Add cscope files to git ignore list

[users/jgh/exim.git] / doc / doc-txt / experimental-spec.txt

diff --git a/doc/doc-txt/experimental-spec.txt b/doc/doc-txt/experimental-spec.txt
index 58854345420d60d97ee58f20cf268f2fd2f41402..6657f63c773f88e513d259f95164f237ef63a58c 100644 (file)
--- a/doc/doc-txt/experimental-spec.txt
+++ b/doc/doc-txt/experimental-spec.txt
@@ -6,97 +6,6 @@ about experimental  features, all  of which  are unstable and
 liable to incompatible change.
 
 
 liable to incompatible change.
 
 

-OCSP Stapling support
---------------------------------------------------------------
-
-X.509 PKI certificates expire and can be revoked; to handle this, the
-clients need some way to determine if a particular certificate, from a
-particular Certificate Authority (CA), is still valid.  There are three
-main ways to do so.
-
-The simplest way is to serve up a Certificate Revocation List (CRL) with
-an ordinary web-server, regenerating the CRL before it expires.  The
-downside is that clients have to periodically re-download a potentially
-huge file from every certificate authority it knows of.
-
-The way with most moving parts at query time is Online Certificate
-Status Protocol (OCSP), where the client verifies the certificate
-against an OCSP server run by the CA.  This lets the CA track all
-usage of the certs.  This requires running software with access to the
-private key of the CA, to sign the responses to the OCSP queries.  OCSP
-is based on HTTP and can be proxied accordingly.
-
-The only widespread OCSP server implementation (known to this writer)
-comes as part of OpenSSL and aborts on an invalid request, such as
-connecting to the port and then disconnecting.  This requires
-re-entering the passphrase each time some random client does this.
-
-The third way is OCSP Stapling; in this, the server using a certificate
-issued by the CA periodically requests an OCSP proof of validity from
-the OCSP server, then serves it up inline as part of the TLS
-negotiation.   This approach adds no extra round trips, does not let the
-CA track users, scales well with number of certs issued by the CA and is
-resilient to temporary OCSP server failures, as long as the server
-starts retrying to fetch an OCSP proof some time before its current
-proof expires.  The downside is that it requires server support.
-
-If Exim is built with EXPERIMENTAL_OCSP and it was built with OpenSSL,
-or with GnuTLS 3.1.3 or later, then it gains a new global option:
-"tls_ocsp_file".
-
-The file specified therein is expected to be in DER format, and contain
-an OCSP proof.  Exim will serve it as part of the TLS handshake.  This
-option will be re-expanded for SNI, if the tls_certificate option
-contains $tls_sni, as per other TLS options.
-
-Exim does not at this time implement any support for fetching a new OCSP
-proof.  The burden is on the administrator to handle this, outside of
-Exim.  The file specified should be replaced atomically, so that the
-contents are always valid.  Exim will expand the "tls_ocsp_file" option
-on each connection, so a new file will be handled transparently on the
-next connection.
-
-Under OpenSSL Exim will check for a valid next update timestamp in the
-OCSP proof; if not present, or if the proof has expired, it will be
-ignored.
-
-Also, given EXPERIMENTAL_OCSP, the smtp transport gains two options:
-- "hosts_require_ocsp"; a host-list for which an OCSP Stapling
-is requested and required for the connection to proceed.  The default
-value is empty.
-- "hosts_request_ocsp"; a host-list for which (additionally) an OCSP
-Stapling is requested (but not necessarily verified).  The default
-value is "*" meaning that requests are made unless configured
-otherwise.
-
-The host(s) should also be in "hosts_require_tls", and
-"tls_verify_certificates" configured for the transport.
-
-For the client to be able to verify the stapled OCSP the server must
-also supply, in its stapled information, any intermediate
-certificates for the chain leading to the OCSP proof from the signer
-of the server certificate.  There may be zero or one such. These
-intermediate certificates should be added to the server OCSP stapling
-file (named by tls_ocsp_file).
-
-Note that the proof only covers the terminal server certificate,
-not any of the chain from CA to it.
-
-At this point in time, we're gathering feedback on use, to determine if
-it's worth adding complexity to the Exim daemon to periodically re-fetch
-OCSP files and somehow handling multiple files.
-
-  A helper script "ocsp_fetch.pl" for fetching a proof from a CA
-  OCSP server is supplied.  The server URL may be included in the
-  server certificate, if the CA is helpful.
-
-  One failure mode seen was the OCSP Signer cert expiring before the end
-  of validity of the OCSP proof. The checking done by Exim/OpenSSL
-  noted this as invalid overall, but the re-fetch script did not.
-
-
-
-

 Brightmail AntiSpam (BMI) suppport
 --------------------------------------------------------------
 
 Brightmail AntiSpam (BMI) suppport
 --------------------------------------------------------------
 


@@ -1108,11 +1017,27 @@ an example, in my connect ACL, I have:
        logwrite = Internal Server Address: $received_ip_address:$received_port
 
 
        logwrite = Internal Server Address: $received_ip_address:$received_port
 
 

-4. Runtime issues to be aware of:
+4. Recommended ACL additions:

    - Since the real connections are all coming from your proxy, and the
      per host connection tracking is done before Proxy Protocol is
      evaluated, smtp_accept_max_per_host must be set high enough to
      handle all of the parallel volume you expect per inbound proxy.
    - Since the real connections are all coming from your proxy, and the
      per host connection tracking is done before Proxy Protocol is
      evaluated, smtp_accept_max_per_host must be set high enough to
      handle all of the parallel volume you expect per inbound proxy.

+   - With the smtp_accept_max_per_host set so high, you lose the ability
+     to protect your server from massive numbers of inbound connections
+     from one IP.  In order to prevent your server from being DOS'd, you
+     need to add a per connection ratelimit to your connect ACL.  I
+     suggest something like this:
+
+  # Set max number of connections per host
+  LIMIT   = 5
+  # Or do some kind of IP lookup in a flat file or database
+  # LIMIT = ${lookup{$sender_host_address}iplsearch{/etc/exim/proxy_limits}}
+
+  defer   message        = Too many connections from this IP right now
+          ratelimit      = LIMIT / 5s / per_conn / strict
+
+
+5. Runtime issues to be aware of:

    - The proxy has 3 seconds (hard-coded in the source code) to send the
      required Proxy Protocol header after it connects.  If it does not,
      the response to any commands will be:
    - The proxy has 3 seconds (hard-coded in the source code) to send the
      required Proxy Protocol header after it connects.  If it does not,
      the response to any commands will be:


@@ -1131,7 +1056,7 @@ an example, in my connect ACL, I have:
      mail programs from working because that would require mail from
      localhost to use Proxy Protocol.  Again, not advised!
 
      mail programs from working because that would require mail from
      localhost to use Proxy Protocol.  Again, not advised!
 

-5. Example of a refused connection because the Proxy Protocol header was
+6. Example of a refused connection because the Proxy Protocol header was

 not sent from a host configured to use Proxy Protocol.  In the example,
 the 3 second timeout occurred (when a Proxy Protocol banner should have
 been sent), the banner was displayed to the user, but all commands are
 not sent from a host configured to use Proxy Protocol.  In the example,
 the 3 second timeout occurred (when a Proxy Protocol banner should have
 been sent), the banner was displayed to the user, but all commands are


@@ -1146,6 +1071,64 @@ QUIT
 221 mail.example.net closing connection
 
 
 221 mail.example.net closing connection
 
 

+DSN Support
+--------------------------------------------------------------
+
+DSN Support tries to add RFC 3461 support to Exim. It adds support for
+*) the additional parameters for MAIL FROM and RCPT TO
+*) RFC complient MIME DSN messages for all of
+   success, failure and delay notifications
+*) dsn_advertise_hosts main option to select which hosts are able
+   to use the extension
+*) dsn_lasthop router switch to end DSN processing
+
+In case of failure reports this means that the last three parts, the message body
+intro, size info and final text, of the defined template are ignored since there is no
+logical place to put them in the MIME message.
+
+All the other changes are made without changing any defaults
+
+Building exim:
+--------------
+
+Define
+EXPERIMENTAL_DSN=YES
+in your Local/Makefile.
+
+Configuration:
+--------------
+All DSNs are sent in MIME format if you built exim with EXPERIMENTAL_DSN=YES
+No option needed to activate it, and no way to turn it off.
+
+Failure and delay DSNs are triggered as usual except a sender used NOTIFY=...
+to prevent them.
+
+Support for Success DSNs is added and activated by NOTIFY=SUCCESS by clients.
+
+Add
+dsn_advertise_hosts = *
+or a more restrictive host_list to announce DSN in EHLO answers
+
+Those hosts can then use NOTIFY,ENVID,RET,ORCPT options.
+
+If a message is relayed to a DSN aware host without changing the envelope
+recipient the options are passed along and no success DSN is generated.
+
+A redirect router will always trigger a success DSN if requested and the DSN
+options are not passed any further.
+
+A success DSN always contains the recipient address as submitted by the
+client as required by RFC. Rewritten addresses are never exposed.
+
+If you used DSN patch up to 1.3 before remove all "dsn_process" switches from
+your routers since you don't need them anymore. There is no way to "gag"
+success DSNs anymore. Announcing DSN means answering as requested.
+
+You can prevent Exim from passing DSN options along to other DSN aware hosts by defining
+dsn_lasthop
+in a router. Exim will then send the success DSN himself if requested as if
+the next hop does not support DSN.
+Adding it to a redirect router makes no difference.

 
 Certificate name checking
 --------------------------------------------------------------
 
 Certificate name checking
 --------------------------------------------------------------