Implement "DNS-0x20 encoding".

This provides extra protection against reply-spoof attacks. Since DNS queries are case-insensitive, it's possible to randomly flip the case of letters in a query and still get the correct answer back. This adds an extra dimension for a cache-poisoning attacker to guess when sending replies in-the-blind since it's expected that the legitimate answer will have the same pattern of upper and lower case as the query, so any replies which don't can be ignored as malicious. The amount of extra entropy clearly depends on the number of a-z and A-Z characters in the query, and this implementation puts a hard limit of 32 bits to make rescource allocation easy. This about doubles entropy over the standard random ID and random port combination.
2025-12-19 18:28:25 +00:00 · 2025-01-19 21:44:19 +00:00
parent 65f9c1aca1
commit 995a16ca0c
6 changed files with 113 additions and 66 deletions
--- a/src/rfc1035.c
+++ b/src/rfc1035.c
@@ -16,26 +16,29 @@

 #include "dnsmasq.h"

-/* isExtract == 1 -> extract name
-   isExtract == 0 -> compare name, case insensitive
-   isExtract == -1 -> compare name, case sensitive
-
+/* EXTR_NAME_EXTRACT -> extract name
+   EXTR_NAME_COMPARE -> compare name, case insensitive
+   EXTR_NAME_NOCASE -> compare name, case sensitive
+   EXTR_NAME_FLIP -> flip 0x20 bits in packet, controlled by bitmap in parm. name may be NULL 
+   
   return = 0 -> error
-   return = 1 -> extract OK, compare OK
+   return = 1 -> extract OK, compare OK, flip OK
   return = 2 -> extract OK, compare failed.
 */
 int extract_name(struct dns_header *header, size_t plen, unsigned char **pp, 
-		 char *name, int isExtract, int extrabytes)
+		 char *name, int func, unsigned int parm)
 {
  unsigned char *cp = (unsigned char *)name, *p = *pp, *p1 = NULL;
  unsigned int j, l, namelen = 0, hops = 0;
-  int retvalue = 1, case_insens = 1;
-
-  if (isExtract == -1)
-    isExtract = case_insens = 0;
-  else if (isExtract)
-    *cp = 0;
-
+  int retvalue = 1, case_insens = 1, isExtract = 0, flip = 0, extrabytes = (int)parm;
+  
+  if (func == EXTR_NAME_EXTRACT)
+    isExtract = 1, *cp = 0;
+  else if (func == EXTR_NAME_NOCASE)
+    case_insens = 0;
+  else if (func == EXTR_NAME_FLIP)
+    flip = 1, extrabytes = 0;
+  
  while (1)
    { 
      unsigned int label_type;
@@ -56,7 +59,7 @@ int extract_name(struct dns_header *header, size_t plen, unsigned char **pp,
 		cp--;
 	      *cp = 0; /* terminate: lose final period */
 	    }
-	  else if (*cp != 0)
+	  else if (!flip && *cp != 0)
 	    retvalue = 2;
 	  
 	  if (p1) /* we jumped via compression */
@@ -95,7 +98,7 @@ int extract_name(struct dns_header *header, size_t plen, unsigned char **pp,
 	  if (!CHECK_LEN(header, p, plen, l))
 	    return 0;
 	  
-	  for(j=0; j<l; j++, p++)
+	  for (j=0; j<l; j++, p++)
 	    if (isExtract)
 	      {
 		unsigned char c = *p;
@@ -108,6 +111,18 @@ int extract_name(struct dns_header *header, size_t plen, unsigned char **pp,
 		else
 		  *cp++ = c; 
 	      }
+	    else if (flip)
+	      {
+		unsigned char c = *p;
+
+		/* parm is unsigned. We only flip up to the first 32 alpha-chars. */
+		if ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z'))
+		  {
+		    if (parm & 1)
+		      *p ^= 0x20;
+		    parm >>= 1;
+		  }
+	      }
 	    else 
 	      {
 		unsigned char c1 = *cp, c2 = *p;
@@ -117,15 +132,15 @@ int extract_name(struct dns_header *header, size_t plen, unsigned char **pp,
 		else 
 		  {
 		    cp++;
-		    if (case_insens && c1 >= 'A' && c1 <= 'Z')
-		      c1 += 'a' - 'A';

 		    if (c1 == NAME_ESCAPE)
 		      c1 = (*cp++)-1;
+		    else if (case_insens && c1 >= 'A' && c1 <= 'Z')
+		      c1 += 'a' - 'A';
 		    
 		    if (case_insens && c2 >= 'A' && c2 <= 'Z')
 		      c2 += 'a' - 'A';
-		     
+		    
 		    if (c1 != c2)
 		      retvalue =  2;
 		  }
@@ -133,7 +148,7 @@ int extract_name(struct dns_header *header, size_t plen, unsigned char **pp,
 	    
 	  if (isExtract)
 	    *cp++ = '.';
-	  else if (*cp != 0 && *cp++ != '.')
+	  else if (!flip && *cp != 0 && *cp++ != '.')
 	    retvalue = 2;
 	}
      else
@@ -409,7 +424,7 @@ int do_doctor(struct dns_header *header, size_t qlen, char *namebuff)
      if (i == ntohs(header->ancount) && !(p = skip_section(p, ntohs(header->nscount), header, qlen)))
 	return done;
      
-      if (!extract_name(header, qlen, &p, namebuff, 1, 10))
+      if (!extract_name(header, qlen, &p, namebuff, EXTR_NAME_EXTRACT, 10))
 	return done; /* bad packet */
      
      GETSHORT(qtype, p); 
@@ -490,7 +505,7 @@ static int find_soa(struct dns_header *header, size_t qlen, char *name, int *sub
  
  for (i = 0; i < ntohs(header->nscount); i++)
    {
-      if (!extract_name(header, qlen, &p, daemon->workspacename, 1, 0))
+      if (!extract_name(header, qlen, &p, daemon->workspacename, EXTR_NAME_EXTRACT, 0))
 	return 0; /* bad packet */
      
      GETSHORT(qtype, p); 
@@ -519,7 +534,7 @@ static int find_soa(struct dns_header *header, size_t qlen, char *name, int *sub
 	      
 	      for (j = 0; j < 2; j++) /* MNAME, RNAME */
 		{
-		  if (!extract_name(header, qlen, &p, daemon->workspacename, 1, 0))
+		  if (!extract_name(header, qlen, &p, daemon->workspacename, EXTR_NAME_EXTRACT, 0))
 		    {
 		      if (!no_cache)
 			blockdata_free(addr.rrblock.rrdata);
@@ -670,7 +685,7 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t

  namep = p = (unsigned char *)(header+1);
  
-  if (ntohs(header->qdcount) != 1 || !extract_name(header, qlen, &p, name, 1, 4))
+  if (ntohs(header->qdcount) != 1 || !extract_name(header, qlen, &p, name, EXTR_NAME_EXTRACT, 4))
    return 2; /* bad packet */
  
  GETSHORT(qtype, p); 
@@ -694,7 +709,7 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
 	  for (j = 0; j < ntohs(header->ancount); j++) 
 	    {
 	      int secflag = 0;
-	      if (!(res = extract_name(header, qlen, &p1, name, 0, 10)))
+	      if (!(res = extract_name(header, qlen, &p1, name, EXTR_NAME_COMPARE, 10)))
 		return 2; /* bad packet */
 	      
 	      GETSHORT(aqtype, p1); 
@@ -732,7 +747,7 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
 		  if (aqtype == T_CNAME)
 		    log_query(secflag | F_CNAME | F_FORWARD | F_UPSTREAM, name, NULL, NULL, 0);
 		  
-		  if (!extract_name(header, qlen, &p1, name, 1, 0))
+		  if (!extract_name(header, qlen, &p1, name, EXTR_NAME_EXTRACT, 0))
 		    return 2;
 		  
 		  if (aqtype == T_CNAME)
@@ -810,7 +825,7 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
 	{
 	  int secflag = 0;
 	  
-	  if (!(res = extract_name(header, qlen, &p1, name, 0, 10)))
+	  if (!(res = extract_name(header, qlen, &p1, name, EXTR_NAME_COMPARE, 10)))
 	    return 2; /* bad packet */
 	  
 	  GETSHORT(aqtype, p1); 
@@ -872,7 +887,7 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
 		}
 	      
 	      namep = p1;
-	      if (!extract_name(header, qlen, &p1, name, 1, 0))
+	      if (!extract_name(header, qlen, &p1, name, EXTR_NAME_EXTRACT, 0))
 		return 2;
 	      
 	      if (qtype != T_CNAME)
@@ -942,7 +957,7 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
 			      /* Name, extract it then re-encode. */
 			      int len;
 			      
-			      if (!extract_name(header, qlen, &p1, name, 1, 0))
+			      if (!extract_name(header, qlen, &p1, name, EXTR_NAME_EXTRACT, 0))
 				{
 				  blockdata_free(addr.rrblock.rrdata);
 				  return 2;
@@ -975,7 +990,7 @@ int extract_addresses(struct dns_header *header, size_t qlen, char *name, time_t
 			} while (desc != -1);
 		      
 		      /* we overwrote the original name, so get it back here. */
-		      if (!extract_name(header, qlen, &tmp, name, 1, 0))
+		      if (!extract_name(header, qlen, &tmp, name, EXTR_NAME_EXTRACT, 0))
 			{
 			  blockdata_free(addr.rrblock.rrdata);
 			  return 2;
@@ -1137,7 +1152,7 @@ void report_addresses(struct dns_header *header, size_t len, u32 mark)
    {
      int aqtype, aqclass, ardlen;
      
-      if (!extract_name(header, len, &p, daemon->namebuff, 1, 10))
+      if (!extract_name(header, len, &p, daemon->namebuff, EXTR_NAME_EXTRACT, 10))
 	return;
      
      if (!CHECK_LEN(header, p, len, 10))
@@ -1155,7 +1170,7 @@ void report_addresses(struct dns_header *header, size_t len, u32 mark)
 	{
 	  if (aqtype == T_CNAME)
 	    {
-	      if (!extract_name(header, len, &p, daemon->workspacename, 1, 0))
+	      if (!extract_name(header, len, &p, daemon->workspacename, EXTR_NAME_EXTRACT, 0))
 		return;
 	      if (safe_name(daemon->namebuff) && safe_name(daemon->workspacename))
 		ubus_event_bcast_connmark_allowlist_resolved(mark, daemon->namebuff, daemon->workspacename, attl);
@@ -1205,7 +1220,7 @@ unsigned int extract_request(struct dns_header *header, size_t qlen, char *name,
  if (!(header->hb3 & HB3_QR) && (ntohs(header->ancount) != 0 || ntohs(header->nscount) != 0))
    return 0; /* non-standard query. */
  
-  if (!extract_name(header, qlen, &p, name, 1, 4))
+  if (!extract_name(header, qlen, &p, name, EXTR_NAME_EXTRACT, 4))
    return 0; /* bad packet */
   
  GETSHORT(qtype, p); 
@@ -1319,7 +1334,7 @@ static int check_bad_address(struct dns_header *header, size_t qlen, struct bogu

  for (i = ntohs(header->ancount); i != 0; i--)
    {
-      if (name && !extract_name(header, qlen, &p, name, 1, 10))
+      if (name && !extract_name(header, qlen, &p, name, EXTR_NAME_EXTRACT, 10))
 	return 0; /* bad packet */

      if (!name && !(p = skip_name(p, header, qlen, 10)))
@@ -1629,7 +1644,7 @@ size_t answer_request(struct dns_header *header, char *limit, size_t qlen,
  nameoffset = p - (unsigned char *)header;
  
  /* now extract name as .-concatenated string into name */
-  if (!extract_name(header, qlen, &p, name, 1, 4))
+  if (!extract_name(header, qlen, &p, name, EXTR_NAME_EXTRACT, 4))
    return 0; /* bad packet */
  
  GETSHORT(qtype, p);