dnsmasq: backport arcount edns0 fix

Don't return arcount=1 if EDNS0 RR won't fit in the packet.

Omitting the EDNS0 RR but setting arcount gives a malformed packet.
Also, don't accept UDP packet size less than 512 in received EDNS0.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

package/network/services/dnsmasq/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=dnsmasq
 PKG_VERSION:=2.77
-PKG_RELEASE:=11
+PKG_RELEASE:=12
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.xz
 PKG_SOURCE_URL:=http://thekelleys.org.uk/dnsmasq/

package/network/services/dnsmasq/patches/030-fix-arcount-edns0-behaviour.patch

@@ -0,0 +1,44 @@
+From a3303e196e5d304ec955c4d63afb923ade66c6e8 Mon Sep 17 00:00:00 2001
+From: Simon Kelley <simon@thekelleys.org.uk>
+Date: Thu, 7 Sep 2017 20:45:00 +0100
+Subject: [PATCH] Don't return arcount=1 if EDNS0 RR won't fit in the packet.
+
+Omitting the EDNS0 RR but setting arcount gives a malformed packet.
+Also, don't accept UDP packet size less than 512 in recieved EDNS0.
+---
+ src/edns0.c   |    5 ++++-
+ src/forward.c |    2 ++
+ 2 files changed, 6 insertions(+), 1 deletion(-)
+
+diff --git a/src/edns0.c b/src/edns0.c
+index 3fde17f..f5b798c 100644
+--- a/src/edns0.c
++++ b/src/edns0.c
+@@ -208,7 +208,10 @@ size_t add_pseudoheader(struct dns_header *header, size_t plen, unsigned char *l
+ 	  free(buff);
+ 	  p += rdlen;
+ 	}
+-      header->arcount = htons(ntohs(header->arcount) + 1);
++      
++      /* Only bump arcount if RR is going to fit */ 
++      if (((ssize_t)optlen) <= (limit - (p + 4)))
++	header->arcount = htons(ntohs(header->arcount) + 1);
+     }
+   
+   if (((ssize_t)optlen) > (limit - (p + 4)))
+diff --git a/src/forward.c b/src/forward.c
+index e3fa94b..942b02d 100644
+--- a/src/forward.c
++++ b/src/forward.c
+@@ -1412,6 +1412,8 @@ void receive_query(struct listener *listen, time_t now)
+ 	 defaults to 512 */
+       if (udp_size > daemon->edns_pktsz)
+ 	udp_size = daemon->edns_pktsz;
++      else if (udp_size < PACKETSZ)
++	udp_size = PACKETSZ; /* Sanity check - can't reduce below default. RFC 6891 6.2.3 */
+     }
+ 
+ #ifdef HAVE_AUTH
+-- 
+1.7.10.4
+