Add ares_init_options() configurability for path to resolv.conf file

Add resolvconf_path to end of struct ares_options with ARES_OPT_RESOLVCONF option
so on Unix-like systems a custom path can be specified.  If no path is specified, 
/etc/resolv.conf is used like normal.

Fix By: Sarat Addepalli @SirR4T 
Fixes Bug: #220 
Review By: Brad House @bradh352

ares.h

@@ -164,6 +164,7 @@ extern "C" {
 #define ARES_OPT_ROTATE         (1 << 14)
 #define ARES_OPT_EDNSPSZ        (1 << 15)
 #define ARES_OPT_NOROTATE       (1 << 16)
+#define ARES_OPT_RESOLVCONF     (1 << 17)
 
 /* Nameinfo flag values */
 #define ARES_NI_NOFQDN                  (1 << 0)
@@ -270,6 +271,7 @@ struct ares_options {
   struct apattern *sortlist;
   int nsort;
   int ednspsz;
+  char *resolvconf_path;
 };
 
 struct hostent;

ares_destroy.c

@@ -36,6 +36,8 @@ void ares_destroy_options(struct ares_options *options)
     ares_free(options->sortlist);
   if(options->lookups)
     ares_free(options->lookups);
+  if(options->resolvconf_path)
+    ares_free(options->resolvconf_path);
 }
 
 void ares_destroy(ares_channel channel)
@@ -85,6 +87,9 @@ void ares_destroy(ares_channel channel)
   if (channel->lookups)
     ares_free(channel->lookups);
 
+  if (channel->resolvconf_path)
+    ares_free(channel->resolvconf_path);
+
   ares_free(channel);
 }
 

ares_init.c

@@ -169,6 +169,7 @@ int ares_init_options(ares_channel *channelptr, struct ares_options *options,
   channel->sock_config_cb_data = NULL;
   channel->sock_funcs = NULL;
   channel->sock_func_cb_data = NULL;
+  channel->resolvconf_path = NULL;
 
   channel->last_server = 0;
   channel->last_timeout_processed = (time_t)now.tv_sec;
@@ -242,6 +243,8 @@ done:
         ares_free(channel->sortlist);
       if(channel->lookups)
         ares_free(channel->lookups);
+      if(channel->resolvconf_path)
+        ares_free(channel->resolvconf_path);
       ares_free(channel);
       return status;
     }
@@ -347,7 +350,7 @@ int ares_save_options(ares_channel channel, struct ares_options *options,
   (*optmask) = (ARES_OPT_FLAGS|ARES_OPT_TRIES|ARES_OPT_NDOTS|
                 ARES_OPT_UDP_PORT|ARES_OPT_TCP_PORT|ARES_OPT_SOCK_STATE_CB|
                 ARES_OPT_SERVERS|ARES_OPT_DOMAINS|ARES_OPT_LOOKUPS|
-                ARES_OPT_SORTLIST|ARES_OPT_TIMEOUTMS);
+                ARES_OPT_SORTLIST|ARES_OPT_TIMEOUTMS|ARES_OPT_RESOLVCONF);
   (*optmask) |= (channel->rotate ? ARES_OPT_ROTATE : ARES_OPT_NOROTATE);
 
   /* Copy easy stuff */
@@ -422,6 +425,13 @@ int ares_save_options(ares_channel channel, struct ares_options *options,
   }
   options->nsort = channel->nsort;
 
+  /* copy path for resolv.conf file */
+  if (channel->resolvconf_path) {
+    options->resolvconf_path = ares_strdup(channel->resolvconf_path);
+    if (!options->resolvconf_path)
+      return ARES_ENOMEM;
+  }
+
   return ARES_SUCCESS;
 }
 
@@ -530,6 +540,14 @@ static int init_by_options(ares_channel channel,
     channel->nsort = options->nsort;
   }
 
+  /* Set path for resolv.conf file, if given. */
+  if ((optmask & ARES_OPT_RESOLVCONF) && !channel->resolvconf_path)
+    {
+      channel->resolvconf_path = ares_strdup(options->resolvconf_path);
+      if (!channel->resolvconf_path && options->resolvconf_path)
+        return ARES_ENOMEM;
+    }
+
   channel->optmask = optmask;
 
   return ARES_SUCCESS;
@@ -1644,6 +1662,7 @@ static int init_by_resolv_conf(ares_channel channel)
     size_t linesize;
     int error;
     int update_domains;
+    const char *resolvconf_path;
 
     /* Don't read resolv.conf and friends if we don't have to */
     if (ARES_CONFIG_CHECK(channel))
@@ -1652,7 +1671,14 @@ static int init_by_resolv_conf(ares_channel channel)
     /* Only update search domains if they're not already specified */
     update_domains = (channel->ndomains == -1);
 
-    fp = fopen(PATH_RESOLV_CONF, "r");
+    /* Support path for resolvconf filename set by ares_init_options */
+    if(channel->resolvconf_path) {
+      resolvconf_path = channel->resolvconf_path;
+    } else {
+      resolvconf_path = PATH_RESOLV_CONF;
+    }
+
+    fp = fopen(resolvconf_path, "r");
     if (fp) {
       while ((status = ares__read_line(fp, &line, &linesize)) == ARES_SUCCESS)
       {
@@ -1663,10 +1689,10 @@ static int init_by_resolv_conf(ares_channel channel)
         else if ((p = try_config(line, "search", ';')) && update_domains)
           status = set_search(channel, p);
         else if ((p = try_config(line, "nameserver", ';')) &&
-                 channel->nservers == -1)
+                channel->nservers == -1)
           status = config_nameserver(&servers, &nservers, p);
         else if ((p = try_config(line, "sortlist", ';')) &&
-                 channel->nsort == -1)
+                channel->nsort == -1)
           status = config_sortlist(&sortlist, &nsort, p);
         else if ((p = try_config(line, "options", ';')))
           status = set_options(channel, p);
@@ -1686,7 +1712,7 @@ static int init_by_resolv_conf(ares_channel channel)
         break;
       default:
         DEBUGF(fprintf(stderr, "fopen() failed with error: %d %s\n",
-                       error, strerror(error)));
+                      error, strerror(error)));
         DEBUGF(fprintf(stderr, "Error opening file: %s\n", PATH_RESOLV_CONF));
         status = ARES_EFILE;
       }
@@ -1954,6 +1980,11 @@ static int init_by_defaults(ares_channel channel)
       ares_free(channel->lookups);
       channel->lookups = NULL;
     }
+
+    if(channel->resolvconf_path) {
+      ares_free(channel->resolvconf_path);
+      channel->resolvconf_path = NULL;
+    }
   }
 
   if(hostname)

ares_init_options.3

@@ -40,6 +40,7 @@ struct ares_options {
   struct apattern *sortlist;
   int nsort;
   int ednspsz;
+  char *resolvconf_path;
 };
 
 int ares_init_options(ares_channel *\fIchannelptr\fP,
@@ -181,6 +182,15 @@ The receive buffer size to set for the socket.
 .B ARES_OPT_EDNSPSZ
 .B int \fIednspsz\fP;
 .br
+.TP 18
+.B ARES_OPT_RESOLVCONF
+.B char *\fIresolvconf_path\fP;
+The path to use for reading the resolv.conf file. The
+.I resolvconf_path
+should be set to a path string, and will be honoured on *nix like systems. The
+default is
+.B /etc/resolv.conf
+.br
 The message size to be advertized in EDNS; only takes effect if the
 .B ARES_FLAG_EDNS
 flag is set.
@@ -259,6 +269,9 @@ c-ares library initialization not yet performed.
 .SH NOTES
 When initializing from
 .B /etc/resolv.conf,
+(or, alternatively when specified by the
+.I resolvconf_path
+path location)
 \fBares_init_options(3)\fP reads the \fIdomain\fP and \fIsearch\fP directives
 to allow lookups of short names relative to the domains specified. The
 \fIdomain\fP and \fIsearch\fP directives override one another. If more that

ares_private.h

@@ -325,6 +325,9 @@ struct ares_channeldata {
 
   const struct ares_socket_functions * sock_funcs;
   void *sock_func_cb_data;
+
+  /* Path for resolv.conf file, configurable via ares_options */
+  char *resolvconf_path;
 };
 
 /* Memory management functions */

test/ares-test-init.cc

@@ -86,6 +86,8 @@ TEST_F(LibraryTest, OptionsChannelInit) {
   opts.lookups = strdup("b");
   optmask |= ARES_OPT_LOOKUPS;
   optmask |= ARES_OPT_ROTATE;
+  opts.resolvconf_path = strdup("/etc/resolv.conf");
+  optmask |= ARES_OPT_RESOLVCONF;
 
   ares_channel channel = nullptr;
   EXPECT_EQ(ARES_SUCCESS, ares_init_options(&channel, &opts, optmask));
@@ -112,6 +114,7 @@ TEST_F(LibraryTest, OptionsChannelInit) {
   EXPECT_EQ(std::string(opts.domains[0]), std::string(opts2.domains[0]));
   EXPECT_EQ(std::string(opts.domains[1]), std::string(opts2.domains[1]));
   EXPECT_EQ(std::string(opts.lookups), std::string(opts2.lookups));
+  EXPECT_EQ(std::string(opts.resolvconf_path), std::string(opts2.resolvconf_path));
 
   ares_destroy_options(&opts);
   ares_destroy_options(&opts2);
@@ -169,6 +172,8 @@ TEST_F(LibraryTest, OptionsChannelAllocFail) {
   opts.lookups = strdup("b");
   optmask |= ARES_OPT_LOOKUPS;
   optmask |= ARES_OPT_ROTATE;
+  opts.resolvconf_path = strdup("/etc/resolv.conf");
+  optmask |= ARES_OPT_RESOLVCONF;
 
   ares_channel channel = nullptr;
   for (int ii = 1; ii <= 8; ii++) {
@@ -396,6 +401,35 @@ CONTAINED_TEST_F(LibraryTest, ContainerFullResolvInit,
   return HasFailure();
 }
 
+// Allow path for resolv.conf to be configurable
+NameContentList myresolvconf = {
+  {"/tmp/myresolv.cnf", " nameserver   1.2.3.4 \n"
+                       "search   first.com second.com\n"
+                       "lookup bind\n"
+                       "options debug ndots:5\n"
+                       "sortlist 1.2.3.4/16 2.3.4.5\n"}};
+CONTAINED_TEST_F(LibraryTest, ContainerMyResolvConfInit,
+                 "myhostname", "mydomain.org", myresolvconf) {
+  char filename[] = "/tmp/myresolv.cnf";
+  ares_channel channel = nullptr;
+  struct ares_options options = {0};
+  options.resolvconf_path = strdup(filename);
+  int optmask = ARES_OPT_RESOLVCONF;
+  EXPECT_EQ(ARES_SUCCESS, ares_init_options(&channel, &options, optmask));
+
+  optmask = 0;
+  free(options.resolvconf_path);
+  options.resolvconf_path = NULL;
+
+  EXPECT_EQ(ARES_SUCCESS, ares_save_options(channel, &options, &optmask));
+  EXPECT_EQ(ARES_OPT_RESOLVCONF, (optmask & ARES_OPT_RESOLVCONF));
+  EXPECT_EQ(std::string(filename), std::string(options.resolvconf_path));
+
+  ares_destroy_options(&options);
+  ares_destroy(channel);
+  return HasFailure();
+}
+
 NameContentList hostconf = {
   {"/etc/resolv.conf", "nameserver 1.2.3.4\n"
                        "sortlist1.2.3.4\n"  // malformed line