From 312723debac9b9c328c9399a191a5fc637f56e73 Mon Sep 17 00:00:00 2001

From: Simon Kelley <simon@thekelleys.org.uk>

Date: Wed, 18 Nov 2020 18:34:55 +0000

Subject: [PATCH] Handle multiple identical near simultaneous DNS queries

 better.

Previously, such queries would all be forwarded

independently. This is, in theory, inefficent but in practise

not a problem, _except_ that is means that an answer for any

of the forwarded queries will be accepted and cached.

An attacker can send a query multiple times, and for each repeat,

another {port, ID} becomes capable of accepting the answer he is

sending in the blind, to random IDs and ports. The chance of a

succesful attack is therefore multiplied by the number of repeats

of the query. The new behaviour detects repeated queries and

merely stores the clients sending repeats so that when the

first query completes, the answer can be sent to all the

clients who asked. Refer: CERT VU#434904.

---

 src/dnsmasq.h |  19 ++++---

 src/forward.c | 144 ++++++++++++++++++++++++++++++++++++++++++--------

 2 files changed, 135 insertions(+), 28 deletions(-)

diff --git a/src/dnsmasq.h b/src/dnsmasq.h

index 9f0af7f..9f5b48b 100644

--- a/src/dnsmasq.h

+++ b/src/dnsmasq.h

@@ -597,21 +597,26 @@ struct hostsfile {

 #define FREC_DO_QUESTION       64

 #define FREC_ADDED_PHEADER    128

 #define FREC_TEST_PKTSZ       256

-#define FREC_HAS_EXTRADATA    512        

+#define FREC_HAS_EXTRADATA    512

+#define FREC_HAS_PHEADER     1024

 #define HASH_SIZE 32 /* SHA-256 digest size */

 struct frec {

-  union mysockaddr source;

-  struct all_addr dest;

+  struct frec_src {

+    union mysockaddr source;

+    struct all_addr dest;

+    unsigned int iface, log_id;

+    unsigned short orig_id;

+    struct frec_src *next;

+  } frec_src;

   struct server *sentto; /* NULL means free */

   struct randfd *rfd4;

 #ifdef HAVE_IPV6

   struct randfd *rfd6;

 #endif

-  unsigned int iface;

-  unsigned short orig_id, new_id;

-  int log_id, fd, forwardall, flags;

+  unsigned short new_id;

+  int fd, forwardall, flags;

   time_t time;

   unsigned char *hash[HASH_SIZE];

 #ifdef HAVE_DNSSEC 

@@ -1007,6 +1012,8 @@ extern struct daemon {

 #endif

   unsigned int local_answer, queries_forwarded, auth_answer;

   struct frec *frec_list;

+  struct frec_src *free_frec_src;

+  int frec_src_count;

   struct serverfd *sfds;

   struct irec *interfaces;

   struct listener *listeners;

diff --git a/src/forward.c b/src/forward.c

index 596f593..a0b1f1d 100644

--- a/src/forward.c

+++ b/src/forward.c

@@ -20,6 +20,8 @@ static struct frec *lookup_frec(unsigned short id, int fd, int family, void *has

 static struct frec *lookup_frec_by_sender(unsigned short id,

 					  union mysockaddr *addr,

 					  void *hash);

+static struct frec *lookup_frec_by_query(void *hash, unsigned int flags);

+

 static unsigned short get_id(void);

 static void free_frec(struct frec *f);

@@ -236,15 +238,28 @@ static int forward_query(int udpfd, union mysockaddr *udpaddr,

   int type = SERV_DO_DNSSEC, norebind = 0;

   struct all_addr *addrp = NULL;

   unsigned int flags = 0;

+  unsigned int fwd_flags = 0;

   struct server *start = NULL;

   void *hash = hash_questions(header, plen, daemon->namebuff);

 #ifdef HAVE_DNSSEC

   int do_dnssec = 0;

 #endif

- unsigned int gotname = extract_request(header, plen, daemon->namebuff, NULL);

+  unsigned int gotname = extract_request(header, plen, daemon->namebuff, NULL);

+  unsigned char *oph = find_pseudoheader(header, plen, NULL, NULL, NULL, NULL);

  (void)do_bit;

-

+  

+  if (header->hb4 & HB4_CD)

+    fwd_flags |= FREC_CHECKING_DISABLED;

+  if (ad_reqd)

+    fwd_flags |= FREC_AD_QUESTION;

+  if (oph)

+    fwd_flags |= FREC_HAS_PHEADER;

+#ifdef HAVE_DNSSEC

+  if (do_bit)

+    fwd_flags |= FREC_DO_QUESTION;

+#endif

+  

   /* may be no servers available. */

   if (forward || (forward = lookup_frec_by_sender(ntohs(header->id), udpaddr, hash)))

     {

@@ -321,6 +336,41 @@ static int forward_query(int udpfd, union mysockaddr *udpaddr,

     }

   else 

     {

+      /* Query from new source, but the same query may be in progress

+	 from another source. If so, just add this client to the

+	 list that will get the reply.

+	 

+	 Note that is the EDNS client subnet option is in use, we can't do this,

+	 as the clients (and therefore query EDNS options) will be different

+	 for each query. The EDNS subnet code has checks to avoid

+	 attacks in this case. */

+      if (!option_bool(OPT_CLIENT_SUBNET) && (forward = lookup_frec_by_query(hash, fwd_flags)))

+	{

+	  if (!daemon->free_frec_src &&

+	      daemon->frec_src_count < daemon->ftabsize &&

+	      (daemon->free_frec_src = whine_malloc(sizeof(struct frec_src))))

+	    {

+	      daemon->frec_src_count++;

+	      daemon->free_frec_src->next = NULL;

+	    }

+	  

+	  /* If we've been spammed with many duplicates, just drop the query. */

+	  if (daemon->free_frec_src)

+	    {

+	      struct frec_src *new = daemon->free_frec_src;

+	      daemon->free_frec_src = new->next;

+	      new->next = forward->frec_src.next;

+	      forward->frec_src.next = new;

+	      new->orig_id = ntohs(header->id);

+	      new->source = *udpaddr;

+	      new->dest = *dst_addr;

+	      new->log_id = daemon->log_id;

+	      new->iface = dst_iface;

+	    }

+	  

+	  return 1;

+	}

+	

       if (gotname)

 	flags = search_servers(now, &addrp, gotname, daemon->namebuff, &type, &domain, &norebind);

@@ -328,22 +378,23 @@ static int forward_query(int udpfd, union mysockaddr *udpaddr,

       do_dnssec = type & SERV_DO_DNSSEC;

 #endif

       type &= ~SERV_DO_DNSSEC;      

-

+      

       if (daemon->servers && !flags)

 	forward = get_new_frec(now, NULL, 0);

       /* table full - flags == 0, return REFUSED */

       if (forward)

 	{

-	  forward->source = *udpaddr;

-	  forward->dest = *dst_addr;

-	  forward->iface = dst_iface;

-	  forward->orig_id = ntohs(header->id);

+	  forward->frec_src.source = *udpaddr;

+	  forward->frec_src.orig_id = ntohs(header->id);

+	  forward->frec_src.dest = *dst_addr;

+	  forward->frec_src.iface = dst_iface;

+	  forward->frec_src.next = NULL;

 	  forward->new_id = get_id();

 	  forward->fd = udpfd;

 	  memcpy(forward->hash, hash, HASH_SIZE);

 	  forward->forwardall = 0;

-	  forward->flags = 0;

+	  forward->flags = fwd_flags;

 	  if (norebind)

 	    forward->flags |= FREC_NOREBIND;

 	  if (header->hb4 & HB4_CD)

@@ -397,9 +448,9 @@ static int forward_query(int udpfd, union mysockaddr *udpaddr,

       size_t edns0_len;

       /* If a query is retried, use the log_id for the retry when logging the answer. */

-      forward->log_id = daemon->log_id;

+      forward->frec_src.log_id = daemon->log_id;

-      edns0_len  = add_edns0_config(header, plen, ((unsigned char *)header) + PACKETSZ, &forward->source, now, &subnet;;

+      edns0_len = add_edns0_config(header, plen, ((unsigned char *)header) + PACKETSZ, &forward->frec_src.source, now, &subnet;;

       if (edns0_len != plen)

 	{

@@ -536,7 +587,7 @@ static int forward_query(int udpfd, union mysockaddr *udpaddr,

 	return 1;

       /* could not send on, prepare to return */ 

-      header->id = htons(forward->orig_id);

+      header->id = htons(forward->frec_src.orig_id);

       free_frec(forward); /* cancel */

     }	  

@@ -773,8 +824,8 @@ void reply_query(int fd, int family, time_t now)

   /* log_query gets called indirectly all over the place, so 

      pass these in global variables - sorry. */

-  daemon->log_display_id = forward->log_id;

-  daemon->log_source_addr = &forward->source;

+  daemon->log_display_id = forward->frec_src.log_id;

+  daemon->log_source_addr = &forward->frec_src.source;

   if (daemon->ignore_addr && RCODE(header) == NOERROR &&

       check_for_ignored_address(header, n, daemon->ignore_addr))

@@ -973,6 +1024,7 @@ void reply_query(int fd, int family, time_t now)

 #ifdef HAVE_IPV6

 		      new->rfd6 = NULL;

 #endif

+		      new->frec_src.next = NULL;

 		      new->flags &= ~(FREC_DNSKEY_QUERY | FREC_DS_QUERY);

 		      new->dependent = forward; /* to find query awaiting new one. */

@@ -1094,9 +1146,11 @@ void reply_query(int fd, int family, time_t now)

       if ((nn = process_reply(header, now, forward->sentto, (size_t)n, check_rebind, no_cache_dnssec, cache_secure, bogusanswer, 

 			      forward->flags & FREC_AD_QUESTION, forward->flags & FREC_DO_QUESTION, 

-			      forward->flags & FREC_ADDED_PHEADER, forward->flags & FREC_HAS_SUBNET, &forward->source)))

+			      forward->flags & FREC_ADDED_PHEADER, forward->flags & FREC_HAS_SUBNET, &forward->frec_src.source)))

 	{

-	  header->id = htons(forward->orig_id);

+	  struct frec_src *src;

+

+	  header->id = htons(forward->frec_src.orig_id);

 	  header->hb4 |= HB4_RA; /* recursion if available */

 #ifdef HAVE_DNSSEC

 	  /* We added an EDNSO header for the purpose of getting DNSSEC RRs, and set the value of the UDP payload size

@@ -1111,9 +1165,22 @@ void reply_query(int fd, int family, time_t now)

 	      nn = resize_packet(header, nn, NULL, 0);

 	    }

 #endif

-	  send_from(forward->fd, option_bool(OPT_NOWILD) || option_bool (OPT_CLEVERBIND), daemon->packet, nn, 

-		    &forward->source, &forward->dest, forward->iface);

+	  for (src = &forward->frec_src; src; src = src->next)

+	    {

+	      header->id = htons(src->orig_id);

+	      

+	      send_from(forward->fd, option_bool(OPT_NOWILD) || option_bool (OPT_CLEVERBIND), daemon->packet, nn, 

+			&src->source, &src->dest, src->iface);

+

+	      if (option_bool(OPT_EXTRALOG) && src != &forward->frec_src)

+		{

+		  daemon->log_display_id = src->log_id;

+		  daemon->log_source_addr = &src->source;

+		  log_query(F_UPSTREAM, "query", NULL, "duplicate");

+		}

+	    }

 	}

+

       free_frec(forward); /* cancel */

     }

 }

@@ -2034,6 +2101,17 @@ void free_rfd(struct randfd *rfd)

 static void free_frec(struct frec *f)

 {

+  struct frec_src *src, *tmp;

+

+   /* add back to freelist of not the record builtin to every frec. */

+  for (src = f->frec_src.next; src; src = tmp)

+    {

+      tmp = src->next;

+      src->next = daemon->free_frec_src;

+      daemon->free_frec_src = src;

+    }

+  

+  f->frec_src.next = NULL;    

   free_rfd(f->rfd4);

   f->rfd4 = NULL;

   f->sentto = NULL;

@@ -2175,17 +2253,39 @@ static struct frec *lookup_frec_by_sender(unsigned short id,

 					  void *hash)

 {

   struct frec *f;

+  struct frec_src *src;

+

+  for (f = daemon->frec_list; f; f = f->next)

+    if (f->sentto &&

+	!(f->flags & (FREC_DNSKEY_QUERY | FREC_DS_QUERY)) &&

+	memcmp(hash, f->hash, HASH_SIZE) == 0)

+      for (src = &f->frec_src; src; src = src->next)

+	if (src->orig_id == id && 

+	    sockaddr_isequal(&src->source, addr))

+	  return f;

+  

+  return NULL;

+}

+

+static struct frec *lookup_frec_by_query(void *hash, unsigned int flags)

+{

+  struct frec *f;

+

+  /* FREC_DNSKEY and FREC_DS_QUERY are never set in flags, so the test below 

+     ensures that no frec created for internal DNSSEC query can be returned here. */

+

+#define FLAGMASK (FREC_CHECKING_DISABLED | FREC_AD_QUESTION | FREC_DO_QUESTION \

+		  | FREC_HAS_PHEADER | FREC_DNSKEY_QUERY | FREC_DS_QUERY)

   for(f = daemon->frec_list; f; f = f->next)

     if (f->sentto &&

-	f->orig_id == id && 

-	memcmp(hash, f->hash, HASH_SIZE) == 0 &&

-	sockaddr_isequal(&f->source, addr))

+	(f->flags & FLAGMASK) == flags &&

+	memcmp(hash, f->hash, HASH_SIZE) == 0)

       return f;

-   

+  

   return NULL;

 }

- 

+

 /* Send query packet again, if we can. */

 void resend_query()

 {

-- 

2.26.2