Linux套接字:为什么以太网MTU即使在通过802.11传输时也是一个限制?

在使用原始和 SOCK_DRAM UDP套接字,我注意到以太网最大传输单元总是表示我可以发送的数据的限制,而不会导致碎片,即使套接字严格绑定到无线接口(使用 bind()

我知道802.11MTU是2346B(正确吗?),比1500B大。但是,如果我尝试传输超过以太网MTU大小(1500 B)的数据,则在使用UDP套接字时会出现碎片和“消息太大”错误( errno 90 , EMSGSIZE )使用时 sendto() 在原始套接字上。

这是一个从原始代码中提取的示例代码,您可以使用它进行编译 gcc 并用于重现我之前描述的问题。

当使用原始套接字时,超过1500 B的消息被明确拒绝,设置 errno

代码如下:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <linux/wireless.h>
#include <sys/socket.h>
#include <linux/if_packet.h>
#include <net/ethernet.h>
#include <arpa/inet.h>
#include <string.h>
#include <sys/ioctl.h>

#define DEVNAME "wlp2s0"
#define DESTINATIONMAC_INITIALIZER {0x9C,0xD2,0x1E,0x20,0x91,0xE5}

#define SIZEOK 1500
#define SIZEWRONG 1501

int main (int argc, char **argv) {
    // wlanLookup() variables, for interface name, source MAC address and return value
    char devname[]=DEVNAME;
    int ifindex;

    int descriptor; // Socket descriptor
    struct sockaddr_ll addrll; // sockaddr_ll address structure

    struct ifreq wifireq;

    struct ether_header etherHeader;

    unsigned char bufferok[SIZEOK];
    unsigned char bufferwrong[SIZEWRONG];

    unsigned char *packetok=NULL;
    unsigned char *packetwrong=NULL;

    // Source and destination MAC addresses
    unsigned char macsrc[6];
    unsigned char macdst[6]=DESTINATIONMAC_INITIALIZER;

    // Size variables
    int sentbytes;
    size_t sizeok_final=sizeof(struct ether_header)+SIZEOK; // Size of the ok buffer + size of struct ether_header
    size_t sizewrong_final=sizeof(struct ether_header)+SIZEWRONG; // Size of the ok buffer + size of struct ether_header

    if(SIZEWRONG<SIZEOK) {
        fprintf(stderr,"Error: in this sample code, SIZEWRONG (%d) must be >= than SIZEOK (%d)\n",SIZEWRONG,SIZEOK);
        exit(EXIT_FAILURE);
    }

    descriptor=socket(AF_PACKET,SOCK_RAW,htons(ETH_P_ALL));

    if(descriptor==-1) {
        perror("socket() error");
        exit(EXIT_FAILURE);
    }

    // Get interface index of the interface
    strncpy(wifireq.ifr_name,devname,IFNAMSIZ);
    if(ioctl(descriptor,SIOCGIFINDEX,&wifireq)!=-1) {
        ifindex=wifireq.ifr_ifindex;
    } else {
        perror("ioctl() error");
        close(descriptor);
        exit(EXIT_FAILURE);
    }

    fprintf(stdout,"Using interface: %s (index: %x)\n",DEVNAME,ifindex);

    // Prepare sockaddr_ll structure
    memset(&addrll,0,sizeof(addrll));
    addrll.sll_ifindex=ifindex;
    addrll.sll_family=AF_PACKET;
    addrll.sll_protocol=htons(ETH_P_ALL);

    // Bind to the wireless interface
    if(bind(descriptor,(struct sockaddr *) &addrll,sizeof(addrll))<0) {
        perror("Cannot bind to interface: bind() error");
        close(descriptor);
        exit(EXIT_FAILURE);
    }

    fprintf(stdout,"Bound to interface: %s (index: %x)\n",DEVNAME,ifindex);

    // Populate both buffers with some data
    for(int i=0;i<SIZEWRONG;i++) {
        if(i<SIZEOK) {
            bufferok[i]=(unsigned char) (i & 15); // Fill each byte with a cyclic sequence 0x00, 0x01, 0x02, ... 0x0F, 0x00, 0x01, ...
        }
        bufferwrong[i]=(unsigned char) (i & 15);
    }

    // Get source MAC address
    strncpy(wifireq.ifr_name,devname,IFNAMSIZ); 
    if(ioctl(descriptor,SIOCGIFHWADDR,&wifireq)!=-1) {
        memcpy(macsrc,wifireq.ifr_hwaddr.sa_data,6);
    } else {
        perror("Cannot get source MAC address: ioctl() error");
        close(descriptor);
    }
    fprintf(stdout,"Source MAC address: %02x:%02x:%02x:%02x:%02x:%02x\n",macsrc[0],macsrc[1],macsrc[2],macsrc[3],macsrc[4],macsrc[5]);
    fprintf(stdout,"Destination MAC address: %02x:%02x:%02x:%02x:%02x:%02x\n",macdst[0],macdst[1],macdst[2],macdst[3],macdst[4],macdst[5]);

    // Fill struct ether_header
    memcpy(etherHeader.ether_dhost,macdst,ETHER_ADDR_LEN);
    memcpy(etherHeader.ether_shost,macsrc,ETHER_ADDR_LEN);
    // Using local experimental ethertype for the sake of this sample code, but the effect is the same for any other EtherType
    etherHeader.ether_type=htons(0x88B5);

    packetok=malloc(sizeok_final*sizeof(unsigned char));
    if(!packetok) {
        perror("malloc() error");
        close(descriptor);
        exit(EXIT_FAILURE);
    }
    packetwrong=malloc(sizewrong_final*sizeof(unsigned char));

    if(!packetwrong) {
        perror("malloc() error");
        free(packetok);
        close(descriptor);
        exit(EXIT_FAILURE);
    }

    // Generate the complete packet buffers
    // Packet OK
    memcpy(packetok,&etherHeader,sizeof(struct ether_header));
    memcpy(packetok+sizeof(struct ether_header),bufferok,sizeok_final);

    // Packet WRONG
    memcpy(packetwrong,&etherHeader,sizeof(struct ether_header));
    memcpy(packetwrong+sizeof(struct ether_header),bufferwrong,sizewrong_final);

    sentbytes=sendto(descriptor,packetok,sizeok_final,0,(struct sockaddr *)&addrll,sizeof(struct sockaddr_ll));

    perror("Packet OK errors (if any)");
    fprintf(stdout,"Packet OK: sent %d bytes.\n",sentbytes);

    sentbytes=sendto(descriptor,packetwrong,sizewrong_final,0,(struct sockaddr *)&addrll,sizeof(struct sockaddr_ll));

    perror("Packet WRONG errors (if any)");
    fprintf(stdout,"Packet WRONG: sent %d bytes.\n",sentbytes);

    close(descriptor);

    return 0;
}

在编译之前,您应该设置 DEVNAME 示例程序应绑定到的接口名称,以及 DESTINATIONMAC_INITIALIZER

我实际上得到了这个输出(在用 sudo ),显示超过1500 B的数据包如何被拒绝:

Using interface: wlp2s0 (index: 3)
Bound to interface: wlp2s0 (index: 3)
Source MAC address: 00:16:ea:4a:bd:7e
Destination MAC address: 9c:d2:1e:20:91:e5
Packet OK errors (if any): Success
Packet OK: sent 1514 bytes.
Packet WRONG errors (if any): Message too long
Packet WRONG: sent -1 bytes.

当启动示例程序三次时,“ok”数据包被目标设备正确接收,而所有“错误”数据包则没有: