Merge branch '835-use-UV_UDP_LINUX_RECVERR-to-detect-destination-unreachable' into 'main'

Set IP(V6)_RECVERR on connect UDP sockets (via libuv)

Closes #835

See merge request isc-projects/bind9!6171

CHANGES

@@ -1,3 +1,8 @@
+5869.	[func]		Enable use of IP(V6)_RECVERR on Linux that allows
+			the kernel to report destination host/network
+			unreachable errors to the userspace application.
+			[GL #4251]
+
 5868.	[cleanup]	Use Daniel Lemire's "nearly divisionless" algorithm
 			for unbiased bounded random numbers, and move
 			re-seeding out of the hot path. [GL !6161]

configure.ac

@@ -562,6 +562,9 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([#include <sys/socket.h>],
 		  [AC_MSG_RESULT([no])
 		   AC_CHECK_DECLS([UV_UDP_RECVMMSG], [], [], [[#include <uv.h>]])])
 
+# libuv recverr support
+AC_CHECK_DECLS([UV_UDP_LINUX_RECVERR], [], [], [[#include <uv.h>]])
+
 # [pairwise: --enable-doh --with-libnghttp2=auto, --enable-doh --with-libnghttp2=yes, --disable-doh]
 AC_ARG_ENABLE([doh],
 	      [AS_HELP_STRING([--disable-doh], [enable DNS over HTTPS, requires libnghttp2 (default=yes)])],

lib/isc/netmgr/udp.c

@@ -882,6 +882,10 @@ udp_connect_direct(isc_nmsocket_t *sock, isc__nm_uvreq_t *req) {
 		uv_bind_flags |= UV_UDP_IPV6ONLY;
 	}
 
+#if HAVE_DECL_UV_UDP_LINUX_RECVERR
+	uv_bind_flags |= UV_UDP_LINUX_RECVERR;
+#endif
+
 	r = uv_udp_bind(&sock->uv_handle.udp, &sock->iface.type.sa,
 			uv_bind_flags);
 	if (r != 0) {