开发环境 VB.Net2005   Windows2003  

'调用方式

        If Logined() Then
            MsgBox("登陆成功")
        Else
            MsgBox("登陆失败")
        End If

'功能函数

Imports System
Imports System.Net
Imports System.IO
Imports System.Text
Imports System.Net.Sockets

    Public Const BLOCK_SIZE = 512
    Private Obj_ClientSocket As Socket
    Private Str_Domain As String = "ftp.lob.cn"          'ftp服务器地址
    Private Str_User As String = "lob"                       'ftp登陆帐号
    Private Str_Password As String = "www.lob.cn"    'ftp登陆密码
    Private Int_RemotePort As Integer = 21              'ftp服务器端口
    Private Int_Reply, Int_Bytes As Int32
    Private Str_Message, Str_Reply As String
    Private Byte_Buffer(BLOCK_SIZE) As Byte
    Private ASCII As Encoding = Encoding.ASCII

  Public Function Logined() As Boolean  '乐博网www.lob.cn会员逍遥原创 

        Obj_ClientSocket = New Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp)
        Dim x_lob As New IPEndPoint(Dns.GetHostEntry(Str_Domain).AddressList(0), Int_RemotePort)
        Try
            Obj_ClientSocket.Connect(x_lob)
        Catch ex As Exception
            MsgBox("无法登陆远程Ftp服务器")
        End Try
        Str_Message = ""
        Str_Reply = ServerReply()
        Int_Reply = Int32.Parse(Str_Reply.Substring(0, 3))
        If (Int_Reply <> 220) Then
            MsgBox(Str_Reply.Substring(4))
        End If

        SendCommand("USER " & Str_User)
        Str_Reply = ServerReply(True)
        Int_Reply = Int32.Parse(Str_Reply.Substring(0, 3))
        If (Not (Int_Reply = 331 Or Int_Reply = 230)) Then
            MsgBox(Str_Reply.Substring(4) )
            Return False
        End If

        If (Int_Reply <> 230) Then
            SendCommand("PASS " & Str_Password)
            Str_Reply = ServerReply(True)
            Int_Reply = Int32.Parse(Str_Reply.Substring(0, 3))
            If (Not (Int_Reply = 230 Or Int_Reply = 202)) Then
                MsgBox(Str_Reply.Substring(4))
                Return False
            End If
        End If

        Return True

    End Function

    Private Function ServerReply(Optional ByVal Bool_Clear As Boolean = False) As String

        Dim Sep As Char = ControlChars.Lf
        Dim Arr_Msg() As String
        If (Bool_Clear) Then
            Str_Message = ""
        End If
        Do While (True)
            Array.Clear(Byte_Buffer, 0, BLOCK_SIZE)
            Int_Bytes = Obj_ClientSocket.Receive(Byte_Buffer, Byte_Buffer.Length, 0)
            Str_Message += ASCII.GetString(Byte_Buffer, 0, Int_Bytes)
            If (Int_Bytes < Byte_Buffer.Length) Then
                Exit Do
            End If
        Loop
        Arr_Msg = Str_Message.Split(Sep)
        If (Str_Message.Length > 2) Then
            Str_Message = Arr_Msg(Arr_Msg.Length - 2)
        Else
            Str_Message = Arr_Msg(0)
        End If
        If (Not (Str_Message.Substring(3, 1).Equals(" "))) Then
            Return ServerReply(True)
        End If
        Return Str_Message

    End Function

    Private Sub SendCommand(ByVal Str_Command As String)

        Str_Command = Str_Command & ControlChars.CrLf
        Dim cmdbytes As Byte() = ASCII.GetBytes(Str_Command)
        Obj_ClientSocket.Send(cmdbytes, cmdbytes.Length, 0)

    End Sub

出处：https://www.cnblogs.com/NetPig/archive/2011/07/03/2096805.html