at the moment the event loop code handles the syncfd directly. it could
add it's own event handler like all the other fds integrated into the
event loop with add_protocol.

could someone test this for me?

Index: dhcpd.c
===================================================================
RCS file: /cvs/src/usr.sbin/dhcpd/dhcpd.c,v
diff -u -p -r1.60 dhcpd.c
--- dhcpd.c	21 Aug 2024 09:19:55 -0000	1.60
+++ dhcpd.c	20 May 2025 04:40:59 -0000
@@ -204,8 +204,7 @@ main(int argc, char *argv[])
 			fatal("setrtable");
 
 	if (syncsend || syncrecv) {
-		syncfd = sync_init(sync_iface, sync_baddr, sync_port);
-		if (syncfd == -1)
+		if (sync_init(sync_iface, sync_baddr, sync_port) == -1)
 			err(1, "sync init");
 	}
 
Index: dispatch.c
===================================================================
RCS file: /cvs/src/usr.sbin/dhcpd/dispatch.c,v
diff -u -p -r1.45 dispatch.c
--- dispatch.c	2 Sep 2023 10:18:45 -0000	1.45
+++ dispatch.c	20 May 2025 04:40:59 -0000
@@ -67,8 +67,6 @@
 #include "log.h"
 #include "sync.h"
 
-extern int syncfd;
-
 struct interface_info *interfaces;
 struct protocol *protocols;
 struct dhcpd_timeout *timeouts;
@@ -309,8 +307,6 @@ dispatch(void)
 
 	for (nfds = 0, l = protocols; l; l = l->next)
 		nfds++;
-	if (syncfd != -1)
-		nfds++;
 	if (nfds > nfds_max) {
 		fds = reallocarray(fds, nfds, sizeof(struct pollfd));
 		if (fds == NULL)
@@ -363,12 +359,6 @@ another:
 		if (i == 0)
 			fatalx("No live interfaces to poll on - exiting.");
 
-		if (syncfd != -1) {
-			/* add syncer */
-			fds[i].fd = syncfd;
-			fds[i].events = POLLIN;
-		}
-
 		/* Wait for a packet or a timeout... */
 		switch (poll(fds, nfds, to_msec)) {
 		case -1:
@@ -392,8 +382,6 @@ another:
 			}
 			++i;
 		}
-		if ((syncfd != -1) && (fds[i].revents & (POLLIN | POLLHUP)))
-			sync_recv();
 		interfaces_invalidated = 0;
 	}
 }
Index: sync.c
===================================================================
RCS file: /cvs/src/usr.sbin/dhcpd/sync.c,v
diff -u -p -r1.25 sync.c
--- sync.c	24 Aug 2024 08:35:24 -0000	1.25
+++ sync.c	20 May 2025 04:40:59 -0000
@@ -61,6 +61,7 @@ struct sync_host {
 };
 LIST_HEAD(synchosts, sync_host) sync_hosts = LIST_HEAD_INITIALIZER(sync_hosts);
 
+void	 sync_recv(struct protocol *);
 void	 sync_send(struct iovec *, int);
 
 int
@@ -223,6 +224,8 @@ sync_init(const char *iface, const char 
 		    sendmcast ? "" : "receive ",
 		    ttl, inet_ntoa(sync_out.sin_addr), port);
 
+	add_protocol("sync", syncfd, sync_recv, NULL);
+
 	return (syncfd);
 
  fail:
@@ -231,7 +234,7 @@ sync_init(const char *iface, const char 
 }
 
 void
-sync_recv(void)
+sync_recv(struct protocol *protocol)
 {
 	struct dhcp_synchdr *hdr;
 	struct sockaddr_in addr;
Index: sync.h
===================================================================
RCS file: /cvs/src/usr.sbin/dhcpd/sync.h,v
diff -u -p -r1.5 sync.h
--- sync.h	4 Oct 2016 22:47:51 -0000	1.5
+++ sync.h	20 May 2025 04:40:59 -0000
@@ -67,9 +67,7 @@ struct dhcp_synctlv_lease {
 #define DHCP_SYNC_END		0x0000
 #define DHCP_SYNC_LEASE		0x0001
 
-extern int	 syncfd;
 extern int	 sync_init(const char *, const char *, u_short);
 extern int	 sync_addhost(const char *, u_short);
-extern void	 sync_recv(void);
 extern void	 sync_lease(struct lease *);
 #endif /* _DHCPD_SYNC */