Calculate the ip udp tcp validation and

The following describes the IPv4 ip udp tcp verification and calculation.
Ip verification and calculation are relatively simple. You only need to calculate the ip protocol header;
The IP header is 20 bytes long;
The udp header is 8 bytes long;
Tcp Header Length: 20-20 ~ 60 bytes;
Udp and tcp checksum not only need to load the entire IP protocol (including
Udp/tcp header and udp/tcp protocol load,
A pseudo-protocol header must be calculated first.
Before calculation, the protocol header verification and field must be cleared.

The structure of the pseudo-protocol header is as follows:
0 7 8 15 16 23 24 31
+ -------- +
| Source address |
+ -------- +
| Destination address |
+ -------- +
| Zero | protocol | udp/tcp length |
+ -------- +

Fields in the pseudo-protocol header are also in network byte order;

The following code snippet shows the specific computing methods in GNU/Linux:

// ================================================ ====================
# Include <stdint. h>

# Include <unistd. h>
# Include <sys/socket. h>
# Include <arpa/inet. h>
# Include <netdb. h>
# Include <netinet/in. h>
# Include <netinet/ip. h>
# Include <netinet/ether. h>
# Include <netinet/udp. h>
# Include <netinet/tcp. h>

Struct pseudo _ head {
Uint32_t saddr;
Uint32_t daddr;
Char zero;
Char proto;
Unsigned short len;
};

Int cal_cksum (char * p)
{
Struct iphdr * ip = (struct iphdr *)
(P + sizeof (struct ether_header ));

Cal_ip_cksum (ip );

Struct tcphdr * tcp;
Struct udphdr * udp;
Char * f = (char *) ip + 20;

Switch (ip-> protocol ){
Case IPPROTO_TCP:
Tcp = (struct tcphdr *) f;
Tcp-> check = 0;
Tcp-> check = cal_udptcp_cksum (
(Unsigned short *) f, ip );
Break;
Case IPPROTO_UDP:
Udp = (struct udphdr *) f;
Udp-> check = 0;
Udp-> check = cal_udptcp_cksum (
(Unsigned short *) f, ip );
Break;
Case IPPROTO_IP:
Break;
Default:
Return 1;
}
Return 0;
}

Inline static void cal_ip_cksum (struct iphdr * ip)
{
Ip-> check = 0;
Ip-> check = in_cksum (unsigned short *) ip,
20, 0 );
}

Inline static unsigned short cal_udptcp_cksum (
Unsigned short * p, struct iphdr * ip)
{
Int len = ntohs (ip-> tot_len)-20;
Struct pseudo-head ph;

Ph. len = htons (len );
Ph. saddr = ip-> saddr;
Ph. daddr = ip-> daddr;
Ph. proto = ip-> protocol;
Ph. zero = 0;

Int sum = cal_sum (
(Unsigned short *) & ph, sizeof (ph ));

Return in_cksum (p, len, sum );
}

Unsigned short in_cksum (unsigned short * p,
Int len, int sum)
{
Int n = len;

While (n> 1 ){
Sum + = * p ++;
N-= 2;
}
If (n = 1)
Sum + = * (unsigned char *) p;

Sum = (sum> 16) + (sum & 0 xffff );
Sum + = (sum> 16 );

Unsigned short ret = ~ Sum;
Return ret;
}

Inline static int cal_sum (unsigned short * p, int len)
{
Int sum = 0;
While (len> 0 ){
Sum + = * p ++;
Len-= 2;
}
Return sum;
}

From the column leeshuheng