require CAP_NET_ADMIN to set in
the first place.  We can allow the same thing on a per-socket basis with
the IP_FREEBIND (or IPV6_FREEBIND) socket option.

Add a --freebind command line argument to enable this socket option on
all listening sockets.

Link: https://bugs.passt.top/show_bug.cgi?id=101
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

conf.c

@@ -836,6 +836,7 @@ static void usage(const char *name, FILE *f, int status)
 		"  --no-ndp		Disable NDP responses\n"
 		"  --no-dhcpv6		Disable DHCPv6 server\n"
 		"  --no-ra		Disable router advertisements\n"
+		"  --freebind		Bind to any address for forwarding\n"
 		"  --no-map-gw		Don't map gateway address to host\n"
 		"  -4, --ipv4-only	Enable IPv4 operation only\n"
 		"  -6, --ipv6-only	Enable IPv6 operation only\n");
@@ -1255,6 +1256,7 @@ void conf(struct ctx *c, int argc, char **argv)
 		{"no-dhcpv6",	no_argument,		&c->no_dhcpv6,	1 },
 		{"no-ndp",	no_argument,		&c->no_ndp,	1 },
 		{"no-ra",	no_argument,		&c->no_ra,	1 },
+		{"freebind",	no_argument,		&c->freebind,	1 },
 		{"no-map-gw",	no_argument,		&no_map_gw,	1 },
 		{"ipv4-only",	no_argument,		NULL,		'4' },
 		{"ipv6-only",	no_argument,		NULL,		'6' },

passt.1

@@ -327,6 +327,16 @@ namespace will be silently dropped.
 Disable Router Advertisements. Router Solicitations coming from guest or target
 namespace will be ignored.
 
+.TP
+.BR \-\-freebind
+Allow any binding address to be specified for \fB-t\fR and \fB-u\fR
+options.  Usually binding addresses must be addresses currently
+configured on the host.  With \fB\-\-freebind\fR, the
+\fBIP_FREEBIND\fR or \fBIPV6_FREEBIND\fR socket option is enabled
+allowing any address to be used.  This is typically used to bind
+addresses which might be configured on the host in future, at which
+point the forwarding will immediately start operating.
+
 .TP
 .BR \-\-map-host-loopback " " \fIaddr
 Translate \fIaddr\fR to refer to the host. Packets from the guest to

passt.h

@@ -225,6 +225,7 @@ struct ip6_ctx {
  * @no_dhcpv6:		Disable DHCPv6 server
  * @no_ndp:		Disable NDP handler altogether
  * @no_ra:		Disable router advertisements
+ * @freebind:		Allow binding of non-local addresses for forwarding
  * @low_wmem:		Low probed net.core.wmem_max
  * @low_rmem:		Low probed net.core.rmem_max
  */
@@ -284,6 +285,7 @@ struct ctx {
 	int no_dhcpv6;
 	int no_ndp;
 	int no_ra;
+	int freebind;
 
 	int low_wmem;
 	int low_rmem;

util.c

@@ -52,6 +52,7 @@ int sock_l4_sa(const struct ctx *c, enum epoll_type type,
 {
 	sa_family_t af = ((const struct sockaddr *)sa)->sa_family;
 	union epoll_ref ref = { .type = type, .data = data };
+	bool freebind = false;
 	struct epoll_event ev;
 	int fd, y = 1, ret;
 	uint8_t proto;
@@ -61,8 +62,11 @@ int sock_l4_sa(const struct ctx *c, enum epoll_type type,
 	case EPOLL_TYPE_TCP_LISTEN:
 		proto = IPPROTO_TCP;
 		socktype = SOCK_STREAM | SOCK_NONBLOCK;
+		freebind = c->freebind;
 		break;
 	case EPOLL_TYPE_UDP_LISTEN:
+		freebind = c->freebind;
+		/* fallthrough */
 	case EPOLL_TYPE_UDP_REPLY:
 		proto = IPPROTO_UDP;
 		socktype = SOCK_DGRAM | SOCK_NONBLOCK;
@@ -127,6 +131,18 @@ int sock_l4_sa(const struct ctx *c, enum epoll_type type,
 		}
 	}
 
+	if (freebind) {
+		int level = af == AF_INET ? IPPROTO_IP : IPPROTO_IPV6;
+		int opt = af == AF_INET ? IP_FREEBIND : IPV6_FREEBIND;
+
+		if (setsockopt(fd, level, opt, &y, sizeof(y))) {
+			err_perror("Failed to set %s on socket %i",
+				   af == AF_INET ? "IP_FREEBIND"
+				                 : "IPV6_FREEBIND",
+				   fd);
+		}
+	}
+
 	if (bind(fd, sa, sl) < 0) {
 		/* We'll fail to bind to low ports if we don't have enough
 		 * capabilities, and we'll fail to bind on already bound ports,