Lower EXIM_CLIENT_DH_MIN_BITS 1024 -> 512.
[exim.git] / doc / doc-txt / GnuTLS-FAQ.txt
1 Using Exim 4.80+ with GnuTLS
2 ============================
4 (1) I'm having problems building with GnuTLS 1, why?
5 (2) What changed? Why?
6 (3) I'm seeing:
7 "(gnutls_handshake): A TLS packet with unexpected length was received"
8 Why?
9 (4) What's the deal with MD5?
10 (5) What happened to gnutls_require_kx / gnutls_require_mac /
11 gnutls_require_protocols?
12 (6) What's the deal with tls_dh_max_bits? What's DH?
13 (7) What's a Priority String?
14 (8) How do I use tls_require_ciphers?
15 (9) How do I test STARTTLS support?
19 (1): I'm having problems building with GnuTLS 1, why?
20 -----------------------------------------------------
22 GnuTLS's library interface has changed and Exim uses the more current
23 interface. Since GnuTLS is security critical code, you should probably update
24 to a supported release.
26 If updating GnuTLS is not an option, then build Exim against OpenSSL instead.
28 If neither is an option, then you might build Exim with the rule
29 "SUPPORT_TLS=yes" commented out in "Local/Makefile", so that your Exim build
30 no longer has TLS support.
32 If you need to keep TLS support, and you can't use OpenSSL, then you'll have
33 to update the GnuTLS you have installed. Sorry.
35 We've tested the build of Exim back as far as GnuTLS 2.8.x; most development
36 work is done with 2.12 and tested on 2.10 and 3.x.
38 If you have to pick a version to upgrade to, use GnuTLS 3.x if available. The
39 GnuTLS developers took advantage of the version bump to add an error code
40 return value which makes debugging some problems a lot easier.
44 (2): What changed? Why?
45 ------------------------
47 The GnuTLS provider integration in Exim was overhauled, rewritten but with
48 some copy/paste, because building Exim against more current releases of GnuTLS
49 was issuing deprecation warnings from the compiler.
51 When a library provider marks up the include files so that some function calls
52 will cause the compiler/linker to emit deprecation warnings, it's time to pay
53 serious attention. A future release might not work at all. Using the new
54 APIs may mean that Exim will *stop* working with older releases of GnuTLS.
55 The GnuTLS support in Exim was overhauled in Exim 4.80. In prior releases,
56 Exim hard-coded a lot of algorithms and constrained what could happen. In
57 Exim 4.79, we added to the hard-coded list just enough to let TLSv1.1 and
58 TLSv1.2 be negotiated, but not actually support the mandatory algorithms of
59 those protocol versions. When Exim's GnuTLS integration was originally
60 written, there was no other choice than to make Exim responsible for a lot of
61 this. In the meantime, GnuTLS has improved.
63 With the rewrite, we started using the current API and leaving a lot more
64 responsibility for TLS decisions to the library.
66 The GnuTLS developers added "priority strings" (see Q7), which provide an
67 interface exposed to the configuration file for a lot of the tuning.
69 The GnuTLS policy is to no longer support MD5 in certificates. Exim had
70 previously been immune to this policy, but no longer. See Q4.
74 (3): I'm seeing "A TLS packet with unexpected length was received". Why?
75 -------------------------------------------------------------------------
77 The most likely reason is that the client dropped the connection during
78 handshake, because their library disliked some aspect of the negotiation.
80 In GnuTLS 2, an EOF on the connection is reported with an error code for
81 packets being too large, and the above is the string returned by the library
82 for that error code. In GnuTLS 3, there's a specific error code for EOF and
83 the diagnostic will be less confusing.
85 Most likely cause is an MD5 hash used in a certificate. See Q4 below.
86 Alternatively, the client dislikes the size of the Diffie-Hellman prime
87 offered by the server; if lowering the value of the "tls_dh_max_bits" Exim
88 option fixes the problem, this was the cause. See Q6.
92 (4): What's the deal with MD5?
93 ------------------------------
95 MD5 is a hash algorithm. Hash algorithms are used to reduce a lot of data
96 down to a fairly short value, which is supposed to be extremely hard to
97 manipulate to get a value of someone's choosing. Signatures, used to attest
98 to identity or integrity, rely upon this manipulation being effectively
99 impossible, because the signature is the result of math upon the hash result.
100 Without hash algorithms, signatures would be longer than the text being
101 signed.
103 MD5 was once very popular. It still is far too popular. Real world attacks
104 have been proven possible against MD5. Including an attack against PKI
105 (Public Key Infrastructure) certificates used for SSL/TLS. In that attack,
106 the attackers got a certificate for one identity but we able to then public a
107 certificate with the same signature but a different identity. This undermines
108 the whole purpose of having certificates.
110 So GnuTLS stopped trusting any certificate with an MD5-based hash used in it.
111 The world has been hurriedly moving away from MD5 in certificates for a while.
112 If you still have such a certificate, you should move too.
114 If you paid someone for your certificate, they should be willing to reissue
115 the certificate with a different algorithm, for no extra money. If they try
116 to charge money to replace their defective product, buy from someone else
117 instead. Part of the reason for paying money on a recurring basis is to cover
118 the ongoing costs of proving a trust relationship, such as providing
119 revocation protocols. This is just another of those ongoing costs you have
120 already paid for.
124 (5): ... gnutls_require_kx / gnutls_require_mac / gnutls_require_protocols?
125 ---------------------------------------------------------------------------
127 These Exim options were used to provide fine-grained control over the TLS
128 negotiation performed by GnuTLS. They required explicit protocol knowledge
129 from Exim, which vastly limited what GnuTLS could do and involved the Exim
130 maintainers in decisions which aren't part of their professional areas of
131 expertise. The need for Exim to be able to do this went away when GnuTLS
132 introduced Priority Strings (see Q7).
134 If you were using these options before, then you're already an expert user and
135 should be able to easily craft a priority string to accomplish your goals.
136 Set the Exim "tls_require_ciphers" value accordingly. There is a main section
137 option of this name, used for Exim receiving inbound connections, and an SMTP
138 driver transport option of this name, used for Exim establishing outbound
139 connections.
143 (6): What's the deal with tls_dh_max_bits? What's DH?
144 ------------------------------------------------------
146 You can avoid all of the tls_dh_max_bits issues if you leave "tls_dhparam"
147 unset, so that you get one of the standard built-in primes used for DH.
150 DH, Diffie-Hellman (or Diffie-Hellman-Merkle, or something naming Williamson)
151 is the common name for a way for two parties to a communication stream to
152 exchange some private random data so that both end up with a shared secret
153 which no evesdropper can get. It does not provide for proof of the identity
154 of either party, so on its own is subject to man-in-the-middle attacks, but is
155 often combined with systems which do provide such proof, improving them by
156 separating the session key (the shared secret) from the long-term identity,
157 and so protecting past communications from a break of the long-term identity.
159 To do this, the server sends to the client a very large prime number; this is
160 in the clear, an attacker can see it. This is not a problem; it's so not a
161 problem, that there are standard named primes which applications can use, and
162 which a future release of Exim will probably support.
164 The size of the prime number affects how difficult it is to break apart the
165 shared secret and decrypt the data. As time passes, the size required to
166 provide protection against an adversary climbs: computers get more powerful,
167 mathematical advances are made, and so on.
169 Estimates of the size needed are published as recommendations by various
170 groups; a good summary of sizes currently recommended, for various
171 cryptographic primitives, is available at:
173 http://www.keylength.com/en/3/
175 The GnuTLS folks think the ECRYPT II advice is good. They know far more of
176 such matters than the Exim folks, we just say "er, what they said".
178 One of the new pieces of the GnuTLS API is a means for an application to ask
179 it for guidance and advice on how large some numbers should be. This is not
180 entirely internal to GnuTLS since generating the numbers is slow, an
181 application might want to use a standard prime, etc. So, in an attempt to get
182 away from being involved in cryptographic policy, and to get rid of a
183 hard-coded "1024" in Exim's source-code, we switched to asking GnuTLS how many
184 bits should be in the prime number generated for use for Diffie-Hellman. To
185 give back to GnuTLS for use We can ask for various sizes, and did not expose
186 this to the administrator but instead just asked for "NORMAL" protection.
187 Literally:
189 dh_bits = gnutls_sec_param_to_pk_bits(GNUTLS_PK_DH, GNUTLS_SEC_PARAM_NORMAL);
191 This API is only available as of GnuTLS 2.12. Prior to that release, we stuck
192 with the old value, for compatibility, so "1024" is still hard-coded.
193 Reviewing the page above, you'll see that this is described as "Short-term
194 protection against medium organizations, medium-term protection against small
195 organizations."
197 So if you are using an old release of GnuTLS, you can either add to
198 Local/Makefile a different value of "EXIM_SERVER_DH_BITS_PRE2_12" or accept
199 that your protection might not be adequate to your needs. We advise updating
200 to a more current GnuTLS release and rebuilding Exim against that.
202 Unfortunately, some TLS libraries have the client side bound how large a DH
203 prime they will accept from the server. The larger the number, the more
204 computation required to work with it and the slower that things get. So they
205 pick what they believe to be reasonable upper bounds, and then typically
206 forget about it for several years.
208 Worse, in TLS the DH negotiation happens after a ciphersuite has been chosen,
209 so if the client dislikes the value then a different ciphersuite avoiding DH
210 can not be negotiated! The client typically drops the connection, resulting
211 in errors to the user and errors in the Exim logs. With GnuTLS 3, you'll see
212 the EOF (End-Of-File) error message in Exim's logs, reported as being part of
213 "gnutls_handshake", but with GnuTLS 2 you'll see a log message about a packet
214 with an unexpected size. Unless the client software is written intelligently
215 enough to be able to adapt and reconnect forbidding DH, the client will never
216 be able to negotiate TLS.
218 This time around, we discovered that the NSS library used by various Mozilla
219 products, Chrome, etc, and most particularly by the Thunderbird mail client,
220 has the lowest cap. In fact, prior to recent updates, their upper limit was
221 lower than the value returned by GnuTLS for "NORMAL". The most recent NSS
222 library release raises this, but the most recent Thunderbird release still has
223 the old limit.
225 So Exim had to get involved in cryptography policy decisions again. We added
226 the "tls_dh_max_bits" global option, to set a number used in both OpenSSL and
227 GnuTLS bindings for Exim. In GnuTLS, it clamps the value returned by
228 gnutls_sec_param_to_pk_bits(), so that if the returned value is larger than
229 tls_dh_max_bits then tls_dh_max_bits would be used instead.
231 Our policy decision was to default the value of tls_dh_max_bits to the maximum
232 supported in the most recent Thunderbird release, and to make this an
233 administrator-available option so that administrators can choose to trade off
234 security versus compatibility by raising it.
236 A future release of Exim may even let the administrator tell GnuTLS to ask for
237 more or less than "NORMAL".
239 To add to the fun, the size of the prime returned by GnuTLS when we call
240 gnutls_dh_params_generate2() is not limited to be the requested size. GnuTLS
241 has a tendency to overshoot. 2237 bit primes are common when 2236 is
242 requested, and higher still have been observed. Further, there is no API to
243 ask how large the prime bundled up inside the parameter is; the most we can do
244 is ask how large the DH prime used in an active TLS session is. Since we're
245 not able to use GnuTLS API calls (and exporting to PKCS3 and then calling
246 OpenSSL routines would be undiplomatic, plus add a library dependency), we're
247 left with no way to actually know the size of the freshly generated DH prime.
249 Thus we check if the the value returned is at least 10 more than the minimum
250 we'll accept as a client (EXIM_CLIENT_DH_MIN_BITS, see below, defaults to
251 512) and if it is, we subtract 10. Then we reluctantly deploy a strategy
252 called "hope". This is not guaranteed to be successful; in the first code
253 pass on this logic, we subtracted 3, asked for 2233 bits and got 2240 in the
254 first test.
256 If you see Thunderbird clients still failing, then as a user who can see into
257 Exim's spool directory, run:
259 $ openssl dhparam -noout -text -in /path/to/spool/gnutls-params-2236 | head
261 Ideally, the first line will read "PKCS#3 DH Parameters: (2236 bit)". If the
262 count is more than 2236, then remove the file and let Exim regenerate it, or
263 generate one yourself and move it into place. Ideally use "openssl dhparam"
264 to generate it, and then wait a very long time; at least this way, the size
265 will be correct.
267 The use of "hope" as a strategy was felt to be unacceptable as a default, so
268 late in the RC series for 4.80, the whole issue was side-stepped. The primes
269 used for DH are publicly revealed; moreover, there are selection criteria for
270 what makes a "good" DH prime. As it happens, there are *standard* primes
271 which can be used, and are specified to be used for certain protocols. So
272 these primes were built into Exim, and by default exim now uses a 2048 bit
273 prime from section 2.2 of RFC 5114.
276 A TLS client does not get to choose the DH prime used, but can choose a
277 minimum acceptable value. For Exim, this is a compile-time constant called
278 "EXIM_CLIENT_DH_MIN_BITS" of 512, which can be overruled in "Local/Makefile".
279 (It should be higher, but some real-world sites are using dangerously small
280 values. Although some might argue that our old size of 1024 was dangerously
281 low; "opinions vary". This is expected to be a configure file option for
282 the Exim 4.81 release.)
286 (7): What's a Priority String?
287 ------------------------------
289 A priority string is a way for a user of GnuTLS to tell GnuTLS how it should
290 make decisions about what to do in TLS; it includes which algorithms to make
291 available for various roles, what compatibility trade-offs to make, which
292 features to enable or disable.
294 It is exposed to the Mail Administrator in Exim's configuration file as the
295 "tls_require_ciphers" option, which exists as a main section option for use in
296 Exim as a server, accepting connections, and as an option on Transports using
297 the SMTP driver, for use in Exim as a client. The main section option is
298 *not* the default for the transport option, they are entirely independent.
299 For both, the default value used by Exim is the string "NORMAL". (This is not
300 the same NORMAL as for DH prime bit size selection in Q6, but a different
301 NORMAL.) See Q8.
303 The current documentation, for the most recent release of GnuTLS, is available
304 online at:
306 http://www.gnu.org/software/gnutls/manual/html_node/Priority-Strings.html
308 Beware that if you are not using the most recent GnuTLS release then this
309 documentation will be wrong for you! You should find the "info" documentation
310 which came with GnuTLS to review the available options. It's under "The TLS
311 Handshake Protocol".
313 $ pinfo --node="Priority Strings" gnutls
315 (This author is unable to persuade the "info" command-line tool to jump
316 straight to the required node, but "pinfo" works.)
318 To trade off some security for more compatibility, you might set a value of
319 "NORMAL:%COMPAT". See the documentation for more, including lowering security
320 even further for more security, forcing clients to use the server's protocol
321 suite, and ways to force selection of particular algorithms.
325 (8): How do I use tls_require_ciphers?
326 --------------------------------------
328 This is the name of two options in Exim. One is a main section option, used
329 by Exim as a server when a client initiates SSL/TLS negotiation, the other is
330 an option on transports which use "driver = smtp", used when Exim initiates
331 SSL/TLS as a client talking to a remote server.
333 The option is expanded and so can take advantage of any variables which have
334 been set. This includes the IP address of the remote side, the port upon
335 which a connection was accepted (when a server), and more. Currently it does
336 not have access to $tls_sni, whether as a client or as a server.
338 This example, for the main section's option, will let the library defaults be
339 permitted on the MX port, where there's probably no identity verification
340 anyway, and lowers security further by increasing compatibility; but this ups
341 the ante on the submission ports where the administrator might have some
342 influence on the choice of clients used:
344 tls_require_ciphers = ${if =={$received_port}{25}\
346 {SECURE128}}
348 Note that during Exim start-up, when this option is sanity-checked, there will
349 be no value of $received_port. In the above example, the checked value will
350 thus be "SECURE128". Be careful to ensure that it always expands safely.
354 (9): How do I test STARTTLS support?
355 ------------------------------------
357 The best command-line client for debugging specifically SSL/TLS which this
358 author has encountered is part of the GnuTLS suite, and is called
359 "gnutls-cli". It's best because it's the only interactive tool which lets the
360 user start TLS handshake exactly when they wish, so can choose to use the
361 STARTTLS command.
363 $ gnutls-cli --starttls --crlf --port 587 mail.example.org
365 After EHLO, to see the capabilities, enter STARTTLS, wait for the response,
366 then send EOF. Typically that's done by typing Ctrl-D at the start of a line.
367 The "gnutls-cli" tool will take over, set up TLS (or fail) and by the time it
368 returns to await more user input, you're using a secure connection and should
369 type your second EHLO.
371 The "--x509cafile" option may be helpful for checking certificates and
372 "--priority" to pass a priority string to the client tool for configuring it.
374 The --crlf is for strict protocol correctness, but Exim doesn't really need
375 it, so "gnutls-cli -s -p 587 mail.example.org" is shorter.
378 For debugging SMTP as a whole, we recommend swaks, "Swiss Army Knife SMTP", by
379 John Jetmore (one of the Exim Maintainers). This has some TLS tuning options;
380 it can be found at:
382 http://www.jetmore.org/john/code/swaks/
385 For OpenSSL, the "openssl s_client" command helps; you can either set up Exim
386 with a listening port which is SSL-on-connect or tell s_client to use
389 For the former, use the "tls_on_connect_ports" option and the
390 "daemon_smtp_ports" option. Most clients for SSL-on-connect use the port
391 which was briefly registered with IANA for this purpose, 465. So you would
392 set something like:
394 daemon_smtp_ports = 25 : 465 : 587
395 tls_on_connect_ports = 465
397 To use s_client with STARTTLS support, use "-starttls smtp" on the
398 command-line. Beware that older versions of OpenSSL did not wait for the SMTP
399 banner before sending EHLO, which will fall afoul of the protocol
400 synchronisation checks in Exim (used to trip up pump-and-dump spammers); also
401 you will not get control of the session until TLS is established. That said,
402 this tool provides more tuning hooks for adjusting how TLS will be set up than
403 most.
405 *BEWARE* that by default, s_client will take any line starting with a capital
406 letter "R" to be a request to initiate TLS renegotiation with the server and
407 the line will not be sent. This may trip up "RCPT TO:<someone@example.org>"
408 lines in SMTP. SMTP is not case-sensitive, so type "rcpt to" instead.
409 Alternatively, invoke s_client with the "-ign_eof" option to disable this
410 R-filtering and a few other features.
413 # END OF FAQ