add launch tool

src/enter_vpn.c

@@ -0,0 +1,36 @@
+#define _GNU_SOURCE
+
+#include <sched.h>
+#include <fcntl.h>
+#include <pwd.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <string.h>
+
+int main() {
+    char shell[128] = {0};
+
+    struct passwd *pw = getpwent();
+    strlcpy(shell, pw->pw_shell, sizeof(shell));
+
+    endpwent();
+
+    int fd = open("/run/vpn/net", 0);
+
+    if(fd > 0) {
+        int failure = setns(fd, CLONE_NEWNET);
+        if(failure) {
+            perror("setns /run/vpn/net");
+        }
+        close(fd);
+        if(failure) {
+            return 1;
+        }
+    } else {
+        perror("open /run/vpn/net");
+        return 1;
+    }
+    execl(shell, shell, NULL);
+    perror(NULL);
+    return 0;
+}