Update and document the privilege restriction.

doc/bird.sgml

@@ -145,10 +145,42 @@ options. The most important ones are:
 
 	<tag>-s <m/name of communication socket/</tag>
 	use given filename for a  socket for communications with the client, default is <it/prefix/<file>/var/run/bird.ctl</file>.
+
+	<tag>-u <m/user/</tag>
+	drop privileges and use that user ID, see the next section for details.
+
+	<tag>-g <m/group/</tag>
+	use that group ID, see the next section for details.
 </descrip>
 
 <p>BIRD writes messages about its work to log files or syslog (according to config).
 
+<sect>Privileges
+
+<p>BIRD, as a routing daemon, uses several privileged operations (like
+setting routing table and using raw sockets). Traditionally, BIRD is
+executed and runs with root privileges, which may be prone to security
+problems. The recommended way is to use a privilege restriction
+(options <cf/-u/, <cf/-g/). In that case BIRD is executed with root
+privileges, but it changes its user and group ID to an unprivileged
+ones, while using Linux capabilities to retain just required
+privileges (capabilities CAP_NET_*). Note that the control socket is
+created before the privileges are dropped, but the config file is read
+after that. The privilege restriction is not implemented in BSD port
+of BIRD.
+
+<p>A nonprivileged user (as an argument to <cf/-u/ options) may be the
+user <cf/nobody/, but it is suggested to use a new dedicated user
+account (like <cf/bird/). The similar considerations apply for
+the group option, but there is one more condition -- the users
+in the same group can use <file/birdc/ to control BIRD.
+
+<p>Finally, there is a possibility to use external tools to run BIRD in
+an environment with restricted privileges. This may need some
+configuration, but it is generally easy -- BIRD needs just the
+standard library, privileges to read the config file and create the
+control socket and the CAP_NET_* capabilities.
+
 <chapt>About routing tables
 
 <p>BIRD has one or more routing tables which may or may not be

sysdep/linux/syspriv.h

@@ -48,15 +48,20 @@ drop_uid(uid_t uid)
     CAP_TO_MASK(CAP_NET_ADMIN) |
     CAP_TO_MASK(CAP_NET_RAW);
 
+  /* change effective user ID to be able to switch to that
+     user ID completely after dropping CAP_SETUID */
   if (seteuid(uid) < 0)
     die("seteuid: %m");
 
+  /* restrict the capabilities */
   if (set_capabilities(caps) < 0)
     die("capset: %m");
 
+  /* keep the capabilities after dropping root ID */
   if (prctl(PR_SET_KEEPCAPS, 1) < 0)
     die("prctl: %m");
 
+  /* completely switch to the unprivileged user ID */
   if (setresuid(uid, uid, uid) < 0)
     die("setresuid: %m");
 }

sysdep/unix/main.c

@@ -383,7 +383,7 @@ cli_connect(sock *s, int size UNUSED)
 }
 
 static void
-cli_init_unix(void)
+cli_init_unix(uid_t use_uid, gid_t use_gid)
 {
   sock *s;
 
@@ -393,6 +393,13 @@ cli_init_unix(void)
   s->rx_hook = cli_connect;
   s->rbsize = 1024;
   sk_open_unix(s, path_control_socket);
+
+  if (use_uid || use_gid)
+    if (chown(path_control_socket, use_uid, use_gid) < 0)
+      die("chown: %m");
+
+  if (chmod(path_control_socket, 0660) < 0)
+    die("chmod: %m");
 }
 
 /*
@@ -503,9 +510,13 @@ get_uid(const char *s)
 {
   struct passwd *pw;
   char *endptr;
-  
+  long int rv;
+
+  if (!s)
+    return 0;
+
   errno = 0;
-  long int rv = strtol(s, &endptr, 10);
+  rv = strtol(s, &endptr, 10);
 
   if (!errno && !*endptr)
     return rv;
@@ -522,9 +533,13 @@ get_gid(const char *s)
 {
   struct group *gr;
   char *endptr;
+  long int rv;
+
+  if (!s)
+    return 0;
   
   errno = 0;
-  long int rv = strtol(s, &endptr, 10);
+  rv = strtol(s, &endptr, 10);
 
   if (!errno && !*endptr)
     return rv;
@@ -601,24 +616,26 @@ main(int argc, char **argv)
     log_init_debug("");
   log_switch(debug_flag, NULL, NULL);
 
-  if (use_group)
-    drop_gid(get_gid(use_group));
-
-  if (use_user)
-    drop_uid(get_uid(use_user));
-
-  if (!parse_and_exit)
-    test_old_bird(path_control_socket);
-
-  DBG("Initializing.\n");
   resource_init();
   olock_init();
   io_init();
   rt_init();
   if_init();
 
+  uid_t use_uid = get_uid(use_user);
+  gid_t use_gid = get_gid(use_group);
+
   if (!parse_and_exit)
-    cli_init_unix();
+  {
+    test_old_bird(path_control_socket);
+    cli_init_unix(use_uid, use_gid);
+  }
+
+  if (use_gid)
+    drop_gid(use_gid);
+
+  if (use_uid)
+    drop_uid(use_uid);
 
   protos_build();
   proto_build(&proto_unix_kernel);