pppd: Introduce net_capable option to unroot pppd for Linux

Introduce USE_LIBCAP option for the linux build (requires libcap).
Provide an option to check that we are capable to admin the network
wihout root via CAP_NET_RAW libcap option.

Signed-off-by: Alexey Andreev <a.andreev@omprussia.ru>

Author: a-andreyev

pppd/Makefile.linux

@@ -78,6 +78,9 @@ PLUGIN=y
 # Enable EAP SRP-SHA1 authentication (requires libsrp)
 #USE_SRP=y
 
+# Use libcap (requires libcap)
+USE_LIBCAP=n
+
 # Use libutil
 USE_LIBUTIL=y
 
@@ -127,7 +130,12 @@ endif
 
 ifneq ($(wildcard $(shell $(CC) --print-sysroot)/usr/include/crypt.h),)
 CFLAGS  += -DHAVE_CRYPT_H=1
-LIBS	+= -lcrypt
+LIBS   += -lcrypt
+endif
+
+ifdef USE_LIBCAP
+CFLAGS  += -DUSE_CAP
+LIBS    += -lcap
 endif
 
 ifdef USE_LIBUTIL

pppd/main.c

@@ -370,11 +370,10 @@ main(argc, argv)
 	setlogmask(LOG_UPTO(LOG_DEBUG));
 
     /*
-     * Check that we are running as root.
+     * Check that we are capable to admin the network.
      */
-    if (geteuid() != 0) {
-	option_error("must be root to run %s, since it is not setuid-root",
-		     argv[0]);
+    if (!net_capable()) {
+    option_error("must be net capable to run the %s", argv[0]);
 	exit(EXIT_NOT_ROOT);
     }
 

pppd/pppd.h

@@ -623,6 +623,7 @@ void sys_init __P((void));	/* Do system-dependent initialization */
 void sys_cleanup __P((void));	/* Restore system state before exiting */
 int  sys_check_options __P((void)); /* Check options specified */
 void sys_close __P((void));	/* Clean up in a child before execing */
+int  net_capable __P((void));	/* Test for any access to the net management */
 int  ppp_available __P((void));	/* Test whether ppp kernel support exists */
 int  get_pty __P((int *, int *, char *, int));	/* Get pty master/slave */
 int  open_ppp_loopback __P((void)); /* Open loopback for demand-dialling */

pppd/sys-linux.c

@@ -144,6 +144,11 @@
 #include <sys/locks.h>
 #endif
 
+#ifdef USE_CAP
+#include <sys/types.h>
+#include <sys/capability.h>
+#endif /* USE_CAP */
+
 #ifdef INET6
 #ifndef _LINUX_IN6_H
 /*
@@ -2242,6 +2247,42 @@ ppp_registered(void)
     return ret;
 }
 
+/***********************************************************
+ *
+ * net_capable - check for any access to the net management
+ */
+
+int net_capable(void)
+{
+	int ok = 0;
+#ifdef USE_CAP
+	/*
+	 * Check that we are capable to admin the network.
+	 */
+	cap_t cap;
+	cap_flag_value_t cap_flag_value;
+	cap = cap_get_pid(getpid());
+	if (cap != 0) {
+		if (cap_get_flag(cap, CAP_NET_RAW, CAP_EFFECTIVE, &cap_flag_value) == 0) {
+			if (cap_flag_value == CAP_SET)
+			ok = 1;
+		}
+		if (cap_get_flag(cap, CAP_NET_RAW, CAP_PERMITTED, &cap_flag_value) == 0) {
+			if (cap_flag_value == CAP_SET)
+			ok = 1;
+		}
+	}
+#else /* USE_CAP */
+	/*
+	 * Check that we are running as root.
+	 */
+	if (geteuid() == 0) {
+		ok = 1;
+	}
+#endif /* USE_CAP */
+	return ok;
+}
+
 /********************************************************************
  *
  * ppp_available - check whether the system has any ppp interfaces