1. Overview of tftp protocol
Simple File Transfer Protocol, a set of standard protocols for file transfer on the Internet, using UDP for transmission.
Features:
① is the application layer protocol
② Implementation based on UDP protocol
③ Data transmission mode
- octet: binary mode (commonly used)
- mail: no longer supported
2. tftp download model
3. tftp protocol analysis
Error code:
0 undefined, error message
1 File not found.
2 Access violation.
3 Disk full or allocation exceeded.
4 illegal TFTP operation.
5 Unknown transfer ID.
6 File already exists.
7 No such user.
8 Unsupported option(s) requested.
4. Summary of TFTP communication process
- The server waits for the client’s request on port 69
- If the server approves the request, it will use the ephemeral port to communicate with the client.
- The number of each packet changes (starts from 1)
- Each data packet must be confirmed by ACK. If a timeout occurs, the last data packet or ACK packet needs to be resent
- The data length is transmitted in 512Byte, and the data less than 512Byte means that the data transmission is over.
5. Client download file (code)
include <stdio.h>
include <head.h>
include <sys/types.h>
include <sys/socket.h>
include <arpa/inet.h>
define PORT 69 //The port number bound by the server
define IP "192.168.1.109" //"192.168.122.120" //IP address of the server
define FILENAME "5.png"
int main(int argc, const char *argv[])
//Create a message socket
int cfd = socket(AF_INET, SOCK_DGRAM, 0);
if(cfd < 0)
{
ERR_MSG("socket");
return -1;
}
printf("cfd = %d\\
",cfd);
// Bind the address information structure of the client to the socket ---> binding is not required
//If not bound, the operating system will bind the client to the IP of the running host and a random port number
//Fill the address information structure of the server to be connected, the real address information structure is formulated according to the address family
//Whoever you want to send to, fill in the address information of the person you want to send it to.
//AF_INET : man 7 ip
struct sockaddr_in sin;
socklen_t addrlen=sizeof(sin);
sin.sin_family = AF_INET; //must fill in AF_INET
sin.sin_port = htons(PORT); //port number: the port number bound by the server
sin.sin_addr.s_addr = inet_addr(IP);//Server bound IP
//Edit read and write requests
char buf[516]="";
char *ptr=buf;
unsigned short *p1=(short*)buf; //Operation code
*p1=htons(1);
char *p2=buf + 2; //File name
strcpy(p2,FILENAME);
char *p3=p2 + strlen(p2); //The first 0
*p3=0;
char *p4=p3 + 1; //Mode
strcpy(p4,"octet");
size_t size=2 + strlen(p2) + 1 + strlen(p4) + 1; //opcode + filename + 0 + mode + 0
size_t res=0;
//Send download request
if(sendto(cfd,buf,size,0,(struct sockaddr*) & amp;sin,sizeof(sin))<0)
{
ERR_MSG("sendto");
return -1;
}
//Create download file and clear it
int fd=open("./1.png",O_WRONLY|O_CREAT|O_TRUNC,0664);
if(fd < 0)
{
perror("open");
return -1;
}
//Cycle send and receive, receive files, send ack
while(1)
{
//receive packet
res = recvfrom(cfd,buf,sizeof(buf),0,(struct sockaddr*) &sin, &addrlen);
if(res < 0)
{
ERR_MSG("recvfrom");
return -1;
}
if(write(fd,buf + 4,res-4) < 0 )
{
perror("write");
return -1;
}
//send ACK
*p1=htons(4);
if(sendto(cfd,buf,4,0,(struct sockaddr*) & amp;sin,addrlen) < 0)
{
ERR_MSG("sendto");
return -1;
}
if(res-4 < 512)
{
printf("download success\\
");
break;
}
}
close(cfd);
close(fd);
return 0;
}
6. [Full] code (client download, upload file)
#include <stdio.h>
#include <head.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <errno.h>
#define PORT 69 //The port number bound by the server
#define IP "192.168.1.107" //IP address of the server
int do_download(int cfd,struct sockaddr_in sin);
int do_upload(int cfd, struct sockaddr_in sin);
int main(int argc, const char *argv[])
{
//Create a report socket
int cfd = socket(AF_INET, SOCK_DGRAM, 0);
if(cfd < 0)
{
ERR_MSG("socket");
return -1;
}
printf("cfd = %d\\
",cfd);
// Bind the address information structure of the client to the socket ---> binding is not required
//If not bound, the operating system will bind the client to the IP of the running host and a random port number
//Fill the address information structure of the server to be connected, the real address information structure is formulated according to the address family
//Whoever you want to send it to, fill in the address information
//AF_INET: man 7 ip
struct sockaddr_in sin;
socklen_t addrlen=sizeof(sin);
sin.sin_family = AF_INET; //must fill in AF_INET
sin.sin_port = htons(PORT); //port number: the port number bound by the server
sin.sin_addr.s_addr = inet_addr(IP); //Server bound IP
char choose =0;
while(1)
{
printf("------------------------\\
");
printf("---------1. Download --------\\
");
printf("---------2. Upload--------\\
");
printf("---------3. Exit-------\\
");
printf("------------------------\\
");
printf("------------------------\\
");
printf("Please enter >>> ");
choose = getchar();
while(getchar() != 10); //Loop to get characters until \\
is encountered to end the loop
switch(choose)
{
case '1':
do_download(cfd,sin);
break;
case '2':
do_upload(cfd,sin);
break;
case '3':
goto END;
break;
default:
printf("Input error! Please re-enter\\
");
}
}
END:
//Close file descriptor
close(cfd);
return 0;
}
int do_download(int cfd, struct sockaddr_in sin)
{
//The package is ready to send the download request
char buf[516]="";
char name[20]="";
printf("Please enter the file name to download >>> ");
scanf("%s",name);
while(getchar()!=10);
unsigned short *p1=(short*)buf; //opcode
*p1=htons(1);
char *p2=buf + 2; //file name
strcpy(p2,name);
char *p3=p2 + strlen(p2); //the first 0
*p3=0;
char *p4=p3 + 1; //mode
strcpy(p4,"octet");
size_t size=2 + strlen(p2) + 1 + strlen(p4) + 1; //opcode + filename + 0 + mode + 0
//Send download request
if(sendto(cfd,buf,sizeof(buf),0,(struct sockaddr*) & amp;sin,sizeof(sin)) < 0)
{
ERR_MSG("sendto");
return -1;
}
//Create the download file and clear it
int fd = -1; // must be initialized to an invalid file descriptor
socklen_t addrlen = sizeof(sin);
ssize_t res = 0;
unsigned short num = 0; //Record the local block number
//Send download request
while(1)
{
\t\t//Receive data
bzero(buf, sizeof(buf));
res = recvfrom(cfd,buf,sizeof(buf),0,(struct sockaddr*) &sin, &addrlen);
if(res < 0)
{
ERR_MSG("recvfrom");
return -1;
}
if(3 == buf[1]) //data packet
{
//Determine whether the block number of the data packet returned by the server is consistent with the locally recorded block number
if(*(unsigned short*)(buf + 2) == htons((num + 1)))
{
num + + ; //Update the block number of the local record
if(-1 == fd)
{
fd=open(name,O_WRONLY|O_CREAT|O_TRUNC,0664);
if(fd < 0)
{
perror("open");
return -1;
}
}
//write data to file
if(write(fd,buf + 4,res-4) < 0 )
{
perror("write");
return -1;
}
//send ACK
buf[1] = 4;
//*p1=htons(4);
if(sendto(cfd,buf,4,0,(struct sockaddr*) & amp;sin,sizeof(sin)) < 0)
{
ERR_MSG("sendto");
return -1;
}
//If the received data is less than 512, jump out of the loop and end the download.
if(res-4 < 512)
{
printf("%s file downloaded\\
",name);
break;
}
}
}
else if(5 == buf[1]) // error package
{
printf("Error: %d %s\\
",ntohs(*(short*)(buf + 2)),buf + 4);
close(fd);
return -1;
}
}
close(fd);
return 0;
}
int do_upload(int cfd, struct sockaddr_in sin)
{
//The package is ready to send the upload request
char buf[516]="";
char name[20]="";
printf("Please enter the file name to upload >>> ");
scanf("%s",name);
while(getchar()!=10);
int fd = open(name,O_RDONLY);
if(fd < 0)
{
if(errno == ENOENT)
{
printf(">>>File does not exist, please re-enter <<<\\
");
return -2;
}
else
{
ERR_MSG("open");
return -1;
}
}
//The package is ready to send the upload request
unsigned short *p1=(short*)buf; //opcode
*p1=htons(2);
char *p2=buf + 2; //file name
strcpy(p2,name);
char *p3=p2 + strlen(p2); //the first 0
*p3=0;
char *p4=p3 + 1; //mode
strcpy(p4,"octet");
size_t size=2 + strlen(p2) + 1 + strlen(p4) + 1; //Operation code + file name + 0 + mode + 0
//Send upload request
if(sendto(cfd,buf,sizeof(buf),0,(struct sockaddr*) & amp;sin,sizeof(sin)) < 0)
{
ERR_MSG("sendto");
return -1;
}
//Receive and send packets in a loop
ssize_t res;
unsigned short num = 0;
socklen_t addrlen = sizeof(sin);
while(1)
{
//Read data from file into buf
bzero(buf, sizeof(buf));
res = recvfrom(cfd,buf,sizeof(buf),0,(struct sockaddr*) & amp;sin, & amp;addrlen);
if(res < 0)
{
ERR_MSG("recvfrom");
return -1;
}
//The range of the opcode is 1-5 because it is network byte order
//So the valid opcode is stored in the high bit, that is, the position of buf[1]
//printf("buf[1] = %d\\
",buf[1]); //4 The server returns the response packet
if(4 == buf[1]) //packet
{
//Determine whether the number of the current data packet is equal to the number of the response packet
//Prevent packet loss or repeated packet receipt during transmission
if(num == ntohs(*(unsigned short*)(buf + 2)))
{
//Modify opcode to data packet
buf[1] = 3;
//fill block number
num + + ;
*(unsigned short*)(buf + 2) = htons(num);
//Read data
res = read(fd,buf + 4,sizeof(buf)-4);
if(res < 0)
{
ERR_MSG("read");
return -1;
}
else if(0 == res)
{
printf("%s file upload completed!\\
",name);
break;
}
//Send packet
//The size of the sent data packet is, the number of bytes read res + opcode 2byte + block number 2bytes
if(sendto(cfd,buf,sizeof(buf),0,(struct sockaddr*) & amp;sin,sizeof(sin)) < 0)
{
ERR_MSG("sendto");
return -1;
}
}
else
{
printf("File upload failed, please check the network environment\\
");
break;
}
}
else if(5 == buf[1]) //Error package
{
printf("Error: %d %s\\
",ntohs(*(short*)(buf + 2)),buf + 4);
close(fd);
return -1;
}
}
close(fd);
return 0;
}
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