diff --git a/daemon/daemon.c b/daemon/daemon.c
index ea4e83e70..51dd51de3 100644
--- a/daemon/daemon.c
+++ b/daemon/daemon.c
@@ -79,6 +79,7 @@
 #include "util/tcp_conn_limit.h"
 #include "util/edns.h"
 #include "services/listen_dnsport.h"
+#include "services/outside_network.h"
 #include "services/cache/rrset.h"
 #include "services/cache/infra.h"
 #include "services/localzone.h"
@@ -813,6 +814,10 @@ daemon_create_workers(struct daemon* daemon)
 		fatal_exit("out of memory during daemon init");
 	numport = daemon_get_shufport(daemon, shufport);
 	verbose(VERB_ALGO, "total of %d outgoing ports available", numport);
+	if(!(daemon->shared_ports = shared_ports_create(daemon->cfg->out_ifs,
+		daemon->cfg->num_out_ifs, daemon->cfg->do_ip4,
+		daemon->cfg->do_ip6, shufport, numport)))
+		fatal_exit("could not setup shared ports: out of memory");
 
 #ifdef HAVE_NGTCP2
 	if (cfg_has_quic(daemon->cfg)) {
@@ -843,10 +848,7 @@ daemon_create_workers(struct daemon* daemon)
 #endif
 	}
 	for(i=0; i<daemon->num; i++) {
-		if(!(daemon->workers[i] = worker_create(daemon, i,
-			shufport+numport*i/daemon->num, 
-			numport*(i+1)/daemon->num - numport*i/daemon->num)))
-			/* the above is not ports/numthr, due to rounding */
+		if(!(daemon->workers[i] = worker_create(daemon, i)))
 			fatal_exit("could not create worker");
 	}
 	/* create per-worker alloc caches if not reusing existing ones. */
@@ -1204,6 +1206,8 @@ daemon_cleanup(struct daemon* daemon)
 	if(!daemon->reuse_cache || daemon->need_to_exit)
 		daemon_clear_allocs(daemon);
 	daemon->num = 0;
+	shared_ports_delete(daemon->shared_ports);
+	daemon->shared_ports = NULL;
 #ifdef USE_DNSTAP
 	dt_delete(daemon->dtenv);
 	daemon->dtenv = NULL;
diff --git a/daemon/daemon.h b/daemon/daemon.h
index 20386d7fc..e6f099629 100644
--- a/daemon/daemon.h
+++ b/daemon/daemon.h
@@ -62,6 +62,7 @@ struct doq_table;
 struct cookie_secrets;
 struct fast_reload_thread;
 struct fast_reload_printq;
+struct shared_ports;
 
 #include "dnstap/dnstap_config.h"
 #ifdef USE_DNSTAP
@@ -97,6 +98,8 @@ struct daemon {
 	int rc_port;
 	/** listening ports for remote control */
 	struct listen_port* rc_ports;
+	/** the shared ports structure, with random ports numbers. */
+	struct shared_ports* shared_ports;
 	/** remote control connections management (for first worker) */
 	struct daemon_remote* rc;
 	/** ssl context for listening to dnstcp over ssl */
diff --git a/daemon/worker.c b/daemon/worker.c
index a5dd9bc02..aa72f4e25 100644
--- a/daemon/worker.c
+++ b/daemon/worker.c
@@ -2225,23 +2225,16 @@ void worker_probe_timer_cb(void* arg)
 }
 
 struct worker*
-worker_create(struct daemon* daemon, int id, int* ports, int n)
+worker_create(struct daemon* daemon, int id)
 {
 	unsigned int seed;
 	struct worker* worker = (struct worker*)calloc(1,
 		sizeof(struct worker));
 	if(!worker)
 		return NULL;
-	worker->numports = n;
-	worker->ports = (int*)memdup(ports, sizeof(int)*n);
-	if(!worker->ports) {
-		free(worker);
-		return NULL;
-	}
 	worker->daemon = daemon;
 	worker->thread_num = id;
 	if(!(worker->cmd = tube_create())) {
-		free(worker->ports);
 		free(worker);
 		return NULL;
 	}
@@ -2249,7 +2242,6 @@ worker_create(struct daemon* daemon, int id, int* ports, int n)
 	if(!(worker->rndstate = ub_initstate(daemon->rand))) {
 		log_err("could not init random numbers.");
 		tube_delete(worker->cmd);
-		free(worker->ports);
 		free(worker);
 		return NULL;
 	}
@@ -2348,14 +2340,14 @@ worker_init(struct worker* worker, struct config_file *cfg,
 		cfg->out_ifs, cfg->num_out_ifs, cfg->do_ip4, cfg->do_ip6,
 		cfg->do_tcp?cfg->outgoing_num_tcp:0, cfg->ip_dscp,
 		worker->daemon->env->infra_cache, worker->rndstate,
-		cfg->use_caps_bits_for_id, worker->ports, worker->numports,
+		cfg->use_caps_bits_for_id,
 		cfg->unwanted_threshold, cfg->outgoing_tcp_mss,
 		&worker_alloc_cleanup, worker,
 		cfg->do_udp || cfg->udp_upstream_without_downstream,
 		worker->daemon->connect_dot_sslctx, cfg->delay_close,
 		cfg->tls_use_sni, dtenv, cfg->udp_connect,
 		cfg->max_reuse_tcp_queries, cfg->tcp_reuse_timeout,
-		cfg->tcp_auth_query_timeout);
+		cfg->tcp_auth_query_timeout, worker->daemon->shared_ports);
 	if(!worker->back) {
 		log_err("could not create outgoing sockets");
 		worker_delete(worker);
@@ -2506,7 +2498,6 @@ worker_delete(struct worker* worker)
 	tube_delete(worker->cmd);
 	comm_timer_delete(worker->stat_timer);
 	comm_timer_delete(worker->env.probe_timer);
-	free(worker->ports);
 	if(worker->thread_num == 0) {
 #ifdef UB_ON_WINDOWS
 		wsvc_desetup_worker(worker);
diff --git a/daemon/worker.h b/daemon/worker.h
index b7bb52fd7..37f3728ef 100644
--- a/daemon/worker.h
+++ b/daemon/worker.h
@@ -104,10 +104,6 @@ struct worker {
 	struct listen_dnsport* front;
 	/** the backside outside network interface to the auth servers */
 	struct outside_network* back;
-	/** ports to be used by this worker. */
-	int* ports;
-	/** number of ports for this worker */
-	int numports;
 	/** the signal handler */
 	struct comm_signal* comsig;
 	/** commpoint to listen to commands. */
@@ -146,11 +142,9 @@ struct worker {
  * with backpointers only. Use worker_init on it later.
  * @param daemon: the daemon that this worker thread is part of.
  * @param id: the thread number from 0.. numthreads-1.
- * @param ports: the ports it is allowed to use, array.
- * @param n: the number of ports.
  * @return: the new worker or NULL on alloc failure.
  */
-struct worker* worker_create(struct daemon* daemon, int id, int* ports, int n);
+struct worker* worker_create(struct daemon* daemon, int id);
 
 /**
  * Initialize worker.
diff --git a/libunbound/libworker.c b/libunbound/libworker.c
index 6e7244c03..d70527f59 100644
--- a/libunbound/libworker.c
+++ b/libunbound/libworker.c
@@ -105,6 +105,7 @@ libworker_delete_env(struct libworker* w)
 	SSL_CTX_free(w->sslctx);
 #endif
 	outside_network_delete(w->back);
+	shared_ports_delete(w->shared_ports);
 }
 
 /** delete libworker struct */
@@ -219,17 +220,25 @@ libworker_setup(struct ub_ctx* ctx, int is_bg, struct ub_event_base* eb)
 		libworker_delete(w);
 		return NULL;
 	}
+	if(!(w->shared_ports = shared_ports_create(cfg->out_ifs,
+		cfg->num_out_ifs, cfg->do_ip4, cfg->do_ip6, ports, numports))) {
+		if(!w->is_bg || w->is_bg_thread) {
+			lock_basic_unlock(&ctx->cfglock);
+		}
+		libworker_delete(w);
+		return NULL;
+	}
 	w->back = outside_network_create(w->base, cfg->msg_buffer_size,
 		(size_t)cfg->outgoing_num_ports, cfg->out_ifs,
 		cfg->num_out_ifs, cfg->do_ip4, cfg->do_ip6, 
 		cfg->do_tcp?cfg->outgoing_num_tcp:0, cfg->ip_dscp,
 		w->env->infra_cache, w->env->rnd, cfg->use_caps_bits_for_id,
-		ports, numports, cfg->unwanted_threshold,
+		cfg->unwanted_threshold,
 		cfg->outgoing_tcp_mss, &libworker_alloc_cleanup, w,
 		cfg->do_udp || cfg->udp_upstream_without_downstream, w->sslctx,
 		cfg->delay_close, cfg->tls_use_sni, NULL, cfg->udp_connect,
 		cfg->max_reuse_tcp_queries, cfg->tcp_reuse_timeout,
-		cfg->tcp_auth_query_timeout);
+		cfg->tcp_auth_query_timeout, w->shared_ports);
 	w->env->outnet = w->back;
 	if(!w->is_bg || w->is_bg_thread) {
 		lock_basic_unlock(&ctx->cfglock);
diff --git a/libunbound/libworker.h b/libunbound/libworker.h
index 42aa5bae3..f527cb093 100644
--- a/libunbound/libworker.h
+++ b/libunbound/libworker.h
@@ -60,6 +60,7 @@ struct tube;
 struct sldns_buffer;
 struct ub_event_base;
 struct query_info;
+struct shared_ports;
 
 /** 
  * The library-worker status structure
@@ -84,6 +85,8 @@ struct libworker {
 	struct comm_base* base;
 	/** the backside outside network interface to the auth servers */
 	struct outside_network* back;
+	/** shared ports structure */
+	struct shared_ports* shared_ports;
 	/** random() table for this worker. */
 	struct ub_randstate* rndstate;
 	/** sslcontext for SSL wrapped DNS over TCP queries */
diff --git a/services/outside_network.c b/services/outside_network.c
index 8034ff60b..4533217ae 100644
--- a/services/outside_network.c
+++ b/services/outside_network.c
@@ -1480,7 +1480,7 @@ portcomm_loweruse(struct outside_network* outnet, struct port_comm* pc)
 	pif = pc->pif;
 	log_assert(pif->inuse > 0);
 #ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
-	pif->avail_ports[pif->avail_total - pif->inuse] = pc->number;
+	shared_ports_return_port(outnet->shared_ports, pif->shpif, pc->number);
 #endif
 	pif->inuse--;
 	pif->out[pc->index] = pif->out[pif->inuse];
@@ -1694,19 +1694,19 @@ create_pending_tcp(struct outside_network* outnet, size_t bufsize)
 }
 
 /** setup an outgoing interface, ready address */
-static int setup_if(struct port_if* pif, const char* addrstr, 
-	int* avail, int numavail, size_t numfd)
+static int setup_if(struct port_if* pif, const char* addrstr, size_t numfd,
+	struct shared_ports* shp)
 {
-#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
-	pif->avail_total = numavail;
-	pif->avail_ports = (int*)memdup(avail, (size_t)numavail*sizeof(int));
-	if(!pif->avail_ports)
-		return 0;
-#endif
 	if(!ipstrtoaddr(addrstr, UNBOUND_DNS_PORT, &pif->addr, &pif->addrlen) &&
 	   !netblockstrtoaddr(addrstr, UNBOUND_DNS_PORT,
 			      &pif->addr, &pif->addrlen, &pif->pfxlen))
 		return 0;
+#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
+	pif->shpif = shared_ports_find_if(shp, &pif->addr, pif->addrlen,
+		pif->pfxlen);
+#else
+	(void)shp;
+#endif
 	pif->maxout = (int)numfd;
 	pif->inuse = 0;
 	pif->out = (struct port_comm**)calloc(numfd, 
@@ -1720,12 +1720,12 @@ struct outside_network*
 outside_network_create(struct comm_base *base, size_t bufsize, 
 	size_t num_ports, char** ifs, int num_ifs, int do_ip4, 
 	int do_ip6, size_t num_tcp, int dscp, struct infra_cache* infra,
-	struct ub_randstate* rnd, int use_caps_for_id, int* availports, 
-	int numavailports, size_t unwanted_threshold, int tcp_mss,
+	struct ub_randstate* rnd, int use_caps_for_id,
+	size_t unwanted_threshold, int tcp_mss,
 	void (*unwanted_action)(void*), void* unwanted_param, int do_udp,
 	void* sslctx, int delayclose, int tls_use_sni, struct dt_env* dtenv,
 	int udp_connect, int max_reuse_tcp_queries, int tcp_reuse_timeout,
-	int tcp_auth_query_timeout)
+	int tcp_auth_query_timeout, struct shared_ports* shared_ports)
 {
 	struct outside_network* outnet = (struct outside_network*)
 		calloc(1, sizeof(struct outside_network));
@@ -1760,6 +1760,7 @@ outside_network_create(struct comm_base *base, size_t bufsize,
 	outnet->do_udp = do_udp;
 	outnet->tcp_mss = tcp_mss;
 	outnet->ip_dscp = dscp;
+	outnet->shared_ports = shared_ports;
 #ifndef S_SPLINT_S
 	if(delayclose) {
 		outnet->delayclose = 1;
@@ -1770,7 +1771,7 @@ outside_network_create(struct comm_base *base, size_t bufsize,
 	if(udp_connect) {
 		outnet->udp_connect = 1;
 	}
-	if(numavailports == 0 || num_ports == 0) {
+	if(num_ports == 0) {
 		log_err("no outgoing ports available");
 		outside_network_delete(outnet);
 		return NULL;
@@ -1831,13 +1832,13 @@ outside_network_create(struct comm_base *base, size_t bufsize,
 	/* allocate interfaces */
 	if(num_ifs == 0) {
 		if(do_ip4 && !setup_if(&outnet->ip4_ifs[0], "0.0.0.0", 
-			availports, numavailports, num_ports)) {
+			num_ports, outnet->shared_ports)) {
 			log_err("malloc failed");
 			outside_network_delete(outnet);
 			return NULL;
 		}
 		if(do_ip6 && !setup_if(&outnet->ip6_ifs[0], "::", 
-			availports, numavailports, num_ports)) {
+			num_ports, outnet->shared_ports)) {
 			log_err("malloc failed");
 			outside_network_delete(outnet);
 			return NULL;
@@ -1848,7 +1849,7 @@ outside_network_create(struct comm_base *base, size_t bufsize,
 		for(i=0; i<num_ifs; i++) {
 			if(str_is_ip6(ifs[i]) && do_ip6) {
 				if(!setup_if(&outnet->ip6_ifs[done_6], ifs[i],
-					availports, numavailports, num_ports)){
+					num_ports, outnet->shared_ports)){
 					log_err("malloc failed");
 					outside_network_delete(outnet);
 					return NULL;
@@ -1857,7 +1858,7 @@ outside_network_create(struct comm_base *base, size_t bufsize,
 			}
 			if(!str_is_ip6(ifs[i]) && do_ip4) {
 				if(!setup_if(&outnet->ip4_ifs[done_4], ifs[i],
-					availports, numavailports, num_ports)){
+					num_ports, outnet->shared_ports)){
 					log_err("malloc failed");
 					outside_network_delete(outnet);
 					return NULL;
@@ -1935,9 +1936,6 @@ outside_network_delete(struct outside_network* outnet)
 				comm_point_delete(pc->cp);
 				free(pc);
 			}
-#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
-			free(outnet->ip4_ifs[i].avail_ports);
-#endif
 			free(outnet->ip4_ifs[i].out);
 		}
 		free(outnet->ip4_ifs);
@@ -1951,9 +1949,6 @@ outside_network_delete(struct outside_network* outnet)
 				comm_point_delete(pc->cp);
 				free(pc);
 			}
-#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
-			free(outnet->ip6_ifs[i].avail_ports);
-#endif
 			free(outnet->ip6_ifs[i].out);
 		}
 		free(outnet->ip6_ifs);
@@ -2163,7 +2158,10 @@ static int
 select_ifport(struct outside_network* outnet, struct pending* pend,
 	int num_if, struct port_if* ifs)
 {
-	int my_if, my_port, fd, portno, inuse, tries=0;
+	int my_if, fd, portno, inuse, tries=0;
+#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
+	int reused;
+#endif
 	struct port_if* pif;
 	/* randomly select interface and port */
 	if(num_if == 0) {
@@ -2177,37 +2175,35 @@ select_ifport(struct outside_network* outnet, struct pending* pend,
 		my_if = ub_random_max(outnet->rnd, num_if);
 		pif = &ifs[my_if];
 #ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
-		if(outnet->udp_connect) {
-			/* if we connect() we cannot reuse fds for a port */
-			if(pif->inuse >= pif->avail_total) {
-				tries++;
-				if(tries < MAX_PORT_RETRY)
-					continue;
-				log_err("failed to find an open port, drop msg");
-				return 0;
-			}
-			my_port = pif->inuse + ub_random_max(outnet->rnd,
-				pif->avail_total - pif->inuse);
-		} else  {
-			my_port = ub_random_max(outnet->rnd, pif->avail_total);
-			if(my_port < pif->inuse) {
-				/* port already open */
-				pend->pc = pif->out[my_port];
-				verbose(VERB_ALGO, "using UDP if=%d port=%d",
-					my_if, pend->pc->number);
-				break;
-			}
+		if(!shared_ports_fetch_random(outnet->shared_ports,
+			pif->shpif, outnet->rnd, outnet->udp_connect,
+			pif->inuse, &portno, &reused)) {
+			tries++;
+			if(tries < MAX_PORT_RETRY)
+				continue;
+			log_err("failed to find an open port, drop msg");
+			return 0;
+		}
+		if(reused) {
+			/* port already open */
+			log_assert(portno < pif->inuse);
+			pend->pc = pif->out[portno];
+			verbose(VERB_ALGO, "using UDP if=%d port=%d",
+				my_if, pend->pc->number);
+			break;
 		}
-		/* try to open new port, if fails, loop to try again */
-		log_assert(pif->inuse < pif->maxout);
-		portno = pif->avail_ports[my_port - pif->inuse];
 #else
-		my_port = portno = 0;
+		portno = 0;
 #endif
+		/* try to open new port, if fails, loop to try again */
 		fd = udp_sockport(&pif->addr, pif->addrlen, pif->pfxlen,
 			portno, &inuse, outnet->rnd, outnet->ip_dscp);
 		if(fd == -1 && !inuse) {
 			/* nonrecoverable error making socket */
+#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
+			shared_ports_return_port(outnet->shared_ports,
+				pif->shpif, portno);
+#endif
 			return 0;
 		}
 		if(fd != -1) {
@@ -2224,6 +2220,11 @@ select_ifport(struct outside_network* outnet, struct pending* pend,
 							pend->addrlen);
 					}
 					sock_close(fd);
+#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
+					shared_ports_return_port(
+						outnet->shared_ports,
+						pif->shpif, portno);
+#endif
 					return 0;
 				}
 			}
@@ -2241,14 +2242,14 @@ select_ifport(struct outside_network* outnet, struct pending* pend,
 
 			/* grab port in interface */
 			pif->out[pif->inuse] = pend->pc;
-#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
-			pif->avail_ports[my_port - pif->inuse] =
-				pif->avail_ports[pif->avail_total-pif->inuse-1];
-#endif
 			pif->inuse++;
 			break;
 		}
 		/* failed, already in use */
+#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
+		shared_ports_return_port(outnet->shared_ports, pif->shpif,
+			portno);
+#endif
 		verbose(VERB_QUERY, "port %d in use, trying another", portno);
 		tries++;
 		if(tries == MAX_PORT_RETRY) {
@@ -3630,13 +3631,16 @@ fd_for_dest(struct outside_network* outnet, struct sockaddr_storage* to_addr,
 {
 	struct sockaddr_storage* addr;
 	socklen_t addrlen;
-	int i, try, pnum, dscp;
+	int i, try, dscp;
 	struct port_if* pif;
 
 	/* create fd */
 	dscp = outnet->ip_dscp;
 	for(try = 0; try<1000; try++) {
 		int port = 0;
+#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
+		int reused = 0;
+#endif
 		int freebind = 0;
 		int noproto = 0;
 		int inuse = 0;
@@ -3665,16 +3669,18 @@ fd_for_dest(struct outside_network* outnet, struct sockaddr_storage* to_addr,
 		addr = &pif->addr;
 		addrlen = pif->addrlen;
 #ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
-		pnum = ub_random_max(outnet->rnd, pif->avail_total);
-		if(pnum < pif->inuse) {
-			/* port already open */
-			port = pif->out[pnum]->number;
-		} else {
-			/* unused ports in start part of array */
-			port = pif->avail_ports[pnum - pif->inuse];
+		if(!shared_ports_fetch_random(outnet->shared_ports,
+			pif->shpif, outnet->rnd, 0, pif->inuse,
+			&port, &reused)) {
+			/* try again, perhaps another interface. */
+			continue;
+		}
+		if(reused) {
+			log_assert(port < pif->inuse);
+			port = pif->out[port]->number;
 		}
 #else
-		pnum = port = 0;
+		port = 0;
 #endif
 		if(addr_is_ip6(to_addr, to_addrlen)) {
 			struct sockaddr_in6 sa = *(struct sockaddr_in6*)addr;
@@ -3689,6 +3695,14 @@ fd_for_dest(struct outside_network* outnet, struct sockaddr_storage* to_addr,
 				(struct sockaddr*)addr, addrlen, 1, &inuse, &noproto,
 				0, 0, 0, NULL, 0, freebind, 0, dscp);
 		}
+#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
+		if(!reused) {
+			/* Return the port to the pool, since the caller does
+			 * not keep track of it, also have done fd, and bind. */
+			shared_ports_return_port(outnet->shared_ports,
+				pif->shpif, port);
+		}
+#endif
 		if(fd != -1) {
 			return fd;
 		}
@@ -3919,11 +3933,7 @@ if_get_mem(struct port_if* pif)
 {
 	size_t s;
 	int i;
-	s = sizeof(*pif) +
-#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
-	    sizeof(int)*pif->avail_total +
-#endif
-		sizeof(struct port_comm*)*pif->maxout;
+	s = sizeof(*pif) + sizeof(struct port_comm*)*pif->maxout;
 	for(i=0; i<pif->inuse; i++)
 		s += sizeof(*pif->out[i]) + 
 			comm_point_get_mem(pif->out[i]->cp);
@@ -4011,3 +4021,237 @@ serviced_get_mem(struct serviced_query* sq)
 	return s;
 }
 
+#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
+/** Setup shared port interface */
+static int shared_ports_setup_if(struct shared_ports_if* shpif, char* str,
+	int* availports, int numavailports)
+{
+	shpif->avail_ports = (int*)memdup(availports,
+		(size_t)numavailports*sizeof(int));
+	if(!shpif->avail_ports)
+		return 0;
+	shpif->avail_total = numavailports;
+	shpif->inuse = 0;
+	shpif->pfxlen = 0;
+	if(!ipstrtoaddr(str, UNBOUND_DNS_PORT, &shpif->addr, &shpif->addrlen) &&
+	   !netblockstrtoaddr(str, UNBOUND_DNS_PORT, &shpif->addr,
+	   &shpif->addrlen, &shpif->pfxlen))
+		return 0;
+	return 1;
+}
+#endif
+
+#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
+/** Allocate shared ports interfaces */
+static int shared_ports_alloc_ifs(struct shared_ports* shp, char** ifs,
+	int num_ifs, int do_ip4, int do_ip6, int* availports,
+	int numavailports)
+{
+#ifndef INET6
+	do_ip6 = 0;
+#endif
+	calc_num46(ifs, num_ifs, do_ip4, do_ip6,
+		&shp->num_ip4, &shp->num_ip6);
+	if(shp->num_ip4 != 0) {
+		if(!(shp->ip4_ifs = (struct shared_ports_if*)calloc(
+			(size_t)shp->num_ip4,
+			sizeof(struct shared_ports_if))))
+			return 0;
+	}
+	if(shp->num_ip6 != 0) {
+		if(!(shp->ip6_ifs = (struct shared_ports_if*)calloc(
+			(size_t)shp->num_ip6,
+			sizeof(struct shared_ports_if))))
+			return 0;
+	}
+	if(num_ifs == 0) {
+		if(do_ip4 && !shared_ports_setup_if(&shp->ip4_ifs[0],
+			"0.0.0.0", availports, numavailports))
+			return 0;
+		if(do_ip6 && !shared_ports_setup_if(&shp->ip6_ifs[0],
+			"::", availports, numavailports))
+			return 0;
+	} else {
+		size_t done_4 = 0, done_6 = 0;
+		int i;
+		for(i=0; i<num_ifs; i++) {
+			if(str_is_ip6(ifs[i]) && do_ip6) {
+				if(!shared_ports_setup_if(&shp->ip6_ifs[done_6],
+					ifs[i], availports, numavailports))
+					return 0;
+				done_6++;
+			}
+			if(!str_is_ip6(ifs[i]) && do_ip4) {
+				if(!shared_ports_setup_if(&shp->ip4_ifs[done_4],
+					ifs[i], availports, numavailports))
+					return 0;
+				done_4++;
+			}
+		}
+	}
+	return 1;
+}
+#endif
+
+struct shared_ports* shared_ports_create(char** ifs, int num_ifs, int do_ip4,
+	int do_ip6, int* availports, int numavailports)
+{
+	struct shared_ports* shp = calloc(1, sizeof(*shp));
+	if(!shp) {
+		log_err("malloc failed");
+		return NULL;
+	}
+	lock_basic_init(&shp->lock);
+	lock_protect(&shp->lock, shp, sizeof(*shp));
+
+#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
+	/* Allocate interfaces */
+	if(!shared_ports_alloc_ifs(shp, ifs, num_ifs, do_ip4, do_ip6,
+		availports, numavailports)) {
+		log_err("malloc failed");
+		shared_ports_delete(shp);
+		return NULL;
+	}
+#else
+	(void)ifs; (void)num_ifs; (void)do_ip4; (void)do_ip6;
+	(void)availports; (void)numavailports;
+#endif
+	return shp;
+}
+
+#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
+/** Delete shared ports interface structure */
+static void shared_ports_if_delete(struct shared_ports_if* shpif)
+{
+	if(!shpif)
+		return;
+	free(shpif->avail_ports);
+}
+#endif
+
+void shared_ports_delete(struct shared_ports* shp)
+{
+#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
+	int i;
+#endif
+	if(!shp)
+		return;
+	lock_basic_destroy(&shp->lock);
+#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
+	for(i=0; i<shp->num_ip4; i++) {
+		shared_ports_if_delete(&shp->ip4_ifs[i]);
+	}
+	free(shp->ip4_ifs);
+	for(i=0; i<shp->num_ip6; i++) {
+		shared_ports_if_delete(&shp->ip6_ifs[i]);
+	}
+	free(shp->ip6_ifs);
+#endif
+	free(shp);
+}
+
+struct shared_ports_if* shared_ports_find_if(struct shared_ports* shp,
+	struct sockaddr_storage* addr, socklen_t addrlen, int pfxlen)
+{
+#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
+	struct shared_ports_if* ret, *ifs = NULL;
+	int i, num_ifs = 0;
+	lock_basic_lock(&shp->lock);
+	if(addr_is_ip6(addr, addrlen)) {
+		ifs = shp->ip6_ifs;
+		num_ifs = shp->num_ip6;
+	} else {
+		ifs = shp->ip4_ifs;
+		num_ifs = shp->num_ip4;
+	}
+	for(i=0; i<num_ifs; i++) {
+		if(sockaddr_cmp(addr, addrlen, &ifs[i].addr,
+			ifs[i].addrlen) == 0
+			&& pfxlen == ifs[i].pfxlen) {
+			ret = &ifs[i];
+			lock_basic_unlock(&shp->lock);
+			return ret;
+		}
+	}
+	lock_basic_unlock(&shp->lock);
+	return NULL;
+#else
+	(void)shp; (void)addr; (void)addrlen; (void)pfxlen;
+	return NULL;
+#endif
+}
+
+int shared_ports_fetch_random(struct shared_ports* shp,
+	struct shared_ports_if* shpif, struct ub_randstate* rnd,
+	int udp_connect, int reusenum, int* port, int* reused)
+{
+#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
+	int portno = 0, my_port = 0;
+	if(!shpif)
+		return 0;
+	lock_basic_lock(&shp->lock);
+	if(udp_connect) {
+		/* if we connect() we cannot reuse fds for a port. */
+		if(shpif->inuse >= shpif->avail_total) {
+			lock_basic_unlock(&shp->lock);
+			return 0;
+		}
+		my_port = ub_random_max(rnd,
+			shpif->avail_total - shpif->inuse);
+	} else {
+		/* select from free ports and open ports on this thread. */
+		if(shpif->inuse >= shpif->avail_total) {
+			lock_basic_unlock(&shp->lock);
+			if(reusenum == 0) {
+				return 0;
+			}
+			my_port = ub_random_max(rnd, reusenum);
+			*port = my_port;
+			*reused = 1;
+			return 1;
+		}
+		my_port = ub_random_max(rnd, shpif->avail_total - shpif->inuse
+			+ reusenum);
+		if(my_port < reusenum) {
+			/* port already open */
+			lock_basic_unlock(&shp->lock);
+			*port = my_port;
+			*reused = 1;
+			return 1;
+		}
+		my_port -= reusenum;
+	}
+	log_assert(shpif->inuse < shpif->avail_total);
+	log_assert(my_port >= 0 && my_port < shpif->avail_total);
+	portno = shpif->avail_ports[my_port];
+	shpif->avail_ports[my_port] =
+		shpif->avail_ports[shpif->avail_total-shpif->inuse-1];
+	shpif->inuse++;
+	lock_basic_unlock(&shp->lock);
+	*port = portno;
+	*reused = 0;
+	return 1;
+#else
+	(void)shp; (void)shpif; (void)rnd; (void)udp_connect;
+	(void)reusenum;
+	*port = 0;
+	*reused = 0;
+	return 1;
+#endif
+}
+
+void shared_ports_return_port(struct shared_ports* shp,
+	struct shared_ports_if* shpif, int port)
+{
+#ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
+	if(!shpif)
+		return;
+	lock_basic_lock(&shp->lock);
+	log_assert(shpif->inuse > 0);
+	shpif->avail_ports[shpif->avail_total - shpif->inuse] = port;
+	shpif->inuse--;
+	lock_basic_unlock(&shp->lock);
+#else
+	(void)shp; (void)shpif; (void)port;
+#endif
+}
diff --git a/services/outside_network.h b/services/outside_network.h
index e30ce92eb..573f3cb02 100644
--- a/services/outside_network.h
+++ b/services/outside_network.h
@@ -70,6 +70,8 @@ struct module_env;
 struct module_qstate;
 struct query_info;
 struct config_file;
+struct shared_ports;
+struct shared_ports_if;
 
 /**
  * Send queries to outside servers and wait for answers from servers.
@@ -119,6 +121,9 @@ struct outside_network {
 	int udp_connect;
 	/** number of udp packets sent. */
 	size_t num_udp_outgoing;
+	/** the shared ports structure, with random ports numbers.
+	 * This is a reference to the member in the daemon structure. */
+	struct shared_ports* shared_ports;
 
 	/** array of outgoing IP4 interfaces */
 	struct port_if* ip4_ifs;
@@ -211,11 +216,8 @@ struct port_if {
 	int pfxlen;
 
 #ifndef DISABLE_EXPLICIT_PORT_RANDOMISATION
-	/** the available ports array. These are unused.
-	 * Only the first total-inuse part is filled. */
-	int* avail_ports;
-	/** the total number of available ports (size of the array) */
-	int avail_total;
+	/** the shared port numbers for this interface. */
+	struct shared_ports_if* shpif;
 #endif
 
 	/** array of the commpoints currently in use. 
@@ -245,6 +247,42 @@ struct port_comm {
 	struct comm_point* cp;
 };
 
+/**
+ * Shared ports, the list of ports shared across threads
+ */
+struct shared_ports {
+	/** mutex on the ports */
+	lock_basic_type lock;
+	/** array of IP4 interfaces */
+	struct shared_ports_if* ip4_ifs;
+	/** number of outgoing IP4 interfaces */
+	int num_ip4;
+	/** array of IP6 interfaces */
+	struct shared_ports_if* ip6_ifs;
+	/** number of outgoing IP6 interfaces */
+	int num_ip6;
+};
+
+/**
+ * Shared ports for an interface.
+ */
+struct shared_ports_if {
+	/** address ready to allocate new socket (except port no). */
+	struct sockaddr_storage addr;
+	/** length of addr field */
+	socklen_t addrlen;
+	/** if a netblock, the prefix */
+	int pfxlen;
+
+	/** the available ports array. These are unused.
+	 * Only the first total-inuse part is filled. */
+	int* avail_ports;
+	/** the total number of available ports (size of the array) */
+	int avail_total;
+	/** the number in use. */
+	int inuse;
+};
+
 /**
  * Reuse TCP connection, still open can be used again.
  */
@@ -551,8 +589,6 @@ struct serviced_query {
  * @param infra: pointer to infra cached used for serviced queries.
  * @param rnd: stored to create random numbers for serviced queries.
  * @param use_caps_for_id: enable to use 0x20 bits to encode id randomness.
- * @param availports: array of available ports. 
- * @param numavailports: number of available ports in array.
  * @param unwanted_threshold: when to take defensive action.
  * @param unwanted_action: the action to take.
  * @param unwanted_param: user parameter to action.
@@ -567,17 +603,18 @@ struct serviced_query {
  * @param max_reuse_tcp_queries: max number of queries on a reuse connection.
  * @param tcp_reuse_timeout: timeout for REUSE entries in milliseconds.
  * @param tcp_auth_query_timeout: timeout in milliseconds for TCP queries to auth servers.
+ * @param shared_ports: the shared_ports structure.
  * @return: the new structure (with no pending answers) or NULL on error.
  */
 struct outside_network* outside_network_create(struct comm_base* base,
 	size_t bufsize, size_t num_ports, char** ifs, int num_ifs,
 	int do_ip4, int do_ip6, size_t num_tcp, int dscp, struct infra_cache* infra, 
-	struct ub_randstate* rnd, int use_caps_for_id, int* availports, 
-	int numavailports, size_t unwanted_threshold, int tcp_mss,
+	struct ub_randstate* rnd, int use_caps_for_id,
+	size_t unwanted_threshold, int tcp_mss,
 	void (*unwanted_action)(void*), void* unwanted_param, int do_udp,
 	void* sslctx, int delayclose, int tls_use_sni, struct dt_env *dtenv,
 	int udp_connect, int max_reuse_tcp_queries, int tcp_reuse_timeout,
-	int tcp_auth_query_timeout);
+	int tcp_auth_query_timeout, struct shared_ports* shared_ports);
 
 /**
  * Delete outside_network structure.
@@ -819,6 +856,54 @@ struct comm_point* outnet_comm_point_for_http(struct outside_network* outnet,
 /** connect tcp connection to addr, 0 on failure */
 int outnet_tcp_connect(int s, struct sockaddr_storage* addr, socklen_t addrlen);
 
+/**
+ * Create new shared ports structure.
+ * @param ifs: interface names (or NULL for default interface).
+ *    These interfaces must be able to access all authoritative servers.
+ * @param num_ifs: number of names in array ifs.
+ * @param do_ip4: service IP4.
+ * @param do_ip6: service IP6.
+ * @param availports: array of available ports.
+ * @param numavailports: number of available ports in array.
+ * @return new, or NULL on failure.
+ */
+struct shared_ports* shared_ports_create(char** ifs, int num_ifs, int do_ip4,
+	int do_ip6, int* availports, int numavailports);
+
+/**
+ * Delete shared ports structure.
+ * @param shp: shared ports structure.
+ */
+void shared_ports_delete(struct shared_ports* shp);
+
+/** Find interface in shared ports. */
+struct shared_ports_if* shared_ports_find_if(struct shared_ports* shp,
+	struct sockaddr_storage* addr, socklen_t addrlen, int pfxlen);
+
+/**
+ * Get a shared port from the list of random ports.
+ * @param shp: shared ports structure.
+ * @param shpif: the shared ports interface.
+ * @param rnd: used to make random numbers.
+ * @param udp_connect: set to true if no reuse is possible.
+ * @param reusenum: number of ports that can be reused (already open).
+ * @param port: the port number is returned.
+ * @param reused: if the port numer is reused, returned.
+ * @return false on failure. That can mean no more free ports to use.
+ */
+int shared_ports_fetch_random(struct shared_ports* shp,
+	struct shared_ports_if* shpif, struct ub_randstate* rnd,
+	int udp_connect, int reusenum, int* port, int* reused);
+
+/**
+ * Return a shared port to the list of random ports.
+ * @param shp: shared ports structure.
+ * @param shpif: the shared ports interface.
+ * @param port: port number to return to be used again.
+ */
+void shared_ports_return_port(struct shared_ports* shp,
+	struct shared_ports_if* shpif, int port);
+
 /** callback for incoming udp answers from the network */
 int outnet_udp_cb(struct comm_point* c, void* arg, int error,
 	struct comm_reply *reply_info);