Initial commit.

Makefile

+CFLAGS=-Wall --pedantic
+
+all: libuserhosts.so
+
+libuserhosts.so: userhosts.c
+	gcc -ldl -shared -fPIC $(CFLAGS) $< -o $@

README

+userhosts - Redirects accesses to /etc/hosts to another file.
+
+Rationale
+---------
+
+The need is to customise hostname (reverse )resolution for a single process
+(and potentially its children), potentially resolving conflicting domain names
+to different addresses and vice-versa.
+
+libnss' HOSTALIASES feature only allows mapping between domain names, not
+between names and addresses, so it is not powerful enough.
+
+Limitations
+-----------
+
+This library relies on LD_PRELOAD mechanism, so it inherits its limitations
+(it will not work on suid binaries, ...).
+
+Requirements
+------------
+
+- make
+- gcc (source uses __attribute__)
+- glibc (source uses RTLD_NEXT)
+
+Building
+--------
+
+Just run "make".
+
+Usage
+-----
+
+$ env HOSTS=/path/to/custom/hosts LD_PRELOAD=/path/to/libuserhosts.so <command>

configure

+

userhosts.c

+/* userhosts - redirect access to /etc/hosts to another file */
+/*
+ * Copyright (C) 2014 Vincent Pelletier <vincent@nexedi.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+
+/* Needed for RTLD_NEXT */
+#define _GNU_SOURCE
+
+#include <string.h>
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <dlfcn.h>
+#include <fcntl.h>
+
+#define ORIGINAL_HOSTS_PATH "/etc/hosts"
+
+static int (*original_open)(const char *, int, ...);
+static FILE *(*original_fopen)(const char *, const char *);
+static const char *replacement_hosts;
+
+/* Call dlsym(RTLD_NEXT, name), abort()'ing with informative message to
+ * stderr if it cannot be found. */
+static inline void *dlsym_or_abort(const char *name) {
+  char *error;
+  void *symbol;
+  dlerror(); /* Clear any previous error */
+  if (NULL == (symbol = dlsym(RTLD_NEXT, name)) && (error = dlerror())) {
+    fprintf(stderr, "Error loading '%s': %s\n", name, error);
+    abort();
+  }
+  return symbol;
+}
+
+static void __attribute__ ((constructor)) init(void) {
+  *(void **)(&original_open) = dlsym_or_abort("open");
+  *(void **)(&original_fopen) = dlsym_or_abort("fopen");
+  replacement_hosts = getenv("HOSTS");
+  if (replacement_hosts == NULL)
+    /* XXX: warn ? fallback on $HOME/???/hosts ? */
+    replacement_hosts = ORIGINAL_HOSTS_PATH;
+}
+
+int open(const char *__file, int __oflag, ...) {
+  if(strcmp(__file, ORIGINAL_HOSTS_PATH) == 0)
+    __file = replacement_hosts;
+  if (__oflag & (O_CREAT | O_TMPFILE)) {
+    va_list ap;
+    mode_t mode;
+
+    va_start(ap, __oflag);
+    mode = va_arg(ap, mode_t);
+    va_end(ap);
+    return (*original_open)(__file, __oflag, mode);
+  }
+  return (*original_open)(__file, __oflag);
+}
+
+FILE *fopen(const char *path, const char *mode) {
+  if (strcmp(path, ORIGINAL_HOSTS_PATH) == 0)
+    path = replacement_hosts;
+  return (*original_fopen)(path, mode);
+}