diff --git a/conf.c b/conf.c
index 14d8ece..2f5d649 100644
--- a/conf.c
+++ b/conf.c
@@ -1245,6 +1245,7 @@ void conf(struct ctx *c, int argc, char **argv)
 	const char *optstring;
 	size_t logsize = 0;
 	char *runas = NULL;
+	long fd_tap_opt;
 	int name, ret;
 	uid_t uid;
 	gid_t gid;
@@ -1260,6 +1261,7 @@ void conf(struct ctx *c, int argc, char **argv)
 	c->tcp.fwd_in.mode = c->tcp.fwd_out.mode = FWD_UNSET;
 	c->udp.fwd_in.mode = c->udp.fwd_out.mode = FWD_UNSET;
 
+	optind = 1;
 	do {
 		name = getopt_long(argc, argv, optstring, options, NULL);
 
@@ -1424,11 +1426,13 @@ void conf(struct ctx *c, int argc, char **argv)
 			break;
 		case 'F':
 			errno = 0;
-			c->fd_tap = strtol(optarg, NULL, 0);
+			fd_tap_opt = strtol(optarg, NULL, 0);
 
-			if (c->fd_tap < 0 || errno)
+			if (errno ||
+			    fd_tap_opt <= STDERR_FILENO || fd_tap_opt > INT_MAX)
 				die("Invalid --fd: %s", optarg);
 
+			c->fd_tap = fd_tap_opt;
 			c->one_off = true;
 			*c->sock_path = 0;
 			break;
diff --git a/isolation.c b/isolation.c
index 4956d7e..45fba1e 100644
--- a/isolation.c
+++ b/isolation.c
@@ -29,7 +29,8 @@
  *
  * Executed immediately after startup, drops capabilities we don't
  * need at any point during execution (or which we gain back when we
- * need by joining other namespaces).
+ * need by joining other namespaces), and closes any leaked file we
+ * might have inherited from the parent process.
  *
  * 2. isolate_user()
  * =================
@@ -166,14 +167,17 @@ static void clamp_caps(void)
 }
 
 /**
- * isolate_initial() - Early, config independent self isolation
+ * isolate_initial() - Early, mostly config independent self isolation
+ * @argc:	Argument count
+ * @argv:	Command line options: only --fd (if present) is relevant here
  *
  * Should:
  *  - drop unneeded capabilities
+ *  - close all open files except for standard streams and the one from --fd
  * Musn't:
  *  - remove filesytem access (we need to access files during setup)
  */
-void isolate_initial(void)
+void isolate_initial(int argc, char **argv)
 {
 	uint64_t keep;
 
@@ -207,6 +211,8 @@ void isolate_initial(void)
 		keep |= BIT(CAP_SETFCAP) | BIT(CAP_SYS_PTRACE);
 
 	drop_caps_ep_except(keep);
+
+	close_open_files(argc, argv);
 }
 
 /**
diff --git a/isolation.h b/isolation.h
index 846b2af..80bb68d 100644
--- a/isolation.h
+++ b/isolation.h
@@ -7,7 +7,7 @@
 #ifndef ISOLATION_H
 #define ISOLATION_H
 
-void isolate_initial(void);
+void isolate_initial(int argc, char **argv);
 void isolate_user(uid_t uid, gid_t gid, bool use_userns, const char *userns,
 		  enum passt_modes mode);
 int isolate_prefork(const struct ctx *c);
diff --git a/passt.c b/passt.c
index ea5bece..4b3c306 100644
--- a/passt.c
+++ b/passt.c
@@ -211,7 +211,7 @@ int main(int argc, char **argv)
 
 	arch_avx2_exec(argv);
 
-	isolate_initial();
+	isolate_initial(argc, argv);
 
 	c.pasta_netns_fd = c.fd_tap = c.pidfile_fd = -1;
 
diff --git a/util.c b/util.c
index 07fb21c..7761bd3 100644
--- a/util.c
+++ b/util.c
@@ -26,6 +26,7 @@
 #include <errno.h>
 #include <stdbool.h>
 #include <linux/errqueue.h>
+#include <getopt.h>
 
 #include "util.h"
 #include "iov.h"
@@ -694,3 +695,43 @@ const char *str_ee_origin(const struct sock_extended_err *ee)
 
 	return "<invalid>";
 }
+
+/**
+ * close_open_files() - Close leaked files, but not --fd, stdin, stdout, stderr
+ * @argc:	Argument count
+ * @argv:	Command line options, as we need to skip any file given via --fd
+ */
+void close_open_files(int argc, char **argv)
+{
+	const struct option optfd[] = { { "fd", required_argument, NULL, 'F' },
+					{ 0 },
+				      };
+	long fd = -1;
+	int name, rc;
+
+	do {
+		name = getopt_long(argc, argv, ":F", optfd, NULL);
+
+		if (name == 'F') {
+			errno = 0;
+			fd = strtol(optarg, NULL, 0);
+
+			if (errno || fd <= STDERR_FILENO || fd > INT_MAX)
+				die("Invalid --fd: %s", optarg);
+		}
+	} while (name != -1);
+
+	if (fd == -1) {
+		rc = close_range(STDERR_FILENO + 1, ~0U, CLOSE_RANGE_UNSHARE);
+	} else if (fd == STDERR_FILENO + 1) { /* Still a single range */
+		rc = close_range(STDERR_FILENO + 2, ~0U, CLOSE_RANGE_UNSHARE);
+	} else {
+		rc = close_range(STDERR_FILENO + 1, fd - 1,
+				 CLOSE_RANGE_UNSHARE);
+		if (!rc)
+			rc = close_range(fd + 1, ~0U, CLOSE_RANGE_UNSHARE);
+	}
+
+	if (rc)
+		die_perror("Failed to close files leaked by parent");
+}
diff --git a/util.h b/util.h
index 83d2b53..cb4d181 100644
--- a/util.h
+++ b/util.h
@@ -183,6 +183,7 @@ int __daemon(int pidfile_fd, int devnull_fd);
 int fls(unsigned long x);
 int write_file(const char *path, const char *buf);
 int write_remainder(int fd, const struct iovec *iov, size_t iovcnt, size_t skip);
+void close_open_files(int argc, char **argv);
 
 /**
  * af_name() - Return name of an address family