0.md

First, compile the shim:

gcc -Wall -O2 -fpic -shared -ldl bind.c -o bind.so

Now, run the demo and notice that it does what it looks like it does:

$ LD_PRELOAD=./bind.so ruby demo.rb
#<TCPServer:fd 5, AF_INET, 0.0.0.0, 8888>

Then, set ISOSPIN_NAMESPACE_IP and try again:

export ISOSPIN_NAMESPACE_IP=127.0.0.1
$ LD_PRELOAD=./bind.so ruby demo.rb
#<TCPServer:fd 5, AF_INET, 127.0.0.1, 8888>

bind.c

// For RTLD_NEXT
#define _GNU_SOURCE

#include <stdio.h>
#include <dlfcn.h>
#include <stdlib.h>
#include <string.h>

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define ENVVARNAME "ISOSPIN_NAMESPACE_IP"

static int (*real_bind)(int sockfd, const struct sockaddr *addr, socklen_t addrlen) = NULL;

int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen) {
  char * target_ip = getenv(ENVVARNAME);
  if (target_ip == NULL) {
    return real_bind(sockfd, addr, addrlen);
  }

  if (addr->sa_family != AF_INET) {
    return real_bind(sockfd, addr, addrlen);
  }

  char *ip = inet_ntoa(((struct sockaddr_in*)addr)->sin_addr);
  if (strcmp(ip, "0.0.0.0") != 0) {
    return real_bind(sockfd, addr, addrlen);
  }

  struct sockaddr shadowed_addr;
  shadowed_addr.sa_family = addr->sa_family;
  if (inet_aton(target_ip, &((struct sockaddr_in*)&shadowed_addr)->sin_addr) == 0) {
    printf("failed to shadow INADDR_ANY bind: address invalid\n");
    exit(255);
  }

  return real_bind(sockfd, &shadowed_addr, addrlen);
}

void __attribute__((constructor)) initialize(void) {
  real_bind = dlsym(RTLD_NEXT, "bind");

  if (real_bind == NULL) {
    printf("dynamic linker failed to find bind(2)\n");
    exit(255);
  }
}

demo.rb

require('socket')

s = TCPServer.new('0.0.0.0', 8888)
puts s.inspect
s.close