safely handle failure to open hosts, services, resolv.conf files

previously, transient failures like fd exhaustion or other
resource-related errors were treated the same as non-existence of
these files, leading to fallbacks or false-negative results. in
particular:

- failure to open hosts resulted in fallback to dns, possibly yielding
  EAI_NONAME for a hostname that should be defined locally, or an
  unwanted result from dns that the hosts file was intended to
  replace.

- failure to open services resulted in EAI_SERVICE.

- failure to open resolv.conf resulted in querying localhost rather
  than the configured nameservers.

now, only permanent errors trigger the fallback behaviors above; all
other errors are reportable to the caller as EAI_SYSTEM.

1 files changed, 11 insertions, 2 deletions

diff --git a/src/network/res_msend.c b/src/network/res_msend.c
index 35f106dd..d0e8e481 100644
--- a/src/network/res_msend.c
+++ b/src/network/res_msend.c
@@ -54,7 +54,15 @@ int __res_msend(int nqueries, const unsigned char *const *queries,
 
 	/* Get nameservers from resolv.conf, fallback to localhost */
 	f = __fopen_rb_ca("/etc/resolv.conf", &_f, _buf, sizeof _buf);
-	if (f) for (nns=0; nns<3 && fgets(line, sizeof line, f); ) {
+	if (!f) switch (errno) {
+	case ENOENT:
+	case ENOTDIR:
+	case EACCES:
+		goto no_resolv_conf;
+	default:
+		return -1;
+	}
+	for (nns=0; nns<3 && fgets(line, sizeof line, f); ) {
 		if (!strncmp(line, "options", 7) && isspace(line[7])) {
 			unsigned long x;
 			char *p, *z;
@@ -92,7 +100,8 @@ int __res_msend(int nqueries, const unsigned char *const *queries,
 			}
 		}
 	}
-	if (f) __fclose_ca(f);
+	__fclose_ca(f);
+no_resolv_conf:
 	if (!nns) {
 		ns[0].sin.sin_family = AF_INET;
 		ns[0].sin.sin_port = htons(53);