diff --git a/usr.sbin/rtsold/rtsol.c b/usr.sbin/rtsold/rtsol.c
--- a/usr.sbin/rtsold/rtsol.c
+++ b/usr.sbin/rtsold/rtsol.c
@@ -81,6 +81,7 @@
 
 #define	_ARGS_MANAGED	managedconf_script, ifi->ifname, rasender
 #define	_ARGS_OTHER	otherconf_script, ifi->ifname, rasender
+#define	_ARGS_ALWAYS	alwaysconf_script, ifi->ifname, rasender
 #define	_ARGS_RESADD	resolvconf_script, "-a", rsid
 #define	_ARGS_RESDEL	resolvconf_script, "-d", rsid
 
@@ -327,6 +328,17 @@
 		if (!ifi->managedconfig)
 			CALL_SCRIPT(OTHER, NULL);
 	}
+
+	/*
+	 * "Always" script.
+	 */
+	if (!ifi->alwaysconfig) {
+		const char *rasender = inet_ntop(AF_INET6, &from.sin6_addr,
+		    ntopbuf, sizeof(ntopbuf));
+		ifi->alwaysconfig = 1;
+		CALL_SCRIPT(ALWAYS, NULL);
+	}
+
 	clock_gettime(CLOCK_MONOTONIC_FAST, &now);
 	newent_rai = 0;
 	rai = find_rainfo(ifi, &from);
diff --git a/usr.sbin/rtsold/rtsold.h b/usr.sbin/rtsold/rtsold.h
--- a/usr.sbin/rtsold/rtsold.h
+++ b/usr.sbin/rtsold/rtsold.h
@@ -75,6 +75,7 @@
 				 * configuration */
 	int otherconfig;	/* need a separate protocol for the "other"
 				 * configuration */
+	int alwaysconfig;
 	int state;
 	int probes;
 	int dadcount;
@@ -160,6 +161,7 @@
 extern int uflag;
 extern const char *managedconf_script;
 extern const char *otherconf_script;
+extern const char *alwaysconf_script;
 extern const char *resolvconf_script;
 extern struct cap_channel *capllflags, *capscript, *capsendmsg, *capsyslog;
 
diff --git a/usr.sbin/rtsold/rtsold.8 b/usr.sbin/rtsold/rtsold.8
--- a/usr.sbin/rtsold/rtsold.8
+++ b/usr.sbin/rtsold/rtsold.8
@@ -29,7 +29,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd December 15, 2021
+.Dd May 2, 2023
 .Dt RTSOLD 8
 .Os
 .\"
@@ -42,6 +42,7 @@
 .Op Fl dDfFimu1
 .Op Fl M Ar script-name
 .Op Fl O Ar script-name
+.Op Fl A Ar script-name
 .Op Fl p Ar pidfile
 .Op Fl R Ar script-name
 .Ar interface ...
@@ -49,6 +50,7 @@
 .Op Fl dDfFimu1
 .Op Fl M Ar script-name
 .Op Fl O Ar script-name
+.Op Fl A Ar script-name
 .Op Fl p Ar pidfile
 .Op Fl R Ar script-name
 .Fl a
@@ -56,12 +58,14 @@
 .Op Fl dDiu
 .Op Fl M Ar script-name
 .Op Fl O Ar script-name
+.Op Fl A Ar script-name
 .Op Fl R Ar script-name
 .Ar interface ...
 .Nm rtsol
 .Op Fl dDiu
 .Op Fl M Ar script-name
 .Op Fl O Ar script-name
+.Op Fl A Ar script-name
 .Op Fl R Ar script-name
 .Fl a
 .\"
@@ -250,6 +254,18 @@
 must be the absolute path from root to the script file, be a regular
 file, and be created by the same owner who runs
 .Nm .
+.It Fl A Ar script-name
+Specifies a supplement script file to always be called for the router
+advertisement.
+.Nm
+will invoke
+.Ar script-name
+with a first argument of the receiving interface name
+and a second argument of the sending router address.
+.Ar script-name
+must be the absolute path from root to the script file, be a regular
+file, and be created by the same owner who runs
+.Nm .
 .It Fl p Ar pidfile
 Writes the process ID of
 .Nm
diff --git a/usr.sbin/rtsold/rtsold.c b/usr.sbin/rtsold/rtsold.c
--- a/usr.sbin/rtsold/rtsold.c
+++ b/usr.sbin/rtsold/rtsold.c
@@ -82,6 +82,7 @@
 
 const char *managedconf_script;
 const char *otherconf_script;
+const char *alwaysconf_script;
 const char *resolvconf_script = "/sbin/resolvconf";
 
 cap_channel_t *capllflags, *capscript, *capsendmsg, *capsyslog;
@@ -126,11 +127,11 @@
 
 	progname = basename(argv[0]);
 	if (strcmp(progname, "rtsold") == 0) {
-		opts = "adDfFim1M:O:p:R:u";
+		opts = "adDfFim1M:O:A:p:R:u";
 		once = 0;
 		pidfilepath = NULL;
 	} else {
-		opts = "adDFiM:O:R:u";
+		opts = "adDFiM:O:A:R:u";
 		fflag = 1;
 		once = 1;
 	}
@@ -167,6 +168,9 @@
 		case 'O':
 			otherconf_script = optarg;
 			break;
+		case 'A':
+			alwaysconf_script = optarg;
+			break;
 		case 'p':
 			pidfilepath = optarg;
 			break;
@@ -204,6 +208,9 @@
 	if (otherconf_script != NULL && *otherconf_script != '/')
 		errx(1, "configuration script (%s) must be an absolute path",
 		    otherconf_script);
+	if (alwaysconf_script != NULL && *alwaysconf_script != '/')
+		errx(1, "configuration script (%s) must be an absolute path",
+		    alwaysconf_script);
 	if (*resolvconf_script != '/')
 		errx(1, "configuration script (%s) must be an absolute path",
 		    resolvconf_script);
@@ -336,7 +343,8 @@
 {
 #ifdef WITH_CASPER
 	const char *const scripts[] =
-	    { resolvconf_script, managedconf_script, otherconf_script };
+	    { resolvconf_script, managedconf_script, otherconf_script,
+	    alwaysconf_script };
 	cap_channel_t *capcasper;
 	nvlist_t *limits;
 
@@ -616,6 +624,7 @@
 				if (probe) {
 					ifi->managedconfig = 0;
 					ifi->otherconfig = 0;
+					ifi->alwaysconfig = 0;
 				}
 				if (probe && mobile_node) {
 					error = cap_probe_defrouters(capsendmsg,
@@ -786,13 +795,17 @@
 
 	if (strcmp(progname, "rtsold") == 0) {
 		fprintf(stderr, "usage: rtsold [-dDfFm1] [-O script-name] "
+		    "[-M script-name ] [-A script-name ] "
 		    "[-p pidfile] [-R script-name] interface ...\n");
 		fprintf(stderr, "usage: rtsold [-dDfFm1] [-O script-name] "
+		    "[-M script-name ] [-A script-name ] "
 		    "[-p pidfile] [-R script-name] -a\n");
 	} else {
 		fprintf(stderr, "usage: rtsol [-dDF] [-O script-name] "
+		    "[-M script-name ] [-A script-name ] "
 		    "[-p pidfile] [-R script-name] interface ...\n");
 		fprintf(stderr, "usage: rtsol [-dDF] [-O script-name] "
+		    "[-M script-name ] [-A script-name ] "
 		    "[-p pidfile] [-R script-name] -a\n");
 	}
 	exit(1);