Date: Fri, 12 May 2006 04:02:23 -0700 (PDT)
From: Aaron Hopkins <lists@die.net>
To: nsd-users@NLnetLabs.nl
Subject: Making accept() not block

NSD currently assumes that selecting on a TCP listen() socket will always
result in accept() not blocking.  However, it doesn't mark its TCP listen()
sockets as non-blocking, so there is probably a race condition at least on
some OSes with either the -N flag (number of servers to fork) set to 2 or
more, or if someone opens a connection and closes it by the time NSD tries
to accept().  This may result in NSD not answering any requests until it
receives its next TCP connection.

There is a simple fix to this, which is to just mark the listen() socket as
non-blocking.  An example patch against NSD 2.3.4 is attached below.

diff -ur nsd-2.3.4/server.c nsd-2.3.4.safer/server.c
--- nsd-2.3.4/server.c	2006-04-06 07:26:35.000000000 -0700
+++ nsd-2.3.4.safer/server.c	2006-05-12 03:44:50.000000000 -0700
@@ -308,6 +308,11 @@
 		}
 #endif
 
+		if (fcntl(nsd->tcp[i].s, F_SETFL, O_NONBLOCK) == -1) {
+			log_msg(LOG_ERR, "fcntl failed: %s", strerror(errno));
+			return -1;
+		}
+
 		/* Bind it... */
 		if (bind(nsd->tcp[i].s, (struct sockaddr *) nsd->tcp[i].addr->ai_addr, nsd->tcp[i].addr->ai_addrlen) != 0) {
 			log_msg(LOG_ERR, "can't bind the socket: %s", strerror(errno));