Initial version

Working ping back and forth with specified data, and data
arrives at the other party.

Currently, the server uses nflog to get the pings, which:
1. requires iptables settings to work;
2. buffers and collects messages for a second.
Both are suboptimal, and I believe raw sockets can improve
on this.

1 files changed, 99 insertions, 0 deletions

diff --git a/server.c b/server.c
new file mode 100644
index 0000000..544c59b
--- /dev/null
+++ b/server.c
@@ -0,0 +1,99 @@
+#include <stdio.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <stdint.h>
+#include <arpa/inet.h>
+#include <netinet/ip.h>
+
+typedef uint8_t u_int8_t;
+typedef uint16_t u_int16_t;
+typedef uint32_t u_int32_t;
+#include <libnetfilter_log/libnetfilter_log.h>
+
+#include "icmp.h"
+#include "util.h"
+
+
+struct state {
+	int socket;
+};
+
+static int icmp_callback(struct nflog_g_handle *gh, struct nfgenmsg *nfmsg, struct nflog_data *ldata, void *state_) {
+	(void)gh; (void)nfmsg;
+
+	struct state *state = (struct state*)state_;
+
+	uint8_t *ip_start;  // received packet, starting at the IP buffer
+	int ip_len = nflog_get_payload(ldata, (char**)&ip_start);
+
+	struct iphdr *hdr = (struct iphdr*)ip_start;
+	int hdr_len = hdr->ihl * 4;
+	uint32_t saddr = hdr->saddr;
+
+	struct icmp_echo *msg = (struct icmp_echo*)(ip_start + hdr_len);
+	int msg_len = ip_len - hdr_len;
+	printf("Received: type %u   code %u   id %hu   seqnum %hu   payload:\n",
+			(unsigned)msg->type, (unsigned)msg->code, msg->id, msg->seqnum);
+	xxd(msg->payload, msg_len - offsetof(struct icmp_echo, payload));
+
+	uint8_t *saddr_bytes = (uint8_t*)&saddr;
+	char ip_address[16];
+	sprintf(ip_address, "%u.%u.%u.%u", saddr_bytes[0], saddr_bytes[1], saddr_bytes[2], saddr_bytes[3]);
+	if (icmp_send_echo_reply(ip_address, msg->id, msg->seqnum, "dank je wel", 11) < 0) {
+		perror("icmp_send_echo_reply");
+	}
+
+	return 0;
+}
+
+
+int main(void) {
+	struct nflog_handle *h = nflog_open();
+	if (!h) {
+		perror("nflog_open");
+		return 1;
+	}
+	if (nflog_unbind_pf(h, AF_INET) < 0) {
+		perror("nflog_unbind_pf");
+		return 1;
+	}
+	if (nflog_bind_pf(h, AF_INET) < 0) {
+		perror("nflog_bind_pf");
+		return 1;
+	}
+
+	struct nflog_g_handle *qh = nflog_bind_group(h, 0);
+	if (!qh) {
+		fprintf(stderr, "nflog_bind_group: no handle for group 0\n");
+		return 1;
+	}
+
+	if (nflog_set_mode(qh, NFULNL_COPY_PACKET, 0xffff) < 0) {
+		fprintf(stderr, "nflog_set_mode: can't set packet copy mode\n");
+		return 1;
+	}
+
+	struct state state;
+	// state.socket = icmp_open_socket();
+	// if (state.socket < 0) {
+	//     perror("icmp_open_socket");
+	//     return 1;
+	// }
+
+	nflog_callback_register(qh, &icmp_callback, &state);
+
+	int fd = nflog_fd(h);
+
+	char buf[4096];
+	while (true) {
+		ssize_t nr = recv(fd, buf, sizeof buf, 0);
+		if (nr < 0) break;
+
+		nflog_handle_packet(h, buf, nr);
+	}
+
+	nflog_unbind_group(qh);
+	nflog_close(h);
+}