In C how to check if a TCP port is available

Question

In C  to use a TcpClient or generally to connect to a socket how can I first check if a certain port is free on my machine   more info  This is the code I use   TcpClient c    I want to check here if port is free  c   new TcpClient ip  port

User · Answer

Thanks for this tip   I needed the same functionality but on the Server side to check if a Port was in use so I modified it to this code    private bool CheckAvailableServerPort int port        LOG InfoFormat  Checking Port  0    port       bool isAvailable   true          Evaluate current system tcp connections  This is the same information provided        by the netstat command line application  just in  Net strongly-typed object        form   We will look through the list  and if our port we would like to use        in our TcpClient is occupied  we will set isAvailable to false      IPGlobalProperties ipGlobalProperties   IPGlobalProperties GetIPGlobalProperties        IPEndPoint   tcpConnInfoArray   ipGlobalProperties GetActiveTcpListeners         foreach  IPEndPoint endpoint in tcpConnInfoArray            if  endpoint Port    port                isAvailable   false              break                       LOG InfoFormat  Port  0  available    1    port  isAvailable        return isAvailable

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When you set up a TCP connection  the 4-tuple  source-ip  source-port  dest-ip  dest-port  has to be unique - this is to ensure packets are delivered to the right place   There is a further restriction on the server side that only one server program can bind to an incoming port number  assuming one IP address  multi-NIC servers have other powers but we don t need to discuss them here    So  at the server end  you    create a socket  bind that socket to a port  listen on that port  accept connections on that port  and there can be multiple connections coming in  one per client     On the client end  it s usually a little simpler    create a socket  open the connection  When a client opens the connection  it specifies the ip address and port of the server  It can specify its source port but usually uses zero which results in the system assigning it a free port automatically    There is no requirement that the destination IP port be unique since that would result in only one person at a time being able to use Google  and that would pretty well destroy their business model   This means you can even do such wondrous things as multi-session FTP since you set up multiple sessions where the only difference is your source port  allowing you to download chunks in parallel  Torrents are a little different in that the destination of each session is usually different   And  after all that waffling  sorry   the answer to your specific question is that you don t need to specify a free port  If you re connecting to a server with a call that doesn t specify your source port  it ll almost certainly be using zero under the covers and the system will give you an unused one

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If I m not very much mistaken  you can use System Network whatever to check   However  this will always incur a race condition   The canonical way of checking is try to listen on that port  If you get an error that port wasn t open   I think this is part of why bind   and listen   are two separate system calls

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netstat  That s a network command line utility which ships with windows  It shows all current established connections and all ports currently being listened to  You can use this program to check  but if you want to do this from code look into the System Net NetworkInformation namespace  It s a new namespace as of 2 0  There s some goodies there  But eventually if you wanna get the same kind of information that s available through the command netstat you ll need to result to P Invoke    Update  System Net NetworkInformation That namespace contains a bunch of classes you can use for figuring out things about the network  I wasn t able to find that old pice of code but I think you can write something similar yourself  A good start is to check out the IP Helper API  Google MSDN for the  GetTcpTable WINAPI function and use P Invoke to enumerate until you have the information you need

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ipGlobalProperties GetActiveTcpConnections   doesn t return connections in Listen State   Port can be used for listening  but with no one connected to it the method described above will not work

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TcpClient c     I want to check here if port is free  c   new TcpClient ip  port           how can I first check if a certain port is free on my machine       I mean that it is not in use by any other application     If an application is using a port others can t use it until it becomes free      Ali   You have misunderstood what s happening here    TcpClient      parameters are of server ip and server port you wish to connect to    The TcpClient selects a transient local port from the available pool to communicate to the server  There s no need to check for the availability of the local port as it is automatically handled by the winsock layer   In case you can t connect to the server using the above code fragment  the problem could be one or more of several   i e  server ip and or port is wrong  remote server not available  etc

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From the avalaible ports i would exclude    active TCP connections  active TCP listeners  active UDP listeners   With the following import   using System Net NetworkInformation    You can use the following function to check if a port is avalaible or not   private bool isPortAvalaible int myPort        var avalaiblePorts   new List lt int gt         var properties   IPGlobalProperties GetIPGlobalProperties            Active connections     var connections   properties GetActiveTcpConnections        avalaiblePorts AddRange connections           Active tcp listners     var endPointsTcp   properties GetActiveTcpListeners        avalaiblePorts AddRange endPointsTcp           Active udp listeners     var endPointsUdp   properties GetActiveUdpListeners        avalaiblePorts AddRange endPointsUdp        foreach  int p in avalaiblePorts           if  p    myPort  return false            return true      I give you a similar function for those who use VB NET   Imports System Net NetworkInformation Private Function isPortAvalaible ByVal myPort As Integer  As Boolean   Dim props As IPGlobalProperties   IPGlobalProperties GetIPGlobalProperties        ignore active connections   Dim tcpConnInfoArray   As TcpConnectionInformation   props GetActiveTcpConnections     For Each tcpi As Net NetworkInformation TcpConnectionInformation In tcpConnInfoArray     If tcpi LocalEndPoint Port   myPort Then       Return False     End If   Next tcpi      ignore active TCP listeners   Dim activeTcpListeners   As Net IPEndPoint   props GetActiveTcpListeners   For Each tcpListener As Net IPEndPoint In activeTcpListeners     If tcpListener Port   myPort Then       Return False     End If   Next tcpListener      ignore active UPD listeners   Dim activeUdpListeners   As Net IPEndPoint   props GetActiveUdpListeners   For Each udpListener As Net IPEndPoint In activeUdpListeners     If udpListener Port   myPort Then       Return False     End If   Next udpListener    Return True End Function

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Check for error code 10048  try       TcpListener tcpListener   new TcpListener ipAddress  portNumber       tcpListener Start      catch SocketException ex        if ex ErrorCode    10048                MessageBox Show  Port     portNumber     is currently in use               return

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string hostname    localhost   int portno   9081  IPAddress ipa    IPAddress  Dns GetHostAddresses hostname  0     try       System Net Sockets Socket sock   new System Net Sockets Socket System Net Sockets AddressFamily InterNetwork  System Net Sockets SocketType Stream  System Net Sockets ProtocolType Tcp       sock Connect ipa  portno       if  sock Connected    true      Port is in use and connection is successful             MessageBox Show  Port is Closed        sock Close       catch  System Net Sockets SocketException ex        if  ex ErrorCode    10061      Port is unused and could not establish connection          MessageBox Show  Port is Open         else         MessageBox Show ex Message

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test connection  ip   port     public void test connection String hostname  int portno      IPAddress ipa    IPAddress Dns GetHostAddresses hostname  0     try       System Net Sockets Socket sock   new System Net Sockets Socket System Net Sockets AddressFamily InterNetwork  System Net Sockets SocketType Stream  System Net Sockets ProtocolType Tcp      sock Connect ipa  portno      if  sock Connected    true         MessageBox Show  Port is in use             sock Close        catch  System Net Sockets SocketException ex       if  ex ErrorCode    10060         MessageBox Show  No connection

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To answer the exact question of finding a free port  which is what I needed in my unit tests  in dotnet core 3 1 I came up this  public static int GetAvailablePort IPAddress ip            TcpListener l   new TcpListener ip  0           l Start            int port     IPEndPoint l LocalEndpoint  Port          l Stop            Log Info   Available port found   port             return port          note  based the comment by  user207421 about port zero I searched and found this and slightly modified it

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thanks for the answer jro   I had to tweak it for my usage   I needed to check if a port was being listened on  and not neccessarily active   For this I replaced   TcpConnectionInformation   tcpConnInfoArray   ipGlobalProperties GetActiveTcpConnections      with   IPEndPoint   objEndPoints   ipGlobalProperties GetActiveTcpListeners         I iterated the array of endpoints checking that my port value was not found

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try this  in my case the port number for the created object wasn t available so I came up with this  IPEndPoint endPoint  int port   1  while  true        try               endPoint   new IPEndPoint IPAddress Any  port           break            catch  SocketException                 port

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Be aware the time window between you make check and the moment you try to make connection some process may take the port - classical TOCTOU  Why don t you just try to connect  If it fails then you know the port is not available

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You don t have to know what ports are open on your local machine to connect to some remote TCP service  unless you want to use a specific local port  but usually that is not the case    Every TCP IP connection is identified by 4 values  remote IP  remote port number  local IP  local port number  but you only need to know remote IP and remote port number to establish a connection    When you create tcp connection using   TcpClient c  c   new TcpClient remote ip  remote port      Your system will automatically assign one of many free local port numbers to your connection  You don t need to do anything  You might also want to check if a remote port is open  but there is no better way to do that than just trying to connect to it

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You say      I mean that it is not in use by any   other application  If an application   is using a port others can t use it   until it becomes free    But you can always connect to a port while others are using it if something s listening there   Otherwise  http port 80 would be a mess   If your      c   new TcpClient ip  port     fails  then nothing s listening there  Otherwise  it will connect  even if some other machine application has a socket open to that ip and port

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Since you re using a TcpClient  that means you re checking open TCP ports  There are lots of good objects available in the System Net NetworkInformation namespace   Use the IPGlobalProperties object to get to an array of TcpConnectionInformation objects  which you can then interrogate about endpoint IP and port      int port   456     lt --- This is your value  bool isAvailable   true       Evaluate current system tcp connections  This is the same information provided     by the netstat command line application  just in  Net strongly-typed object     form   We will look through the list  and if our port we would like to use     in our TcpClient is occupied  we will set isAvailable to false   IPGlobalProperties ipGlobalProperties   IPGlobalProperties GetIPGlobalProperties     TcpConnectionInformation   tcpConnInfoArray   ipGlobalProperties GetActiveTcpConnections      foreach  TcpConnectionInformation tcpi in tcpConnInfoArray        if  tcpi LocalEndPoint Port  port            isAvailable   false       break               At this point  if isAvailable is true  we can proceed accordingly

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public static bool TestOpenPort int Port                var tcpListener   default TcpListener            try                       var ipAddress   Dns GetHostEntry  localhost   AddressList 0                tcpListener   new TcpListener ipAddress  Port               tcpListener Start                 return true                    catch  SocketException                              finally                       if  tcpListener    null                  tcpListener Stop                       return false

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You re on the wrong end of the Intertube   It is the server that can have only one particular port open   Some code     IPAddress ipAddress   Dns GetHostEntry  localhost   AddressList 0     try       TcpListener tcpListener   new TcpListener ipAddress  666       tcpListener Start          catch  SocketException ex        MessageBox Show ex Message   kaboom          Fails with      Only one usage of each socket address  protocol network address port  is normally permitted

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