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Asked: 2026-05-28T06:29:32+00:00

If I print out the header field values of the IP header sent on

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If I print out the header field values of the IP header sent on a the RAW socket with the following parameters and default settings, I see that the protocol number is set to ‘2’ which belongs to that of IGMP. 

 sock = socket(PF_INET, SOCK_RAW, AF_INET);

However, if I access the packet using TCP header, I get good values for most of the fields, except that data offset gets a wrong value. Also, I am not able to read the header correctly using IGMP headers on the Linux platform (Ubuntu).

For reading out the header:

printf("\n");
printf("IP Header\n");
printf("   |-IP Version        : %d\n",(unsigned int)iph->version);
printf("   |-IP Header Length  : %d DWORDS or %d Bytes\n",(unsigned int)iph->ihl,  ((unsigned int)(iph->ihl))*4);
printf("   |-Type Of Service   : %d\n",(unsigned int)iph->tos);
printf("   |-IP Total Length   : %d  Bytes(Size of Packet)\n",ntohs(iph->tot_len));
printf("   |-Identification    : %d\n",ntohs(iph->id));
printf("   |-TTL      : %d\n",(unsigned int)iph->ttl);
printf("   |-Protocol : %d\n",(unsigned int)iph->protocol);
printf("   |-Checksum : %d\n",ntohs(iph->check));
printf("   |-Source IP        : %s\n",inet_ntoa(source.sin_addr));
printf("   |-Destination IP   : %s\n",inet_ntoa(dest.sin_addr));

// IGMP Print
printf("\n");
printf("IGMP Header\n");
printf("   |-IGMP Type        : %d\n",(unsigned int)head->igmp_type);
printf("   |-IGMP Code        : %d\n",(unsigned int)head->igmp_code);
printf("   |-IGMP Checksum    : %d\n",(unsigned int)head->igmp_cksum);
printf("   |-IGMP Address     : %s\n",inet_ntoa(head->igmp_group.sin_addr);

This gives me error for IGMP Packet:

error: dereferencing pointer to incomplete type

For all 4 fields.

If I treat it as TCP header, it works fine.

I am using RAW socket to communicate between a daemon and a CLI as its management agent.

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1 Answer

  1. Editorial Team

    I think you’ve seriously failed to understand what raw sockets are for. They’re a tool to let you manually assemble or inspect the contents of IP packets, including the higher-level headers inside the IP packet that tell whether it’s TCP, UDP, ICMP, IGMP, etc. They are not a means of communication to be used directly, but a means to hack around with existing higher-level means of communication at a lower level.

    For your application you almost certainly want TCP or UDP sockets.

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