//Hook function for outgoing packets
static unsigned int hook_func_out(unsigned int hooknum, struct sk_buff *skb, const struct net_device *in, const struct net_device *out, int (*okfn)(struct sk_buff *))
{
struct iphdr *ip_header=NULL; //IP header structure
struct tcphdr *tcp_header=NULL; //TCP header structure
unsigned short int src_port; //TCP source port
ip_header=(struct iphdr *)skb_network_header(skb);
//The packet is not ip packet (e.g. ARP or others)
if (!ip_header)
{
return NF_ACCEPT;
}
if(ip_header->protocol==IPPROTO_TCP) //TCP
{
tcp_header = (struct tcphdr *)((__u32 *)ip_header+ ip_header->ihl);
src_port=htons((unsigned short int) tcp_header->source);
if(src_port==5001)
{
tcp_modify_timestamp(skb,get_tsval());
}
}
return NF_ACCEPT;
}
//Modify incoming TCP packets and return RTT sample value
u32 tcp_modify_incoming(struct sk_buff *skb, u32 time)
{
struct iphdr *ip_header=NULL; //IP header structure
struct tcphdr *tcp_header=NULL; //TCP header structure
unsigned short tcp_header_len=0; //TCP header length
u8 *tcp_opt=NULL; //TCP option pointer
u32 *tsecr=NULL; //TCP Timestamp echo reply pointer
unsigned short tcplen=0; //TCP packet length
u8 tcp_opt_value=0; //TCP option pointer value
u32 rtt=0; //Sample RTT
//If we can not modify this packet, return 0
if(skb_linearize(skb)!=0)
{
return 0;
}
//Get IP header
ip_header=(struct iphdr *)skb_network_header(skb);
//Get TCP header on the base of IP header
tcp_header=(struct tcphdr *)((__u32 *)ip_header+ ip_header->ihl);
//Get TCP header length
tcp_header_len=(unsigned short)(tcp_header->doff*4);
//Minimum TCP header length=20(Raw TCP header)+10(TCP Timestamp option)
if(tcp_header_len<30)
{
return 0;
}
//TCP option offset=IP header pointer+IP header length+TCP header length
tcp_opt=(u8*)ip_header+ip_header->ihl*4+20;
//printk(KERN_INFO "TCP header length is %hu\n",tcp_header_len);
while(1)
{
//If pointer has moved out off the range of TCP option, stop current loop
if(tcp_opt-(u8*)tcp_header>=tcp_header_len)
{
break;
}
//Get value of current byte
tcp_opt_value=*tcp_opt;
if(tcp_opt_value==0x01)//No-Operation (NOP)
{
//Move to next byte
tcp_opt++;
}
else if(tcp_opt_value==0x08) //TCP option kind: Timestamp (8)
{
//Get pointer to Timestamp echo reply (TSecr)
tsecr=(u32*)(tcp_opt+6);
//Get one RTT sample
rtt=get_tsval()-ntohl(*tsecr);
//printk(KERN_INFO "Echo back value: %u\n",ntohl(*tsecr));
//Modify TCP TSecr back to jiffies
//Don't disturb TCP. Wrong TCP timestamp echo reply may reset TCP connections
*tsecr=htonl(time);
break;
}
else //Other TCP options (e.g. MSS(2))
{
//Move to next byte to get length of this TCP option
tcp_opt++;
//Get length of this TCP option
tcp_opt_value=*tcp_opt;
//Move to next TCP option
tcp_opt=tcp_opt+1+((unsigned int)tcp_opt_value-2);
}
}
//TCP length=Total length - IP header length
//tcplen=(ip_header->tot_len)-(ip_header->ihl<<2);
tcplen=skb->len-(ip_header->ihl<<2);
tcp_header->check=0;
tcp_header->check = csum_tcpudp_magic(ip_header->saddr, ip_header->daddr,tcplen, ip_header->protocol,csum_partial((char *)tcp_header, tcplen, 0));
skb->ip_summed = CHECKSUM_UNNECESSARY;
return rtt;
}
