af_packet raw server client program with Epoll system call =========================================================== .. tab-set:: .. tab-item:: AF_PACKET RAW * 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 `_ * `send `_ * `recv `_ .. 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_raw_socket: .. tab-set:: .. tab-item:: AF_PACKET RAW 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_RAW, htons(ETH_P_ALL))`` do? .. dropdown:: See Answer This call creates a raw socket at the data link layer, allowing capturing of all Ethernet frames, including those not destined for the local host. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow How can I handle errors when creating a raw socket for packet capturing? .. 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 Are there specific privileges required to create a raw packet socket? .. dropdown:: See Answer Yes, typically, root or superuser privileges are required to create a raw packet socket for security reasons. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow What kind of errors can occur when using a raw packet socket? .. 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 raw packet socket? .. 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 raw packet sockets? .. dropdown:: See Answer ``EPERM`` (Operation not permitted) typically indicates insufficient privileges. Ensure the program has the necessary permissions to create raw packet sockets. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow How do I handle errors when using ``recv`` and ``send`` to capture packets and send packets with a raw socket? .. dropdown:: See Answer Check the return value of ``recv`` and ``send``. 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 raw 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 capture packets on a specific network interface with a raw packet socket? .. dropdown:: See Answer Yes, use the ``bind`` function to associate the raw 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 a common practice for handling timeouts in socket programming? .. dropdown:: See Answer Use non-blocking sockets with functions like epoll() or poll(). .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow Can you use a TCP socket (SOCK_STREAM) for sending and receiving data concurrently between a client and server? .. dropdown:: See Answer Yes, TCP sockets support bidirectional communication. .. 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_raw_socket_server_sequence_diagram: .. tab-set:: .. tab-item:: Step 1: Sequence Diagram for SERVER.c .. plantuml:: @startuml !theme spacelab start :socket(AF_PACKET, SOCK_RAW, 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) :recv(server_socket, buffer, BUFFER_SIZE, 0); :send(server_socket, buffer, BUFFER_SIZE, 0); else (no) endif endwhile (CTRL+c) :(void)close(server_socket); stop @enduml .. _epoll_af_packet_raw_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 * Recv data_packet * Send data_packet * Close socket * ``socket()`` is used to create a new socket. For example, .. code-block:: c server_socket = socket(AF_PACKET, SOCK_RAW, 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); * ``recv`` is used in network programming to receive data from a connected socket. For example, .. code-block:: c len = recv(server_socket, buffer, BUFFER_SIZE, 0); * ``send`` is used in network programming to send data over a connected socket. For example, .. code-block:: c ret = send(server_socket, buffer, BUFFER_SIZE, 0); * ``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_raw_htons_ETH_P_ALL/server/server.c :language: c :emphasize-lines: 53, 54, 55, 70, 71, 72, 85, 95, 96, 104, 105, 116, 117, 136, 137, 149 .. _epoll_af_packet_raw_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 RAW socket 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_raw_socket_client_sequence_diagram: .. tab-set:: .. tab-item:: Step 4: Sequence Diagram for CLIENT.c .. plantuml:: @startuml !theme spacelab start :socket(AF_PACKET, SOCK_RAW, 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) :send(client_socket, buffer, BUFFER_SIZE, 0); :epoll_wait(epoll_fd, events, MAX_EVENTS, -1); if (events[0].data.fd == client_socket) then (yes) :recv(client_socket, buffer, BUFFER_SIZE, 0); else (no) endif endwhile (CTRL+c) :(void)close(client_socket); stop @enduml .. _epoll_af_packet_raw_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 * Bind Socket * Epoll create1 * Epoll_ctl * Epoll_wait * Send data_packet * Recv data_packet * Close socket * ``socket`` is used to create a new socket. For example, .. code-block:: c client_socket = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL)); * ``bind()`` is used to associate the socket with a specific address and port. For example, .. code-block:: c ret = bind(client_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 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); * ``send`` is used in network programming to send data over a connected socket. For example, .. code-block:: c ret = send(client_socket, buffer, BUFFER_SIZE, 0); * ``recv`` is used in network programming to receive data from a connected socket. For example, .. code-block:: c len = recv(client_socket, buffer, BUFFER_SIZE, 0); * ``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_raw_htons_ETH_P_ALL/client/client.c :language: c :emphasize-lines: 52, 53, 54, 69, 70, 71, 79, 89, 90, 99, 100, 112, 113, 123, 124, 137 .. _epoll_af_packet_raw_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 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_RAW, 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_RAW, 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_raw_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.. recv Receive data from a connected socket. send Send data over a connected socket. ============== =========================================================================================== .. card:: See Also * Previous topic * :doc:`../../../sockets/raw_af_packet_raw_htons_ETH_P_ALL/poll/poll` * Current topic * :doc:`../../../sockets/raw_af_packet_raw_htons_ETH_P_ALL/epoll/epoll` * Next topic * :doc:`../../../sockets/raw_af_packet_udp_htons_ETH_P_ALL` * Other sockets * :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`