tls_dh_min_bits smtp transport option

Could not find an API for use with OpenSSL, so GnuTLS only

7 files changed, 34 insertions, 13 deletions

diff --git a/src/src/buildconfig.c b/src/src/buildconfig.c
index 62114fc09..f3390cb75 100644
--- a/src/src/buildconfig.c
+++ b/src/src/buildconfig.c
@@ -847,16 +847,17 @@ else if (isgroup)
         }
 
       /* how many bits Exim, as a client, demands must be in D-H */
-      /* as of GnuTLS 2.12.x, we ask for "normal" for D-H PK; before that, we
-      specify the number of bits.  We've stuck with the historical value, but
-      it can be overridden. */
-      else if ((strcmp(name, "EXIM_CLIENT_DH_MIN_BITS") == 0) ||
+      /* 1024 is a historical figure; some sites actually use lower, so we
+      permit the value to be lowered "dangerously" low, but not "insanely"
+      low.  Though actually, 1024 is becoming "dangerous". */
+      else if ((strcmp(name, "EXIM_CLIENT_DH_MIN_MIN_BITS") == 0) ||
+               (strcmp(name, "EXIM_CLIENT_DH_DEFAULT_MIN_BITS") == 0) ||
                (strcmp(name, "EXIM_SERVER_DH_BITS_PRE2_12") == 0))
         {
         long nv;
         char *end;
         nv = strtol(value, &end, 10);
-        if (end != value && *end == '\0' && nv >= 1000 && nv < 50000)
+        if (end != value && *end == '\0' && nv >= 512 && nv < 500000)
           {
           fprintf(new, "%s\n", value);
           }
diff --git a/src/src/config.h.defaults b/src/src/config.h.defaults
index 92a4cd348..f02aef12c 100644
--- a/src/src/config.h.defaults
+++ b/src/src/config.h.defaults
@@ -49,7 +49,8 @@ it's a default value. */
 #define EXIMDB_LOCK_TIMEOUT          60
 #define EXIMDB_LOCKFILE_MODE       0640
 #define EXIMDB_MODE                0640
-#define EXIM_CLIENT_DH_MIN_BITS
+#define EXIM_CLIENT_DH_MIN_MIN_BITS         512
+#define EXIM_CLIENT_DH_DEFAULT_MIN_BITS    1024
 #define EXIM_GNUTLS_LIBRARY_LOG_LEVEL
 #define EXIM_SERVER_DH_BITS_PRE2_12
 #define EXIM_PERL
diff --git a/src/src/functions.h b/src/src/functions.h
index fa9d5585e..2758a4aec 100644
--- a/src/src/functions.h
+++ b/src/src/functions.h
@@ -27,7 +27,7 @@ extern const char *
                std_dh_prime_named(const uschar *);
 extern int     tls_client_start(int, host_item *, address_item *, uschar *,
                  uschar *, uschar *, uschar *, uschar *, uschar *, uschar *,
-                 int);
+                 int, int);
 extern void    tls_close(BOOL);
 extern int     tls_feof(void);
 extern int     tls_ferror(void);
diff --git a/src/src/tls-gnu.c b/src/src/tls-gnu.c
index c8bf634bc..cf315b6d1 100644
--- a/src/src/tls-gnu.c
+++ b/src/src/tls-gnu.c
@@ -1536,6 +1536,7 @@ Arguments:
   verify_certs      file for certificate verify
   verify_crl        CRL for verify
   require_ciphers   list of allowed ciphers or NULL
+  dh_min_bits       minimum number of bits acceptable in server's DH prime
   timeout           startup timeout
 
 Returns:            OK/DEFER/FAIL (because using common functions),
@@ -1547,7 +1548,7 @@ tls_client_start(int fd, host_item *host,
     address_item *addr ARG_UNUSED, uschar *dhparam ARG_UNUSED,
     uschar *certificate, uschar *privatekey, uschar *sni,
     uschar *verify_certs, uschar *verify_crl,
-    uschar *require_ciphers, int timeout)
+    uschar *require_ciphers, int dh_min_bits, int timeout)
 {
 int rc;
 const char *error;
@@ -1559,7 +1560,17 @@ rc = tls_init(host, certificate, privatekey,
     sni, verify_certs, verify_crl, require_ciphers, &state);
 if (rc != OK) return rc;
 
-gnutls_dh_set_prime_bits(state->session, EXIM_CLIENT_DH_MIN_BITS);
+if (dh_min_bits < EXIM_CLIENT_DH_MIN_MIN_BITS)
+  {
+  DEBUG(D_tls)
+    debug_printf("WARNING: tls_dh_min_bits far too low, clamping %d up to %d\n",
+        dh_min_bits, EXIM_CLIENT_DH_MIN_MIN_BITS);
+  dh_min_bits = EXIM_CLIENT_DH_MIN_MIN_BITS;
+  }
+
+DEBUG(D_tls) debug_printf("Setting D-H prime minimum acceptable bits to %d\n",
+    dh_min_bits);
+gnutls_dh_set_prime_bits(state->session, dh_min_bits);
 
 if (verify_certs == NULL)
   {
diff --git a/src/src/tls-openssl.c b/src/src/tls-openssl.c
index 22c0730c3..fdcb95ef2 100644
--- a/src/src/tls-openssl.c
+++ b/src/src/tls-openssl.c
@@ -1233,6 +1233,8 @@ Argument:
   verify_certs     file for certificate verify
   crl              file containing CRL
   require_ciphers  list of allowed ciphers
+  dh_min_bits      minimum number of bits acceptable in server's DH prime
+                   (unused in OpenSSL)
   timeout          startup timeout
 
 Returns:           OK on success
@@ -1244,7 +1246,7 @@ int
 tls_client_start(int fd, host_item *host, address_item *addr, uschar *dhparam,
   uschar *certificate, uschar *privatekey, uschar *sni,
   uschar *verify_certs, uschar *crl,
-  uschar *require_ciphers, int timeout)
+  uschar *require_ciphers, int dh_min_bits ARG_UNUSED, int timeout)
 {
 static uschar txt[256];
 uschar *expciphers;
diff --git a/src/src/transports/smtp.c b/src/src/transports/smtp.c
index f9f225fca..b3856f553 100644
--- a/src/src/transports/smtp.c
+++ b/src/src/transports/smtp.c
@@ -129,6 +129,8 @@ optionlist smtp_transport_options[] = {
       (void *)offsetof(smtp_transport_options_block, tls_certificate) },
   { "tls_crl",              opt_stringptr,
       (void *)offsetof(smtp_transport_options_block, tls_crl) },
+  { "tls_dh_min_bits",      opt_int,
+      (void *)offsetof(smtp_transport_options_block, tls_dh_min_bits) },
   { "tls_privatekey",       opt_stringptr,
       (void *)offsetof(smtp_transport_options_block, tls_privatekey) },
   { "tls_require_ciphers",  opt_stringptr,
@@ -195,9 +197,11 @@ smtp_transport_options_block smtp_transport_option_defaults = {
   NULL,                /* gnutls_require_kx */
   NULL,                /* gnutls_require_mac */
   NULL,                /* gnutls_require_proto */
+  NULL,                /* tls_sni */
   NULL,                /* tls_verify_certificates */
-  TRUE,                /* tls_tempfail_tryclear */
-  NULL                 /* tls_sni */
+  EXIM_CLIENT_DH_DEFAULT_MIN_BITS,
+                       /* tls_dh_min_bits */
+  TRUE                 /* tls_tempfail_tryclear */
 #endif
 #ifndef DISABLE_DKIM
  ,NULL,                /* dkim_canon */
@@ -1136,6 +1140,7 @@ if (tls_offered && !suppress_tls &&
       ob->tls_verify_certificates,
       ob->tls_crl,
       ob->tls_require_ciphers,
+      ob->tls_dh_min_bits,
       ob->command_timeout);
 
     /* TLS negotiation failed; give an error. From outside, this function may
diff --git a/src/src/transports/smtp.h b/src/src/transports/smtp.h
index 621cb6ba9..17b75cf3b 100644
--- a/src/src/transports/smtp.h
+++ b/src/src/transports/smtp.h
@@ -52,9 +52,10 @@ typedef struct {
   uschar *gnutls_require_kx;
   uschar *gnutls_require_mac;
   uschar *gnutls_require_proto;
+  uschar *tls_sni;
   uschar *tls_verify_certificates;
+  int     tls_dh_min_bits;
   BOOL    tls_tempfail_tryclear;
-  uschar *tls_sni;
   #endif
   #ifndef DISABLE_DKIM
   uschar *dkim_domain;