udp.c: Do not send NAKs in response to NAKs

If an incoming packet isn't name-addressed and has an invalid
destination site id, udp.c would send a NAK.  This is not a good idea
- if somehow the source site id was wrong too, it will result in a NAK
storm.

This is a security vulnerability as it can be used by an attacker to
trigger an unending NAK storm.

Also, improve the message printed when a NAK is sent by udp.c because
no site wanted to handle it.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

diff --git a/udp.c b/udp.c
index 77be5b1786f7868ad3691b24be8fa35c3936695d..c83e618fb6ca91876dcb1307c2bac516ba35a06a 100644 (file)
--- a/udp.c
+++ b/udp.c
@@ -133,17 +133,26 @@ static void udp_afterpoll(void *state, struct pollfd *fds, int nfds)
                    }
                }
                if (!done) {
-                   uint32_t source,dest;
-                   /* Manufacture and send NAK packet */
-                   source=get_uint32(st->rbuf->start); /* Us */
-                   dest=get_uint32(st->rbuf->start+4); /* Them */
-                   Message(M_INFO,"udp (port %d): sending NAK\n",st->port);
-                   buffer_init(st->rbuf,0);
-                   buf_append_uint32(st->rbuf,dest);
-                   buf_append_uint32(st->rbuf,source);
-                   buf_append_uint32(st->rbuf,LABEL_NAK);
-                   sendto(st->fd, st->rbuf->start, st->rbuf->size, 0,
-                          (struct sockaddr *)&from, sizeof(from));
+                   uint32_t msgtype;
+                   if (st->rbuf->size>12 /* prevents traffic amplification */
+                       && ((msgtype=get_uint32(st->rbuf->start+8))
+                           != LABEL_NAK)) {
+                       uint32_t source,dest;
+                       /* Manufacture and send NAK packet */
+                       source=get_uint32(st->rbuf->start); /* Us */
+                       dest=get_uint32(st->rbuf->start+4); /* Them */
+                       Message(M_INFO,"udp (port %d, peer %s):"
+                               " %08"PRIx32"<-%08"PRIx32": %08"PRIx32":"
+                               " unwanted/incorrect, sending NAK\n",
+                               st->port, saddr_to_string(&from),
+                               dest, source, msgtype);
+                       buffer_init(st->rbuf,0);
+                       buf_append_uint32(st->rbuf,dest);
+                       buf_append_uint32(st->rbuf,source);
+                       buf_append_uint32(st->rbuf,LABEL_NAK);
+                       sendto(st->fd, st->rbuf->start, st->rbuf->size, 0,
+                              (struct sockaddr *)&from, sizeof(from));
+                   }
                    BUF_FREE(st->rbuf);
                }
                BUF_ASSERT_FREE(st->rbuf);