setsockopt HDRINCL 'error code 22, Invalid arg' OpenBSD 3.3 using gcc 2.95.3

Hi all. Im having problems allowing the kernel to let me include my own ip header.

I am writing a network/firewall and protocol analysis tool for my work.

Running on x86/OpenBSD 3.3 platform using gcc 2.95.3 with no special opts apart from debugging.

The src compiles ok, but when run; 'perror' recieves error code '22' 'invalid arg' from setsockopt ( i presume ) and I cannot understand why???

I have tested the prog bypassing the setsockopt function and it works ok, alas with the kernel ip header included ;(

Please forgive me if its a glaringly obvious error on my part as I am returning to c & *nix after a few years in the wilderness.

N.B Apologies, but my code seems to have lost all its tabs when i posted it



Many thanks for any help offered

Chris

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#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <string.h>
#include <netinet/tcp.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <errno.h>
#include <unistd.h>
#include "decs.h"  

int main( void )
{
    unsigned int tport;    
    unsigned short uno, s;
    struct iphead *iphead;
    struct tcphead *tcphead;
    struct sockaddr_in dst;
    char dgram[BUFFER];
    tport = 1500;
    uno = 1;

    if( getuid() ) {
        printf("ALERT: You need to be r00t\n");
        exit( 0 );
    }

    if ( !( s = socket( AF_INET, SOCK_RAW, IPPROTO_TCP ) ) )
        perror("socket error");

    // cast dgram to struct and point *iphead at element
    iphead = ( struct iphead *) dgram;
    // as above but with size of ip_header offset
    tcphead = ( struct tcphead *) dgram + sizeof( struct iphead );

    dst.sin_family = AF_INET;
    dst.sin_port = htons( tport );
    dst.sin_addr.s_addr = inet_addr( "192.168.0.7" );
    memset( dgram, 0, BUFFER );

    // custom values
    iphead->ip_hl = 5;
    iphead->ip_v = 4;
    iphead->ip_tos = 0;
    iphead->ip_len = BUFFER;
    iphead->ip_id = htonl( 1234 );
    iphead->ip_off = 0;
    iphead->ip_ttl = 255;
    iphead->ip_p = 6; // tcp
    iphead->ip_sum = 0; // set to 0 by default
    iphead->ip_src.s_addr = inet_addr( "192.168.0.1" );
    iphead->ip_dst.s_addr = dst.sin_addr.s_addr;
    tcphead->th_sport = htons( 1234 );
    tcphead->th_dport = htons( tport );
    tcphead->th_seq = random();
    tcphead->th_ack = 0;
    tcphead->th_x2 = 0;
    tcphead->th_off = 0;
    tcphead->th_flags = TH_SYN; // init new connection
    tcphead->th_win = htonl( MAX_WINDOW );
    tcphead->th_sum = 0;
    tcphead->th_urp = 0; // no urgency flags

    // generate chksum
    iphead->ip_sum = chksum( ( unsigned short *)dgram, iphead->ip_len >> 1 );

    // header check
    if ( setsockopt( s,
    IPPROTO_IP,
    IP_HDRINCL,
    &uno,
    sizeof( uno ) ) < 0 ) {
    perror( "Alert: setsockopt cannot set HDRINCL" );
    exit( 1 );
    }

    // kill with control-c
    while( 1 ) {
        if( sendto( s,
        dgram,
        sizeof( dgram ),
        0,
        ( struct sockaddr * ) &dst,
        sizeof( dst ) ) < 0 ) {
            perror( "Alert: sendto failed" );
            exit( 1 );
        } else {
            printf( "." );
        }
    }
exit( 0 );
}


--------------------------------------------------

// Decs.h

#ifndef H_DECS
#define H_DECS
#define BUFFER sizeof( struct iphead ) + sizeof( struct tcphead )
#define MAX_WINDOW 65535

__BEGIN_DECLS
unsigned short chksum( unsigned short *, int );
void chkhdr( unsigned short );
__END_DECLS

/* initial command line opts */
typedef struct opts {
unsigned char *cfg; // absolute path
unsigned char *host; // dest host
unsigned short int port; // dest port
unsigned short int pkts; // number of packets to send
}cli_opts;

struct iphead {
unsigned char ip_hl, ip_v; // 4 bits respectively
unsigned char ip_tos; // limo or taxi???
unsigned short int ip_len; // total len of header and data
unsigned short int ip_id; // help assemble fragments
unsigned short ip_off; // 3bit control flags, 13bit offset. Do i frag?
unsigned char ip_ttl; // --ttl each hop. if 0 then destroy
unsigned char ip_p; // next level protocol
unsigned short int ip_sum; // header checksum
    struct in_addr ip_src, ip_dst; // source and dest
};

struct tcphead {
unsigned short int th_sport; // source port
unsigned short int th_dport; // destination port
unsigned int th_seq; // seq number of first data octet in segment
unsigned int th_ack; // next seq num expected to recieve
unsigned char th_x2, th_off; /* x2 reserved for future use ( must be 0 )
* data_off indicates where data begins */
unsigned char th_flags; /* TH_URG: urgent, TH_ACK: yup, TH_PSH, do not
                                    * buffer segment push straight thro stack
                                    * TH_RST: tell peer to terminate
                                    * TH_SYN: init new connect
                                    * TH_FIN: close connection
                                    * control bits */
unsigned short int th_win; /* amount of data octets to be accepted
                                        * including original seq_num octet */
unsigned short int th_sum; // initial value = 0
unsigned short int th_urp; // only used if TH_URG is set in tcp_flags
};


/* standard BSD checksum */
unsigned short chksum ( unsigned short *addr, int len ) {
    register int sum = 0;
    u_short answer = 0;
    register u_short *w = addr;
    register int nleft = len;

    /* 32 bit accumulator 'sum', add sequential 16 bit words to it '*w'.
    * At the end fold back all the carry bits from top 16 bits into
    * lower 16 bits */
    while ( nleft > 1 ) {
    sum += *w++;
    nleft -= 2;
    }


    // mop up odd byte, if necessary
    if ( nleft == 1 ) {
    *( u_char * ) ( &answer ) = *( u_char * )w ;
     sum += answer;
    }

    // add back carry outs from top 16 bits to lower 16 bits
    sum = ( sum >> 16 ) + ( sum + 0xffff ); // add hi 16 to low 16
    sum += ( sum >> 16 ); // add carry
    answer = ~sum; // truncate to 16bits
    return ( answer );
    }

    // check kernel hasnt inserted header
    void chkhdr( unsigned short sockd ) {

    }

#endif
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