在使用libnet发送TCP SYN数据时，一般默认不设置options信息，即没有mss和时间戳。
在某些IDC机房，这些数据会被拦截掉。出现的现象是使用telnet访问没有问题，但是libnet发送SYN数据包没有回应。需要在发包的时候设置options信息。libnet提供了libnet_builf_tcp_options来进行设置。
uint8_t payload[20] = {0x00};
static int set_options_payload(uint8_t *payload,uint32_t timeval,uint16_t mss)
{
int len = 0;
//set mss value
payload[len++] = 0x02;
payload[len++] = 0x04;
payload[len++] = (uint8_t)((mss >> 8) & 0xFF);
payload[len++] = (uint8_t)(mss & 0xFF);
//set SACK
payload[len++] = 0x04;
payload[len++] = 0x02;
//set timeval
payload[len++] = 0x08;
payload[len++] = 0x0a;
payload[len++] = (uint8_t)((timeval >> 24) & 0xFF);
payload[len++] = (uint8_t)((timeval >> 16) & 0xFF);
payload[len++] = (uint8_t)((timeval >> 8) & 0xFF);
payload[len++] = (uint8_t)(timeval & 0xFF);
len+=4;
//set NOP
payload[len++] = 0x01;
//set Window scale
payload[len++] = 0x03;
payload[len++] = 0x03;
payload[len] = 0x07;
return 0;
}
r = libnet_build_tcp_options(
payload,
20,
l,
0);使用libnet_build_tcp_options设置options数据
原文：http://zhuminghui.blog.51cto.com/3838075/1795265