diff --git a/pkgs/base/info.rkt b/pkgs/base/info.rkt
index 9abf7e4a2c..5fe297f3cd 100644
--- a/pkgs/base/info.rkt
+++ b/pkgs/base/info.rkt
@@ -12,7 +12,7 @@
 
 (define collection 'multi)
 
-(define version "6.6.0.4")
+(define version "6.6.0.5")
 
 (define deps `("racket-lib"
                ["racket" #:version ,version]))
diff --git a/pkgs/racket-doc/scribblings/foreign/misc.scrbl b/pkgs/racket-doc/scribblings/foreign/misc.scrbl
index 03670206ef..d35e7b2a46 100644
--- a/pkgs/racket-doc/scribblings/foreign/misc.scrbl
+++ b/pkgs/racket-doc/scribblings/foreign/misc.scrbl
@@ -37,12 +37,23 @@ to create mock foreign functions for testing).
 
 @history[#:changed "6.4.0.9"]{Added the one-argument variant.}}
 
-@defproc[(lookup-errno [sym (or/c 'EINTR 'EEXIST 'EAGAIN)])
-         exact-integer?]{
+@defproc[(lookup-errno [sym symbol?])
+         (or/c exact-integer? #f)]{
 
-Returns a platform-specific value corresponding to a Posix @tt{errno}
-symbol. The set of supported symbols is likely to expand in the
-future.}
+Returns a platform-specific positive integer corresponding to a POSIX
+@tt{errno} code, or @racket[#f] if the code is unknown. A code's value
+is known if the code is one of the recognized symbols described below
+@emph{and} the code was defined by the @tt{"errno.h"} header used to
+compile Racket. Note that the contents of @tt{"errno.h"} vary based on
+platform and compiler.
+
+The recognized symbols currently consist of the 81 codes defined by
+@hyperlink["http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/errno.h.html"]{IEEE
+Std 1003.1, 2013 Edition} (also known as POSIX.1), including
+@racket['EINTR], @racket['EEXIST], and @racket['EAGAIN].
+
+@history[#:changed "6.6.0.5" @elem{Relaxed the contract and added
+support for more symbols.}]}
 
 
 @defproc[(cast [v any/c] [from-type ctype?] [to-type ctype?]) any/c]{
diff --git a/racket/src/foreign/foreign.c b/racket/src/foreign/foreign.c
index a986854dca..4e9c54c82c 100644
--- a/racket/src/foreign/foreign.c
+++ b/racket/src/foreign/foreign.c
@@ -4326,16 +4326,337 @@ static Scheme_Object *foreign_saved_errno(int argc, Scheme_Object *argv[])
 static Scheme_Object *foreign_lookup_errno(int argc, Scheme_Object *argv[])
 {
   Scheme_Object *v = argv[0];
-  if (SCHEME_SYMBOLP(v) && !SCHEME_SYM_WEIRDP(v)) {
-    if (!strcmp("EINTR", SCHEME_SYM_VAL(v)))
-      return scheme_make_integer(EINTR);
-    if (!strcmp("EEXIST", SCHEME_SYM_VAL(v)))
-      return scheme_make_integer(EEXIST);
-    if (!strcmp("EAGAIN", SCHEME_SYM_VAL(v)))
-      return scheme_make_integer(EAGAIN);
+  if (!SCHEME_SYMBOLP(v)) {
+    scheme_wrong_contract(MYNAME, "symbol?", 0, argc, argv);
+    return NULL;
   }
-  scheme_wrong_contract(MYNAME, "(or/c 'EINTR 'EEXIST 'EAGAIN)",0, argc, argv);
-  return NULL;
+  if (SCHEME_SYM_WEIRDP(v))
+    return scheme_false;
+  #ifdef E2BIG
+  if (!strcmp("E2BIG", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(E2BIG);
+  #endif
+  #ifdef EACCES
+  if (!strcmp("EACCES", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EACCES);
+  #endif
+  #ifdef EADDRINUSE
+  if (!strcmp("EADDRINUSE", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EADDRINUSE);
+  #endif
+  #ifdef EADDRNOTAVAIL
+  if (!strcmp("EADDRNOTAVAIL", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EADDRNOTAVAIL);
+  #endif
+  #ifdef EAFNOSUPPORT
+  if (!strcmp("EAFNOSUPPORT", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EAFNOSUPPORT);
+  #endif
+  #ifdef EAGAIN
+  if (!strcmp("EAGAIN", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EAGAIN);
+  #endif
+  #ifdef EALREADY
+  if (!strcmp("EALREADY", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EALREADY);
+  #endif
+  #ifdef EBADF
+  if (!strcmp("EBADF", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EBADF);
+  #endif
+  #ifdef EBADMSG
+  if (!strcmp("EBADMSG", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EBADMSG);
+  #endif
+  #ifdef EBUSY
+  if (!strcmp("EBUSY", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EBUSY);
+  #endif
+  #ifdef ECANCELED
+  if (!strcmp("ECANCELED", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ECANCELED);
+  #endif
+  #ifdef ECHILD
+  if (!strcmp("ECHILD", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ECHILD);
+  #endif
+  #ifdef ECONNABORTED
+  if (!strcmp("ECONNABORTED", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ECONNABORTED);
+  #endif
+  #ifdef ECONNREFUSED
+  if (!strcmp("ECONNREFUSED", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ECONNREFUSED);
+  #endif
+  #ifdef ECONNRESET
+  if (!strcmp("ECONNRESET", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ECONNRESET);
+  #endif
+  #ifdef EDEADLK
+  if (!strcmp("EDEADLK", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EDEADLK);
+  #endif
+  #ifdef EDESTADDRREQ
+  if (!strcmp("EDESTADDRREQ", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EDESTADDRREQ);
+  #endif
+  #ifdef EDOM
+  if (!strcmp("EDOM", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EDOM);
+  #endif
+  #ifdef EDQUOT
+  if (!strcmp("EDQUOT", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EDQUOT);
+  #endif
+  #ifdef EEXIST
+  if (!strcmp("EEXIST", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EEXIST);
+  #endif
+  #ifdef EFAULT
+  if (!strcmp("EFAULT", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EFAULT);
+  #endif
+  #ifdef EFBIG
+  if (!strcmp("EFBIG", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EFBIG);
+  #endif
+  #ifdef EHOSTUNREACH
+  if (!strcmp("EHOSTUNREACH", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EHOSTUNREACH);
+  #endif
+  #ifdef EIDRM
+  if (!strcmp("EIDRM", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EIDRM);
+  #endif
+  #ifdef EILSEQ
+  if (!strcmp("EILSEQ", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EILSEQ);
+  #endif
+  #ifdef EINPROGRESS
+  if (!strcmp("EINPROGRESS", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EINPROGRESS);
+  #endif
+  #ifdef EINTR
+  if (!strcmp("EINTR", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EINTR);
+  #endif
+  #ifdef EINVAL
+  if (!strcmp("EINVAL", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EINVAL);
+  #endif
+  #ifdef EIO
+  if (!strcmp("EIO", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EIO);
+  #endif
+  #ifdef EISCONN
+  if (!strcmp("EISCONN", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EISCONN);
+  #endif
+  #ifdef EISDIR
+  if (!strcmp("EISDIR", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EISDIR);
+  #endif
+  #ifdef ELOOP
+  if (!strcmp("ELOOP", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ELOOP);
+  #endif
+  #ifdef EMFILE
+  if (!strcmp("EMFILE", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EMFILE);
+  #endif
+  #ifdef EMLINK
+  if (!strcmp("EMLINK", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EMLINK);
+  #endif
+  #ifdef EMSGSIZE
+  if (!strcmp("EMSGSIZE", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EMSGSIZE);
+  #endif
+  #ifdef EMULTIHOP
+  if (!strcmp("EMULTIHOP", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EMULTIHOP);
+  #endif
+  #ifdef ENAMETOOLONG
+  if (!strcmp("ENAMETOOLONG", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENAMETOOLONG);
+  #endif
+  #ifdef ENETDOWN
+  if (!strcmp("ENETDOWN", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENETDOWN);
+  #endif
+  #ifdef ENETRESET
+  if (!strcmp("ENETRESET", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENETRESET);
+  #endif
+  #ifdef ENETUNREACH
+  if (!strcmp("ENETUNREACH", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENETUNREACH);
+  #endif
+  #ifdef ENFILE
+  if (!strcmp("ENFILE", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENFILE);
+  #endif
+  #ifdef ENOBUFS
+  if (!strcmp("ENOBUFS", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOBUFS);
+  #endif
+  #ifdef ENODATA
+  if (!strcmp("ENODATA", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENODATA);
+  #endif
+  #ifdef ENODEV
+  if (!strcmp("ENODEV", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENODEV);
+  #endif
+  #ifdef ENOENT
+  if (!strcmp("ENOENT", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOENT);
+  #endif
+  #ifdef ENOEXEC
+  if (!strcmp("ENOEXEC", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOEXEC);
+  #endif
+  #ifdef ENOLCK
+  if (!strcmp("ENOLCK", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOLCK);
+  #endif
+  #ifdef ENOLINK
+  if (!strcmp("ENOLINK", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOLINK);
+  #endif
+  #ifdef ENOMEM
+  if (!strcmp("ENOMEM", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOMEM);
+  #endif
+  #ifdef ENOMSG
+  if (!strcmp("ENOMSG", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOMSG);
+  #endif
+  #ifdef ENOPROTOOPT
+  if (!strcmp("ENOPROTOOPT", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOPROTOOPT);
+  #endif
+  #ifdef ENOSPC
+  if (!strcmp("ENOSPC", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOSPC);
+  #endif
+  #ifdef ENOSR
+  if (!strcmp("ENOSR", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOSR);
+  #endif
+  #ifdef ENOSTR
+  if (!strcmp("ENOSTR", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOSTR);
+  #endif
+  #ifdef ENOSYS
+  if (!strcmp("ENOSYS", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOSYS);
+  #endif
+  #ifdef ENOTCONN
+  if (!strcmp("ENOTCONN", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOTCONN);
+  #endif
+  #ifdef ENOTDIR
+  if (!strcmp("ENOTDIR", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOTDIR);
+  #endif
+  #ifdef ENOTEMPTY
+  if (!strcmp("ENOTEMPTY", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOTEMPTY);
+  #endif
+  #ifdef ENOTRECOVERABLE
+  if (!strcmp("ENOTRECOVERABLE", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOTRECOVERABLE);
+  #endif
+  #ifdef ENOTSOCK
+  if (!strcmp("ENOTSOCK", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOTSOCK);
+  #endif
+  #ifdef ENOTSUP
+  if (!strcmp("ENOTSUP", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOTSUP);
+  #endif
+  #ifdef ENOTTY
+  if (!strcmp("ENOTTY", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENOTTY);
+  #endif
+  #ifdef ENXIO
+  if (!strcmp("ENXIO", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ENXIO);
+  #endif
+  #ifdef EOPNOTSUPP
+  if (!strcmp("EOPNOTSUPP", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EOPNOTSUPP);
+  #endif
+  #ifdef EOVERFLOW
+  if (!strcmp("EOVERFLOW", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EOVERFLOW);
+  #endif
+  #ifdef EOWNERDEAD
+  if (!strcmp("EOWNERDEAD", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EOWNERDEAD);
+  #endif
+  #ifdef EPERM
+  if (!strcmp("EPERM", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EPERM);
+  #endif
+  #ifdef EPIPE
+  if (!strcmp("EPIPE", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EPIPE);
+  #endif
+  #ifdef EPROTO
+  if (!strcmp("EPROTO", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EPROTO);
+  #endif
+  #ifdef EPROTONOSUPPORT
+  if (!strcmp("EPROTONOSUPPORT", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EPROTONOSUPPORT);
+  #endif
+  #ifdef EPROTOTYPE
+  if (!strcmp("EPROTOTYPE", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EPROTOTYPE);
+  #endif
+  #ifdef ERANGE
+  if (!strcmp("ERANGE", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ERANGE);
+  #endif
+  #ifdef EROFS
+  if (!strcmp("EROFS", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EROFS);
+  #endif
+  #ifdef ESPIPE
+  if (!strcmp("ESPIPE", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ESPIPE);
+  #endif
+  #ifdef ESRCH
+  if (!strcmp("ESRCH", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ESRCH);
+  #endif
+  #ifdef ESTALE
+  if (!strcmp("ESTALE", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ESTALE);
+  #endif
+  #ifdef ETIME
+  if (!strcmp("ETIME", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ETIME);
+  #endif
+  #ifdef ETIMEDOUT
+  if (!strcmp("ETIMEDOUT", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ETIMEDOUT);
+  #endif
+  #ifdef ETXTBSY
+  if (!strcmp("ETXTBSY", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(ETXTBSY);
+  #endif
+  #ifdef EWOULDBLOCK
+  if (!strcmp("EWOULDBLOCK", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EWOULDBLOCK);
+  #endif
+  #ifdef EXDEV
+  if (!strcmp("EXDEV", SCHEME_SYM_VAL(v)))
+    return scheme_make_integer(EXDEV);
+  #endif
+  return scheme_false;
 }
 #undef MYNAME
 
diff --git a/racket/src/foreign/foreign.rktc b/racket/src/foreign/foreign.rktc
index b9da5337ab..6d7adf6666 100755
--- a/racket/src/foreign/foreign.rktc
+++ b/racket/src/foreign/foreign.rktc
@@ -3469,27 +3469,112 @@ static void save_errno_values(int kind)
   }
 }
 
+@(define standard-errnos
+   ;; Error codes defined by IEEE Std 1003.1, 2013 Edition
+   ;; (http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/errno.h.html)
+   ;; Linux, Mac, *BSD, MinGW all seem to support most (but not necessarily all)
+   ;; of these codes.
+   '(E2BIG
+     EACCES
+     EADDRINUSE
+     EADDRNOTAVAIL
+     EAFNOSUPPORT
+     EAGAIN
+     EALREADY
+     EBADF
+     EBADMSG
+     EBUSY
+     ECANCELED
+     ECHILD
+     ECONNABORTED
+     ECONNREFUSED
+     ECONNRESET
+     EDEADLK
+     EDESTADDRREQ
+     EDOM
+     EDQUOT
+     EEXIST
+     EFAULT
+     EFBIG
+     EHOSTUNREACH
+     EIDRM
+     EILSEQ
+     EINPROGRESS
+     EINTR
+     EINVAL
+     EIO
+     EISCONN
+     EISDIR
+     ELOOP
+     EMFILE
+     EMLINK
+     EMSGSIZE
+     EMULTIHOP
+     ENAMETOOLONG
+     ENETDOWN
+     ENETRESET
+     ENETUNREACH
+     ENFILE
+     ENOBUFS
+     ENODATA
+     ENODEV
+     ENOENT
+     ENOEXEC
+     ENOLCK
+     ENOLINK
+     ENOMEM
+     ENOMSG
+     ENOPROTOOPT
+     ENOSPC
+     ENOSR
+     ENOSTR
+     ENOSYS
+     ENOTCONN
+     ENOTDIR
+     ENOTEMPTY
+     ENOTRECOVERABLE
+     ENOTSOCK
+     ENOTSUP
+     ENOTTY
+     ENXIO
+     EOPNOTSUPP
+     EOVERFLOW
+     EOWNERDEAD
+     EPERM
+     EPIPE
+     EPROTO
+     EPROTONOSUPPORT
+     EPROTOTYPE
+     ERANGE
+     EROFS
+     ESPIPE
+     ESRCH
+     ESTALE
+     ETIME
+     ETIMEDOUT
+     ETXTBSY
+     EWOULDBLOCK
+     EXDEV))
+
 @cdefine[lookup-errno 1 #:kind immed]{
   Scheme_Object *v = argv[0];
-  @(let* ([errnos '(EINTR EEXIST EAGAIN)]
-          [syms (string-append
-                 "(or/c "
-                 (let loop ([errnos errnos])
-                   (if (null? (cdr errnos))   
-                       (format "'~a" (car errnos))
-                       (format "'~a ~a" (car errnos) (loop (cdr errnos)))))
-                 ")")])
+  @(let ()
     @list{
-      if (SCHEME_SYMBOLP(v) && !SCHEME_SYM_WEIRDP(v)) {
-        @(add-newlines
-          (for/list ([e errnos])
-            @string-append{
-              if (!strcmp("@symbol->string[e]", SCHEME_SYM_VAL(v)))
-                return scheme_make_integer(@symbol->string[e]);
-            }))
+      if (!SCHEME_SYMBOLP(v)) {
+        scheme_wrong_contract(MYNAME, "symbol?", 0, argc, argv);
+        return NULL;
       }
-      scheme_wrong_contract(MYNAME, "@syms",0, argc, argv);
-      return NULL;
+      if (SCHEME_SYM_WEIRDP(v))
+        return scheme_false;
+      @(add-newlines
+        (for/list ([e standard-errnos])
+          @string-append{
+            #ifdef @symbol->string[e]
+            if (!strcmp("@symbol->string[e]", SCHEME_SYM_VAL(v)))
+              return scheme_make_integer(@symbol->string[e]);
+            #endif
+          }))
+      return scheme_false;
     })
 }
 
diff --git a/racket/src/racket/src/schvers.h b/racket/src/racket/src/schvers.h
index af114434af..959ac21bfc 100644
--- a/racket/src/racket/src/schvers.h
+++ b/racket/src/racket/src/schvers.h
@@ -13,12 +13,12 @@
    consistently.)
 */
 
-#define MZSCHEME_VERSION "6.6.0.4"
+#define MZSCHEME_VERSION "6.6.0.5"
 
 #define MZSCHEME_VERSION_X 6
 #define MZSCHEME_VERSION_Y 6
 #define MZSCHEME_VERSION_Z 0
-#define MZSCHEME_VERSION_W 4
+#define MZSCHEME_VERSION_W 5
 
 #define MZSCHEME_VERSION_MAJOR ((MZSCHEME_VERSION_X * 100) + MZSCHEME_VERSION_Y)
 #define MZSCHEME_VERSION_MINOR ((MZSCHEME_VERSION_Z * 1000) + MZSCHEME_VERSION_W)