[PATCH 2/2] udp: support traceroute for IPv4

Now that ICMP pass-through from socket-to-tap is in place, it is
easy to support UDP based traceroute functionality in direction
tap-to-socket.

We fix that for IPv4 in this commit.

Signed-off-by: Jon Maloy <jmaloy(a)redhat.com>
---
 packet.h |  2 ++
 tap.c    |  8 ++++++--
 udp.c    | 10 +++++++++-
 udp.h    |  3 ++-
 4 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/packet.h b/packet.h
index d099f02..e406c6d 100644
--- a/packet.h
+++ b/packet.h
@@ -9,6 +9,8 @@
 /* Maximum size of a single packet stored in pool, including headers */
 #define PACKET_MAX_LEN	UINT16_MAX
 
+#define DEFAULT_TTL 64
+
 /**
  * struct pool - Generic pool of packets stored in a buffer
  * @buf:	Buffer storing packet descriptors,
diff --git a/tap.c b/tap.c
index 182a115..7d0970d 100644
--- a/tap.c
+++ b/tap.c
@@ -544,6 +544,7 @@ PACKET_POOL_DECL(pool_l4, UIO_MAXIOV, pkt_buf);
  * @dest:	Destination port
  * @saddr:	Source address
  * @daddr:	Destination address
+ * @ttl:	Time to live
  * @msg:	Array of messages that can be handled in a single call
  */
 static struct tap4_l4_t {
@@ -555,6 +556,8 @@ static struct tap4_l4_t {
 	struct in_addr saddr;
 	struct in_addr daddr;
 
+	uint8_t ttl;
+
 	struct pool_l4_t p;
 } tap4_l4[TAP_SEQS /* Arbitrary: TAP_MSGS in theory, so limit in users */];
 
@@ -776,6 +779,7 @@ resume:
 		(seq)->dest		= (uh)->dest;			\
 		(seq)->saddr.s_addr	= (iph)->saddr;			\
 		(seq)->daddr.s_addr	= (iph)->daddr;			\
+		(seq)->ttl		= (iph)->ttl;				\
 	} while (0)
 
 		if (seq && L4_MATCH(iph, uh, seq) && seq->p.count < UIO_MAXIOV)
@@ -824,7 +828,7 @@ append:
 			for (k = 0; k < p->count; )
 				k += udp_tap_handler(c, PIF_TAP, AF_INET,
 						     &seq->saddr, &seq->daddr,
-						     p, k, now);
+						     seq->ttl, p, k, now);
 		}
 	}
 
@@ -1007,7 +1011,7 @@ append:
 			for (k = 0; k < p->count; )
 				k += udp_tap_handler(c, PIF_TAP, AF_INET6,
 						     &seq->saddr, &seq->daddr,
-						     p, k, now);
+						     DEFAULT_TTL, p, k, now);
 		}
 	}
 
diff --git a/udp.c b/udp.c
index 271e570..1ee289c 100644
--- a/udp.c
+++ b/udp.c
@@ -844,6 +844,7 @@ void udp_reply_sock_handler(const struct ctx *c, union epoll_ref ref,
  * @af:		Address family, AF_INET or AF_INET6
  * @saddr:	Source address
  * @daddr:	Destination address
+ * @ttl:	TTL for packets to be sent in this call
  * @p:		Pool of UDP packets, with UDP headers
  * @idx:	Index of first packet to process
  * @now:	Current timestamp
@@ -854,7 +855,8 @@ void udp_reply_sock_handler(const struct ctx *c, union epoll_ref ref,
  */
 int udp_tap_handler(const struct ctx *c, uint8_t pif,
 		    sa_family_t af, const void *saddr, const void *daddr,
-		    const struct pool *p, int idx, const struct timespec *now)
+		    uint8_t ttl, const struct pool *p, int idx,
+		    const struct timespec *now)
 {
 	const struct flowside *toside;
 	struct mmsghdr mm[UIO_MAXIOV];
@@ -933,6 +935,12 @@ int udp_tap_handler(const struct ctx *c, uint8_t pif,
 		mm[i].msg_hdr.msg_controllen = 0;
 		mm[i].msg_hdr.msg_flags = 0;
 
+		if (ttl <= 30) {
+			if (setsockopt(s, IPPROTO_IP, IP_TTL,
+				       &ttl, sizeof(ttl)) < 0)
+				perror("setsockopt (IP_TTL)");
+		}
+
 		count++;
 	}
 
diff --git a/udp.h b/udp.h
index de2df6d..041fad4 100644
--- a/udp.h
+++ b/udp.h
@@ -15,7 +15,8 @@ void udp_reply_sock_handler(const struct ctx *c, union epoll_ref ref,
 			    uint32_t events, const struct timespec *now);
 int udp_tap_handler(const struct ctx *c, uint8_t pif,
 		    sa_family_t af, const void *saddr, const void *daddr,
-		    const struct pool *p, int idx, const struct timespec *now);
+		    uint8_t  ttl, const struct pool *p, int idx,
+		    const struct timespec *now);
 int udp_sock_init(const struct ctx *c, int ns, const union inany_addr *addr,
 		  const char *ifname, in_port_t port);
 int udp_init(struct ctx *c);
-- 
2.48.1