pppd: Check CAP_NET_RAW capability on Linux rather than requiring euid 0

Introduce USE_LIBCAP option turned on by default for the linux build.
Provide an option to check that we are capable to admin the network wihout root
via CAP_NET_RAW libcap option. Requires libcap library.
Fallback to geteuid method in case of Solaris and Linux without libcap.

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

Author: a-andreyev

pppd/Makefile.linux

@@ -79,6 +79,9 @@ PLUGIN=y
 # Enable EAP SRP-SHA1 authentication (requires libsrp)
 #USE_SRP=y
 
+# Use libcap (requires libcap)
+#USE_LIBCAP=y
+
 # Use libutil; test if logwtmp is declared in <utmp.h> to detect
 ifeq ($(shell echo '\#include <utmp.h>' | $(CC) -E - 2>/dev/null | grep -q logwtmp && echo yes),yes)
 USE_LIBUTIL=y
@@ -148,6 +151,11 @@ CFLAGS  += -DHAVE_CRYPT_H=1
 LIBS	+= -lcrypt
 endif
 
+ifdef USE_LIBCAP
+CFLAGS  += -DUSE_CAP
+LIBS    += -lcap
+endif
+
 ifdef USE_LIBUTIL
 CFLAGS	+= -DHAVE_LOGWTMP=1
 LIBS	+= -lutil

pppd/main.c

@@ -360,11 +360,10 @@ main(int argc, char *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 have CAP_NET_RAW or root privilege to run %s", argv[0]);
 	exit(EXIT_NOT_ROOT);
     }
 

pppd/pppd.h

@@ -622,6 +622,7 @@ void sys_init(void);	/* Do system-dependent initialization */
 void sys_cleanup(void);	/* Restore system state before exiting */
 int  sys_check_options(void); /* Check options specified */
 void sys_close(void);	/* Clean up in a child before execing */
+int  net_capable((void));	/* Test for any access to the net management */
 int  ppp_available(void);	/* Test whether ppp kernel support exists */
 int  get_pty(int *, int *, char *, int);	/* Get pty master/slave */
 int  open_ppp_loopback(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
 /*
@@ -2329,6 +2334,42 @@ 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

pppd/sys-solaris.c

@@ -852,6 +852,20 @@ daemon(int nochdir, int noclose)
 }
 #endif
 
+/***********************************************************
+ *
+ * net_capable - check for any access to the net management
+ */
+
+int net_capable(void)
+{
+    int ok = 0;
+    if (geteuid() == 0) {
+        ok = 1;
+    }
+    return ok;
+}
+
 /*
  * ppp_available - check whether the system has any ppp interfaces
  */