flow, tcp: Flow based NAT and port forwarding for TCP

Currently the code to translate host side addresses and ports to guest side
addresses and ports, and vice versa, is scattered across the TCP code.
This includes both port redirection as controlled by the -t and -T options,
and our special case NAT controlled by the --no-map-gw option.

Gather this logic into fwd_nat_from_*() functions for each input
interface in fwd.c which take protocol and address information for the
initiating side and generates the pif and address information for the
forwarded side.  This performs any NAT or port forwarding needed.

We create a flow_target() helper which applies those forwarding functions
as needed to automatically move a flow from INI to TGT state.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 148 insertions, 0 deletions

diff --git a/fwd.c b/fwd.c
index d3f1798..3288b0d 100644
--- a/fwd.c
+++ b/fwd.c
@@ -25,6 +25,7 @@
 #include "fwd.h"
 #include "passt.h"
 #include "lineread.h"
+#include "flow_table.h"
 
 /* See enum in kernel's include/net/tcp_states.h */
 #define UDP_LISTEN	0x07
@@ -154,3 +155,150 @@ void fwd_scan_ports_init(struct ctx *c)
 				   &c->tcp.fwd_out, &c->tcp.fwd_in);
 	}
 }
+
+/**
+ * fwd_nat_from_tap() - Determine to forward a flow from the tap interface
+ * @c:		Execution context
+ * @proto:	Protocol (IP L4 protocol number)
+ * @ini:	Flow address information of the initiating side
+ * @tgt:	Flow address information on the target side (updated)
+ *
+ * Return: pif of the target interface to forward the flow to, PIF_NONE if the
+ *         flow cannot or should not be forwarded at all.
+ */
+uint8_t fwd_nat_from_tap(const struct ctx *c, uint8_t proto,
+			 const struct flowside *ini, struct flowside *tgt)
+{
+	(void)proto;
+
+	tgt->eaddr = ini->faddr;
+	tgt->eport = ini->fport;
+
+	if (!c->no_map_gw) {
+		if (inany_equals4(&tgt->eaddr, &c->ip4.gw))
+			tgt->eaddr = inany_loopback4;
+		else if (inany_equals6(&tgt->eaddr, &c->ip6.gw))
+			tgt->eaddr = inany_loopback6;
+	}
+
+	/* The relevant addr_out controls the host side source address.  This
+	 * may be unspecified, which allows the kernel to pick an address.
+	 */
+	if (inany_v4(&tgt->eaddr))
+		tgt->faddr = inany_from_v4(c->ip4.addr_out);
+	else
+		tgt->faddr.a6 = c->ip6.addr_out;
+
+	/* Let the kernel pick a host side source port */
+	tgt->fport = 0;
+
+	return PIF_HOST;
+}
+
+/**
+ * fwd_nat_from_splice() - Determine to forward a flow from the splice interface
+ * @c:		Execution context
+ * @proto:	Protocol (IP L4 protocol number)
+ * @ini:	Flow address information of the initiating side
+ * @tgt:	Flow address information on the target side (updated)
+ *
+ * Return: pif of the target interface to forward the flow to, PIF_NONE if the
+ *         flow cannot or should not be forwarded at all.
+ */
+uint8_t fwd_nat_from_splice(const struct ctx *c, uint8_t proto,
+			    const struct flowside *ini, struct flowside *tgt)
+{
+	if (!inany_is_loopback(&ini->eaddr) ||
+	    (!inany_is_loopback(&ini->faddr) && !inany_is_unspecified(&ini->faddr))) {
+		char estr[INANY_ADDRSTRLEN], fstr[INANY_ADDRSTRLEN];
+
+		debug("Non loopback address on %s: [%s]:%hu -> [%s]:%hu",
+		      pif_name(PIF_SPLICE),
+		      inany_ntop(&ini->eaddr, estr, sizeof(estr)), ini->eport,
+		      inany_ntop(&ini->faddr, fstr, sizeof(fstr)), ini->fport);
+		return PIF_NONE;
+	}
+
+	if (inany_v4(&ini->eaddr))
+		tgt->eaddr = inany_loopback4;
+	else
+		tgt->eaddr = inany_loopback6;
+
+	/* Preserve the specific loopback adddress used, but let the kernel pick
+	 * a source port on the target side
+	 */
+	tgt->faddr = ini->eaddr;
+	tgt->fport = 0;
+
+	tgt->eport = ini->fport;
+	if (proto == IPPROTO_TCP)
+		tgt->eport += c->tcp.fwd_out.delta[tgt->eport];
+
+	/* Let the kernel pick a host side source port */
+	tgt->fport = 0;
+
+	return PIF_HOST;
+}
+
+/**
+ * fwd_nat_from_host() - Determine to forward a flow from the host interface
+ * @c:		Execution context
+ * @proto:	Protocol (IP L4 protocol number)
+ * @ini:	Flow address information of the initiating side
+ * @tgt:	Flow address information on the target side (updated)
+ *
+ * Return: pif of the target interface to forward the flow to, PIF_NONE if the
+ *         flow cannot or should not be forwarded at all.
+ */
+uint8_t fwd_nat_from_host(const struct ctx *c, uint8_t proto,
+			  const struct flowside *ini, struct flowside *tgt)
+{
+	/* Common for spliced and non-spliced cases */
+	tgt->eport = ini->fport;
+	if (proto == IPPROTO_TCP)
+		tgt->eport += c->tcp.fwd_in.delta[tgt->eport];
+
+	if (c->mode == MODE_PASTA && inany_is_loopback(&ini->eaddr) &&
+	    proto == IPPROTO_TCP) {
+		/* spliceable */
+
+		/* Preserve the specific loopback adddress used, but let the
+		 * kernel pick a source port on the target side
+		 */
+		tgt->faddr = ini->eaddr;
+		tgt->fport = 0;
+
+		if (inany_v4(&ini->eaddr))
+			tgt->eaddr = inany_loopback4;
+		else
+			tgt->eaddr = inany_loopback6;
+		return PIF_SPLICE;
+	}
+
+	tgt->faddr = ini->eaddr;
+	tgt->fport = ini->eport;
+
+	if (inany_is_loopback4(&tgt->faddr) ||
+	    inany_is_unspecified4(&tgt->faddr) ||
+	    inany_equals4(&tgt->faddr, &c->ip4.addr_seen)) {
+		tgt->faddr = inany_from_v4(c->ip4.gw);
+	} else if (inany_is_loopback6(&tgt->faddr) ||
+		   inany_equals6(&tgt->faddr, &c->ip6.addr_seen) ||
+		   inany_equals6(&tgt->faddr, &c->ip6.addr)) {
+		if (IN6_IS_ADDR_LINKLOCAL(&c->ip6.gw))
+			tgt->faddr.a6 = c->ip6.gw;
+		else
+			tgt->faddr.a6 = c->ip6.addr_ll;
+	}
+
+	if (inany_v4(&tgt->faddr)) {
+		tgt->eaddr = inany_from_v4(c->ip4.addr_seen);
+	} else {
+		if (inany_is_linklocal6(&tgt->faddr))
+			tgt->eaddr.a6 = c->ip6.addr_ll_seen;
+		else
+			tgt->eaddr.a6 = c->ip6.addr_seen;
+	}
+
+	return PIF_TAP;
+}