src/connredir.c

   1 /*
   2
   3 connredir.c
   4
   5 USAGE
   6
   7         LD_PRELOAD=$PWD/connredir.so CONNREDIR_ORIG_IP=169.254.169.254 CONNREDIR_ORIG_PORT=80 CONNREDIR_TO_IP=127.0.0.1 CONNREDIR_TO_PORT=8080 wget ...
   8
   9         LD_PRELOAD=$PWD/connredir.so CONNREDIR_ORIG_IP=127.0.0.1 CONNREDIR_ORIG_PORT=25 CONNREDIR_TO=stdout sendmail ...
  10
  11 DESCRIPTION
  12
  13         This shared library is intended to extend connect(2) standard library function by ip/port override capability, so you control
  14         programs, where to connect, without modifying or configuring themself.
  15
  16         On each connect() calls, it checks that the destination IP and port match to the ones in CONNREDIR_ORIG_IP and CONNREDIR_ORIG_PORT
  17         environment variables. Then changes them to the IP and port numbers in CONNREDIR_TO_IP and CONNREDIR_TO_PORT respectively.
  18
  19         Leave CONNREDIR_ORIG_PORT unset to override all ports.
  20         Leave CONNREDIR_TO_PORT unset to not override the destination port number.
  21
  22         If CONNREDIR_TO is set, then it's evaluated before CONNREDIR_TO_IP.
  23         Valid values for CONNREDIR_TO: stdout.
  24         When CONNREDIR_TO is 'stdout', connredir replaces the socket's FD, which is about to be connected, to the STDOUT, so you can
  25         make your program communicate on STDIO instead of INET. Useful when combined with socat(1).
  26
  27 COMPATIBILITY
  28
  29         inet sockets (ipv4)
  30         SOCK_STREAM (tcp)
  31
  32 ENVIRONMENT VARIABLES
  33
  34         CONNREDIR_ORIG_IP
  35         CONNREDIR_ORIG_PORT
  36         CONNREDIR_TO
  37         CONNREDIR_TO_IP
  38         CONNREDIR_TO_PORT
  39
  40 COMPILE
  41
  42         gcc -D_GNU_SOURCE -ldl -shared -fPIC -o connredir.so connredir.c
  43 */
  44
  45 #include <stdio.h>
  46 #include <stdlib.h>
  47 #include <sys/types.h>
  48 #include <sys/socket.h>
  49 #include <netinet/in.h>
  50 #include <arpa/inet.h>
  51 #include <dlfcn.h>
  52 #include <errno.h>
  53
  54 void _connredir_ip_parse_error(const char* s)
  55 {
  56         fprintf(stderr, "connredir: failed to parse ip address: %s\n", s);
  57 }
  58
  59 int connect(int sockfd, const struct sockaddr_in *orig_sockaddr, socklen_t addrlen)
  60 {
  61         struct in_addr orig_ip;
  62         char *orig_ip_str;
  63         char *orig_port_str;
  64         in_port_t orig_port = 0;
  65
  66         char *redir_to;
  67
  68         char *to_ip_str;
  69         struct in_addr to_ip;
  70         char *to_port_str;
  71         in_port_t to_port = 0;
  72
  73         struct sockaddr_in to_sockaddr;
  74         int (*orig_connect)(int, const struct sockaddr_in *, socklen_t) = dlsym(RTLD_NEXT, "connect");
  75
  76
  77         if(orig_sockaddr->sin_family != AF_INET) goto stdlib;
  78
  79         orig_ip_str = getenv("CONNREDIR_ORIG_IP");
  80         if(orig_ip_str == NULL) goto stdlib;
  81
  82         if(inet_aton(orig_ip_str, &orig_ip) == 0)
  83         {
  84                 _connredir_ip_parse_error(orig_ip_str);
  85                 goto stdlib;
  86         }
  87         if(ntohl(orig_sockaddr->sin_addr.s_addr) != ntohl(orig_ip.s_addr)) goto stdlib;
  88
  89         orig_port_str = getenv("CONNREDIR_ORIG_PORT");
  90         if(orig_port_str != NULL) orig_port = atoi(orig_port_str);
  91         if(orig_port != 0 && orig_port != ntohs(orig_sockaddr->sin_port)) goto stdlib;
  92
  93
  94         redir_to = getenv("CONNREDIR_TO");
  95         if(redir_to != NULL)
  96         {
  97                 if(strcmp(redir_to, "stdout")==0)
  98                 {
  99                         if(dup2(fileno(stdout), sockfd) == -1) { perror("connredir: dup2"); goto stdlib; }
 100                         fprintf(stderr, "connredir: redirecting %s:%d -> stdout\n", inet_ntoa(orig_sockaddr->sin_addr), ntohs(orig_sockaddr->sin_port));
 101                         return 0;
 102                 }
 103                 else
 104                 {
 105                         fprintf(stderr, "connredir: unknown redirection '%s'\n", redir_to);
 106                         goto stdlib;
 107                 }
 108         }
 109
 110         to_ip_str = getenv("CONNREDIR_TO_IP");
 111         if(to_ip_str == NULL) goto stdlib;
 112
 113         if(inet_aton(to_ip_str, &to_ip) == 0)
 114         {
 115                 _connredir_ip_parse_error(to_ip_str);
 116                 goto stdlib;
 117         }
 118
 119         to_port_str = getenv("CONNREDIR_TO_PORT");
 120         if(to_port_str != NULL) to_port = atoi(to_port_str);
 121
 122         to_sockaddr.sin_family = AF_INET;
 123         to_sockaddr.sin_port = to_port == 0 ? orig_sockaddr->sin_port : htons(to_port);
 124         to_sockaddr.sin_addr = to_ip;
 125
 126         fprintf(stderr, "connredir: redirecting %s:%d -> ", inet_ntoa(orig_sockaddr->sin_addr), ntohs(orig_sockaddr->sin_port));
 127         fprintf(stderr, "%s:%d\n", inet_ntoa(to_sockaddr.sin_addr), ntohs(to_sockaddr.sin_port));
 128         orig_sockaddr = &to_sockaddr;
 129
 130         stdlib:
 131         return orig_connect(sockfd, orig_sockaddr, addrlen);
 132 }