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POHow can I set the TCP options for sending packets?
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2012-09-06T13:37:20.587
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2013-04-03T17:01:33.413
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I'm programming software which can send tcp packets to an host. I'm able to create a packet with IP header, TCP Headers and data but I can't manage how to add TCP options like MSS, NOP, STACK, Window scaling or Timestamp. I mean i'm not able to add options to the TCP header, calculate the correct checksum to send a good TCP packet to the host. I can just send correct TCP packets without TCP options. Do you think I'm on the correct patch? Could somebody please help me? <pre><code>/* TCP Header structure */ struct tcphdr { u_int16_t th_sport; /* source port */ u_int16_t th_dport; /* destination port */ u_int32_t th_seq; /* sequence number */ u_int32_t th_ack; /* acknowledgement number */ #if __BYTE_ORDER == __LITTLE_ENDIAN u_int8_t th_x2:4; /* (unused) */ u_int8_t th_off:4; /* data offset */ #endif #if __BYTE_ORDER == __BIG_ENDIAN u_int8_t th_off:4; /* data offset */ u_int8_t th_x2:4; /* (unused) */ #endif u_int8_t th_flags; # define TH_FIN 0x01 # define TH_SYN 0x02 # define TH_RST 0x04 # define TH_PUSH 0x08 # define TH_ACK 0x10 # define TH_URG 0x20 # define TH_ECE 0x40 # define TH_CWR 0x80 u_int16_t th_win; /* window */ u_int16_t th_sum; /* checksum */ u_int16_t th_urp; /* urgent pointer */ }; struct tcp_option_mss { uint8_t kind; /* 2 */ uint8_t len; /* 4 */ uint16_t mss; } __attribute__((packed)); struct tcphdr_mss { struct tcphdr tcphdr; struct tcp_option_mss mss; }; /* IP Header structure */ struct ip { #if __BYTE_ORDER == __LITTLE_ENDIAN unsigned int ip_hl:4; /* header length */ unsigned int ip_v:4; /* version */ #endif #if __BYTE_ORDER == __BIG_ENDIAN unsigned int ip_v:4; /* version */ unsigned int ip_hl:4; /* header length */ #endif u_int8_t ip_tos; /* type of service */ u_short ip_len; /* total length */ u_short ip_id; /* identification */ u_short ip_off; /* fragment offset field */ # define IP_RF 0x8000 /* reserved fragment flag */ # define IP_DF 0x4000 /* dont fragment flag */ # define IP_MF 0x2000 /* more fragments flag */ # define IP_OFFMASK 0x1fff /* mask for fragmenting bits */ u_int8_t ip_ttl; /* time to live */ u_int8_t ip_p; /* protocol */ u_short ip_sum; /* checksum */ struct in_addr ip_src, ip_dst; /* source and dest address */ }; int send_packet(int sock, long dest_ip , long source_ip, long port, u_int8_t th_flags, unsigned long seq, unsigned long ack, unsigned long port1, unsigned char * data, unsigned long data_i) { char * packet; struct ip * pkt_ip; struct tcphdr * pkt_tcp; struct tcphdr_mss * tcp_header; struct sockaddr_in sin; packet = malloc(sizeof(struct ip) + sizeof(struct tcphdr_mss) + data_i); if (packet == NULL) { if (ECHO) fprintf(stderr, "Error in allocating memory\n"); exit(EXIT_FAILURE); } memset(packet, 0, sizeof(struct ip) + sizeof(struct tcphdr_mss)); pkt_ip = (struct ip *) packet; pkt_tcp = (struct tcphdr *) (packet + sizeof(struct ip)); pkt_tcp->th_sport = htons(port1); pkt_tcp->th_dport = htons(port); pkt_tcp->th_seq = htonl(seq); pkt_tcp->th_ack = htonl(ack); pkt_tcp->th_off = sizeof(struct tcphdr) / 4 + 1; pkt_tcp->th_flags = th_flags; pkt_tcp->th_win = htons(32768); pkt_tcp->th_sum = 0; tcp_header = malloc(sizeof(struct tcphdr)); tcp_header->tcphdr = *pkt_tcp; tcp_header->mss.kind = 2; tcp_header->mss.len = 4; tcp_header->mss.mss = htons(32000); pkt_ip->ip_v = 4; pkt_ip->ip_hl = sizeof(struct ip) >> 2; pkt_ip->ip_tos = 0; pkt_ip->ip_len = htons(sizeof(struct ip) + sizeof(struct tcphdr) + data_i); if (ipid > 65000) ipid = 0; ipid++; pkt_ip->ip_id = ipid; pkt_ip->ip_off = 0; pkt_ip->ip_ttl = 64; pkt_ip->ip_p = IPPROTO_TCP ; pkt_ip->ip_sum = 0; pkt_ip->ip_src.s_addr = source_ip; pkt_ip->ip_dst.s_addr = dest_ip; pkt_ip->ip_sum = checksum((unsigned short*)pkt_ip, sizeof( struct ip) ); pkt_tcp->th_sum = in_cksum_tcp(pkt_ip->ip_src.s_addr, pkt_ip->ip_dst.s_addr, (unsigned short *) pkt_tcp, sizeof(struct tcphdr_mss), data, data_i); memcpy(((char *)pkt_tcp + sizeof(struct tcphdr_mss)), data, data_i); memset(&sin, 0, sizeof(sin)); sin.sin_family = AF_INET; sin.sin_addr.s_addr = pkt_ip->ip_dst.s_addr; if (sendto(sock, packet, sizeof(struct ip) + sizeof(struct tcphdr_mss) + data_i, 0, (struct sockaddr *) &sin, sizeof(sin)) < 0) { perror("sendto"); free(packet); return -1; } free(packet); return 0; } </code></pre>
Tags
<c><networking><tcp>
Title
How can I set the TCP options for sending packets?
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