[GPRS] NS: Respond to GRE keepalive messages

GRE has the strange notion of keepalive messages being encapsulated IPv4
packets adressed back to the sender.  Since we actually really only care
about frame relay, this is a bit strange.  However, we'll do some sanity
checks and send it back through our GRE socket...

1 files changed, 67 insertions, 1 deletions

diff --git a/openbsc/src/gprs/gprs_ns_frgre.c b/openbsc/src/gprs/gprs_ns_frgre.c
index 94f937455..e22b0c724 100644
--- a/openbsc/src/gprs/gprs_ns_frgre.c
+++ b/openbsc/src/gprs/gprs_ns_frgre.c
@@ -41,12 +41,67 @@
 #include <openbsc/gprs_ns.h>
 
 #define GRE_PTYPE_FR	0x6559
+#define GRE_PTYPE_IPv4	0x0800
+#define GRE_PTYPE_KAR	0x0000	/* keepalive response */
 
 struct gre_hdr {
 	uint16_t flags;
 	uint16_t ptype;
 } __attribute__ ((packed));
 
+/* IPv4 messages inside the GRE tunnel might be GRE keepalives */
+static int handle_rx_gre_ipv4(struct bsc_fd *bfd, struct msgb *msg,
+				struct iphdr *iph, struct gre_hdr *greh)
+{
+	struct gprs_ns_inst *nsi = bfd->data;
+	int gre_payload_len;
+	struct iphdr *inner_iph;
+	struct gre_hdr *inner_greh;
+	struct sockaddr_in daddr;
+	struct in_addr ia;
+
+	gre_payload_len = msg->len - (iph->ihl*4 + sizeof(*greh));
+
+	inner_iph = (struct iphdr *) (uint8_t *)greh + sizeof(*greh);
+
+	if (gre_payload_len < inner_iph->ihl*4 + sizeof(*inner_greh)) {
+		LOGP(DNS, LOGL_ERROR, "GRE keepalive too short\n");
+		return -EIO;
+	}
+
+	if (inner_iph->saddr != iph->daddr ||
+	    inner_iph->daddr != iph->saddr) {
+		LOGP(DNS, LOGL_ERROR,
+			"GRE keepalive with wrong tunnel addresses\n");
+		return -EIO;
+	}
+
+	if (inner_iph->protocol != IPPROTO_GRE) {
+		LOGP(DNS, LOGL_ERROR, "GRE keepalive with wrong protocol\n");
+		return -EIO;
+	}
+
+	inner_greh = (struct gre_hdr *) ((uint8_t *)iph + iph->ihl*4);
+	if (inner_greh->ptype != htons(GRE_PTYPE_KAR)) {
+		LOGP(DNS, LOGL_ERROR, "GRE keepalive inner GRE type != 0\n");
+		return -EIO;
+	}
+
+	/* Actually send the response back */
+
+	daddr.sin_family = AF_INET;
+	daddr.sin_addr.s_addr = inner_iph->daddr;
+	daddr.sin_port = IPPROTO_GRE;
+
+	ia.s_addr = iph->saddr;
+	LOGP(DNS, LOGL_DEBUG, "GRE keepalive from %s, responding\n",
+		inet_ntoa(ia));
+
+	return sendto(nsi->frgre.fd.fd, inner_greh,
+		      gre_payload_len - inner_iph->ihl*4, 0,
+		      (struct sockaddr *)&daddr, sizeof(daddr));
+}
+
 static struct msgb *read_nsfrgre_msg(struct bsc_fd *bfd, int *error,
 					struct sockaddr_in *saddr)
 {
@@ -95,11 +150,22 @@ static struct msgb *read_nsfrgre_msg(struct bsc_fd *bfd, int *error,
 		LOGP(DNS, LOGL_NOTICE, "Unknown GRE flags 0x%04x\n",
 			ntohs(greh->flags));
 	}
-	if (greh->ptype != htons(GRE_PTYPE_FR)) {
+
+	switch (ntohs(greh->ptype)) {
+	case GRE_PTYPE_IPv4:
+		/* IPv4 messages might be GRE keepalives */
+		*error = handle_rx_gre_ipv4(bfd, msg, iph, greh);
+		goto out_err;
+		break;
+	case GRE_PTYPE_FR:
+		/* continue as usual */
+		break;
+	default:
 		LOGP(DNS, LOGL_NOTICE, "Unknown GRE protocol 0x%04x != FR\n",
 			ntohs(greh->ptype));
 		*error = -EIO;
 		goto out_err;
+		break;
 	}
 
 	if (msg->len < sizeof(*greh) + 2) {