如何使用socat代理拦截Unix域套接字发送辅助数据？

时间：2016-08-17 18:35:47

标签： linux sockets proxy unix-socket socat

我有两个程序，一个服务器和一个客户端。服务器打开文件，向其写入数据，然后通过unix域套接字将其文件描述符发送到客户端。一切正常，直到我介绍一个socat代理。

socat -x -v UNIX-LISTEN:/tmp/unixSockSendFe,mode=775,reuseaddr,fork UNIX-CONNECT:/tmp/unixSockSendFd

解释
服务器侦听/tmp/unixSockSendFd，socat连接到它（UNIX-CONNECT:/tmp/unixSockSendFd），并创建另一个Unix域套接字（UNIX-LISTEN:/tmp/unixSockSendFe,mode=775,reuseaddr,fork），客户端连接到该套接字。客户端和服务器之间的任何通信都通过socat进行中继，它会打印以二进制（-x选项）和ascii（-v选项）形式发送的字节。

如果我不使用socat，并且客户端直接连接到服务器（在/tmp/unixSockSendFd套接字上），一切正常，但当socat用作代理时，客户端崩溃时出现分段错误。

服务器 

/*Server code - sendfd.c*/
#include <sys/socket.h>
#include <sys/un.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <errno.h>

char *socket_path = "/tmp/unixSockSendFd";
char *file="/tmp/abcd.txt" ;/*file whose fd is to be sent*/
int sendfd(int sock, int fd);
int recvfd(int s);
char data[]="sahil\0";

int main(int argc, char *argv[]) {
  struct sockaddr_un addr;
  char buf[100];
        buf[0]='\n';
  int fd,rc,confd;

int fd_to_send;
int temp,len;
temp=1;

fd_to_send=open(file,O_TRUNC|O_RDWR|O_CREAT,S_IRWXU|S_IRWXG|S_IRWXO);
if(fd_to_send==-1)
{
perror("file open error");
return -1;
}

  if (argc > 1) socket_path=argv[1];

  if ( (fd = socket(AF_UNIX, SOCK_STREAM, 0)) == -1) {
    perror("socket error");
    exit(-1);
  }

  memset(&addr, 0, sizeof(addr));
  addr.sun_family = AF_UNIX;
  if (*socket_path == '\0') {
    *addr.sun_path = '\0';
    strncpy(addr.sun_path+1, socket_path+1, sizeof(addr.sun_path)-2);
  } else {
    strncpy(addr.sun_path, socket_path, sizeof(addr.sun_path)-1);
  }
unlink(socket_path);

if(bind(fd,(struct sockaddr*)&addr,sizeof(addr))==-1){
perror("bind error");
return -1;
}

/*Writing data to file before sending fd*/
len=write(fd_to_send,data,(int)strlen(data));
fsync(fd_to_send);
printf("(len=%d)data written in file(content between ## marks) ##%s##\n",len,data);

listen(fd,1);
for(;;){

confd=accept(fd,NULL,NULL);
if(confd==-1)
{
perror("accept error");
continue;
}
else{
printf("new client connected ... sending fd ... \n");
sendfd(confd,fd_to_send);
close(confd);
}
}

  return 0;
}

int sendfd(int sock, int fd)
{
  struct msghdr hdr;
  struct iovec data;

  char cmsgbuf[CMSG_SPACE(sizeof(int))];

  char dummy = '*';
  data.iov_base = &dummy;
  data.iov_len = sizeof(dummy);

  memset(&hdr, 0, sizeof(hdr));
  hdr.msg_name = NULL;
  hdr.msg_namelen = 0;
  hdr.msg_iov = &data;
  hdr.msg_iovlen = 1;
  hdr.msg_flags = 0;

  hdr.msg_control = cmsgbuf;
  hdr.msg_controllen = CMSG_LEN(sizeof(int));

  struct cmsghdr* cmsg = CMSG_FIRSTHDR(&hdr);
  cmsg->cmsg_len   = CMSG_LEN(sizeof(int));
  cmsg->cmsg_level = SOL_SOCKET;
  cmsg->cmsg_type  = SCM_RIGHTS;

  *(int*)CMSG_DATA(cmsg) = fd;
//memcpy((CMSG_DATA(cmsg)), &fd, sizeof(fd)); -- from ivshmem server code - this too works instead of previous line 

  int n = sendmsg(sock, &hdr, 0);

  if(n == -1)
    printf("sendmsg() failed: %s (socket fd = %d)\n", strerror(errno), sock);

  return n;
}


int recvfd(int s)
{
        int n;
        int fd;
        char buf[1];
        struct iovec iov;
        struct msghdr msg;
        struct cmsghdr *cmsg;
        char cms[CMSG_SPACE(sizeof(int))];

        iov.iov_base = buf;
        iov.iov_len = 1;

        memset(&msg, 0, sizeof msg);
        msg.msg_name = 0;
        msg.msg_namelen = 0;
        msg.msg_iov = &iov;
        msg.msg_iovlen = 1;

        msg.msg_control = (caddr_t)cms;
        msg.msg_controllen = sizeof cms;

        if((n=recvmsg(s, &msg, 0)) < 0)
                return -1;
        if(n == 0){
                perror("unexpected EOF");
                return -1;
        }
        cmsg = CMSG_FIRSTHDR(&msg);
        memmove(&fd, CMSG_DATA(cmsg), sizeof(int));
        return fd;
}

客户端 

/*Client code - recvfd.c*/
#include <sys/socket.h>
#include <sys/un.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <errno.h>

char *socket_path = "/tmp/unixSockSendFe";
int sendfd(int sock, int fd);
int recvfd(int s);
int fd_received;

int main(int argc, char *argv[]) {
  struct sockaddr_un addr;
  char buf[100];
        buf[0]='\n';
  int fd,rc,confd;

int temp,len;
temp=1;

  if (argc > 1) socket_path=argv[1];

  if ( (fd = socket(AF_UNIX, SOCK_STREAM, 0)) == -1) {
    perror("socket error");
    exit(-1);
  }

  memset(&addr, 0, sizeof(addr));
  addr.sun_family = AF_UNIX;
  if (*socket_path == '\0') {
    *addr.sun_path = '\0';
    strncpy(addr.sun_path+1, socket_path+1, sizeof(addr.sun_path)-2);
  } else {
    strncpy(addr.sun_path, socket_path, sizeof(addr.sun_path)-1);
  }

if(connect(fd,(struct sockaddr*)&addr,sizeof(addr))==-1)
{
perror("connect error");
exit(-1);
}

fd_received=recvfd(fd);
lseek(fd_received,0,SEEK_SET);
len=read(fd_received,buf,5);
if(len<0)
{
perror("read error");
}
printf("(fd_received=%d,len=%d) first %d characters read from the file whoes fd was received(content within ##) ##%.*s##\n",fd_received,len,5,5,buf);



  return 0;
}

int sendfd(int sock, int fd)
{
  struct msghdr hdr;
  struct iovec data;

  char cmsgbuf[CMSG_SPACE(sizeof(int))];

  char dummy = '*';
  data.iov_base = &dummy;
  data.iov_len = sizeof(dummy);

  memset(&hdr, 0, sizeof(hdr));
  hdr.msg_name = NULL;
  hdr.msg_namelen = 0;
  hdr.msg_iov = &data;
  hdr.msg_iovlen = 1;
  hdr.msg_flags = 0;

  hdr.msg_control = cmsgbuf;
  hdr.msg_controllen = CMSG_LEN(sizeof(int));

  struct cmsghdr* cmsg = CMSG_FIRSTHDR(&hdr);
  cmsg->cmsg_len   = CMSG_LEN(sizeof(int));
  cmsg->cmsg_level = SOL_SOCKET;
  cmsg->cmsg_type  = SCM_RIGHTS;

  *(int*)CMSG_DATA(cmsg) = fd;

  int n = sendmsg(sock, &hdr, 0);

  if(n == -1)
    printf("sendmsg() failed: %s (socket fd = %d)\n", strerror(errno), sock);

  return n;
}


int recvfd(int s)
{
        int n;
        int fd;
        char buf[1];
        struct iovec iov;
        struct msghdr msg;
        struct cmsghdr *cmsg;
        char cms[CMSG_SPACE(sizeof(int))];

        iov.iov_base = buf;
        iov.iov_len = 1;

        memset(&msg, 0, sizeof msg);
        msg.msg_name = 0;
        msg.msg_namelen = 0;
        msg.msg_iov = &iov;
        msg.msg_iovlen = 1;

        msg.msg_control = (caddr_t)cms;
        msg.msg_controllen = sizeof cms;

        if((n=recvmsg(s, &msg, 0)) < 0)
                return -1;
        if(n == 0){
                perror("unexpected EOF");
                return -1;
        }
        cmsg = CMSG_FIRSTHDR(&msg);
        memmove(&fd, CMSG_DATA(cmsg), sizeof(int));
        return fd;
}

在运行客户端（recvfd）时出现分段错误。

./recvfd          
[1]    6104 segmentation fault (core dumped)  ./recvfd

以下是使用coredump运行gdb的行

Core was generated by `./recvfd'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x0000000000400cf9 in recvfd (s=3) at recvfd.c:146
146         memmove(&fd, CMSG_DATA(cmsg), sizeof(int));

这是核心转储 - Link。

我想在发送文件描述符时嗅探两个进程之间发生的通信。我无法弄清楚为什么客户端在使用socat运行时会崩溃，但在没有它的情况下运行时也不会崩溃。

更新1

使用socat来嗅探在完善的开源项目的两个进程之间进行的通信（ ivshmem - 用于在运行的虚拟机之间共享内存，也是英特尔DPDK的一部分， Link），我观察了以下内容。

  • 使用socat
    • 时没有任何进程崩溃
  • 使用socat时，文件描述符未正确发送，并且未添加到收件人进程。
  • 如果未使用socat，并且两个进程直接连接，则会正确发送文件描述符，并将其添加到收件人进程。

0 个答案:

没有答案
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