使用 FTP 命令下载带套接字的文件

Question

我使用的是专有编程语言，它没有内置 FTP 功能。因此我使用 sockets.

sHandle := SocketOpen('ftp.Whosebug.net', 21);
SocketReadString(sHandle, answer);

retW := SocketWriteString(sHandle, 'user user1673665' & CHR(13) & CHR(10));
SocketReadString(sHandle, answer);
retW := SocketWriteString(sHandle, 'pass !@#$%^&*' & CHR(13) & CHR(10));
SocketReadString(sHandle, answer);

retW := SocketWriteString(sHandle, 'cwd update' & CHR(13) & CHR(10));
SocketReadString(sHandle, answer);
retW := SocketWriteString(sHandle, 'retr update.txt' & CHR(13) & CHR(10));
SocketReadString(sHandle, answer);

SocketClose(sHandle);

这些是我从 FTP 服务器收到的答案：

220-Welcome to Whosebug FTP
220 Unauthorized access is illegal!
331 Password required for user1673665
230 Logged on
250 CWD successful. "/update" is current directory.

但是为什么我在retr update.txt之后得到这个错误：

503 Bad sequence of commands.

我正在逐步调试代码。因此响应时间应该不是问题。

Answer 1

RETR 命令之前必须至少有 PASV 或 PORT 才能设置数据连接。

请参阅 RFC 959，尤其是 “3.2. 建立数据连接”部分

使用 PASV 时，您必须打开服务器在 227 响应中返回的端口的连接。

另请注意，FTP 命令应以大写形式发送。

Answer 2

Why does retr update.txt not work?

因为您没有遵循 FTP 协议的规范。数据传输使用单独的 TCP 连接完成，需要使用 PASV、EPSV、PORT 或 EPRT 命令预先设置。有关详细信息，请参阅标准（这就是它们的用途），即 RFC 959 and RFC 2428.

Answer 3

感谢 Martin Prikryl 和 Steffen Ullrich 的帮助。我将在此处添加带有注释的工作代码。也许它对某人有用。也许有必要添加睡眠命令，因为代码运行速度比服务器响应速度快。

# Open control channel
sHandle := SocketOpen('ftp.Whosebug.net', 21);
SocketReadString(sHandle, answer);
SocketWriteString(sHandle, 'USER user1673665' & CHR(13) & CHR(10));
SocketReadString(sHandle, answer);
SocketWriteString(sHandle, 'PASS !@#$%^&*' & CHR(13) & CHR(10));
SocketReadString(sHandle, answer);
SocketWriteString(sHandle, 'CWD update' & CHR(13) & CHR(10));
SocketReadString(sHandle, answer);
# Enter passive mode and receive data channel adress and port
SocketWriteString(sHandle, 'PASV' & CHR(13) & CHR(10));
SocketReadString(sHandle, answer);
# Create adress for data channel
IF answer <> NOVALUE THEN
  # Split answer on character , and ( and ) and whitespace
  retPASV[] := StrSplit(answer, ',() ');
ENDIF;
CASE retPASV[1]
    # Passive mode is 227
  IS = 227 DO
    connect := retPASV[6] & '.' & retPASV[7] & '.' & retPASV[8] & '.' & retPASV[9];
  IS DO
ENDCASE;
# Create port for data channel
# Port is secondlast number * 256 + last number from PASV reply
port := String2Num(retPASV[10]) * 256 + String2Num(retPASV[11]);
# Open data channel on sHandle2
sHandle2 := SocketOpen(connect, port);
SocketReadString(sHandle2, answer2);
# Download file on control channel
SocketWriteString(sHandle, 'RETR update.txt' & CHR(13) & CHR(10));
SocketReadString(sHandle, Antwort);
# Read file on data channel - result of textfile in string answer2
SocketReadString(sHandle2, answer2);
SocketClose(sHandle);

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