##dhcp-probe-03-drop-privs.patch - add option to change uid after setup
--- src/dhcp_probe.c.orig	2024-04-20 21:59:54.841088526 +0200
+++ src/dhcp_probe.c	2024-04-20 22:23:32.870073070 +0200
@@ -26,6 +26,9 @@
 #include "report.h"
 #include "utils.h"
 
+#include <sys/types.h>
+#include <pwd.h>
+
 #ifndef lint
 static const char rcsid[] = "dhcp_probe version " VERSION;
 static const char copyright[] = "Copyright 2000-2021, The Trustees of Princeton University.  All rights reserved.";
@@ -50,6 +53,8 @@ char *capture_file = NULL;
 int snaplen = CAPTURE_BUFSIZE;
 int socket_receive_timeout_feature = 0;
 int keep_pcap = 0;
+int drop_privs = 0;
+char *username = NULL;
 
 char *prog = NULL;
 char *logfile_name = NULL;
@@ -183,6 +188,41 @@ reset_pcap()
 	pd = NULL;
 }
 
+/* drop privileges */
+void 
+drop_privileges(const char *username)
+{
+	struct passwd *pw;
+	pw = getpwnam(username);
+	if (pw == NULL) {
+		report(LOG_ERR, "getpwnam: %s", get_errmsg());
+		my_exit(1, 1, 1);
+	}
+	if (debug > 1)
+		report(LOG_INFO, "changing to uid %d gid %d", pw->pw_uid, pw->pw_gid);
+	
+	if (setregid(pw->pw_gid, pw->pw_gid)) {
+		report(LOG_ERR, "setregid: %s", get_errmsg());
+		my_exit(1, 1, 1);
+	}
+	if (setreuid(pw->pw_uid, pw->pw_uid)) {
+		report(LOG_ERR, "setreuid: %s", get_errmsg());
+		my_exit(1, 1, 1);
+	}
+}
+
+void write_pidfile(void)
+{
+	FILE *pid_fp;
+	if ((pid_fp = open_for_writing(pid_file)) == NULL) {
+		report(LOG_ERR, "could not open pid file %s for writing", pid_file);
+		my_exit(1, 0, 1);
+	} else {
+		fprintf(pid_fp, "%d\n", (int) getpid());
+		fclose(pid_fp);
+	}
+}
+
 int 
 main(int argc, char **argv)
 {
@@ -190,7 +230,6 @@ main(int argc, char **argv)
 	extern char *optarg;
 	extern int optind, opterr, optopt;
 	struct sigaction sa;
-	FILE *pid_fp;
 	char *cwd = CWD;
 
 	int write_packet_len;
@@ -210,7 +249,7 @@ main(int argc, char **argv)
 	else 
 		prog = argv[0];
 
-	while ((c = getopt(argc, argv, "c:d:fhkl:o:p:Q:s:Tvw:")) != EOF) {
+	while ((c = getopt(argc, argv, "c:d:fhkl:o:p:Q:s:Tu:vw:")) != EOF) {
 		switch (c) {
 			case 'c':
 				if (optarg[0] != '/') {
@@ -285,6 +324,10 @@ main(int argc, char **argv)
 				}
 				break;
 			}
+			case 'u':
+				drop_privs = 1;
+				username = optarg;
+				break;
 			case 'T':
 				socket_receive_timeout_feature = 1;
 				break;
@@ -353,16 +396,6 @@ main(int argc, char **argv)
 		my_exit(1, 0, 1);
 	}
 
-
-	/* write pid file as soon as possible after (possibly) forking */
-	if ((pid_fp = open_for_writing(pid_file)) == NULL) {
-		report(LOG_ERR, "could not open pid file %s for writing", pid_file);
-		my_exit(1, 0, 1);
-	} else {
-		fprintf(pid_fp, "%d\n", (int) getpid());
-		fclose(pid_fp);
-	}
-
 	if (! read_configfile(config_file)) {
 		my_exit(1, 1, 1);
 	}
@@ -547,6 +580,12 @@ main(int argc, char **argv)
 	if (keep_pcap)
 		init_pcap(need_promiscuous());
 
+	if (drop_privs)
+		drop_privileges(username);
+
+	/* write the pid file after dropping privileges to be able to remove it later */
+	write_pidfile();
+
 	while (1) { /* MAIN EVENT LOOP */
 		libnet_t *l;						/* to iterate through libnet context queue */
 		/* struct pcap_stat ps;	*/			/* to hold pcap stats */
@@ -1332,6 +1371,7 @@ usage(void)
 	fprintf(stderr, "   -Q vlan_id                     tag outgoing frames with an 802.1Q VLAN ID\n");
 	fprintf(stderr, "   -s capture_bufsize             override default capture bufsize [%d]\n", CAPTURE_BUFSIZE);
 	fprintf(stderr, "   -T                             enable the socket receive timeout feature\n");
+	fprintf(stderr, "   -u username                    change uid after setup (use with -k\n");
 	fprintf(stderr, "   -v                             display version number then exit\n");
 	fprintf(stderr, "   -w cwd                         override default working directory [%s]\n", CWD);
 	fprintf(stderr, "   interface_name                 name of ethernet interface\n");