feat: add icmp_set_dont_fragment and next_mtu extraction

- Add icmp_set_dont_fragment() using IP_MTU_DISCOVER/IP_PMTUDISC_DO
- Add next_mtu field to icmp_offending_packet_t, populated from the
  outer ICMP header on type=3/code=4 (frag needed) replies
- Fix extract_offending: read MTU from outer ICMP, not embedded packet
- Update test to match corrected extraction semantics
- Clean up checksum odd-byte handling with a static zero sentinel

includes/icmp.h

@@ -30,6 +30,7 @@ typedef struct icmp_offending_packet {
 	uint8_t  protocol;
 	uint8_t  icmp_type;
 	uint8_t  icmp_code;
+	uint16_t next_mtu;	/* MTU from outer ICMP (type=3, code=4) */
 	union {
 		struct {
 			uint16_t id;
@@ -74,6 +75,9 @@ int icmp_reply_id_seq(const icmp_reply_t *reply, uint16_t *id,
 int icmp_error_extract_offending(const icmp_reply_t *reply,
 								icmp_offending_packet_t *offending);
 
+/* Socket options */
+int icmp_set_dont_fragment(icmp_handle_t *h);
+
 /* Error handling */
 const char *icmp_strerror(const icmp_handle_t *h);
 int icmp_should_retry(const icmp_handle_t *h);

sources.mk

@@ -8,6 +8,7 @@ SRCS += $(SRC_DIR)/error/strerror.c
 SRCS += $(SRC_DIR)/error/should_retry.c
 SRCS += $(SRC_DIR)/socket/create.c
 SRCS += $(SRC_DIR)/socket/configure.c
+SRCS += $(SRC_DIR)/socket/set_dont_fragment.c
 SRCS += $(SRC_DIR)/handle/create.c
 SRCS += $(SRC_DIR)/handle/destroy.c
 SRCS += $(SRC_DIR)/handle/get_fd.c

src/recv/api/extract_offending.c

@@ -37,6 +37,12 @@ icmp_error_extract_offending(const icmp_reply_t *reply,
 
 	hdr = get_embedded_icmp(reply, ip_hdr_len);
 	extract_icmp_fields(hdr, offending);
+	offending->next_mtu = 0;
+	if (is_frag_needed(reply->type, reply->code) && NULL != reply->ip_payload) {
+		const struct icmp_header *outer;
+		outer = (const struct icmp_header *)reply->ip_payload;
+		offending->next_mtu = ntohs(outer->un.frag.mtu);
+	}
 	return 0;
 }
 
@@ -92,8 +98,4 @@ extract_icmp_fields(const struct icmp_header *hdr, icmp_offending_packet_t *offe
 		offending->rest.echo.id = ntohs(hdr->un.echo.id);
 		offending->rest.echo.seq = ntohs(hdr->un.echo.seq);
 	}
-	else if (is_frag_needed(hdr->type, hdr->code))
-	{
-		offending->rest.frag.mtu = ntohs(hdr->un.frag.mtu);
-	}
 }

src/socket/set_dont_fragment.c

@@ -0,0 +1,26 @@
+#include <sys/socket.h>
+#include <netinet/in.h>
+#include <errno.h>
+#include <string.h>
+
+#include "internal/icmp_internal.h"
+
+int
+icmp_set_dont_fragment(struct icmp_handle *h)
+{
+	int			val;
+	int			saved_errno;
+
+	if (NULL == h)
+		return -1;
+	val = IP_PMTUDISC_DO;
+	if (0 > setsockopt(h->fd, IPPROTO_IP, 
+				IP_MTU_DISCOVER, &val, sizeof(val)))
+	{
+		saved_errno = errno;
+		icmp_set_error_fmt(h, ICMP_ERR_SOCKET,
+				"Failed to set DF bit: %s", strerror(saved_errno));
+		return -1;
+	}
+	return 0;
+}

src/utils/checksum.c

@@ -41,8 +41,10 @@ sum_words(const uint8_t *data, size_t len)
 		}
 	}
 
-	/* Handle odd byte if present */
+	/* Handle odd byte if present: point to last byte or a zero sentinel */
-	sum += ((uint32_t)(*ptr << 8)) & (uint32_t)(-(len & 1));
+	static const uint8_t	zero = 0;
+	ptr = (len & 1) ? ptr : &zero;
+	sum += (uint32_t)(*ptr << 8);
 
 	return sum;
 }

tests/recv/test_extract_offending.c

@@ -213,27 +213,37 @@ Test(extract_offending, ip_header_with_options)
 	cr_assert_eq(offending.rest.echo.seq, 0xAAAA);
 }
 
-Test(extract_offending, embedded_frag_needed_mtu)
+Test(extract_offending, frag_needed_mtu_from_outer_icmp)
 {
-	uint8_t buffer[28];
+	/* outer ICMP header (8 bytes): type=3, code=4, unused=0, mtu=1400 */
-	icmp_reply_t reply = make_error_reply(ICMP_TYPE_TIME_EXCEEDED, buffer, sizeof(buffer));
+	uint8_t outer[8];
+	/* embedded IP (20 bytes) + embedded ICMP echo request (8 bytes) */
+	uint8_t embedded[28];
+	icmp_reply_t reply;
 
-	fill_ip_header(buffer, "10.0.0.1", "10.0.0.2", 1);
+	memset(outer, 0, sizeof(outer));
-	/* Embedded packet is itself a Dest Unreachable / Frag Needed */
+	outer[0] = ICMP_TYPE_DEST_UNREACHABLE;
-	buffer[20] = ICMP_TYPE_DEST_UNREACHABLE;
+	outer[1] = ICMP_CODE_FRAG_NEEDED;
-	buffer[21] = ICMP_CODE_FRAG_NEEDED;
+	*(uint16_t *)(outer + 6) = htons(1400); /* next-hop MTU */
-	buffer[22] = 0;
+
-	buffer[23] = 0;
+	fill_ip_header(embedded, "10.0.0.1", "10.0.0.2", 1);
-	*(uint16_t *)(buffer + 24) = 0; /* unused */
+	fill_icmp_header(embedded, ICMP_TYPE_ECHO_REQUEST, 0, 0x1234, 0x0001);
-	*(uint16_t *)(buffer + 26) = htons(1400); /* mtu in network order */
+
+	memset(&reply, 0, sizeof(reply));
+	reply.type       = ICMP_TYPE_DEST_UNREACHABLE;
+	reply.code       = ICMP_CODE_FRAG_NEEDED;
+	reply.payload    = embedded;
+	reply.payload_len = sizeof(embedded);
+	reply.ip_payload = outer;
 
 	icmp_offending_packet_t offending;
 	int ret = icmp_error_extract_offending(&reply, &offending);
 
 	cr_assert_eq(ret, 0);
-	cr_assert_eq(offending.icmp_type, ICMP_TYPE_DEST_UNREACHABLE);
+	cr_assert_eq(offending.icmp_type, ICMP_TYPE_ECHO_REQUEST);
-	cr_assert_eq(offending.icmp_code, ICMP_CODE_FRAG_NEEDED);
+	cr_assert_eq(offending.rest.echo.id, 0x1234);
-	cr_assert_eq(offending.rest.frag.mtu, 1400, "MTU should be in host byte order");
+	cr_assert_eq(offending.rest.echo.seq, 0x0001);
+	cr_assert_eq(offending.next_mtu, 1400, "MTU should be in host byte order");
 }
 
 Test(extract_offending, redirect_with_tcp_packet)