CAP_NET_RAW为什么不能与SO_BINDTODEVICE一起使用？

Question

我有以下简单的测试程序来创建一个UDP套接字并将其绑定到一个特定的接口，SO_BINDTODEVICE这样我就可以bind()这么INADDR_ANY来收到UDP该接口上专门播放。

//filename: bindtest.c
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>

#define MY_PORT (333)
#define MY_DEVICE "enp0s3"

#define BUFFERSIZE (1000)

/* global variables */
int sock;
struct sockaddr_in sa;
struct sockaddr_in my_addr;
char buffer[BUFFERSIZE];

int main(int argc, char *argv[])
{
  unsigned int echolen, clientlen;
  int rc, n;
  char opt_buffer[1000];
  struct protoent *udp_protoent;
  struct timeval receive_timeout;
  int optval;
  socklen_t opt_length;
  sleep(1);
  /* Create the UDP socket */
  if ((sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0)
  {
    printf ("%s: failed to create UDP socket (%s) \n",
        argv[0], strerror(errno));
    exit (EXIT_FAILURE);
  }
  printf ("UDP socket created\n");

  /* set the recvfrom timeout value */
  receive_timeout.tv_sec = 5;
  receive_timeout.tv_usec = 0;
  rc=setsockopt(sock, SOL_SOCKET, SO_RCVTIMEO, &receive_timeout, sizeof(receive_timeout));
  if (rc != 0)
  {
     printf ("%s: could not set SO_RCVTIMEO (%s)\n",
        argv[0], strerror(errno));
     exit (EXIT_FAILURE);
  }
  printf ("set timeout to time [s]: %d time [ms]: %d\n", receive_timeout.tv_sec, receive_timeout.tv_usec);
  /* allow broadcast messages for the socket */
  int true = 1;
  rc=setsockopt(sock, SOL_SOCKET, SO_BROADCAST, &true, sizeof(true));
  if (rc != 0)
  {
     printf ("%s: could not set SO_BROADCAST (%s)\n",
        argv[0], strerror(errno));
     exit (EXIT_FAILURE);
  }
  printf ("set SO_BROADCAST worked\n");
  /* bind to a specific interface */
  char device[] = MY_DEVICE;
  rc=setsockopt(sock, SOL_SOCKET, SO_BINDTODEVICE, device, sizeof(device));
  if (rc != 0)
  {
     printf ("%s: could not set SO_BINDTODEVICE (%s)\n",
        argv[0], strerror(errno));
     exit (EXIT_FAILURE);
  }
  printf ("SO_BINDTODEVICE worked\n");

  /* bind my own Port */
  my_addr.sin_family = AF_INET;
  my_addr.sin_addr.s_addr = INADDR_ANY;
  my_addr.sin_port = htons(MY_PORT);
  rc = bind (sock, (struct sockaddr *) &my_addr, sizeof(my_addr));
  if (rc < 0)
  {
     printf ("%s: could not bind port (%s)\n",
        argv[0], strerror(errno));
     exit (EXIT_FAILURE);
  }
  printf ("bind() worked\n");
  sa.sin_family = AF_INET;
  sa.sin_addr.s_addr = INADDR_BROADCAST;
  sa.sin_port = htons(MY_PORT);

  char data[20];
  sprintf(data,"FOOBAR");
  int res = sendto(sock, &data, strlen(data), 0, (struct sockaddr*)&sa, sizeof(sa));
  if(res < 0){
    printf("could not send\n");
  } else {
    printf("data sent\n");
  }


  close(sock);
  printf ("socket closed\n");

  exit(0);
}

当我以非root用户身份运行该程序时，得到以下输出：

$ ./bindtest 
UDP socket created
set timeout to time [s]: 5 time [ms]: 0
set SO_BROADCAST worked
./bindtest: could not set SO_BINDTODEVICE (Operation not permitted)

这很合乎逻辑，因为我不是root，这SO_BINDTODEVICE是特权操作。但是据CAP_NET_RAW我从Linux内核的以下代码片段了解，它包含在该功能中：

$ ./bindtest 
UDP socket created
set timeout to time [s]: 5 time [ms]: 0
set SO_BROADCAST worked
./bindtest: could not set SO_BINDTODEVICE (Operation not permitted)

好吧，当我这样做：

$ getcap bindtest
$ sudo setcap cap_net_raw+ep bindtest
$ getcap bindtest
bindtest = cap_net_raw+ep

我得到相同的错误输出：

$ ./bindtest 
UDP socket created
set timeout to time [s]: 5 time [ms]: 0
set SO_BROADCAST worked
./bindtest: could not set SO_BINDTODEVICE (Operation not permitted)

当然，它的工作方式如下root：

$ sudo ./bindtest
UDP socket created
set timeout to time [s]: 5 time [ms]: 0
set SO_BROADCAST worked
SO_BINDTODEVICE worked
bind() worked
data sent
socket closed

那么为什么他们不按预期工作呢？

Answer 1

该代码是正确的，getcap/ 的用法setcap是正确的，因此必须有其他原因阻止它正常工作。

确实是因为所有这些操作都是在/home/user此系统上安装的，并且带有nosuid其他选件。

因此，简单地将二进制文件移动到例如/usr/bin/未安装的任何其他部分上nosuid就可以按预期工作。

（虽然你还需要CAP_NET_BIND_SERVICE为bind()与端口333的工作作为例子）