For DH, use standard primes from RFCs

8 files changed, 882 insertions, 38 deletions

diff --git a/src/OS/Makefile-Base b/src/OS/Makefile-Base
index de387e027..281294558 100644
--- a/src/OS/Makefile-Base
+++ b/src/OS/Makefile-Base
@@ -312,7 +312,7 @@ OBJ_EXIM = acl.o child.o crypt16.o daemon.o dbfn.o debug.o deliver.o \
         os.o parse.o queue.o \
         rda.o readconf.o receive.o retry.o rewrite.o rfc2047.o \
         route.o search.o sieve.o smtp_in.o smtp_out.o spool_in.o spool_out.o \
-        store.o string.o tls.o tod.o transport.o tree.o verify.o \
+        std-crypto.o store.o string.o tls.o tod.o transport.o tree.o verify.o \
         $(OBJ_LOOKUPS) \
         local_scan.o $(EXIM_PERL) $(OBJ_WITH_CONTENT_SCAN) \
         $(OBJ_WITH_OLD_DEMIME) $(OBJ_EXPERIMENTAL)
@@ -575,6 +575,7 @@ smtp_in.o:       $(HDRS) smtp_in.c
 smtp_out.o:      $(HDRS) smtp_out.c
 spool_in.o:      $(HDRS) spool_in.c
 spool_out.o:     $(HDRS) spool_out.c
+std-crypto.o:    $(HDRS) std-crypto.c
 store.o:         $(HDRS) store.c
 string.o:        $(HDRS) string.c
 tls.o:           $(HDRS) tls.c tls-gnu.c tls-openssl.c
diff --git a/src/README.UPDATING b/src/README.UPDATING
index a15bd418e..6a820bc7c 100644
--- a/src/README.UPDATING
+++ b/src/README.UPDATING
@@ -142,6 +142,21 @@ Exim version 4.80
    fail completely.  (The check is not done as root, to ensure that problems
    here are not made worse by the check).
 
+ * The "tls_dhparam" option has been updated, so that it can now specify a
+   path or an identifier for a standard DH prime from one of a few RFCs.
+   The default for OpenSSL is no longer to not use DH but instead to use
+   one of these standard primes.  The default for GnuTLS is no longer to use
+   a file in the spool directory, but to use that same standard prime.
+   The option is now used by GnuTLS too.  If it points to a path, then
+   GnuTLS will use that path, instead of a file in the spool directory;
+   GnuTLS will attempt to create it if it does not exist.
+
+   To preserve the previous behaviour of generating files in the spool
+   directory, set "tls_dhparam = historic".  Since prior releases of Exim
+   ignored tls_dhparam when using GnuTLS, this can safely be done before
+   the upgrade.
+
+
 
 Exim version 4.77
 -----------------
diff --git a/src/scripts/MakeLinks b/src/scripts/MakeLinks
index 166a25f88..62d248a3c 100755
--- a/src/scripts/MakeLinks
+++ b/src/scripts/MakeLinks
@@ -228,6 +228,7 @@ ln -s ../src/smtp_in.c         smtp_in.c
 ln -s ../src/smtp_out.c        smtp_out.c
 ln -s ../src/spool_in.c        spool_in.c
 ln -s ../src/spool_out.c       spool_out.c
+ln -s ../src/std-crypto.c      std-crypto.c
 ln -s ../src/store.c           store.c
 ln -s ../src/string.c          string.c
 ln -s ../src/tls.c             tls.c
diff --git a/src/src/functions.h b/src/src/functions.h
index 160f5133e..29e7db2bd 100644
--- a/src/src/functions.h
+++ b/src/src/functions.h
@@ -21,6 +21,10 @@ extern uschar *init_perl(uschar *);
 
 
 #ifdef SUPPORT_TLS
+extern const char *
+               std_dh_prime_default(void);
+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);
diff --git a/src/src/std-crypto.c b/src/src/std-crypto.c
new file mode 100644
index 000000000..3f0fec897
--- /dev/null
+++ b/src/src/std-crypto.c
@@ -0,0 +1,521 @@
+/*************************************************
+*     Exim - an Internet mail transport agent    *
+*************************************************/
+
+/* Copyright (c) Phil Pennock 2012
+ * But almost everything here is fixed published constants from RFCs, so also:
+ * Copyright (C) The Internet Society (2003)
+ * Copyright (C) The IETF Trust (2008)
+ * Most of the text in RFC referencing comments is copy/paste from RFC,
+ * as is undoubtedly the intention.
+ * The constants are generated from that text using util/gen_pkcs3.c invoked
+ * with the -C option.
+ */
+
+/* See the file NOTICE for conditions of use and distribution. */
+
+#include "exim.h"
+
+#ifndef SUPPORT_TLS
+static void dummy(int x) { dummy(x-1); }
+#else
+
+/* The IETF defines standard primes as "Modular Exponential (MODP) Groups" for
+use in IKE in RFC 2409 and 3526, and then some more, "for Use with IETF
+Standards" in RFC 5114.  These have been thoroughly reviewed as meeting
+certain eligibility criteria, which is more than can be said for primes
+generated quickly on no particular criteria.
+
+Any prime used in TLS is disclosed publicly, and if the security of your
+session depends upon the prime being secret, then one of three situations
+holds:
+ (1) the prime is too small
+ (2) the prime is flawed, use one of these instead
+ (3) you know of fundamental cryptanalytic breaks not currently publicly known
+     to the cryptographic community.
+*/
+
+/* RFC 2409 MODP IKE_id=1 generator=2 bits=768
+   The prime is: 2^768 - 2 ^704 - 1 + 2^64 * { [2^638 pi] + 149686 }
+   Its hexadecimal value is
+         FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
+         29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
+         EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
+         E485B576 625E7EC6 F44C42E9 A63A3620 FFFFFFFF FFFFFFFF
+*/
+static const char dh_ike_1_pem[] =
+"-----BEGIN DH PARAMETERS-----\n"
+"MGYCYQD//////////8kP2qIhaMI0xMZii4DcHNEpAk4IimfMdAILvqY7E5siUUoI\n"
+"eY40BN3vlRmzzTpDGzArCm3yXxQ3T+E1bW1RwkXkhbV2Yl5+xvRMQummOjYg////\n"
+"//////8CAQI=\n"
+"-----END DH PARAMETERS-----\n";
+
+/* RFC 2409 MODP IKE_id=2 generator=2 bits=1024
+   The prime is 2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 }.
+   Its hexadecimal value is
+
+         FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
+         29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
+         EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
+         E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
+         EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE65381
+         FFFFFFFF FFFFFFFF
+*/
+static const char dh_ike_2_pem[] =
+"-----BEGIN DH PARAMETERS-----\n"
+"MIGHAoGBAP//////////yQ/aoiFowjTExmKLgNwc0SkCTgiKZ8x0Agu+pjsTmyJR\n"
+"Sgh5jjQE3e+VGbPNOkMbMCsKbfJfFDdP4TVtbVHCReSFtXZiXn7G9ExC6aY37WsL\n"
+"/1y29Aa37e44a/taiZ+lrp8kEXxLH+ZJKGZR7OZTgf//////////AgEC\n"
+"-----END DH PARAMETERS-----\n";
+
+/* RFC 2409; id=3 and id=4 are EC2N, not yet supported here */
+
+/* RFC 3526 MODP IKE_id=5 generator=2 bits=1536
+   The prime is: 2^1536 - 2^1472 - 1 + 2^64 * { [2^1406 pi] + 741804 }
+   Its hexadecimal value is:
+
+      FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
+      29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
+      EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
+      E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
+      EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D
+      C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F
+      83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
+      670C354E 4ABC9804 F1746C08 CA237327 FFFFFFFF FFFFFFFF
+*/
+static const char dh_ike_5_pem[] =
+"-----BEGIN DH PARAMETERS-----\n"
+"MIHHAoHBAP//////////yQ/aoiFowjTExmKLgNwc0SkCTgiKZ8x0Agu+pjsTmyJR\n"
+"Sgh5jjQE3e+VGbPNOkMbMCsKbfJfFDdP4TVtbVHCReSFtXZiXn7G9ExC6aY37WsL\n"
+"/1y29Aa37e44a/taiZ+lrp8kEXxLH+ZJKGZR7ORbPcIAfLihY78FmNpINhxV05pp\n"
+"Fj+o/STPX4NlXSPco62WHGLzViCFUrue1SkHcJaWbWcMNU5KvJgE8XRsCMojcyf/\n"
+"/////////wIBAg==\n"
+"-----END DH PARAMETERS-----\n";
+
+/* RFC 3526 MODP IKE_id=14 generator=2 bits=2048
+   This prime is: 2^2048 - 2^1984 - 1 + 2^64 * { [2^1918 pi] + 124476 }
+   Its hexadecimal value is:
+
+      FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
+      29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
+      EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
+      E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
+      EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D
+      C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F
+      83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
+      670C354E 4ABC9804 F1746C08 CA18217C 32905E46 2E36CE3B
+      E39E772C 180E8603 9B2783A2 EC07A28F B5C55DF0 6F4C52C9
+      DE2BCBF6 95581718 3995497C EA956AE5 15D22618 98FA0510
+      15728E5A 8AACAA68 FFFFFFFF FFFFFFFF
+*/
+static const char dh_ike_14_pem[] =
+"-----BEGIN DH PARAMETERS-----\n"
+"MIIBCAKCAQEA///////////JD9qiIWjCNMTGYouA3BzRKQJOCIpnzHQCC76mOxOb\n"
+"IlFKCHmONATd75UZs806QxswKwpt8l8UN0/hNW1tUcJF5IW1dmJefsb0TELppjft\n"
+"awv/XLb0Brft7jhr+1qJn6WunyQRfEsf5kkoZlHs5Fs9wgB8uKFjvwWY2kg2HFXT\n"
+"mmkWP6j9JM9fg2VdI9yjrZYcYvNWIIVSu57VKQdwlpZtZww1Tkq8mATxdGwIyhgh\n"
+"fDKQXkYuNs474553LBgOhgObJ4Oi7Aeij7XFXfBvTFLJ3ivL9pVYFxg5lUl86pVq\n"
+"5RXSJhiY+gUQFXKOWoqsqmj//////////wIBAg==\n"
+"-----END DH PARAMETERS-----\n";
+
+/* RFC 3526 MODP IKE_id=15 generator=2 bits=3072
+   This prime is: 2^3072 - 2^3008 - 1 + 2^64 * { [2^2942 pi] + 1690314 }
+   Its hexadecimal value is:
+
+      FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
+      29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
+      EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
+      E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
+      EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D
+      C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F
+      83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
+      670C354E 4ABC9804 F1746C08 CA18217C 32905E46 2E36CE3B
+      E39E772C 180E8603 9B2783A2 EC07A28F B5C55DF0 6F4C52C9
+      DE2BCBF6 95581718 3995497C EA956AE5 15D22618 98FA0510
+      15728E5A 8AAAC42D AD33170D 04507A33 A85521AB DF1CBA64
+      ECFB8504 58DBEF0A 8AEA7157 5D060C7D B3970F85 A6E1E4C7
+      ABF5AE8C DB0933D7 1E8C94E0 4A25619D CEE3D226 1AD2EE6B
+      F12FFA06 D98A0864 D8760273 3EC86A64 521F2B18 177B200C
+      BBE11757 7A615D6C 770988C0 BAD946E2 08E24FA0 74E5AB31
+      43DB5BFC E0FD108E 4B82D120 A93AD2CA FFFFFFFF FFFFFFFF
+*/
+static const char dh_ike_15_pem[] =
+"-----BEGIN DH PARAMETERS-----\n"
+"MIIBiAKCAYEA///////////JD9qiIWjCNMTGYouA3BzRKQJOCIpnzHQCC76mOxOb\n"
+"IlFKCHmONATd75UZs806QxswKwpt8l8UN0/hNW1tUcJF5IW1dmJefsb0TELppjft\n"
+"awv/XLb0Brft7jhr+1qJn6WunyQRfEsf5kkoZlHs5Fs9wgB8uKFjvwWY2kg2HFXT\n"
+"mmkWP6j9JM9fg2VdI9yjrZYcYvNWIIVSu57VKQdwlpZtZww1Tkq8mATxdGwIyhgh\n"
+"fDKQXkYuNs474553LBgOhgObJ4Oi7Aeij7XFXfBvTFLJ3ivL9pVYFxg5lUl86pVq\n"
+"5RXSJhiY+gUQFXKOWoqqxC2tMxcNBFB6M6hVIavfHLpk7PuFBFjb7wqK6nFXXQYM\n"
+"fbOXD4Wm4eTHq/WujNsJM9cejJTgSiVhnc7j0iYa0u5r8S/6BtmKCGTYdgJzPshq\n"
+"ZFIfKxgXeyAMu+EXV3phXWx3CYjAutlG4gjiT6B05asxQ9tb/OD9EI5LgtEgqTrS\n"
+"yv//////////AgEC\n"
+"-----END DH PARAMETERS-----\n";
+
+/* RFC 3526 MODP IKE_id=16 generator=2 bits=4096
+   This prime is: 2^4096 - 2^4032 - 1 + 2^64 * { [2^3966 pi] + 240904 }
+   Its hexadecimal value is:
+
+      FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
+      29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
+      EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
+      E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
+      EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D
+      C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F
+      83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
+      670C354E 4ABC9804 F1746C08 CA18217C 32905E46 2E36CE3B
+      E39E772C 180E8603 9B2783A2 EC07A28F B5C55DF0 6F4C52C9
+      DE2BCBF6 95581718 3995497C EA956AE5 15D22618 98FA0510
+      15728E5A 8AAAC42D AD33170D 04507A33 A85521AB DF1CBA64
+      ECFB8504 58DBEF0A 8AEA7157 5D060C7D B3970F85 A6E1E4C7
+      ABF5AE8C DB0933D7 1E8C94E0 4A25619D CEE3D226 1AD2EE6B
+      F12FFA06 D98A0864 D8760273 3EC86A64 521F2B18 177B200C
+      BBE11757 7A615D6C 770988C0 BAD946E2 08E24FA0 74E5AB31
+      43DB5BFC E0FD108E 4B82D120 A9210801 1A723C12 A787E6D7
+      88719A10 BDBA5B26 99C32718 6AF4E23C 1A946834 B6150BDA
+      2583E9CA 2AD44CE8 DBBBC2DB 04DE8EF9 2E8EFC14 1FBECAA6
+      287C5947 4E6BC05D 99B2964F A090C3A2 233BA186 515BE7ED
+      1F612970 CEE2D7AF B81BDD76 2170481C D0069127 D5B05AA9
+      93B4EA98 8D8FDDC1 86FFB7DC 90A6C08F 4DF435C9 34063199
+      FFFFFFFF FFFFFFFF
+*/
+static const char dh_ike_16_pem[] =
+"-----BEGIN DH PARAMETERS-----\n"
+"MIICCAKCAgEA///////////JD9qiIWjCNMTGYouA3BzRKQJOCIpnzHQCC76mOxOb\n"
+"IlFKCHmONATd75UZs806QxswKwpt8l8UN0/hNW1tUcJF5IW1dmJefsb0TELppjft\n"
+"awv/XLb0Brft7jhr+1qJn6WunyQRfEsf5kkoZlHs5Fs9wgB8uKFjvwWY2kg2HFXT\n"
+"mmkWP6j9JM9fg2VdI9yjrZYcYvNWIIVSu57VKQdwlpZtZww1Tkq8mATxdGwIyhgh\n"
+"fDKQXkYuNs474553LBgOhgObJ4Oi7Aeij7XFXfBvTFLJ3ivL9pVYFxg5lUl86pVq\n"
+"5RXSJhiY+gUQFXKOWoqqxC2tMxcNBFB6M6hVIavfHLpk7PuFBFjb7wqK6nFXXQYM\n"
+"fbOXD4Wm4eTHq/WujNsJM9cejJTgSiVhnc7j0iYa0u5r8S/6BtmKCGTYdgJzPshq\n"
+"ZFIfKxgXeyAMu+EXV3phXWx3CYjAutlG4gjiT6B05asxQ9tb/OD9EI5LgtEgqSEI\n"
+"ARpyPBKnh+bXiHGaEL26WyaZwycYavTiPBqUaDS2FQvaJYPpyirUTOjbu8LbBN6O\n"
+"+S6O/BQfvsqmKHxZR05rwF2ZspZPoJDDoiM7oYZRW+ftH2EpcM7i16+4G912IXBI\n"
+"HNAGkSfVsFqpk7TqmI2P3cGG/7fckKbAj030Nck0BjGZ//////////8CAQI=\n"
+"-----END DH PARAMETERS-----\n";
+
+/* RFC 3526 MODP IKE_id=17 generator=2 bits=6144
+   This prime is: 2^6144 - 2^6080 - 1 + 2^64 * { [2^6014 pi] + 929484 }
+   Its hexadecimal value is:
+
+   FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08
+   8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B
+   302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9
+   A637ED6B 0BFF5CB6 F406B7ED EE386BFB 5A899FA5 AE9F2411 7C4B1FE6
+   49286651 ECE45B3D C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8
+   FD24CF5F 83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
+   670C354E 4ABC9804 F1746C08 CA18217C 32905E46 2E36CE3B E39E772C
+   180E8603 9B2783A2 EC07A28F B5C55DF0 6F4C52C9 DE2BCBF6 95581718
+   3995497C EA956AE5 15D22618 98FA0510 15728E5A 8AAAC42D AD33170D
+   04507A33 A85521AB DF1CBA64 ECFB8504 58DBEF0A 8AEA7157 5D060C7D
+   B3970F85 A6E1E4C7 ABF5AE8C DB0933D7 1E8C94E0 4A25619D CEE3D226
+   1AD2EE6B F12FFA06 D98A0864 D8760273 3EC86A64 521F2B18 177B200C
+   BBE11757 7A615D6C 770988C0 BAD946E2 08E24FA0 74E5AB31 43DB5BFC
+   E0FD108E 4B82D120 A9210801 1A723C12 A787E6D7 88719A10 BDBA5B26
+   99C32718 6AF4E23C 1A946834 B6150BDA 2583E9CA 2AD44CE8 DBBBC2DB
+   04DE8EF9 2E8EFC14 1FBECAA6 287C5947 4E6BC05D 99B2964F A090C3A2
+   233BA186 515BE7ED 1F612970 CEE2D7AF B81BDD76 2170481C D0069127
+   D5B05AA9 93B4EA98 8D8FDDC1 86FFB7DC 90A6C08F 4DF435C9 34028492
+   36C3FAB4 D27C7026 C1D4DCB2 602646DE C9751E76 3DBA37BD F8FF9406
+   AD9E530E E5DB382F 413001AE B06A53ED 9027D831 179727B0 865A8918
+   DA3EDBEB CF9B14ED 44CE6CBA CED4BB1B DB7F1447 E6CC254B 33205151
+   2BD7AF42 6FB8F401 378CD2BF 5983CA01 C64B92EC F032EA15 D1721D03
+   F482D7CE 6E74FEF6 D55E702F 46980C82 B5A84031 900B1C9E 59E7C97F
+   BEC7E8F3 23A97A7E 36CC88BE 0F1D45B7 FF585AC5 4BD407B2 2B4154AA
+   CC8F6D7E BF48E1D8 14CC5ED2 0F8037E0 A79715EE F29BE328 06A1D58B
+   B7C5DA76 F550AA3D 8A1FBFF0 EB19CCB1 A313D55C DA56C9EC 2EF29632
+   387FE8D7 6E3C0468 043E8F66 3F4860EE 12BF2D5B 0B7474D6 E694F91E
+   6DCC4024 FFFFFFFF FFFFFFFF
+*/
+static const char dh_ike_17_pem[] =
+"-----BEGIN DH PARAMETERS-----\n"
+"MIIDCAKCAwEA///////////JD9qiIWjCNMTGYouA3BzRKQJOCIpnzHQCC76mOxOb\n"
+"IlFKCHmONATd75UZs806QxswKwpt8l8UN0/hNW1tUcJF5IW1dmJefsb0TELppjft\n"
+"awv/XLb0Brft7jhr+1qJn6WunyQRfEsf5kkoZlHs5Fs9wgB8uKFjvwWY2kg2HFXT\n"
+"mmkWP6j9JM9fg2VdI9yjrZYcYvNWIIVSu57VKQdwlpZtZww1Tkq8mATxdGwIyhgh\n"
+"fDKQXkYuNs474553LBgOhgObJ4Oi7Aeij7XFXfBvTFLJ3ivL9pVYFxg5lUl86pVq\n"
+"5RXSJhiY+gUQFXKOWoqqxC2tMxcNBFB6M6hVIavfHLpk7PuFBFjb7wqK6nFXXQYM\n"
+"fbOXD4Wm4eTHq/WujNsJM9cejJTgSiVhnc7j0iYa0u5r8S/6BtmKCGTYdgJzPshq\n"
+"ZFIfKxgXeyAMu+EXV3phXWx3CYjAutlG4gjiT6B05asxQ9tb/OD9EI5LgtEgqSEI\n"
+"ARpyPBKnh+bXiHGaEL26WyaZwycYavTiPBqUaDS2FQvaJYPpyirUTOjbu8LbBN6O\n"
+"+S6O/BQfvsqmKHxZR05rwF2ZspZPoJDDoiM7oYZRW+ftH2EpcM7i16+4G912IXBI\n"
+"HNAGkSfVsFqpk7TqmI2P3cGG/7fckKbAj030Nck0AoSSNsP6tNJ8cCbB1NyyYCZG\n"
+"3sl1HnY9uje9+P+UBq2eUw7l2zgvQTABrrBqU+2QJ9gxF5cnsIZaiRjaPtvrz5sU\n"
+"7UTObLrO1Lsb238UR+bMJUszIFFRK9evQm+49AE3jNK/WYPKAcZLkuzwMuoV0XId\n"
+"A/SC185udP721V5wL0aYDIK1qEAxkAscnlnnyX++x+jzI6l6fjbMiL4PHUW3/1ha\n"
+"xUvUB7IrQVSqzI9tfr9I4dgUzF7SD4A34KeXFe7ym+MoBqHVi7fF2nb1UKo9ih+/\n"
+"8OsZzLGjE9Vc2lbJ7C7yljI4f+jXbjwEaAQ+j2Y/SGDuEr8tWwt0dNbmlPkebcxA\n"
+"JP//////////AgEC\n"
+"-----END DH PARAMETERS-----\n";
+
+/* RFC 3526 MODP IKE_id=18 generator=2 bits=8192
+   This prime is: 2^8192 - 2^8128 - 1 + 2^64 * { [2^8062 pi] + 4743158 }
+   Its hexadecimal value is:
+
+      FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
+      29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
+      EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
+      E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
+      EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D
+      C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F
+      83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
+      670C354E 4ABC9804 F1746C08 CA18217C 32905E46 2E36CE3B
+      E39E772C 180E8603 9B2783A2 EC07A28F B5C55DF0 6F4C52C9
+      DE2BCBF6 95581718 3995497C EA956AE5 15D22618 98FA0510
+      15728E5A 8AAAC42D AD33170D 04507A33 A85521AB DF1CBA64
+      ECFB8504 58DBEF0A 8AEA7157 5D060C7D B3970F85 A6E1E4C7
+      ABF5AE8C DB0933D7 1E8C94E0 4A25619D CEE3D226 1AD2EE6B
+      F12FFA06 D98A0864 D8760273 3EC86A64 521F2B18 177B200C
+      BBE11757 7A615D6C 770988C0 BAD946E2 08E24FA0 74E5AB31
+      43DB5BFC E0FD108E 4B82D120 A9210801 1A723C12 A787E6D7
+      88719A10 BDBA5B26 99C32718 6AF4E23C 1A946834 B6150BDA
+      2583E9CA 2AD44CE8 DBBBC2DB 04DE8EF9 2E8EFC14 1FBECAA6
+      287C5947 4E6BC05D 99B2964F A090C3A2 233BA186 515BE7ED
+      1F612970 CEE2D7AF B81BDD76 2170481C D0069127 D5B05AA9
+      93B4EA98 8D8FDDC1 86FFB7DC 90A6C08F 4DF435C9 34028492
+      36C3FAB4 D27C7026 C1D4DCB2 602646DE C9751E76 3DBA37BD
+      F8FF9406 AD9E530E E5DB382F 413001AE B06A53ED 9027D831
+      179727B0 865A8918 DA3EDBEB CF9B14ED 44CE6CBA CED4BB1B
+      DB7F1447 E6CC254B 33205151 2BD7AF42 6FB8F401 378CD2BF
+      5983CA01 C64B92EC F032EA15 D1721D03 F482D7CE 6E74FEF6
+      D55E702F 46980C82 B5A84031 900B1C9E 59E7C97F BEC7E8F3
+      23A97A7E 36CC88BE 0F1D45B7 FF585AC5 4BD407B2 2B4154AA
+      CC8F6D7E BF48E1D8 14CC5ED2 0F8037E0 A79715EE F29BE328
+      06A1D58B B7C5DA76 F550AA3D 8A1FBFF0 EB19CCB1 A313D55C
+      DA56C9EC 2EF29632 387FE8D7 6E3C0468 043E8F66 3F4860EE
+      12BF2D5B 0B7474D6 E694F91E 6DBE1159 74A3926F 12FEE5E4
+      38777CB6 A932DF8C D8BEC4D0 73B931BA 3BC832B6 8D9DD300
+      741FA7BF 8AFC47ED 2576F693 6BA42466 3AAB639C 5AE4F568
+      3423B474 2BF1C978 238F16CB E39D652D E3FDB8BE FC848AD9
+      22222E04 A4037C07 13EB57A8 1A23F0C7 3473FC64 6CEA306B
+      4BCBC886 2F8385DD FA9D4B7F A2C087E8 79683303 ED5BDD3A
+      062B3CF5 B3A278A6 6D2A13F8 3F44F82D DF310EE0 74AB6A36
+      4597E899 A0255DC1 64F31CC5 0846851D F9AB4819 5DED7EA1
+      B1D510BD 7EE74D73 FAF36BC3 1ECFA268 359046F4 EB879F92
+      4009438B 481C6CD7 889A002E D5EE382B C9190DA6 FC026E47
+      9558E447 5677E9AA 9E3050E2 765694DF C81F56E8 80B96E71
+      60C980DD 98EDD3DF FFFFFFFF FFFFFFFF
+*/
+static const char dh_ike_18_pem[] =
+"-----BEGIN DH PARAMETERS-----\n"
+"MIIECAKCBAEA///////////JD9qiIWjCNMTGYouA3BzRKQJOCIpnzHQCC76mOxOb\n"
+"IlFKCHmONATd75UZs806QxswKwpt8l8UN0/hNW1tUcJF5IW1dmJefsb0TELppjft\n"
+"awv/XLb0Brft7jhr+1qJn6WunyQRfEsf5kkoZlHs5Fs9wgB8uKFjvwWY2kg2HFXT\n"
+"mmkWP6j9JM9fg2VdI9yjrZYcYvNWIIVSu57VKQdwlpZtZww1Tkq8mATxdGwIyhgh\n"
+"fDKQXkYuNs474553LBgOhgObJ4Oi7Aeij7XFXfBvTFLJ3ivL9pVYFxg5lUl86pVq\n"
+"5RXSJhiY+gUQFXKOWoqqxC2tMxcNBFB6M6hVIavfHLpk7PuFBFjb7wqK6nFXXQYM\n"
+"fbOXD4Wm4eTHq/WujNsJM9cejJTgSiVhnc7j0iYa0u5r8S/6BtmKCGTYdgJzPshq\n"
+"ZFIfKxgXeyAMu+EXV3phXWx3CYjAutlG4gjiT6B05asxQ9tb/OD9EI5LgtEgqSEI\n"
+"ARpyPBKnh+bXiHGaEL26WyaZwycYavTiPBqUaDS2FQvaJYPpyirUTOjbu8LbBN6O\n"
+"+S6O/BQfvsqmKHxZR05rwF2ZspZPoJDDoiM7oYZRW+ftH2EpcM7i16+4G912IXBI\n"
+"HNAGkSfVsFqpk7TqmI2P3cGG/7fckKbAj030Nck0AoSSNsP6tNJ8cCbB1NyyYCZG\n"
+"3sl1HnY9uje9+P+UBq2eUw7l2zgvQTABrrBqU+2QJ9gxF5cnsIZaiRjaPtvrz5sU\n"
+"7UTObLrO1Lsb238UR+bMJUszIFFRK9evQm+49AE3jNK/WYPKAcZLkuzwMuoV0XId\n"
+"A/SC185udP721V5wL0aYDIK1qEAxkAscnlnnyX++x+jzI6l6fjbMiL4PHUW3/1ha\n"
+"xUvUB7IrQVSqzI9tfr9I4dgUzF7SD4A34KeXFe7ym+MoBqHVi7fF2nb1UKo9ih+/\n"
+"8OsZzLGjE9Vc2lbJ7C7yljI4f+jXbjwEaAQ+j2Y/SGDuEr8tWwt0dNbmlPkebb4R\n"
+"WXSjkm8S/uXkOHd8tqky34zYvsTQc7kxujvIMraNndMAdB+nv4r8R+0ldvaTa6Qk\n"
+"ZjqrY5xa5PVoNCO0dCvxyXgjjxbL451lLeP9uL78hIrZIiIuBKQDfAcT61eoGiPw\n"
+"xzRz/GRs6jBrS8vIhi+Dhd36nUt/osCH6HloMwPtW906Bis89bOieKZtKhP4P0T4\n"
+"Ld8xDuB0q2o2RZfomaAlXcFk8xzFCEaFHfmrSBld7X6hsdUQvX7nTXP682vDHs+i\n"
+"aDWQRvTrh5+SQAlDi0gcbNeImgAu1e44K8kZDab8Am5HlVjkR1Z36aqeMFDidlaU\n"
+"38gfVuiAuW5xYMmA3Zjt09///////////wIBAg==\n"
+"-----END DH PARAMETERS-----\n";
+
+/* RFC 5114 IKE_id=22
+2.1.  1024-bit MODP Group with 160-bit Prime Order Subgroup
+
+   The hexadecimal value of the prime is:
+
+   p = B10B8F96 A080E01D DE92DE5E AE5D54EC 52C99FBC FB06A3C6
+       9A6A9DCA 52D23B61 6073E286 75A23D18 9838EF1E 2EE652C0
+       13ECB4AE A9061123 24975C3C D49B83BF ACCBDD7D 90C4BD70
+       98488E9C 219A7372 4EFFD6FA E5644738 FAA31A4F F55BCCC0
+       A151AF5F 0DC8B4BD 45BF37DF 365C1A65 E68CFDA7 6D4DA708
+       DF1FB2BC 2E4A4371
+
+   The hexadecimal value of the generator is:
+
+   g = A4D1CBD5 C3FD3412 6765A442 EFB99905 F8104DD2 58AC507F
+       D6406CFF 14266D31 266FEA1E 5C41564B 777E690F 5504F213
+       160217B4 B01B886A 5E91547F 9E2749F4 D7FBD7D3 B9A92EE1
+       909D0D22 63F80A76 A6A24C08 7A091F53 1DBF0A01 69B6A28A
+       D662A4D1 8E73AFA3 2D779D59 18D08BC8 858F4DCE F97C2A24
+       855E6EEB 22B3B2E5
+
+   The generator generates a prime-order subgroup of size:
+
+   q = F518AA87 81A8DF27 8ABA4E7D 64B7CB9D 49462353
+*/
+static const char dh_ike_22_pem[] =
+"-----BEGIN DH PARAMETERS-----\n"
+"MIIBCAKBgQCxC4+WoIDgHd6S3l6uXVTsUsmfvPsGo8aaap3KUtI7YWBz4oZ1oj0Y\n"
+"mDjvHi7mUsAT7LSuqQYRIySXXDzUm4O/rMvdfZDEvXCYSI6cIZpzck7/1vrlZEc4\n"
+"+qMaT/VbzMChUa9fDci0vUW/N982XBpl5oz9p21NpwjfH7K8LkpDcQKBgQCk0cvV\n"
+"w/00EmdlpELvuZkF+BBN0lisUH/WQGz/FCZtMSZv6h5cQVZLd35pD1UE8hMWAhe0\n"
+"sBuIal6RVH+eJ0n01/vX07mpLuGQnQ0iY/gKdqaiTAh6CR9THb8KAWm2oorWYqTR\n"
+"jnOvoy13nVkY0IvIhY9Nzvl8KiSFXm7rIrOy5Q==\n"
+"-----END DH PARAMETERS-----\n";
+
+/* RFC 5114 IKE_id=23
+2.2.  2048-bit MODP Group with 224-bit Prime Order Subgroup
+
+   The hexadecimal value of the prime is:
+
+   p =  AD107E1E 9123A9D0 D660FAA7 9559C51F A20D64E5 683B9FD1
+        B54B1597 B61D0A75 E6FA141D F95A56DB AF9A3C40 7BA1DF15
+        EB3D688A 309C180E 1DE6B85A 1274A0A6 6D3F8152 AD6AC212
+        9037C9ED EFDA4DF8 D91E8FEF 55B7394B 7AD5B7D0 B6C12207
+        C9F98D11 ED34DBF6 C6BA0B2C 8BBC27BE 6A00E0A0 B9C49708
+        B3BF8A31 70918836 81286130 BC8985DB 1602E714 415D9330
+        278273C7 DE31EFDC 7310F712 1FD5A074 15987D9A DC0A486D
+        CDF93ACC 44328387 315D75E1 98C641A4 80CD86A1 B9E587E8
+        BE60E69C C928B2B9 C52172E4 13042E9B 23F10B0E 16E79763
+        C9B53DCF 4BA80A29 E3FB73C1 6B8E75B9 7EF363E2 FFA31F71
+        CF9DE538 4E71B81C 0AC4DFFE 0C10E64F
+
+   The hexadecimal value of the generator is:
+
+   g =  AC4032EF 4F2D9AE3 9DF30B5C 8FFDAC50 6CDEBE7B 89998CAF
+        74866A08 CFE4FFE3 A6824A4E 10B9A6F0 DD921F01 A70C4AFA
+        AB739D77 00C29F52 C57DB17C 620A8652 BE5E9001 A8D66AD7
+        C1766910 1999024A F4D02727 5AC1348B B8A762D0 521BC98A
+        E2471504 22EA1ED4 09939D54 DA7460CD B5F6C6B2 50717CBE
+        F180EB34 118E98D1 19529A45 D6F83456 6E3025E3 16A330EF
+        BB77A86F 0C1AB15B 051AE3D4 28C8F8AC B70A8137 150B8EEB
+        10E183ED D19963DD D9E263E4 770589EF 6AA21E7F 5F2FF381
+        B539CCE3 409D13CD 566AFBB4 8D6C0191 81E1BCFE 94B30269
+        EDFE72FE 9B6AA4BD 7B5A0F1C 71CFFF4C 19C418E1 F6EC0179
+        81BC087F 2A7065B3 84B890D3 191F2BFA
+
+   The generator generates a prime-order subgroup of size:
+
+   q =  801C0D34 C58D93FE 99717710 1F80535A 4738CEBC BF389A99
+        B36371EB
+*/
+static const char dh_ike_23_pem[] =
+"-----BEGIN DH PARAMETERS-----\n"
+"MIICCgKCAQEArRB+HpEjqdDWYPqnlVnFH6INZOVoO5/RtUsVl7YdCnXm+hQd+VpW\n"
+"26+aPEB7od8V6z1oijCcGA4d5rhaEnSgpm0/gVKtasISkDfJ7e/aTfjZHo/vVbc5\n"
+"S3rVt9C2wSIHyfmNEe002/bGugssi7wnvmoA4KC5xJcIs7+KMXCRiDaBKGEwvImF\n"
+"2xYC5xRBXZMwJ4Jzx94x79xzEPcSH9WgdBWYfZrcCkhtzfk6zEQyg4cxXXXhmMZB\n"
+"pIDNhqG55YfovmDmnMkosrnFIXLkEwQumyPxCw4W55djybU9z0uoCinj+3PBa451\n"
+"uX7zY+L/ox9xz53lOE5xuBwKxN/+DBDmTwKCAQEArEAy708tmuOd8wtcj/2sUGze\n"
+"vnuJmYyvdIZqCM/k/+OmgkpOELmm8N2SHwGnDEr6q3OddwDCn1LFfbF8YgqGUr5e\n"
+"kAGo1mrXwXZpEBmZAkr00CcnWsE0i7inYtBSG8mK4kcVBCLqHtQJk51U2nRgzbX2\n"
+"xrJQcXy+8YDrNBGOmNEZUppF1vg0Vm4wJeMWozDvu3eobwwasVsFGuPUKMj4rLcK\n"
+"gTcVC47rEOGD7dGZY93Z4mPkdwWJ72qiHn9fL/OBtTnM40CdE81Wavu0jWwBkYHh\n"
+"vP6UswJp7f5y/ptqpL17Wg8ccc//TBnEGOH27AF5gbwIfypwZbOEuJDTGR8r+g==\n"
+"-----END DH PARAMETERS-----\n";
+
+/* RFC 5114 IKE_id=24
+2.3.  2048-bit MODP Group with 256-bit Prime Order Subgroup
+
+   The hexadecimal value of the prime is:
+
+   p = 87A8E61D B4B6663C FFBBD19C 65195999 8CEEF608 660DD0F2
+       5D2CEED4 435E3B00 E00DF8F1 D61957D4 FAF7DF45 61B2AA30
+       16C3D911 34096FAA 3BF4296D 830E9A7C 209E0C64 97517ABD
+       5A8A9D30 6BCF67ED 91F9E672 5B4758C0 22E0B1EF 4275BF7B
+       6C5BFC11 D45F9088 B941F54E B1E59BB8 BC39A0BF 12307F5C
+       4FDB70C5 81B23F76 B63ACAE1 CAA6B790 2D525267 35488A0E
+       F13C6D9A 51BFA4AB 3AD83477 96524D8E F6A167B5 A41825D9
+       67E144E5 14056425 1CCACB83 E6B486F6 B3CA3F79 71506026
+       C0B857F6 89962856 DED4010A BD0BE621 C3A3960A 54E710C3
+       75F26375 D7014103 A4B54330 C198AF12 6116D227 6E11715F
+       693877FA D7EF09CA DB094AE9 1E1A1597
+
+   The hexadecimal value of the generator is:
+
+   g = 3FB32C9B 73134D0B 2E775066 60EDBD48 4CA7B18F 21EF2054
+       07F4793A 1A0BA125 10DBC150 77BE463F FF4FED4A AC0BB555
+       BE3A6C1B 0C6B47B1 BC3773BF 7E8C6F62 901228F8 C28CBB18
+       A55AE313 41000A65 0196F931 C77A57F2 DDF463E5 E9EC144B
+       777DE62A AAB8A862 8AC376D2 82D6ED38 64E67982 428EBC83
+       1D14348F 6F2F9193 B5045AF2 767164E1 DFC967C1 FB3F2E55
+       A4BD1BFF E83B9C80 D052B985 D182EA0A DB2A3B73 13D3FE14
+       C8484B1E 052588B9 B7D2BBD2 DF016199 ECD06E15 57CD0915
+       B3353BBB 64E0EC37 7FD02837 0DF92B52 C7891428 CDC67EB6
+       184B523D 1DB246C3 2F630784 90F00EF8 D647D148 D4795451
+       5E2327CF EF98C582 664B4C0F 6CC41659
+
+   The generator generates a prime-order subgroup of size:
+
+   q = 8CF83642 A709A097 B4479976 40129DA2 99B1A47D 1EB3750B
+       A308B0FE 64F5FBD3
+*/
+static const char dh_ike_24_pem[] =
+"-----BEGIN DH PARAMETERS-----\n"
+"MIICCQKCAQEAh6jmHbS2Zjz/u9GcZRlZmYzu9ghmDdDyXSzu1ENeOwDgDfjx1hlX\n"
+"1Pr330VhsqowFsPZETQJb6o79Cltgw6afCCeDGSXUXq9WoqdMGvPZ+2R+eZyW0dY\n"
+"wCLgse9Cdb97bFv8EdRfkIi5QfVOseWbuLw5oL8SMH9cT9twxYGyP3a2Osrhyqa3\n"
+"kC1SUmc1SIoO8TxtmlG/pKs62DR3llJNjvahZ7WkGCXZZ+FE5RQFZCUcysuD5rSG\n"
+"9rPKP3lxUGAmwLhX9omWKFbe1AEKvQvmIcOjlgpU5xDDdfJjddcBQQOktUMwwZiv\n"
+"EmEW0iduEXFfaTh3+tfvCcrbCUrpHhoVlwKCAQA/syybcxNNCy53UGZg7b1ITKex\n"
+"jyHvIFQH9Hk6GguhJRDbwVB3vkY//0/tSqwLtVW+OmwbDGtHsbw3c79+jG9ikBIo\n"
+"+MKMuxilWuMTQQAKZQGW+THHelfy3fRj5ensFEt3feYqqrioYorDdtKC1u04ZOZ5\n"
+"gkKOvIMdFDSPby+Rk7UEWvJ2cWTh38lnwfs/LlWkvRv/6DucgNBSuYXRguoK2yo7\n"
+"cxPT/hTISEseBSWIubfSu9LfAWGZ7NBuFVfNCRWzNTu7ZODsN3/QKDcN+StSx4kU\n"
+"KM3GfrYYS1I9HbJGwy9jB4SQ8A741kfRSNR5VFFeIyfP75jFgmZLTA9sxBZZ\n"
+"-----END DH PARAMETERS-----\n";
+
+
+/* ========================================================================= */
+
+struct dh_constant {
+  const char *label;
+  const char *pem;
+};
+
+/* KEEP SORTED ALPHABETICALLY;
+ * duplicate PEM are okay, if we want aliases, but names must be alphabetical */
+static struct dh_constant dh_constants[] = {
+    { "default", dh_ike_23_pem },
+    { "ike1", dh_ike_1_pem },
+    { "ike14", dh_ike_14_pem },
+    { "ike15", dh_ike_15_pem },
+    { "ike16", dh_ike_16_pem },
+    { "ike17", dh_ike_17_pem },
+    { "ike18", dh_ike_18_pem },
+    { "ike2", dh_ike_2_pem },
+    { "ike22", dh_ike_22_pem },
+    { "ike23", dh_ike_23_pem },
+    { "ike24", dh_ike_24_pem },
+    { "ike5", dh_ike_5_pem },
+};
+static const int dh_constants_count =
+  sizeof(dh_constants) / sizeof(struct dh_constant);
+
+
+/* A policy decision; in absence of any other data, use a 2048 bit prime,
+ * pick the first one from the latest RFC providing such. */
+const char *
+std_dh_prime_default(void)
+{
+  return dh_ike_23_pem;
+}
+
+
+const char *
+std_dh_prime_named(const uschar *name)
+{
+  int first, last;
+  char *search_name = CS string_copylc(US name);
+
+  first = 0;
+  last = dh_constants_count;
+  while (last > first) {
+    int middle = (first + last)/2;
+    int c = strcmp(search_name, dh_constants[middle].label);
+    if (c == 0)
+      return dh_constants[middle].pem;
+    else if (c > 0)
+      first = middle + 1;
+    else
+      last = middle;
+  }
+  return NULL;
+}
+
+#endif /* SUPPORT_TLS */
+/* EOF */
diff --git a/src/src/tls-gnu.c b/src/src/tls-gnu.c
index e2fcb3338..7aab309e6 100644
--- a/src/src/tls-gnu.c
+++ b/src/src/tls-gnu.c
@@ -358,9 +358,6 @@ file is never present. If two processes both compute some new parameters, you
 waste a bit of effort, but it doesn't seem worth messing around with locking to
 prevent this.
 
-Argument:
-  host       NULL for server, server for client (for error handling)
-
 Returns:     OK/DEFER/FAIL
 */
 
@@ -370,8 +367,12 @@ init_server_dh(void)
 int fd, rc;
 unsigned int dh_bits;
 gnutls_datum m;
-uschar filename[PATH_MAX];
+uschar filename_buf[PATH_MAX];
+uschar *filename = NULL;
 size_t sz;
+uschar *exp_tls_dhparam;
+BOOL use_file_in_spool = FALSE;
+BOOL use_fixed_file = FALSE;
 host_item *host = NULL; /* dummy for macros */
 
 DEBUG(D_tls) debug_printf("Initialising GnuTLS server params.\n");
@@ -379,6 +380,46 @@ DEBUG(D_tls) debug_printf("Initialising GnuTLS server params.\n");
 rc = gnutls_dh_params_init(&dh_server_params);
 exim_gnutls_err_check(US"gnutls_dh_params_init");
 
+m.data = NULL;
+m.size = 0;
+
+if (!expand_check(tls_dhparam, US"tls_dhparam", &exp_tls_dhparam))
+  return DEFER;
+
+if (!exp_tls_dhparam)
+  {
+  DEBUG(D_tls) debug_printf("Loading default hard-coded DH params\n");
+  m.data = US std_dh_prime_default();
+  m.size = Ustrlen(m.data);
+  }
+else if (Ustrcmp(exp_tls_dhparam, "historic") == 0)
+  use_file_in_spool = TRUE;
+else if (Ustrcmp(exp_tls_dhparam, "none") == 0)
+  {
+  DEBUG(D_tls) debug_printf("Requested no DH parameters.\n");
+  return OK;
+  }
+else if (exp_tls_dhparam[0] != '/')
+  {
+  m.data = US std_dh_prime_named(exp_tls_dhparam);
+  if (m.data == NULL)
+    return tls_error(US"No standard prime named", CS exp_tls_dhparam, NULL);
+  m.size = Ustrlen(m.data);
+  }
+else
+  {
+  use_fixed_file = TRUE;
+  filename = exp_tls_dhparam;
+  }
+
+if (m.data)
+  {
+  rc = gnutls_dh_params_import_pkcs3(dh_server_params, &m, GNUTLS_X509_FMT_PEM);
+  exim_gnutls_err_check(US"gnutls_dh_params_import_pkcs3");
+  DEBUG(D_tls) debug_printf("Loaded fixed standard D-H parameters\n");
+  return OK;
+  }
+
 #ifdef HAVE_GNUTLS_SEC_PARAM_CONSTANTS
 /* If you change this constant, also change dh_param_fn_ext so that we can use a
 different filename and ensure we have sufficient bits. */
@@ -404,9 +445,13 @@ if (dh_bits > tls_dh_max_bits)
   dh_bits = tls_dh_max_bits;
   }
 
-if (!string_format(filename, sizeof(filename),
-      "%s/gnutls-params-%d", spool_directory, dh_bits))
-  return tls_error(US"overlong filename", NULL, NULL);
+if (use_file_in_spool)
+  {
+  if (!string_format(filename_buf, sizeof(filename_buf),
+        "%s/gnutls-params-%d", spool_directory, dh_bits))
+    return tls_error(US"overlong filename", NULL, NULL);
+  filename = filename_buf;
+  }
 
 /* Open the cache file for reading and if successful, read it and set up the
 parameters. */
diff --git a/src/src/tls-openssl.c b/src/src/tls-openssl.c
index ebc5a6255..43b79634e 100644
--- a/src/src/tls-openssl.c
+++ b/src/src/tls-openssl.c
@@ -275,60 +275,85 @@ DEBUG(D_tls) debug_printf("SSL info: %s\n", SSL_state_string_long(s));
 /* If dhparam is set, expand it, and load up the parameters for DH encryption.
 
 Arguments:
-  dhparam   DH parameter file
+  dhparam   DH parameter file or fixed parameter identity string
   host      connected host, if client; NULL if server
 
 Returns:    TRUE if OK (nothing to set up, or setup worked)
 */
 
 static BOOL
-init_dh(uschar *dhparam, host_item *host)
+init_dh(SSL_CTX *sctx, uschar *dhparam, host_item *host)
 {
-BOOL yield = TRUE;
 BIO *bio;
 DH *dh;
 uschar *dhexpanded;
+const char *pem;
 
 if (!expand_check(dhparam, US"tls_dhparam", &dhexpanded))
   return FALSE;
 
-if (dhexpanded == NULL) return TRUE;
-
-if ((bio = BIO_new_file(CS dhexpanded, "r")) == NULL)
+if (dhexpanded == NULL || *dhexpanded == '\0')
   {
-  tls_error(string_sprintf("could not read dhparams file %s", dhexpanded),
-    host, (uschar *)strerror(errno));
-  yield = FALSE;
+  bio = BIO_new_mem_buf(CS std_dh_prime_default(), -1);
   }
-else
+else if (dhexpanded[0] == '/')
   {
-  if ((dh = PEM_read_bio_DHparams(bio, NULL, NULL, NULL)) == NULL)
+  bio = BIO_new_file(CS dhexpanded, "r");
+  if (bio == NULL)
     {
     tls_error(string_sprintf("could not read dhparams file %s", dhexpanded),
-      host, NULL);
-    yield = FALSE;
+          host, US strerror(errno));
+    return FALSE;
     }
-  else
+  }
+else
+  {
+  if (Ustrcmp(dhexpanded, "none") == 0)
     {
-    if ((8*DH_size(dh)) > tls_dh_max_bits)
-      {
-      DEBUG(D_tls)
-        debug_printf("dhparams file %d bits, is > tls_dh_max_bits limit of %d",
-            8*DH_size(dh), tls_dh_max_bits);
-      }
-    else
-      {
-      SSL_CTX_set_tmp_dh(ctx, dh);
-      DEBUG(D_tls)
-        debug_printf("Diffie-Hellman initialized from %s with %d-bit key\n",
-          dhexpanded, 8*DH_size(dh));
-      }
-    DH_free(dh);
+    DEBUG(D_tls) debug_printf("Requested no DH parameters.\n");
+    return TRUE;
+    }
+
+  pem = std_dh_prime_named(dhexpanded);
+  if (!pem)
+    {
+    tls_error(string_sprintf("Unknown standard DH prime \"%s\"", dhexpanded),
+        host, US strerror(errno));
+    return FALSE;
     }
+  bio = BIO_new_mem_buf(CS pem, -1);
+  }
+
+dh = PEM_read_bio_DHparams(bio, NULL, NULL, NULL);
+if (dh == NULL)
+  {
   BIO_free(bio);
+  tls_error(string_sprintf("Could not read tls_dhparams \"%s\"", dhexpanded),
+      host, NULL);
+  return FALSE;
   }
 
-return yield;
+/* Even if it is larger, we silently return success rather than cause things
+ * to fail out, so that a too-large DH will not knock out all TLS; it's a
+ * debatable choice. */
+if ((8*DH_size(dh)) > tls_dh_max_bits)
+  {
+  DEBUG(D_tls)
+    debug_printf("dhparams file %d bits, is > tls_dh_max_bits limit of %d",
+        8*DH_size(dh), tls_dh_max_bits);
+  }
+else
+  {
+  SSL_CTX_set_tmp_dh(sctx, dh);
+  DEBUG(D_tls)
+    debug_printf("Diffie-Hellman initialized from %s with %d-bit prime\n",
+      dhexpanded ? dhexpanded : US"default", 8*DH_size(dh));
+  }
+
+DH_free(dh);
+BIO_free(bio);
+
+return TRUE;
 }
 
 
@@ -619,6 +644,9 @@ OCSP information. */
 rc = tls_expand_session_files(ctx_sni, cbinfo);
 if (rc != OK) return SSL_TLSEXT_ERR_NOACK;
 
+rc = init_dh(ctx_sni, cbinfo->dhparam, NULL);
+if (rc != OK) return SSL_TLSEXT_ERR_NOACK;
+
 DEBUG(D_tls) debug_printf("Switching SSL context.\n");
 SSL_set_SSL_CTX(s, ctx_sni);
 
@@ -779,7 +807,7 @@ else
 
 /* Initialize with DH parameters if supplied */
 
-if (!init_dh(dhparam, host)) return DEFER;
+if (!init_dh(ctx, dhparam, host)) return DEFER;
 
 /* Set up certificate and key (and perhaps OCSP info) */
 
diff --git a/src/util/gen_pkcs3.c b/src/util/gen_pkcs3.c
new file mode 100644
index 000000000..ae7e7610a
--- /dev/null
+++ b/src/util/gen_pkcs3.c
@@ -0,0 +1,229 @@
+/* Copyright (C) 2012 Phil Pennock.
+ * This is distributed as part of Exim and licensed under the GPL.
+ * See the file "NOTICE" for more details.
+ */
+
+/* Build with:
+ * c99 $(pkg-config --cflags openssl) gen_pkcs3.c $(pkg-config --libs openssl)
+ */
+
+#include <ctype.h>
+#include <errno.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include <openssl/bio.h>
+#include <openssl/bn.h>
+#include <openssl/dh.h>
+#include <openssl/err.h>
+#include <openssl/pem.h>
+
+extern const char *__progname;
+
+
+void __attribute__((__noreturn__)) __attribute__((__format__(printf, 1, 2)))
+die(const char *fmt, ...)
+{
+  va_list ap;
+
+  fflush(NULL);
+  fprintf(stderr, "%s: ", __progname);
+  va_start(ap, fmt);
+  vfprintf(stderr, fmt, ap);
+  va_end(ap);
+  fprintf(stderr, "\n");
+  fflush(stderr);
+  exit(1);
+}
+
+
+void __attribute__((__noreturn__))
+die_openssl_err(const char *msg)
+{
+  char err_string[250];
+  unsigned long e;
+
+  ERR_error_string_n(ERR_get_error(), err_string, sizeof(err_string));
+  die("%s: %s", msg, err_string);
+}
+
+
+static BIGNUM *
+bn_from_text(const char *text)
+{
+  BIGNUM *b;
+  char *p, *spaceless;
+  const char *q, *end;
+  size_t len;
+  int rc;
+
+  len = strlen(text);
+  spaceless = malloc(len);
+  if (!spaceless)
+    die("malloc(%zu) failed: %s", len, strerror(errno));
+
+  for (p = spaceless, q = text, end = text + len;
+       q < end;
+       ++q) {
+    if (!isspace(*q))
+      *p++ = *q;
+  }
+
+  b = NULL;
+  rc = BN_hex2bn(&b, spaceless);
+
+  if (rc != p - spaceless)
+    die("BN_hex2bn did not convert entire input; took %d of %z bytes",
+        rc, p - spaceless);
+
+  return b;
+}
+
+
+static void
+our_dh_check(DH *dh)
+{
+  int rc, errflags = 0;
+
+  rc = DH_check(dh, &errflags);
+  if (!rc) die_openssl_err("DH_check() could not be performed");;
+
+  /* We ignore DH_UNABLE_TO_CHECK_GENERATOR because some of the invocations
+   * deliberately provide generators other than 2 or 5. */
+
+  if (errflags & DH_CHECK_P_NOT_SAFE_PRIME)
+    die("DH_check(): p not a safe prime");
+  if (errflags & DH_NOT_SUITABLE_GENERATOR)
+    die("DH_check(): g not suitable as generator");
+}
+
+
+static void
+emit_c_format_dh(FILE *stream, DH *dh)
+{
+  BIO *bio;
+  long length;
+  char *data, *end, *p, *nl;
+
+  bio = BIO_new(BIO_s_mem());
+  PEM_write_bio_DHparams(bio, dh);
+  length = BIO_get_mem_data(bio, &data);
+  if (!length)
+    die("no data in memory BIO to format for printing");
+  if (length < 0)
+    die("grr, negative length memory not supported");
+  end = data + length;
+
+  for (p = data; p < end; /**/) {
+    nl = strchr(p, '\n');
+    if (!nl) {
+      fprintf(stream, "\"%s\\n\"\n/* missing final newline */\n", p);
+      break;
+    }
+    *nl = '\0';
+    fprintf(stream, "\"%s\\n\"\n", p);
+    p = nl + 1;
+  }
+}
+
+
+void __attribute__((__noreturn__))
+usage(FILE *stream, int exitcode)
+{
+  fprintf(stream, "Usage: %s [-CPcst] <dh_p> <dh_g>\n"
+"Both dh_p and dh_g should be hex strings representing the numbers\n"
+"They may contain whitespace.\n"
+"\n"
+" -C      show C string form of PEM result\n"
+" -P      do not show PEM\n"
+" -c      run OpenSSL DH_check() on the DH object\n"
+" -s      show the parsed p and g\n"
+" -t      show text form of certificate\n"
+
+      , __progname);
+  exit(exitcode);
+}
+
+
+int
+main(int argc, char *argv[])
+{
+  BIGNUM *p, *g;
+  DH *dh;
+  int ch;
+  bool perform_dh_check = false;
+  bool show_c_form = false;
+  bool show_numbers = false;
+  bool show_pem = true;
+  bool show_text = false;
+
+  while ((ch = getopt(argc, argv, "CPcsth")) != -1) {
+    switch (ch) {
+      case 'C':
+        show_c_form = true;
+        break;
+      case 'P':
+        show_pem = false;
+        break;
+      case 'c':
+        perform_dh_check = true;
+        break;
+      case 's':
+        show_numbers = true;
+        break;
+      case 't':
+        show_text = true;
+        break;
+
+      case 'h':
+        usage(stdout, 0);
+      case '?':
+        die("Unknown option or missing argument -%c", optopt);
+      default:
+        die("Unhandled option -%c", ch);
+    }
+  }
+
+  optind -= 1;
+  argc -= optind;
+  argv += optind;
+
+  if (argc != 3) {
+    fprintf(stderr, "argc: %d\n", argc);
+    usage(stderr, 1);
+  }
+
+  p = bn_from_text(argv[1]);
+  g = bn_from_text(argv[2]);
+
+  if (show_numbers) {
+    printf("p = ");
+    BN_print_fp(stdout, p);
+    printf("\ng = ");
+    BN_print_fp(stdout, g);
+    printf("\n");
+  }
+
+  dh = DH_new();
+  dh->p = p;
+  dh->g = g;
+
+  if (perform_dh_check)
+    our_dh_check(dh);
+
+  if (show_text)
+    DHparams_print_fp(stdout, dh);
+
+  if (show_pem) {
+    if (show_c_form)
+      emit_c_format_dh(stdout, dh);
+    else
+      PEM_write_DHparams(stdout, dh);
+  }
+
+  DH_free(dh); /* should free p & g too */
+  return 0;
+}