Using a C++ TCP client socket on a specific network interface Linux/Unix

Question

I have the following code which by default connects to interface "eth0" which is a 1G NIC, but I would like to connect using "eth5", which is a 10G NIC.

 class TCPClientSocket {
  protected:
    int socket_file_descriptor_;

  public:

    TCPClientSocket ( ) 
      : socket_file_descriptor_ ( -1 )
    {
      /* socket creation */
      socket_file_descriptor_ = socket ( AF_INET, SOCK_STREAM, 0 );
      if ( socket_file_descriptor_ < 0 ) { exit(1); }
    }

    void Connect ( const std::string & _ors_ip_, const int _ors_port_ ) {
      struct sockaddr_in ors_Addr_ ;
      bzero ( &ors_Addr_, sizeof ( ors_Addr_ ) ) ;
      ors_Addr_.sin_family = AF_INET;
      ors_Addr_.sin_port = htons ( _ors_port_ );
      inet_pton ( AF_INET, _ors_ip_.c_str(), &(ors_Addr_.sin_addr) );

      if ( connect ( socket_file_descriptor_, (struct sockaddr *) &ors_Addr_, sizeof(struct sockaddr_in) ) < 0 ) {
        fprintf ( stderr, "connect() failed on %s:%d\n", _ors_ip_.c_str( ), _ors_port_ );
        close ( socket_file_descriptor_ );
        socket_file_descriptor_ = -1;
      }
    }

    inline int WriteN ( const unsigned int _len_, const void * _src_ ) const {
      if ( socket_file_descriptor_ != -1 ) {
        return write ( socket_file_descriptor_, _src_, _len_ );
      }
      return -1;
    }

    inline int ReadN ( const unsigned int _len_, void * _dest_ ) const {
      if ( socket_file_descriptor_ != -1 ) {
        return read ( socket_file_descriptor_, _dest_, _len_ );
      }
      return -1;
    }

    inline bool IsOpen ( ) const { return ( socket_file_descriptor_ != -1 ) ; }
    inline int socket_file_descriptor() const { return socket_file_descriptor_; }    
    void Close ( ) {
      if ( socket_file_descriptor_ != -1 ) {
        shutdown ( socket_file_descriptor_, SHUT_RDWR );
        close ( socket_file_descriptor_ );
        socket_file_descriptor_ = -1;
      }
    }
  };

possible duplicate of How do I dynamically bind a socket to only one network interface? — NG.
– NG., Commented Aug 14, 2011 at 14:55
Could you add more details to the scenario ? @SB I am not (yet) sure it's a duplicate of that question. — cnicutar
– cnicutar, Commented Aug 14, 2011 at 15:00
@cnicular The way we use this class is just instantiate it and call connect. I was hoping to just change the class so that it automatically uses the "eth5" NIC. As a next change we would then add an argument to the instantiation which allowed the user to control which NIC to use. — Humble Debugger
– Humble Debugger, Commented Aug 14, 2011 at 15:06
Out of interest why not solve it out side of the application with a metric on the interfaces? — Flexo - Save the data dump
– Flexo - Save the data dump ♦, Commented Aug 14, 2011 at 15:29
@awoodland I did not understand the "metric on the interfaces" ? — Humble Debugger
– Humble Debugger, Commented Aug 14, 2011 at 15:30

user339024 · Accepted Answer · 2011-08-14 15:00:28Z

8

According to the information here you can use setsockopt() to achieve this as follows:

char* interface = "eth5";
setsockopt( socket_file_descriptor_, SOL_SOCKET, SO_BINDTODEVICE, interface, 4 );

The final parameter, 4, represents the number of bytes in the interface variable.

answered Aug 14, 2011 at 15:00

user339024

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2 Comments

Avner Over a year ago

Should probably use sizeof(interface) instead of 4 for that last argument.

rido Over a year ago

@Avner You are wrong. interface contain an address to the string literal "eth5". It in not an array itself. It is a pointer to an array. So, its size will be sizeof(char *) which is the pointer size on that machine. As far as I know, we have three option here: 1. make interface a char array. and pass the sizeof(interface). 2. Using constant 4 is Ok, But magic numbers are not welcome most of the time. strlen(interface) is an another option. 3. Use the macro IFNAMSIZ.

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