af_packet udp server client program with Epoll system call =========================================================== .. tab-set:: .. tab-item:: AF_PACKET UDP * In this program, you are going to learn .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow * How to create a Socket ? .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow * How to bind a socket ? .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow * How to send a data ? .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow * How to recv a data ? .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow * How to use socket APIs ? * `socket `_ * `bind `_ * `epoll_create1 `_ * `epoll_ctl `_ * `epoll_wait `_ * `sendto `_ * `recvfrom `_ .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow Topics in this section, * :ref:`AF_PACKET RAW SOCKET FAQs ` * :ref:`Step 1: Sequence Diagram for SERVER.c ` * :ref:`Step 2: Program for Server.c ` * :ref:`Step 3: Compile and Execute Server.c ` * :ref:`Step 4: Sequence Diagram for CLIENT.c ` * :ref:`Step 5: Program for Client.c ` * :ref:`Step 6: Compile and Execute Client.c ` * :ref:`Summary ` .. _epoll_af_packet_udp_socket: .. tab-set:: .. tab-item:: AF_PACKET UDP SOCKET : FAQs .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow Let us answer few basic questions in this socket .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow What does ``socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_ALL))`` do? .. dropdown:: See Answer This call creates a packet socket that allows receiving all Ethernet frames, regardless of destination address, using the ``recvfrom`` function. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow How can I handle errors when creating a packet socket for receiving all data? .. dropdown:: See Answer Check the return value of the ``socket`` function. If it returns -1, use ``perror`` to print a descriptive error message. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow What kind of errors can occur when using a packet socket for receiving data? .. dropdown:: See Answer Common errors include permission-related errors ``(EACCES)``, socket creation failures ``(ENOMEM)``, or invalid arguments ``(EINVAL)``. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow How do I handle errors when binding a packet socket for receiving data? .. dropdown:: See Answer Check the return value of the ``bind`` function. If it returns -1, handle the error by printing a message or taking appropriate corrective action based on the error code. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow What is the significance of error code ``EPERM`` when dealing with packet sockets? .. dropdown:: See Answer ``EPERM`` (Operation not permitted) typically indicates insufficient privileges. Ensure the program has the necessary permissions to create packet sockets. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow How do I handle errors when using ``recvfrom`` to receive data with a packet socket? .. dropdown:: See Answer Check the return value of ``recvfrom``. If it returns -1, handle the error by printing a message or taking appropriate corrective action. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow Is it important to close the packet socket on error? .. dropdown:: See Answer Yes, closing the socket is crucial to release system resources. Always follow error-handling best practices and close sockets on error. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow Is it possible to receive packets only from a specific network interface with a packet socket? .. dropdown:: See Answer Yes, use the ``bind`` function to associate the packet socket with a specific network interface. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow How do I handle errors related to socket file descriptor management? .. dropdown:: See Answer When closing sockets, check for errors using ``close``. Handle errors by printing messages or taking corrective actions. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow What is the primary purpose of the epoll system call? .. dropdown:: See Answer To efficiently monitor multiple file descriptors for I/O events .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow What types of file descriptors can be monitored using epoll? .. dropdown:: See Answer sockets, files, timerfd, socketpair, message_queue, Namedpipes and shared_memory. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow What data structure is used by epoll to store events? .. dropdown:: See Answer Hash table .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow How do you handle errors when using the epoll system call? .. dropdown:: See Answer Check the return value for -1 to detect errors, Use perror to print error messages. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow How does epoll handle a set of file descriptors with different states (e.g., reading, writing, exception)? .. dropdown:: See Answer Create the epoll Instance: Before monitoring file descriptors, the application creates an epoll instance using the epoll_create system call. .. code-block:: c int epoll_fd = epoll_create1(0); Register File Discriptors: The application registers file descriptors with the epoll instance using the epoll_ctl system call. It specifies the file descriptor, the events it is interested in (EPOLLIN for readability, EPOLLOUT for writability, etc.), and a user-defined data associated with the file descriptor. .. code-block:: c struct epoll_event event; event.events = EPOLLIN | EPOLLOUT; // Interested in readability and writability event.data.fd = my_file_descriptor; // File descriptor to monitor epoll_ctl(epoll_fd, EPOLL_CTL_ADD, my_file_descriptor, &event); Wait for Events: The application enters a loop where it calls epoll_wait to wait for events. This call blocks until one or more registered file descriptors become ready or until a timeout occurs. .. code-block:: c #define MAX_EVENTS 10 struct epoll_event events[MAX_EVENTS]; int num_events = epoll_wait(epoll_fd, events, MAX_EVENTS, timeout_ms); Modify or Remove File Descriptors: The application can dynamically modify or remove file descriptors from the epoll set using the epoll_ctl system call. For example, to modify events for an existing file descriptor: .. code-block:: c struct epoll_event new_event; new_event.events = EPOLLOUT; // Modify to be interested in writability epoll_ctl(epoll_fd, EPOLL_CTL_MOD, my_file_descriptor, &new_event); To remove a file descriptor from the epoll set: .. code-block:: c epoll_ctl(epoll_fd, EPOLL_CTL_DEL, my_file_descriptor, NULL); .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow How does epoll Checking Ready File Descriptors? .. dropdown:: See Answer After epoll_wait returns, the application iterates through the returned events to identify which file descriptors are ready and for what types of events. .. code-block:: c for (int i = 0; i < num_events; ++i) { if (events[i].events & EPOLLIN) { // File descriptor i is ready for reading } if (events[i].events & EPOLLOUT) { // File descriptor i is ready for writing } // Check other events if needed (e.g., EPOLLERR, EPOLLHUP) } .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow What does it mean if epoll returns 0? .. dropdown:: See Answer No file descriptors are ready within the specified timeout. .. _epoll_af_packet_udp_socket_server_sequence_diagram: .. tab-set:: .. tab-item:: Step 1: Sequence Diagram for SERVER.c .. plantuml:: @startuml !theme spacelab start :socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_ALL)); :bind(server_socket, (struct sockaddr*)&server_addr, sizeof(server_addr)); :epoll_fd = epoll_create1(0); :epoll_ctl(epoll_fd, EPOLL_CTL_ADD, server_socket, &event); while (while(1)) is (yes) :epoll_wait(epoll_fd, events, MAX_EVENTS, -1); if (events[0].data.fd == server_socket) then (yes) :recvfrom(server_socket, buffer, BUFFER_SIZE, 0, (struct sockaddr*)&client_addr, &client_addr_len); :sendto(server_socket, buffer, strlen(buffer), 0, (struct sockaddr*)&client_addr, client_addr_len); else (no) endif endwhile (CTRL+c) :(void)close(server_socket); stop @enduml .. _epoll_af_packet_udp_socket_server_code: .. tab-set:: .. tab-item:: Step 2: Program for Server.c * There are many functions used in socket. We can classify those functions based on functionalities. * Create Socket * Bind Socket * Epoll create1 * Epoll_ctl * Epoll_wait * Recvfrom data_packet * Sendto data_packet * Close socket * ``socket()`` is used to create a new socket. For example, .. code-block:: c server_socket = socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_ALL)); * ``bind()`` is used to associate the socket with a specific address and port. For example, .. code-block:: c ret = bind(server_socket, (struct sockaddr*)&server_addr, sizeof(server_addr)); * ``epoll_create1()`` creating an epoll instance using epoll_create1, The size parameter is an advisory hint for the kernel regarding the number of file descriptors expected to be monitored, For example, .. code-block:: c epoll_fd = epoll_create1(0)); * ``epoll_ctl()`` After creating an epoll instance, file descriptors are added to it using epoll_ctl, For example, .. code-block:: c ret = epoll_ctl(epoll_fd, EPOLL_CTL_ADD, server_socket, &event); * ``epoll_wait()`` The application then enters a loop where it waits for events using epoll_wait, For example, .. code-block:: c ready_fds = epoll_wait(epoll_fd, events, MAX_EVENTS, -1); * ``recvfrom`` is commonly used with UDP sockets, where communication is connectionless. it provides information about the source (sender) of the data, including the sender's IP address and port number. For example, .. code-block:: c ret = recvfrom(server_socket, buffer, BUFFER_SIZE, 0, (struct sockaddr*)&client_addr, &client_addr_len); * ``sendto`` is used to send the encoded message to the specified server address and port using a UDP socket. For example, .. code-block:: c ret = sendto(server_socket, buffer, strlen(buffer), 0, (struct sockaddr*)&client_addr, client_addr_len); * ``close`` is used to close the socket To free up system resources associated with the socket. For example, .. code-block:: c (void)close(server_socket); * See the full program below, .. literalinclude:: raw_af_packet_udp_htons_ETH_P_ALL/server/server.c :language: c :emphasize-lines: 53, 54, 55, 70, 71, 72, 85, 95, 96, 104, 105, 116, 117, 118, 119, 133, 134, 135, 136, 149 .. _epoll_af_packet_udp_socket_server_side_compile_and_execute: .. tab-set:: .. tab-item:: Step 3: Compile and Execute Server.c .. code-block:: c :linenos: :emphasize-lines: 1, 3 $ gcc -o server server.c $ sudo ./server UDP is listening Received: hello server! sending message = HELLO Received: hello server! sending message = HELLO Received: HELLO sending message = HELLO Received: hello server! sending message = HELLO Received: hello server! sending message = HELLO Received: HELLO sending message = HELLO Received: hello server! ^CCaught sigINT! .. _epoll_af_packet_udp_socket_client_sequence_diagram: .. tab-set:: .. tab-item:: Step 4: Sequence Diagram for CLIENT.c .. plantuml:: @startuml !theme spacelab start :socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_ALL)); :bind(client_socket, (struct sockaddr*)&server_addr, sizeof(server_addr)); :epoll_fd = epoll_create1(0); :epoll_ctl(epoll_fd, EPOLL_CTL_ADD, client_socket, &event); while (while(1)) is (yes) :sendto(client_socket, buffer, strlen(buffer), 0, (struct sockaddr*)&server_addr, sizeof(server_addr)); :epoll_wait(epoll_fd, events, MAX_EVENTS, -1); if (events[0].data.fd == client_socket) then (yes) :recvfrom(client_socket, buffer, BUFFER_SIZE, 0, NULL, NULL); else (no) endif endwhile (CTRL+c) :(void)close(client_socket); stop @enduml .. _epoll_af_packet_udp_socket_client_code: .. tab-set:: .. tab-item:: Step 5: Program for Client.c * There are many functions used in socket. We can classify those functions based on functionalities. * Create Socket * Epoll create1 * Epoll_ctl * Epoll_wait * Sendto data_packet * Recvfrom data_packet * close socket * ``socket`` is used to create a new socket. For example, .. code-block:: c client_socket = socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_ALL)); * ``epoll_create1()`` creating an epoll instance using epoll_create1, The size parameter is an advisory hint for the kernel regarding the number of file descriptors expected to be monitored, For example, .. code-block:: c epoll_fd = epoll_create1(0)); * ``epoll_ctl()`` After creating an epoll instance, file descriptors are added to it using epoll_ctl, For example, .. code-block:: c ready_fds = epoll_ctl(epoll_fd, EPOLL_CTL_ADD, client_socket, &event); * ``epoll_wait()`` The application then enters a loop where it waits for events using epoll_wait, For example, .. code-block:: c ret = epoll_wait(epoll_fd, events, MAX_EVENTS, -1); * ``sendto`` is used to send the encoded message to the specified server address and port using a UDP socket. For example, .. code-block:: c ret = sendto(client_socket, buffer, strlen(buffer), 0, (struct sockaddr*)&server_addr, sizeof(server_addr)); * ``recvfrom`` is commonly used with UDP sockets, where communication is connectionless. it provides information about the source (sender) of the data, including the sender's IP address and port number. For example, .. code-block:: c ret = recvfrom(client_socket, buffer, BUFFER_SIZE, 0, NULL, NULL); * ``close`` is used to close the socket To free up system resources associated with the socket. For example, .. code-block:: c (void)close(client_socket); * See the full program below, .. literalinclude:: raw_af_packet_udp_htons_ETH_P_ALL/client/client.c :language: c :emphasize-lines: 52, 53, 54, 69, 79, 80, 89, 90, 91, 92, 102, 103, 112, 113, 126 .. _epoll_af_packet_udp_socket_client_side_compile_and_execute: .. tab-set:: .. tab-item:: Step 6: Compile and Execute Client.c .. code-block:: c :linenos: :emphasize-lines: 1, 3 $ gcc -o client client.c $ sudo ./client Sentbuffer : hello server! Received : hello server! Sentbuffer : hello server! Received : HELLO Sentbuffer : hello server! Received : HELLO Sentbuffer : hello server! Received : HELLO Sentbuffer : hello server! Received : HELLO Sentbuffer : hello server! Received : HELLO Sentbuffer : hello server! Received : HELLO Sentbuffer : hello server! Received : HELLO ^CCaught sigINT! .. tab-set:: .. tab-item:: Enhanced Socket Flexibility with ``AF_PACKET`` and ``PF_PACKET`` Domains .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **Default Domain:** By default, the socket is configured to work in the ``AF_PACKET`` domain, handling all types of network data. **Additional Domain Support:** We expand the socket's capabilities to also function in the ``PF_PACKET`` domain, allowing it to operate similarly to ``AF_PACKET``. **Socket Creation:** We set up a network connection point known as a socket using ``socket(PF_PACKET, SOCK_DGRAM, IPPROTO_UDP)``. **Working Scenario:** Despite the change in domain to ``PF_PACKET``, the socket continues to operate the same way, handling general network data. .. tab-set:: .. tab-item:: Enhanced Protocol Flexibility in Socket Configuration .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **Default Protocol Support:** By default, the socket is configured to support the capture of all Ethernet frames ``(ETH_P_ALL protocol)``. **Additional Protocol:** The socket is designed to seamlessly support an additional protocol, namely ``ETH_P_PAE``. **Socket Creation:** A socket is created using the ``socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_PAE))`` call. **Working Scenario:** Despite the change in protocol to ``ETH_P_PAE``, the overall working scenario of the socket remains consistent. .. _epoll_af_packet_udp_Summary: .. tab-set:: .. tab-item:: Summary ============== ================================================================================================================== Socket API Learning ============== ================================================================================================================== socket Create a new socket bind Associate the socket with a specific address and port epoll handles a set of file descriptors with different states, such as reading, writing, and exceptions, by using the struct epoll_event structure and the associated event flags.. recvfrom It provides information about the source (sender) of the data, including the sender's IP address and port number. sendto Send the encoded message to the specified server address and port using a UDP socket. ============== ================================================================================================================== .. card:: See Also * Previous topic * :doc:`../../../sockets/raw_af_packet_udp_htons_ETH_P_ALL/poll/poll` * Current topic * :doc:`../../../sockets/raw_af_packet_udp_htons_ETH_P_ALL/epoll/epoll` * Next topic * :doc:`../../../sockets/raw_af_packet_tcp_htons_ETH_P_ALL` * Other IPCs * :doc:`../../../Message_queues/Message_queues` * :doc:`../../../NamedPipes/NamedPipes` * :doc:`../../../Netlink/Netlink` * :doc:`../../../Shared_Memory/Shared_Memory` * :doc:`../../../Shared_Memory_2_FDS/Shared_Memory_2_FDS` * :doc:`../../../SocketPair/SocketPair` * :doc:`../../../Timerfd/Timerfd`