应用层协议实现系列——FTP服务器之设计与实现
Posted 心砚thu
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在实现了HTTP服务器之后，本人打算再实现一个FTP服务器。由于FTP协议与HTTP一样都位于应用层，所以实现原理也类似。在这里把实现的原理和源码分享给大家。
首先需要明确的是FTP协议中涉及命令端口和数据端口，即每个客户端通过命令端口向服务器发送命令（切换目录、删除文件等），通过数据端口从服务器接收数据（目录列表、下载上传文件等）。这就要求对每个连接都必须同时维护两个端口，如果使用类似于上一篇文章中的多路IO就会复杂很多，因此本文采用了类似Apache的多进程机制，即对每个连接创建一个单独的进程进行管理。
接下来简要说明一下FTP协议的通信流程，Ftp服务器向客户端发送的消息主要由两部分组成，第一部分是状态码（与HTTP类似），第二部分是具体内容（可以为空），两部分之间以空格分隔，如“220 TS FTP Server ready”就告诉了客户端已经连接上了服务器；客户端向服务器发送的命令也由两部分组成，第一部分是命令字符串，第二部分是具体内容（可以为空），两部分之间也以空格分隔，如“USER anonymous”就指定了登录FTP服务器的用户名。以一个登录FTP服务器并获取目录列表的流程为例：
220 TS FTP Server ready...
USER anonymous
331 Password required for anonymous
PASS chrome@example.com
530 Not logged in,password error.
QUIT
221 Goodbye
USER zhaoxy
331 Password required for zhaoxy
PASS 123
230 User zhaoxy logged in
SYST
215 UNIX Type: L8
PWD
257 "/" is current directory.
TYPE I
200 Type set to I
PASV
227 Entering Passive Mode (127,0,0,1,212,54)
SIZE /
550 File not found
PASV
227 Entering Passive Mode (127,0,0,1,212,56)
CWD /
250 CWD command successful. "/" is current directory.
LIST -l
150 Opening data channel for directory list.
16877 8 501 20 272 4 8 114 .
16877 29 501 20 986 4 8 114 ..
33188 1 501 20 6148 3 28 114 .DS_Store
16877 4 501 20 136 2 27 114 css
33279 1 501 20 129639543 6 14 113 haha.pdf
16877 11 501 20 374 2 27 114 images
33261 1 501 20 11930 3 9 114 index.html
16877 6 501 20 204 2 28 114 js
226 Transfer ok.
QUIT
221 GoodbyeFTP协议中主要有两种格式，二进制和ASCII码，两种格式的主要区别在于换行，二进制格式不会对数据进行任何处理，而ASCII码格式会将回车换行转换为本机的回车字符，比如Unix下是\\n,Windows下是\\r\\n，Mac下是\\r。一般图片和执行文件必须用二进制格式，CGI脚本和普通HTML文件必须用ASCII码格式。
在确定了传输格式之后，客户端会设定传输模式，Passive被动模式或Active主动模式。在被动模式下，服务器会再创建一个套接字绑定到一个空闲端口上并开始监听，同时将本机ip和端口号（h1,h2,h3,h4,p1,p2，其中p1*256+p2等于端口号）发送到客户端。当之后需要传输数据的时候，服务器会通过150状态码通知客户端，客户端收到之后会连接到之前指定的端口并等待数据。传输完成之后，服务器会发送226状态码告诉客户端传输成功。如果客户端不需要保持长连接的话，此时可以向服务器发送QUIT命令断开连接。在主动模式下，流程与被动模式类似，只是套接字由客户端创建并监听，服务器连接到客户端的端口上进行数据传输。
以下是main函数中的代码：
#include <iostream>
#include "define.h"
#include "CFtpHandler.h"
#include <sys/types.h>
#include <sys/socket.h>
int main(int argc, const char * argv[])
int port = 2100;
int listenFd = startup(port);
//ignore SIGCHLD signal, which created by child process when exit, to avoid zombie process
signal(SIGCHLD,SIG_IGN);
while (1)
int newFd = accept(listenFd, (struct sockaddr *)NULL, NULL);
if (newFd == -1)
//when child process exit, it'll generate a signal which will cause the parent process accept failed.
//If happens, continue.
if (errno == EINTR) continue;
printf("accept error: %s(errno: %d)\\n",strerror(errno),errno);
//timeout of recv
struct timeval timeout = 3,0;
setsockopt(newFd, SOL_SOCKET, SO_RCVTIMEO, (char *)&timeout,sizeof(struct timeval));
int pid = fork();
//fork error
if (pid < 0)
printf("fork error: %s(errno: %d)\\n",strerror(errno),errno);
//child process
else if (pid == 0)
//close useless socket
close(listenFd);
send(newFd, TS_FTP_STATUS_READY, strlen(TS_FTP_STATUS_READY), 0);
CFtpHandler handler(newFd);
int freeTime = 0;
while (1)
char buff[256];
int len = (int)recv(newFd, buff, sizeof(buff), 0);
//connection interruption
if (len == 0) break;
//recv timeout return -1
if (len < 0)
freeTime += 3;
//max waiting time exceed
if (freeTime >= 30)
break;
else
continue;
buff[len] = '\\0';
//reset free time
freeTime = 0;
if (handler.handleRequest(buff))
break;
close(newFd);
std::cout<<"exit"<<std::endl;
exit(0);
//parent process
else
//close useless socket
close(newFd);
close(listenFd);
return 0;
代码中先创建了套接字并绑定到指定端口上，然后进入循环开始监听端口。每监听到一个新的连接就fork出一个子进程。子进程向客户端发送欢迎信息后进入循环处理客户端发送过来的命令，直到收到QUIT命令或者连接超时退出循环。以上代码中需要注意三个地方，一是子进程在退出之后会向父进程发送SIGCHLD信号，如果父进程不进行处理（调用wait或忽略）就会导致子进程变为僵尸进程，本文中采用的是忽略的方式；二是accept函数在父进程收到信号时会直接返回，因此需要判断如果返回是由于信号则继续循环，不fork，否则会无限创建子进程；三是在fork之后需要将不使用的套接字关闭，比如父进程需要关闭新的连接套接字，而子进程需要关闭监听套接字，避免套接字无法完全关闭。
最后通过CFtpHandler类中的handleRequest方法处理客户端的命令，部分代码如下：
//handle client request
bool CFtpHandler::handleRequest(char *buff)
stringstream recvStream;
recvStream<<buff;
cout<<buff;
string command;
recvStream>>command;
bool isClose = false;
string msg;
//username
if (command == COMMAND_USER)
recvStream>>username;
msg = TS_FTP_STATUS_PWD_REQ(username);
//password
else if (command == COMMAND_PASS)
recvStream>>password;
if (username == "zhaoxy" && password == "123")
msg = TS_FTP_STATUS_LOG_IN(username);
else
msg = TS_FTP_STATUS_PWD_ERROR;
//quit
else if (command == COMMAND_QUIT)
msg = TS_FTP_STATUS_BYE;
isClose = true;
//system type
else if (command == COMMAND_SYST)
msg = TS_FTP_STATUS_SYSTEM_TYPE;
//current directory
else if (command == COMMAND_PWD)
msg = TS_FTP_STATUS_CUR_DIR(currentPath);
//transmit type
else if (command == COMMAND_TYPE)
recvStream>>type;
msg = TS_FTP_STATUS_TRAN_TYPE(type);
//passive mode
else if (command == COMMAND_PASSIVE)
int port = 0;
if (m_dataFd)
close(m_dataFd);
m_dataFd = startup(port);
stringstream stream;
stream<<TS_FTP_STATUS_PASV<<port/256<<","<<port%256<<")";
msg = stream.str();
//active passive mode
m_isPassive = true;
//active mode
else if (command == COMMAND_PORT)
string ipStr;
recvStream>>ipStr;
char ipC[32];
strcpy(ipC, ipStr.c_str());
char *ext = strtok(ipC, ",");
m_clientPort = 0; m_clientIp = 0;
m_clientIp = atoi(ext);
int count = 0;
//convert string to ip address and port number
//be careful, the ip should be network endianness
while (1)
if ((ext = strtok(NULL, ","))==NULL)
break;
switch (++count)
case 1:
case 2:
case 3:
m_clientIp |= atoi(ext)<<(count*8);
break;
case 4:
m_clientPort += atoi(ext)*256;
break;
case 5:
m_clientPort += atoi(ext);
break;
default:
break;
msg = TS_FTP_STATUS_PORT_SUCCESS;
//file size
else if (command == COMMAND_SIZE)
recvStream>>fileName;
string filePath = ROOT_PATH+currentPath+fileName;
long fileSize = filesize(filePath.c_str());
if (fileSize)
stringstream stream;
stream<<TS_FTP_STATUS_FILE_SIZE<<fileSize;
msg = stream.str();
else
msg = TS_FTP_STATUS_FILE_NOT_FOUND;
//change directory
else if (command == COMMAND_CWD)
string tmpPath;
recvStream>>tmpPath;
string dirPath = ROOT_PATH+tmpPath;
if (isDirectory(dirPath.c_str()))
currentPath = tmpPath;
msg = TS_FTP_STATUS_CWD_SUCCESS(currentPath);
else
msg = TS_FTP_STATUS_CWD_FAILED(currentPath);
//show file list
else if (command == COMMAND_LIST || command == COMMAND_MLSD)
string param;
recvStream>>param;
msg = TS_FTP_STATUS_OPEN_DATA_CHANNEL;
sendResponse(m_connFd, msg);
int newFd = getDataSocket();
//get files in directory
string dirPath = ROOT_PATH+currentPath;
DIR *dir = opendir(dirPath.c_str());
struct dirent *ent;
struct stat s;
stringstream stream;
while ((ent = readdir(dir))!=NULL)
string filePath = dirPath + ent->d_name;
stat(filePath.c_str(), &s);
struct tm tm = *gmtime(&s.st_mtime);
//list with -l param
if (param == "-l")
stream<<s.st_mode<<" "<<s.st_nlink<<" "<<s.st_uid<<" "<<s.st_gid<<" "<<setw(10)<<s.st_size<<" "<<tm.tm_mon<<" "<<tm.tm_mday<<" "<<tm.tm_year<<" "<<ent->d_name<<endl;
else
stream<<ent->d_name<<endl;
closedir(dir);
//send file info
string fileInfo = stream.str();
cout<<fileInfo;
send(newFd, fileInfo.c_str(), fileInfo.size(), 0);
//close client
close(newFd);
//send transfer ok
msg = TS_FTP_STATUS_TRANSFER_OK;
//send file
else if (command == COMMAND_RETRIEVE)
recvStream>>fileName;
msg = TS_FTP_STATUS_TRANSFER_START(fileName);
sendResponse(m_connFd, msg);
int newFd = getDataSocket();
//send file
std::ifstream file(ROOT_PATH+currentPath+fileName);
file.seekg(0, std::ifstream::beg);
while(file.tellg() != -1)
char *p = new char[1024];
bzero(p, 1024);
file.read(p, 1024);
int n = (int)send(newFd, p, 1024, 0);
if (n < 0)
cout<<"ERROR writing to socket"<<endl;
break;
delete p;
file.close();
//close client
close(newFd);
//send transfer ok
msg = TS_FTP_STATUS_FILE_SENT;
//receive file
else if (command == COMMAND_STORE)
recvStream>>fileName;
msg = TS_FTP_STATUS_UPLOAD_START;
sendResponse(m_connFd, msg);
int newFd = getDataSocket();
//receive file
ofstream file;
file.open(ROOT_PATH+currentPath+fileName, ios::out | ios::binary);
char buff[1024];
while (1)
int n = (int)recv(newFd, buff, sizeof(buff), 0);
if (n<=0) break;
file.write(buff, n);
file.close();
//close client
close(newFd);
//send transfer ok
msg = TS_FTP_STATUS_FILE_RECEIVE;
//get support command
else if (command == COMMAND_FEAT)
stringstream stream;
stream<<"211-Extension supported"<<endl;
stream<<COMMAND_SIZE<<endl;
stream<<"211 End"<<endl;;
msg = stream.str();
//get parent directory
else if (command == COMMAND_CDUP)
if (currentPath != "/")
char path[256];
strcpy(path, currentPath.c_str());
char *ext = strtok(path, "/");
char *lastExt = ext;
while (ext!=NULL)
ext = strtok(NULL, "/");
if (ext) lastExt = ext;
currentPath = currentPath.substr(0, currentPath.length()-strlen(lastExt)-1);
msg = TS_FTP_STATUS_CDUP(currentPath);
//delete file
else if (command == COMMAND_DELETE)
recvStream>>fileName;
//delete file
if (remove((ROOT_PATH+currentPath+fileName).c_str()) == 0)
msg = TS_FTP_STATUS_DELETE;
else
printf("delete error: %s(errno: %d)\\n",strerror(errno),errno);
msg = TS_FTP_STATUS_DELETE_FAILED;
//other
else if (command == COMMAND_NOOP || command == COMMAND_OPTS)
msg = TS_FTP_STATUS_OK;
sendResponse(m_connFd, msg);
return isClose;
另外，以上代码中的PORT命令处理时涉及对ip地址的解析，需要注意本机字节顺序和网络字节顺序的区别，如127.0.0.1转换成整数应逆序转换，以网络字节顺序存到s_addr变量中。
以上源码已经上传到GitHub中，感兴趣的朋友可以前往下载。
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