dhcpsd: Only start privileged helper when waiting for interfaces

src/dhcp.c

@@ -368,9 +368,20 @@ dhcp_outputipudp(struct interface *ifp, const struct in_addr *src,
 	in_cksum(&udp, sizeof(udp), &sum);
 	udp.uh_sum = in_cksum(bootp, len, &sum);
 
-	nbytes = priv_sendbpf(ifp, iov, ARRAYCOUNT(iov));
-	if (nbytes == -1)
-		logerr("%s: priv_sendbpf: %s", __func__, ifp->if_name);
+	if (ctx->dhcp_ctx->ctx_options & DHCPSD_WAITIF) {
+		nbytes = priv_sendbpf(ifp, iov, ARRAYCOUNT(iov));
+		if (nbytes == -1)
+			logerr("%s: priv_sendbpf: %s", __func__, ifp->if_name);
+	} else if (ifp->if_bpf != NULL) {
+		nbytes = ifp->if_output(ifp, ifp->if_bpf->bpf_fd, iov,
+		    ARRAYCOUNT(iov));
+		if (nbytes == -1)
+			logerr("%s: if_output: %s", __func__, ifp->if_name);
+	} else {
+		errno = ENXIO;
+		logerr("%s: %s", __func__, ifp->if_name);
+		nbytes = -1;
+	}
 	return nbytes;
 }
 

src/dhcpsd.8.in

@@ -24,7 +24,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd April 16, 2026
+.Dd April 21, 2026
 .Dt DHCPSD 8
 .Os
 .Sh NAME
@@ -87,6 +87,8 @@ to work with dynamically created interfaces that are in the ready state.
 If an interface is marked down or departs then
 .Nm
 will deactivate the interface.
+This requires a privileged helper process to open a BPF socket for each
+interface when required.
 .El
 .Sh NOTES
 .Nm

src/dhcpsd.c

@@ -34,6 +34,7 @@
 #include <net/if.h>
 
 #include <errno.h>
+#include <fcntl.h>
 #include <grp.h>
 #include <ifaddrs.h>
 #include <pwd.h>
@@ -324,8 +325,14 @@ dhcpsd_configure_pools(struct interface *ifp)
 			continue;
 		/* XXX When we grow DHCPv6 only open BPF if we configure
 		 * a DHCPv4 pool. */
-		if (priv_openbpf(ifp) == -1)
-			return -1;
+		if (ctx->ctx_options & DHCPSD_WAITIF) {
+			if (priv_openbpf(ifp) == -1)
+				return -1;
+		} else {
+			ifp->if_bpf = bpf_open(ifp, bpf_bootp, O_WRONLY);
+			if (ifp->if_bpf == NULL)
+				return -1;
+		}
 		/* First plugin with config wins */
 		npools += n;
 		break;
@@ -429,12 +436,16 @@ main(int argc, char **argv)
 	ctx.ctx_argc = argc;
 	ctx.ctx_argv = argv;
 
-	if (priv_init(&ctx) == NULL) {
-		logerr("%s: priv_init", __func__);
-		goto exit;
+	/* Waiting for interfaces requires a privileged helper
+	 * to open BPF sockets. */
+	if (ctx.ctx_options & DHCPSD_WAITIF) {
+		if (priv_init(&ctx) == NULL) {
+			logerr("%s: priv_init", __func__);
+			goto exit;
+		}
+		if (ctx.ctx_options & DHCPSD_RUN)
+			goto open_pf_inet;
 	}
-	if (ctx.ctx_options & DHCPSD_RUN)
-		goto open_pf_inet;
 
 	if (unpriv_init(&ctx) == NULL) {
 		logerr("%s: unpriv_init", __func__);

src/if.c

@@ -233,7 +233,8 @@ if_learnifaces(struct ctx *ctx)
 		if_update(ifp, ifa->ifa_addr);
 		if_update_output(ifp);
 		if (if_update_mtu(ifp) == -1) {
-			logerr("%s: if_update_mtu: %s", __func__, ifa->ifa_name);
+			logerr("%s: if_update_mtu: %s", __func__,
+			    ifa->ifa_name);
 			free(ifp);
 			continue;
 		}
@@ -384,10 +385,12 @@ if_free(struct interface *ifp)
 	srv_if_free = options & DHCPSD_MAIN && !(options & DHCPSD_EXITING);
 
 	if (srv_if_free) {
-		if (options & DHCPSD_WAITIF)
+		if (options & DHCPSD_WAITIF) {
 			loginfox("%s: deactiving interface", ifp->if_name);
-		if (priv_freeif(ifp) == -1 && errno != ESRCH)
-			logerr("%s: priv_freeif: %s", __func__, ifp->if_name);
+			if (priv_freeif(ifp) == -1 && errno != ESRCH)
+				logerr("%s: priv_freeif: %s", __func__,
+				    ifp->if_name);
+		}
 		if (unpriv_freeif(ifp) == -1 && errno != ESRCH)
 			logerr("%s: unpriv_freeif: %s", __func__, ifp->if_name);
 	}