[PATCH 07/21] fragmentation: Fix fragmentation field check

[Ian Jackson]

When an incoming packet is for secnet itself, secnet checks the
fragmentation field in the IP header.

(Contrary to the spec, secnet discards fragmented packets addressed to
its own private address; however, this is a tolerable defect as secnet
never sends packets of its own apart from ICMP errors and ICMP Echo
Reply.)

However, secnet would incorrectly check the reserved flag bit in the
16-bit fragmentation superfield.  Fix this.  Also introduce some
manifest constants for the bits in the 16-bit fragmentation
superfield.

Signed-off-by: Ian Jackson <ijackson at chiark.greenend.org.uk>
---
 netlink.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/netlink.c b/netlink.c
index 794bffe..61841b0 100644
--- a/netlink.c
+++ b/netlink.c
@@ -193,6 +193,10 @@ struct iphdr {
     uint16_t   tot_len;
     uint16_t   id;
     uint16_t   frag_off;
+#define IPHDR_FRAG_OFF  ((uint16_t)0x1fff)
+#define IPHDR_FRAG_MORE ((uint16_t)0x2000)
+#define IPHDR_FRAG_DONT ((uint16_t)0x4000)
+/*                 reserved        0x8000 */
     uint8_t    ttl;
     uint8_t    protocol;
     uint16_t   check;
@@ -305,7 +309,7 @@ static bool_t netlink_icmp_may_reply(struct buffer_if *buf)
 	}
     }
     /* How do we spot broadcast destination addresses? */
-    if (ntohs(iph->frag_off)&0x1fff) return False; /* Non-initial fragment */
+    if (ntohs(iph->frag_off)&IPHDR_FRAG_OFF) return False;
     source=ntohl(iph->saddr);
     if (source==0) return False;
     if ((source&0xff000000)==0x7f000000) return False;
@@ -599,7 +603,7 @@ static void netlink_packet_local(struct netlink *st,
     }
     h=(struct icmphdr *)buf->start;
 
-    if ((ntohs(h->iph.frag_off)&0xbfff)!=0) {
+    if ((ntohs(h->iph.frag_off)&(IPHDR_FRAG_OFF|IPHDR_FRAG_MORE))!=0) {
 	Message(M_WARNING,"%s: fragmented packet addressed to secnet; "
 		"ignoring it\n",st->name);
 	BUF_FREE(buf);
-- 
1.7.10.4