Qt raw socket


  • Datagrams seen in Wireshark, not received by Qt UDP Socket
  • A Guide to Using Raw Sockets
  • Qt Documentation
  • Ping is a basic Internet tool that allows a user to verify that a particular IP address exists and can accept requests. We provide nothing but the best curated videos and practice problems for our students. Check out the C Foundation Course and master the C language from basic to advanced level. Wait no more, start learning today! Since IP does not have any inbuilt mechanism for sending error and control messages. It is used for reporting errors and management queries. Example of Ubuntu Ping ping www.

    The packet has a TTL time-to-live value determining max number of router hops. If the packet does not reach, then the sender is noted back with the error. The gethostbyname function converts a normal human readable website and returns a structure of type hostent which contains IP address in form of binary dot notation and also address type. Some ping programs like the one given with ubuntu support reverse DNS lookup.

    Reverse DNS lookup is performed using getnameinfo , and it converts dot notation IP address to hostname. This interrupt is caught by an interrupt handler which just sets our pinging looping condition to false. Here comes the main ping sending loop. We have to: Set the ttl option to a value in the socket TTL value is set to limit the number of hops a packet can make. Set the timeout of the recv function If timeout is not set, recv will wait forever, halting the loop.

    Set id to pid of process Fill msg part randomly. Calculate checksum and fill it in checksum field. Send the packet Wait for it to be received.

    The main problem here is that the packet received does not mean that that the destination is working. Echo reply means destination is OK. Echo reply is sent from destination OS kernel. This is the list of all types and codes.

    If you have no knowledge of the Linux kernel, yet are interested in the contents of network packets, raw sockets are the answer. A raw socket is used to receive raw packets. This means packets received at the Ethernet layer will directly pass to the raw socket. Figure 1: Graphical demonstration of a raw socket A raw socket vs other sockets Other sockets like stream sockets and data gram sockets receive data from the transport layer that contains no headers but only the payload.

    If applications running on the same machine or on different machines are communicating, then they are only exchanging data. The purpose of a raw socket is absolutely different.

    A raw socket allows an application to directly access lower level protocols, which means a raw socket receives un-extracted packets see Figure 2. There is no need to provide the port and IP address to a raw socket, unlike in the case of stream and datagram sockets. Figure 2: Graphical demonstration of how a raw socket works compared to other sockets Network packets and packet sniffers When an application sends data into the network, it is processed by various network layers.

    Before sending data, it is wrapped in various headers of the network layer. The wrapped form of data, which contains all the information like the source and destination address, is called a network packet see Figure 3. Figure 3: A generic representation of a network packet Figure 4: Network Packet for internet Protocol When we connect to the Internet, we receive network packets, and our machine extracts all network layer headers and sends data to a particular application.

    For example, when we type www. By default, a machine receives those packets that have the same destination address as that of the machine, and this mode is called the non-promiscuous mode. But if we want to receive all the packets, we have to switch into the promiscuous mode.

    We can go into the promiscuous mode with the help of ioctls. If we are interested in the contents or the structure of the headers of different network layers, we can access these with the help of a packet sniffer. There are various packet sniffers available for Linux, like Wireshark.

    There is a command line sniffer called tcpdump, which is also a very good packet sniffer. And if we want to make our own packet sniffer, it can easily be done if we know the basics of C and networking.

    A packet sniffer with a raw socket To develop a packet sniffer, you first have to open a raw socket. So, during the execution of the program, you have to be the root user. We can also use the recv api. But recvfrom provides additional information. Extracting the Ethernet header Now that we have the network packets in our buffer, we will get information about the Ethernet header.

    The Ethernet header contains the physical address of the source and destination, or the MAC address and protocol of the receiving packet. Later, we will consider the next header as the IP header. Note 1: The physical address is 6 bytes. Note 2: We can also direct the output to a file for better understanding. Extracting the IP header The IP layer gives various pieces of information like the source and destination IP address, the transport layer protocol, etc.

    The structure of the IP header is defined in the ip. These structures provide the port number of the source and destination. With the help of the port number, the system gives data to a particular application see Figures 7 and 8.

    The size of the IP header varies from 20 bytes to 60 bytes. Note: If your machine is little endian, you have to use ntohs because the network uses the big endian scheme. Extracting data After the transport layer header, there is data payload remaining. For this, we will move the pointer to the data, and then print. The second way is more efficient and will make your program machine-independent, which means you should not enter ifconfig in each machine. Linux supports some standard ioctls to configure network devices.

    They pass an ifreq structure, which means that if you want to know some information about the network, like the interface index or interface name, you can use ioctl and it will fill the value of the ifreq structure passed as a third argument. In short, the ifreq structure is a way to get and set the network configuration. It is defined in the if.

    Figure Structure of ifreq Figure Graphical representation of packets with their structure and payload Getting the index of the interface to send a packet There may be various interfaces in your machine like loopback, wired interface and wireless interface.

    So you have to decide the interface through which we can send our packet. After deciding on the interface, you have to get the index of that interface.

    First, take a buffer in which you will place all information like the Ethernet header, IP header, UDP header and data. That buffer will be your packet. Data after the IP header is called the payload for the IP header and, in the same way, data after the Ethernet header is called the payload for the Ethernet header.

    So, some field of the iphdr structure will get the value after filling the UDP header field. Assign values to the fields of the udphdr structure. For this, increment the sendbuff pointer by the size of the Ethernet and the IP headers. So, first, you have to know the UDP payload, which is the actual data that will be sent. A checksum is used for error checking of the header.

    When the packet arrives at the router, it calculates the checksum, and if the calculated checksum does not match with the checksum field of the header, the router will drop the packet; and if it matches, the router will decrement the time to the live field by one, and forward it.

    To calculate the checksum, sum up all the bit words of the IP header and if there is any carry, add it again to get a bit word. To check whether our checksum is correct, use the above algorithm.

    And in another machine, or in your destination machine, run the packet sniffer program as the root user and analyse the data that you are sending. What to do next We made a packet sniffer as well as a packet sender, but this is a user space task. Now lets try the same things in kernel space. Note: you can download the complete code here.

    If we are interested in the contents or the structure of the headers of different network layers, we can access these with the help of a packet sniffer. There are various packet sniffers available for Linux, like Wireshark. There is a command line sniffer called tcpdump, which is also a very good packet sniffer.

    And if we want to make our own packet sniffer, it can easily be done if we know the basics of C and networking. A packet sniffer with a raw socket To develop a packet sniffer, you first have to open a raw socket. So, during the execution of the program, you have to be the root user.

    We can also use the recv api. But recvfrom provides additional information. Extracting the Ethernet header Now that we have the network packets in our buffer, we will get information about the Ethernet header. The Ethernet header contains the physical address of the source and destination, or the MAC address and protocol of the receiving packet.

    Later, we will consider the next header as the IP header. Note 1: The physical address is 6 bytes. Note 2: We can also direct the output to a file for better understanding. Extracting the IP header The IP layer gives various pieces of information like the source and destination IP address, the transport layer protocol, etc.

    The structure of the IP header is defined in the ip. These structures provide the port number of the source and destination. With the help of the port number, the system gives data to a particular application see Figures 7 and 8. The size of the IP header varies from 20 bytes to 60 bytes.

    Datagrams seen in Wireshark, not received by Qt UDP Socket

    Note: If your machine is little endian, you have to use ntohs because the network uses the big endian scheme. Extracting data After the transport layer header, there is data payload remaining.

    Whenever one or more datagrams arrive, QUdpSocket emits the readyRead signal. Call QUdpSocket::readDatagram to read the datagram. QUdpSocket also supports multicasting.

    A Guide to Using Raw Sockets

    QHostInfo provides a static function that lets you perform such a lookup yourself. By calling QHostInfo::lookupHost with a host name, a QObject pointer, and a slot signature, QHostInfo will perform the name lookup and invoke the given slot when the results are ready. The actual lookup is done in a separate thread, making use of the operating system's own methods for performing name lookups. QHostInfo also provides a static function called QHostInfo::fromName that takes the host name as argument and returns the results.

    Qt Documentation

    In this case, the name lookup is performed in the same thread as the caller. Calling this function in a GUI thread may cause your user interface to freeze while the function blocks as it performs the lookup. Support for Network Proxies Network communication with Qt can be performed through proxies, which direct or filter network traffic between local and remote connections.

    Individual proxies are represented by the QNetworkProxy class, which is used to describe and configure the connection to a proxy. Proxying can be enabled on a per-socket basis or for all network communication in an application. A newly opened socket can be made to use a proxy by calling its QAbstractSocket::setProxy function before it is connected. Application-wide proxying can be enabled for all subsequent socket connections through the use of the QNetworkProxy::setApplicationProxy function.

    Proxy factories are used to create policies for proxy use. QNetworkProxyFactory supplies proxies based on queries for specific proxy types.

    An application-wide policy for proxying can be implemented by passing a factory to QNetworkProxyFactory::setApplicationProxyFactory and a custom proxying policy can be created by subclassing QNetworkProxyFactory ; see the class documentation for details. Documentation contributions included herein are the copyrights of their respective owners. Qt and respective logos are trademarks of The Qt Company Ltd.

    It is used for reporting errors and management queries. Example of Ubuntu Ping ping www. The packet has a TTL time-to-live value determining max number of router hops.

    If the packet does not reach, then the sender is noted back with the error. The gethostbyname function converts a normal human readable website and returns a structure of type hostent which contains IP address in form of binary dot notation and also address type.

    Some ping programs like the one given with ubuntu support reverse DNS lookup. Reverse DNS lookup is performed using getnameinfoand it converts dot notation IP address to hostname.


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