建立tap设备的c的代码

tapper.c

#include <stdio.h>
#include <unistd.h>
#include <sys/socket.h>
#include <linux/if.h>
#include <linux/if_tun.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/select.h>
#include <err.h>
#include <arpa/inet.h> /* inet_addr() & co. */
#include <netinet/in.h> /* INET_ADDRSTRLEN */
/* for uint16_t */
#include <stdint.h>
#define ERR_CANT_TAP 1
#define ERR_OPEN_ERR 2
#define ERR_READ_ERR 3
#define ERR_WRITE_ERR 4
static int primary_fd; /* a copy of the filedescriptor for tap device */
static int enable_tuntap_headers = 0; /* CLI option - will we permit tuntap protocol headers */
static int enable_tapper_headers = 0; /* CLI option - will we enable tapper's headers */
static int enable_verbose = 0; /* CLI option - print out extra debug info on stderr */
//static const char DEFAULT_IP[] = "10.0.2.2";
static const char DEFAULT_IP[] = "10.0.2.1";
static const char DEFAULT_NETMASK[] = "255.255.0.0";
#define VERBOSE(x, ...) if(enable_verbose >= 1) { warnx("NOTE: " x, ##__VA_ARGS__); }
#define DEBUG(x, ...) if(enable_verbose >= 2) { warnx("DEBUG: " x, ##__VA_ARGS__); }
/* following function creates a tap device */
/* it also returns device name in 'newdev' */
int createTap(char *newdev, int bufferlen)
{
    struct ifreq ifr;
    int fd=open("/dev/net/tun", O_RDWR); /* open the tap device */
    if(fd<0) /* handle error */
        err(ERR_CANT_TAP, "Could not create a TAP device");
    memset(&ifr, 0, sizeof(ifr)); /* initialize the structure specifying data about net device */
    ifr.ifr_flags=IFF_TAP; /* we want a tap device, not a tun device */
    if(!enable_tuntap_headers)
        /* by default we won't include tuntap driver's headers */
        ifr.ifr_flags|=IFF_NO_PI;
    if((ioctl(fd, TUNSETIFF, (void*)&ifr))<0) /* tell our device what we want it to do */
    {
        close(fd); /* if we failed close and abort */
        err(ERR_CANT_TAP, "Could not create a TAP device because of ioctl()");
    }
    strncpy(newdev, ifr.ifr_name, bufferlen-1); /* return the generated device name */
    newdev[bufferlen-1] = '\0';
    return fd;
}
/* when we exit we want to clean up */
void atex(void)
{
    close(primary_fd); /* close the tap network device */
}
/* usage */
void usage(void)
{
    printf("usage:\n"
                   "\n"
                   "  ./tapper [--tuntap-headers] [--tapper-headers]"
                   "           [--ip-address %s] [--netmask %s] [--randomize-ip]"
                   "           [-v] [stdinfile stdoutfile]\n"
                   "\n",
           DEFAULT_IP, DEFAULT_NETMASK);
    exit(0);
}
int main(int argc, char ** argv)
{
#define BUFFERSIZE 2000 /* must be larger than MTU - 1500 */
#define DEVNAMESIZE 25
    int fd; /* file descriptor of the tap device; shorthand for primary_fd */
    char devname[DEVNAMESIZE]={0}; /* initialize device name (where we'll pick up the generated dev name)
                to zeros so it doesn't look like we're sending a device name in*/
    char buf[BUFFERSIZE]; /* buffer for receiving stuff */
    in_addr_t ip = inet_addr(DEFAULT_IP);
    in_addr_t netmask = inet_addr(DEFAULT_NETMASK);
    char ip_s[INET_ADDRSTRLEN];
    char netmask_s[INET_ADDRSTRLEN];
    int argoff; /* argument offset; used for parsing CLI */
    for(argoff = 1; argoff < argc; argoff++)
    {
        if(!strcmp(argv[argoff], "-h"))
            usage();
        else if(!strcmp(argv[argoff], "--tuntap-headers"))
            enable_tuntap_headers = 1;
        else if(!strcmp(argv[argoff], "--tapper-headers"))
            enable_tapper_headers = 1;
        else if(!strcmp(argv[argoff], "-v"))
            enable_verbose++;
        else if(!strcmp(argv[argoff], "--ip-address")){
            if(argoff == argc-1)
                usage();
            ip = inet_addr(argv[++argoff]);
            if(ip == INADDR_NONE)
                errx(12, "%s: malformed ip address", argv[argoff]);
            DEBUG("ip address: %s", inet_ntoa(*(struct in_addr*)&ip));
        }else if(!strcmp(argv[argoff], "--netmask")){
            if(argoff == argc-1)
                usage();
            netmask = inet_addr(argv[++argoff]);
            if(netmask == INADDR_NONE)
                errx(13, "%s: malformed netmask address", argv[argoff]);
            DEBUG("netmask: %s", inet_ntoa(*(struct in_addr*)&netmask));
        }else if(!strcmp(argv[argoff], "--randomize-ip")){
            srand(time(NULL)+devname[3]); /* we want our ip address to depend on time, and on last
                             symbol in device name. dev name is always a three-letter 'tap'
                             + number, and let's just presume it's a single digit num */
            uint32_t *ip_int = (uint32_t*)&ip;
            uint32_t *netmask_int = (uint32_t*)&netmask;
            uint32_t local = (uint32_t)rand() & ~*netmask_int;
            if(local == 0)
                local = 1;
            *ip_int |= local;
            DEBUG("randomized ip address: %s", inet_ntoa(*(struct in_addr*)&ip));
        }
        else
            break;
    }
    VERBOSE("verbosity: %d", enable_verbose);
    if(enable_tuntap_headers) VERBOSE("permitting tuntap headers");
    if(enable_tapper_headers) VERBOSE("using tapper headers");
    if(argc - argoff >= 2)
    {
        /* we can receive two arguments:
           - file we'll use for reading in place of stdin
           - file we'll use for writing in place of stdout */
        close(0);
        close(1);
        VERBOSE("using %s for input and %s for output", argv[argoff], argv[argoff+1]);
        if(!fopen(argv[argoff], "r"))
            err(10, "fopen(%s, r)", argv[argoff]);
        if(!fopen(argv[argoff+1], "w"))
            err(11, "fopen(%s, w)", argv[argoff+1]);
    }
    /* get ip address and netmask as strings */
    inet_ntop(AF_INET, &ip, ip_s, INET_ADDRSTRLEN);
    inet_ntop(AF_INET, &netmask, netmask_s, INET_ADDRSTRLEN);
    ip_s[INET_ADDRSTRLEN-1] = '\0';
    netmask_s[INET_ADDRSTRLEN-1] = '\0';
    /* let's create a tap device */
    primary_fd=createTap(devname, DEVNAMESIZE);
    /* configure ip address and netmask */
    snprintf(buf, sizeof(buf), "ifconfig %s inet %s netmask %s up", devname,
             ip_s, netmask_s);
    VERBOSE("configuring ip and netmask: %s", buf);
    system(buf);
    fd=primary_fd; /* store primary_fd into a shorthand */
    if (fd<0) /* error with creating tap? tough luck, let's bail out */
        err(ERR_OPEN_ERR, "open()");
    atexit(atex); /* when the loop is aborted, cleanup */
    while(1){
        int readies, i;
        /*
        since we're trying to create a twoway tunnel between stdio and the tap device
        we need to do monitoring via select(). we simply don't know which one will
        send us data first.
        */
        fd_set fds;
        FD_ZERO(&fds); /* init set of file descriptors */
        FD_SET(fd,&fds); /* add tap device to set */
        FD_SET(0,&fds); /* add stdin to set */

        readies=select(fd+1, &fds, NULL, NULL, NULL); /* monitor the set. the first param is
                            max fd we monitor +1 (as usual with select()).
                            here that's fd. */
        if(readies<=0) err(readies, "Not ready"); /* we passed a timeoutless select() with 0
                            active fds? ouch. */
        for(i=0;i<2;i++){
            /* some arcane magic to make the loop simple. i was lazy to cut paste code.
            basically first the fd_int is stdin (0) and fd_oth is our tap device (fd).
            then the fd_int is tap device (fd) and fd_oth is the stdout (1). */
            int fd_int=i*fd;
            int fd_oth=abs(fd-i*fd);
            if(!fd_oth) fd_oth=1;

            /* is the currently monitored fd_int (first stdin, then tap) actually
               the one causing select() to unblock? */
            if(FD_ISSET(fd_int, &fds)){
                if(!enable_tapper_headers || fd_int != 0){
                    /* yay! that one has something to say. let's read as much as
                       possible for our buffer. num of actually read bytes is
                       stored in ret, unless it's -1 which is error */
                    ssize_t ret = read(fd_int, buf, BUFFERSIZE);
                    if (!ret)
                    {
                        errx(100, "read(): nothing to read expecting data");
                        continue;
                    }
                    if (ret < 0) err(ERR_READ_ERR, "read(%d) for data", fd_int);
                    /* using headers? then the fd_oth is the stdout. first,
                       send the number of bytes we'll dispatch */
                    if(enable_tapper_headers){
                        uint16_t size = (uint16_t)htons(ret);

                        /* copying because we want to do one write, not two */
                        char * with_headers = malloc(sizeof(size) + ret);
                        memcpy(with_headers, &size, sizeof(size));
                        memcpy(with_headers + sizeof(size), buf, ret);
                        if(write(fd_oth, with_headers, sizeof(size) + ret)<0)
                            err(ERR_WRITE_ERR, "write(%d)", fd_oth);
                        DEBUG("wrote %lu bytes", sizeof(size) + ret);
                        free(with_headers);
                    }else{
                        /* write ret bytes into fd_oth; that's all the bytes we read */
                        if(write(fd_oth, buf, ret)<0)
                            err(ERR_WRITE_ERR, "write(%d)", fd_oth);
                    }
                }else{
                    /* new codepath: buffer stdin until filled with enough data */
                    /* only executed for stdin, and if tapper headers are enabled */
                    static uint16_t expected_size = 0;
                    static size_t current_buffer_content_size = 0;
                    if(!expected_size){
                        ssize_t ret = read(fd_int, &expected_size, sizeof(uint16_t));
                        if(!ret)
                        {
                            errx(101, "read(): nothing to read expecting message size");
                            continue;
                        }
                        if (ret < 0) err(ERR_READ_ERR, "read(%d) for message size", fd_int);

                        expected_size = ntohs(expected_size);
                        DEBUG("now expecting %d bytes", expected_size);
                    }else{
                        size_t bytes_left = expected_size - current_buffer_content_size;
                        ssize_t ret = read(fd_int, buf + current_buffer_content_size, bytes_left);
                        if(!ret){
                            errx(102, "read(): nothing to read expecting buffer content");
                            continue;
                        }
                        if (ret < 0) err(ERR_READ_ERR, "read(%d) for buffer content", fd_int);
                        current_buffer_content_size += ret;
                        DEBUG("received %lu bytes; buffer now %lu/%hu", ret, current_buffer_content_size, expected_size);
                        if(current_buffer_content_size == expected_size){
                            DEBUG("got all content");
                            /* write ret bytes into fd_oth; that's all the bytes we read */
                            if(write(fd_oth, buf, ret)<0)
                                err(ERR_WRITE_ERR, "write(%d)", fd_oth);
                            current_buffer_content_size = 0;
                            expected_size = 0;
                        }
                    }
                }
            }
        }
    }
    /* never happens */
    return 0;
}