我的代码
https://github.com/liuqun/demo-linux-tun-virtual-netcard/blob/master/demo-1-ioctl/src/main.c

代码片段1

以下代码节选自"openV屁N"的源码tun.c文件:

openV屁N封装了一组读写函数如下:

• open_tun(dev_name, dev_type, dev_node, tuntap上下文)打开Tun虚拟网卡
• close_tun(tuntap上下文)
• n_bytes = write_tun(tuntap上下文, buf, len)
• n_bytes = read_tun(tuntap上下文, buf, len)

int write_tun(struct tuntap *tt, uint8_t *buf, int len)
{return write(tt->fd, buf, len);
}

int read_tun(struct tuntap *tt, uint8_t *buf, int len)
{return read(tt->fd, buf, len);
}

void open_tun(const char *dev, const char *dev_type, const char *dev_node, struct tuntap *tt)
{struct ifreq ifr;/** We handle --dev null specially, we do not open /dev/null for this.*/if (tt->type == DEV_TYPE_NULL){open_null(tt);}else{/** Process --dev-node*/const char *node = dev_node;if (!node){node = "/dev/net/tun";}/** Open the interface*/if ((tt->fd = open(node, O_RDWR)) < 0){msg(M_ERR, "ERROR: Cannot open TUN/TAP dev %s", node);}/** Process --tun-ipv6*/CLEAR(ifr);ifr.ifr_flags = IFF_NO_PI;#if defined(IFF_ONE_QUEUE) && defined(SIOCSIFTXQLEN)ifr.ifr_flags |= IFF_ONE_QUEUE;
#endif/** Figure out if tun or tap device*/if (tt->type == DEV_TYPE_TUN){ifr.ifr_flags |= IFF_TUN;}else if (tt->type == DEV_TYPE_TAP){ifr.ifr_flags |= IFF_TAP;}else{msg(M_FATAL, "I don't recognize device %s as a tun or tap device",dev);}/** Set an explicit name, if --dev is not tun or tap*/if (strcmp(dev, "tun") && strcmp(dev, "tap")){strncpynt(ifr.ifr_name, dev, IFNAMSIZ);}/** Use special ioctl that configures tun/tap device with the parms* we set in ifr*/if (ioctl(tt->fd, TUNSETIFF, (void *) &ifr) < 0){msg(M_ERR, "ERROR: Cannot ioctl TUNSETIFF %s", dev);}msg(M_INFO, "TUN/TAP device %s opened", ifr.ifr_name);/** Try making the TX send queue bigger*/
#if defined(IFF_ONE_QUEUE) && defined(SIOCSIFTXQLEN)if (tt->options.txqueuelen){struct ifreq netifr;int ctl_fd;if ((ctl_fd = socket(AF_INET, SOCK_DGRAM, 0)) >= 0){CLEAR(netifr);strncpynt(netifr.ifr_name, ifr.ifr_name, IFNAMSIZ);netifr.ifr_qlen = tt->options.txqueuelen;if (ioctl(ctl_fd, SIOCSIFTXQLEN, (void *) &netifr) >= 0){msg(D_OSBUF, "TUN/TAP TX queue length set to %d", tt->options.txqueuelen);}else{msg(M_WARN | M_ERRNO, "Note: Cannot set tx queue length on %s", ifr.ifr_name);}close(ctl_fd);}else{msg(M_WARN | M_ERRNO, "Note: Cannot open control socket on %s", ifr.ifr_name);}}
#endif /* if defined(IFF_ONE_QUEUE) && defined(SIOCSIFTXQLEN) */set_nonblock(tt->fd);set_cloexec(tt->fd);tt->actual_name = string_alloc(ifr.ifr_name, NULL);}return;
}

void close_tun(struct tuntap *tt)
{if (tt){if (tt->type != DEV_TYPE_NULL && tt->did_ifconfig){struct argv argv = argv_new();struct gc_arena gc = gc_new();#ifdef ENABLE_IPROUTEif (is_tun_p2p(tt)){argv_printf(&argv,"%s addr del dev %s local %s peer %s",iproute_path,tt->actual_name,print_in_addr_t(tt->local, 0, &gc),print_in_addr_t(tt->remote_netmask, 0, &gc));}else{argv_printf(&argv,"%s addr del dev %s %s/%d",iproute_path,tt->actual_name,print_in_addr_t(tt->local, 0, &gc),netmask_to_netbits2(tt->remote_netmask));}
#else  /* ifdef ENABLE_IPROUTE */argv_printf(&argv,"%s %s 0.0.0.0",IFCONFIG_PATH,tt->actual_name);
#endif /* ifdef ENABLE_IPROUTE */argv_msg(M_INFO, &argv);openvpn_execve_check(&argv, NULL, 0, "Linux ip addr del failed");if (tt->did_ifconfig_ipv6_setup){const char *ifconfig_ipv6_local = print_in6_addr(tt->local_ipv6, 0, &gc);#ifdef ENABLE_IPROUTEargv_printf(&argv, "%s -6 addr del %s/%d dev %s",iproute_path,ifconfig_ipv6_local,tt->netbits_ipv6,tt->actual_name);argv_msg(M_INFO, &argv);openvpn_execve_check(&argv, NULL, 0, "Linux ip -6 addr del failed");
#else  /* ifdef ENABLE_IPROUTE */argv_printf(&argv,"%s %s del %s/%d",IFCONFIG_PATH,tt->actual_name,ifconfig_ipv6_local,tt->netbits_ipv6);argv_msg(M_INFO, &argv);openvpn_execve_check(&argv, NULL, 0, "Linux ifconfig inet6 del failed");
#endif}argv_reset(&argv);gc_free(&gc);}close_tun_generic(tt);free(tt);}
}

#define TUNNEL_TYPE(tt) ((tt) ? ((tt)->type) : DEV_TYPE_UNDEF)
#define TUNNEL_TOPOLOGY(tt) ((tt) ? ((tt)->topology) : TOP_UNDEF)/** Define a TUN/TAP dev.*/
struct tuntap
{int type; /* DEV_TYPE_x as defined in proto.h */int topology; /* one of the TOP_x values */bool did_ifconfig_setup;bool did_ifconfig_ipv6_setup;bool did_ifconfig;bool persistent_if;         /* if existed before, keep on program end */struct tuntap_options options; /* options set on command line */char *actual_name; /* actual name of TUN/TAP dev, usually including unit number *//* number of TX buffers */int txqueuelen;/* ifconfig parameters */in_addr_t local;in_addr_t remote_netmask;in_addr_t broadcast;struct in6_addr local_ipv6;struct in6_addr remote_ipv6;int netbits_ipv6;#ifdef _WIN32HANDLE hand;struct overlapped_io reads;struct overlapped_io writes;struct rw_handle rw_handle;/* used for setting interface address via IP Helper API* or DHCP masquerade */bool ipapi_context_defined;ULONG ipapi_context;ULONG ipapi_instance;in_addr_t adapter_netmask;/* Windows adapter index for TAP-Windows adapter,* ~0 if undefined */DWORD adapter_index;int standby_iter;
#else  /* ifdef _WIN32 */int fd; /* file descriptor for TUN/TAP dev */
#endif#ifdef TARGET_SOLARISint ip_fd;
#endif#ifdef HAVE_NET_IF_UTUN_Hbool is_utun;
#endif/* used for printing status info only */unsigned int rwflags_debug;/* Some TUN/TAP drivers like to be ioctled for mtu* after open */int post_open_mtu;
};

代码片段2

节选自Linux内核文档提供的一段Tun/Tap用户层样例代码(多处细节已经调整)
https://www.kernel.org/doc/Documentation/networking/tuntap.txt

// 样例代码
#include <linux/if.h>
#include <linux/if_tun.h>int open_tun_and_select_type(char devName[IFNAMSIZ], int devType)//devName字符串指针不能为NULL,但可以是全0x00即'\0'{struct ifreq ifr;int fd;int err;if( (fd = open("/dev/net/tun", O_RDWR)) < 0 )return old_style__open_tun_and_select_type(devName, devType);memset(&ifr, 0, sizeof(ifr));/* Flags: IFF_TUN   - TUN device (no Ethernet headers) *        IFF_TAP   - TAP device  **        IFF_NO_PI - Do not provide packet information  */ ifr.ifr_flags = (short) devType; if( *devName ) {memcpy(ifr.ifr_name, devName, IFNAMSIZ);ifr.ifr_name[IFNAMSIZ-1] = '\0';}if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ){close(fd);return err;}memcpy(devName, ifr.ifr_name, IFNAMSIZ);return fd;}

• Linux标准struct ifreq结构体的定义位于<linux/if.h>, 细节如下:
  http://man7.org/linux/man-pages/man7/netdevice.7.html
  https://github.com/torvalds/linux/blob/master/include/uapi/linux/if.h

#define IFNAMSIZ    16

// 该结构体用于修改网卡的各种参数, 包括MAC地址/IP地址/MTU最大值/Metric跃点数等
// 之前的程序我们仅仅用到 ifr.ifr_flags=IFF_TUN切换Tun/Tap网卡的类型struct ifreq {char ifr_name[IFNAMSIZ]; /* Interface name */union {struct sockaddr ifr_addr;struct sockaddr ifr_dstaddr;struct sockaddr ifr_broadaddr;struct sockaddr ifr_netmask;struct sockaddr ifr_hwaddr;short           ifr_flags;int             ifr_ifindex;int             ifr_metric;int             ifr_mtu;struct ifmap    ifr_map;char            ifr_slave[IFNAMSIZ];char            ifr_newname[IFNAMSIZ];char           *ifr_data;};};

数据格式

tun_read() / tun_write()读写操作的数据包结构如下(摘自Linux内核文档):

frame_format: - Flags [2 bytes]- Proto [2 bytes]- Raw protocol(IP, IPv6, etc) frame.

https://www.kernel.org/doc/Documentation/networking/tuntap.txt