[PATCH v4] udp: support traceroute - dev

3 Apr 2025

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  in this commit.

Signed-off-by: Jon Maloy &lt;jmaloy(a)redhat.com&gt;
---
v2: - Using ancillary data instead of setsockopt to transfer outgoing
      TTL.
    - Support IPv6
v3: - Storing ttl per packet instead of per flow. This may not be
      elegant, but much less intrusive than changing the flow
      criteria. This eliminates the need for the extra, flow-changing
      patch we introduced in v2.
v4: - Going back to something similar to the original solution, but
      storing current ttl in struct udp_flow, plus ensuring that all
      packets in a struct tap4_l4_t/tap6_l4_t instance, have the same
      ttl. After input from David Gibson.
---
 packet.h   |  2 ++
 tap.c      | 18 ++++++++++++++----
 udp.c      | 17 ++++++++++++++++-
 udp.h      |  3 ++-
 udp_flow.c |  1 +
 udp_flow.h |  1 +
 6 files changed, 36 insertions(+), 6 deletions(-)

diff --git a/packet.h b/packet.h
index c94780a..e84e123 100644
--- a/packet.h
+++ b/packet.h
@@ -11,6 +11,8 @@
 /* Maximum size of a single packet stored in pool, including headers */
 #define PACKET_MAX_LEN	((size_t)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 3a6fcbe..e65d592 100644
--- a/tap.c
+++ b/tap.c
@@ -563,6 +563,7 @@ 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 {
@@ -574,6 +575,8 @@ 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 */];
 
@@ -586,6 +589,7 @@ static struct tap4_l4_t {
  * @dest:	Destination port
  * @saddr:	Source address
  * @daddr:	Destination address
+ * @hop_limit:	Hop limit
  * @msg:	Array of messages that can be handled in a single call
  */
 static struct tap6_l4_t {
@@ -598,6 +602,8 @@ static struct tap6_l4_t {
 	struct in6_addr saddr;
 	struct in6_addr daddr;
 
+	uint8_t hop_limit;
+
 	struct pool_l4_t p;
 } tap6_l4[TAP_SEQS /* Arbitrary: TAP_MSGS in theory, so limit in users */];
 
@@ -786,7 +792,8 @@ resume:
 #define L4_MATCH(iph, uh, seq)							\
 	((seq)->protocol == (iph)->protocol &&					\
 	 (seq)->source   == (uh)->source    && (seq)->dest  == (uh)->dest
&&	\
-	 (seq)->saddr.s_addr == (iph)->saddr && (seq)->daddr.s_addr ==
(iph)->daddr)
+	 (seq)->saddr.s_addr == (iph)->saddr &&				\
+	 (seq)->daddr.s_addr == (iph)->daddr && (seq)->ttl == (iph)->ttl)
 
 #define L4_SET(iph, uh, seq)						\
 	do {								\
@@ -795,6 +802,7 @@ 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)
@@ -843,7 +851,7 @@ 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);
 		}
 	}
 
@@ -966,7 +974,8 @@ resume:
 		 (seq)->dest == (uh)->dest                 &&		\
 		 (seq)->flow_lbl == ip6_get_flow_lbl(ip6h) &&		\
 		 IN6_ARE_ADDR_EQUAL(&(seq)->saddr, saddr)  &&		\
-		 IN6_ARE_ADDR_EQUAL(&(seq)->daddr, daddr))
+		 IN6_ARE_ADDR_EQUAL(&(seq)->daddr, daddr)  &&		\
+		 (seq)->hop_limit == (ip6h)->hop_limit)
 
 #define L4_SET(ip6h, proto, uh, seq)					\
 	do {								\
@@ -976,6 +985,7 @@ resume:
 		(seq)->flow_lbl	= ip6_get_flow_lbl(ip6h);		\
 		(seq)->saddr	= *saddr;				\
 		(seq)->daddr	= *daddr;				\
+		(seq)->hop_limit = (ip6h)->hop_limit;			\
 	} while (0)
 
 		if (seq && L4_MATCH(ip6h, proto, uh, seq) &&
@@ -1026,7 +1036,7 @@ append:
 			for (k = 0; k < p->count; )
 				k += udp_tap_handler(c, PIF_TAP, AF_INET6,
 						     &seq->saddr, &seq->daddr,
-						     p, k, now);
+						     seq->hop_limit, p, k, now);
 		}
 	}
 
diff --git a/udp.c b/udp.c
index 39431d7..bc93292 100644
--- a/udp.c
+++ b/udp.c
@@ -849,6 +849,7 @@ fail:
  * @af:		Address family, AF_INET or AF_INET6
  * @saddr:	Source address
  * @daddr:	Destination address
+ * @ttl:	TTL or hop limit 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
@@ -859,7 +860,8 @@ fail:
  */
 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];
@@ -938,6 +940,19 @@ 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 != uflow->ttl[tosidx.sidei]) {
+			uflow->ttl[tosidx.sidei] = ttl;
+			if (af == AF_INET) {
+				if (setsockopt(s, IPPROTO_IP, IP_TTL,
+					       &ttl, sizeof(ttl)) < 0)
+					perror("setsockopt (IP_TTL)");
+			} else {
+				if (setsockopt(s, IPPROTO_IPV6, IPV6_HOPLIMIT,
+					       &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);
diff --git a/udp_flow.c b/udp_flow.c
index bf4b896..39372c2 100644
--- a/udp_flow.c
+++ b/udp_flow.c
@@ -137,6 +137,7 @@ static flow_sidx_t udp_flow_new(const struct ctx *c, union flow
*flow,
 	uflow = FLOW_SET_TYPE(flow, FLOW_UDP, udp);
 	uflow->ts = now->tv_sec;
 	uflow->s[INISIDE] = uflow->s[TGTSIDE] = -1;
+	uflow->ttl[INISIDE] = uflow->ttl[TGTSIDE] = DEFAULT_TTL;
 
 	if (s_ini >= 0) {
 		/* When using auto port-scanning the listening port could go
diff --git a/udp_flow.h b/udp_flow.h
index 9a1b059..606ac08 100644
--- a/udp_flow.h
+++ b/udp_flow.h
@@ -21,6 +21,7 @@ struct udp_flow {
 	bool closed :1;
 	time_t ts;
 	int s[SIDES];
+	uint8_t ttl[SIDES];
 };
 
 struct udp_flow *udp_at_sidx(flow_sidx_t sidx);
-- 
2.48.1


    