dns: follow resolv.conf if it is a symlink for 'rc-manager=file'

Until before 1.2.0, NetworkManager would always write resolv.conf as file, but
if /etc/resolv.conf was a symlink, it would follow the link instead of
replacing it with a file ([1], [2]).

With 1.2.0, we initially dropped that behavior and added a new 'rc-manager=none'
which writes resolv.conf to /var/run/NetworkManager and symlinks resolv.conf [3].
In case resolv.conf being already a symlink to another target, it would
not be replaced [4].
Later, we added 'rc-manager=file', which always writes /etc/resolv.conf as
file [5].

With 1.4.0, we will rename 'rc-manager=none' to 'rc-manager=symlink' [6].

This commit now fixes 'rc-manager=file' to restores the pre-1.2 behavior
and follow symlinks.

[1] 5761e328b81ce8894c2657ce0994ba401923ba35
[2] https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/324233
[3] 4805be2ed27b71a6099477d86dbc109adb41b819
[4] 583568e12f9e580cd2903811637c9f9b7a2f1088
[5] 288799713dc78bc45e2b0a9cf41d228f5d95315f
[6] cd6a469668028fbc347919ed3580275f9894a1f2

https://github.com/NetworkManager/NetworkManager/pull/7

2 files changed, 13 insertions, 5 deletions

diff --git a/man/NetworkManager.conf.xml b/man/NetworkManager.conf.xml
index 23038dbbc1..a6614e3393 100644
--- a/man/NetworkManager.conf.xml
+++ b/man/NetworkManager.conf.xml
@@ -329,7 +329,9 @@ no-auto-default=*
         by pointing the link <filename>/etc/resolv.conf</filename> to
         somewhere else.</para>
         <para><literal>file</literal>: NetworkManager will write
-        <filename>/etc/resolv.conf</filename> as file.</para>
+        <filename>/etc/resolv.conf</filename> as file. If it finds
+        a symlink, it will follow the symlink and update the target
+        instead.</para>
         <para><literal>resolvconf</literal>: NetworkManager will run
         resolvconf to update the DNS configuration.</para>
         <para><literal>netconfig</literal>: NetworkManager will run
diff --git a/src/dns-manager/nm-dns-manager.c b/src/dns-manager/nm-dns-manager.c
index dfeece6240..8134f2f4a4 100644
--- a/src/dns-manager/nm-dns-manager.c
+++ b/src/dns-manager/nm-dns-manager.c
@@ -674,6 +674,8 @@ update_resolv_conf (NMDnsManager *self,
 	gs_free char *content = NULL;
 	SpawnResult write_file_result = SR_SUCCESS;
 	int errsv;
+	const char *rc_path = _PATH_RESCONF;
+	nm_auto_free char *rc_path_real = NULL;
 
 	/* If we are not managing /etc/resolv.conf and it points to
 	 * MY_RESOLV_CONF, don't write the private DNS configuration to
@@ -697,18 +699,22 @@ update_resolv_conf (NMDnsManager *self,
 	if (rc_manager == NM_DNS_MANAGER_RESOLV_CONF_MAN_FILE) {
 		GError *local = NULL;
 
+		rc_path_real = realpath (rc_path, NULL);
+		if (rc_path_real)
+			rc_path = rc_path_real;
+
 		/* we first write to /etc/resolv.conf directly. If that fails,
 		 * we still continue to write to runstatedir but remember the
 		 * error. */
-		if (!g_file_set_contents (_PATH_RESCONF, content, -1, &local)) {
+		if (!g_file_set_contents (rc_path, content, -1, &local)) {
 			_LOGT ("update-resolv-conf: write to %s failed (rc-manager=%s, %s)",
-			       _PATH_RESCONF, _rc_manager_to_string (rc_manager), local->message);
+			       rc_path, _rc_manager_to_string (rc_manager), local->message);
 			write_file_result = SR_ERROR;
 			g_propagate_error (error, local);
 			error = NULL;
 		} else {
 			_LOGT ("update-resolv-conf: write to %s succeeded (rc-manager=%s)",
-			       _PATH_RESCONF, _rc_manager_to_string (rc_manager));
+			       rc_path, _rc_manager_to_string (rc_manager));
 		}
 	}
 
@@ -766,7 +772,7 @@ update_resolv_conf (NMDnsManager *self,
 
 	if (rc_manager == NM_DNS_MANAGER_RESOLV_CONF_MAN_FILE) {
 		_LOGT ("update-resolv-conf: write internal file %s succeeded (rc-manager=%s)",
-		       _PATH_RESCONF, _rc_manager_to_string (rc_manager));
+		       rc_path, _rc_manager_to_string (rc_manager));
 		return write_file_result;
 	}