dereferencing pointer to incomplete type ‘struct tcphdr’?

Question

I'm trying to compile "Example 11-9. Source code to the SYNplescan tool" from this NetworkSecurityTools book on Ubuntu 18.04: http://books.gigatux.nl/mirror/networksecuritytools/0596007949/networkst-CHP-11-SECT-4.html

But it says

error: dereferencing pointer to incomplete type ‘struct tcphdr’
     if (tcp->th_flags == 0x14)
            ^~

How do I fix this?

Answer 1

People change and includes come and go, after the following changes:

@@ -1,9 +1,12 @@
+#define _DEFAULT_SOURCE 1
 #define _BSD_SOURCE 1
 #include <stdio.h>
 #include <unistd.h>
 #include <time.h>
 #include <libnet.h>
 #include <pcap.h>
+#include <netinet/tcp.h>
+#include <netinet/ip.h>
 
 int answer = 0;            /* flag for scan timeout */
 
@@ -42,7 +45,7 @@
 int
 main (int argc, char *argv[])
 {
-  char *device = NULL;        /* device for sniffing/sending */
+  const char *device = NULL;        /* device for sniffing/sending */
   char o;            /* for option processing */
   in_addr_t ipaddr;        /* ip address to scan */
   u_int32_t myipaddr;        /* ip address of this host */

I was able to compile with:

gcc -Wall 1.c -lnet -lpcap

with no compiler messages. I guess that once netinet/tcp.h was included by libnet.h or maybe by pcap.h - seems not be the case anymore and you have to include netinet/tcp.h yourself for struct tcphdr.