SRS (Sender Rewriting Scheme) Support (using libsrs_alt)
--------------------------------------------------------------
-See also below, for an alternative native support implementation.
+See also the main docs, for an alternative native support implementation.
-Exim currently includes SRS support via Miles Wilton's
+Exim can be built with SRS support using Miles Wilton's
libsrs_alt library. The current version of the supported
library is 0.5, there are reports of 1.0 working.
Unpack the tarball, then refer to MTAs/README.EXIM
to proceed. You need to set
-EXPERIMENTAL_SRS=yes
+EXPERIMENTAL_SRS_ALT=yes
in your Local/Makefile.
+The built-in support, included by SUPPORT_SRS,
+shuold *not* be enabled if you wish to use the libsrs_alt
+version.
+
The following main-section options become available:
srs_config string
srs_hashlength int
-SRS (Sender Rewriting Scheme) Support (native)
---------------------------------------------------------------
-This is less full-featured than the libsrs_alt version above.
-
-The Exim build needs to be done with this in Local/Makefile:
-EXPERIMENTAL_SRS_NATIVE=yes
-
-The following are provided:
-- an expansion item "srs_encode"
- This takes three arguments:
- - a site SRS secret
- - the return_path
- - the pre-forwarding domain
-
-- an expansion condition "inbound_srs"
- This takes two arguments: the local_part to check, and a site SRS secret.
- If the secret is zero-length, only the pattern of the local_part is checked.
- The $srs_recipient variable is set as a side-effect.
-
-- an expansion variable $srs_recipient
- This gets the original return_path encoded in the SRS'd local_part
-
-- predefined macros _HAVE_SRS and _HAVE_NATIVE_SRS
-
-Sample usage:
-
- #macro
- SRS_SECRET = <pick something unique for your site for this>
-
- #routers
-
- outbound:
- driver = dnslookup
- # if outbound, and forwarding has been done, use an alternate transport
- domains = ! +my_domains
- transport = ${if eq {$local_part@$domain} \
- {$original_local_part@$original_domain} \
- {remote_smtp} {remote_forwarded_smtp}}
-
- inbound_srs:
- driver = redirect
- senders = :
- domains = +my_domains
- # detect inbound bounces which are SRS'd, and decode them
- condition = ${if inbound_srs {$local_part} {SRS_SECRET}}
- data = $srs_recipient
-
- inbound_srs_failure:
- driver = redirect
- senders = :
- domains = +my_domains
- # detect inbound bounces which look SRS'd but are invalid
- condition = ${if inbound_srs {$local_part} {}}
- allow_fail
- data = :fail: Invalid SRS recipient address
-
- #... further routers here
-
-
- # transport; should look like the non-forward outbound
- # one, plus the max_rcpt and return_path options
- remote_forwarded_smtp:
- driver = smtp
- # modify the envelope from, for mails that we forward
- max_rcpt = 1
- return_path = ${srs_encode {SRS_SECRET} {$return_path} {$original_domain}}
-
-
-
-
DCC Support
--------------------------------------------------------------
Distributed Checksum Clearinghouse; http://www.rhyolite.com/dcc/
Note that non-RFC-documented field names and data types are used.
-LMDB Lookup support
--------------------
-LMDB is an ultra-fast, ultra-compact, crash-proof key-value embedded data store.
-It is modeled loosely on the BerkeleyDB API. You should read about the feature
-set as well as operation modes at https://symas.com/products/lightning-memory-mapped-database/
-
-LMDB single key lookup support is provided by linking to the LMDB C library.
-The current implementation does not support writing to the LMDB database.
-
-Visit https://github.com/LMDB/lmdb to download the library or find it in your
-operating systems package repository.
-
-If building from source, this description assumes that headers will be in
-/usr/local/include, and that the libraries are in /usr/local/lib.
-
-1. In order to build exim with LMDB lookup support add or uncomment
-
-EXPERIMENTAL_LMDB=yes
-
-to your Local/Makefile. (Re-)build/install exim. exim -d should show
-Experimental_LMDB in the line "Support for:".
-
-EXPERIMENTAL_LMDB=yes
-LDFLAGS += -llmdb
-# CFLAGS += -I/usr/local/include
-# LDFLAGS += -L/usr/local/lib
-
-The first line sets the feature to include the correct code, and
-the second line says to link the LMDB libraries into the
-exim binary. The commented out lines should be uncommented if you
-built LMDB from source and installed in the default location.
-Adjust the paths if you installed them elsewhere, but you do not
-need to uncomment them if an rpm (or you) installed them in the
-package controlled locations (/usr/include and /usr/lib).
-
-2. Create your LMDB files, you can use the mdb_load utility which is
-part of the LMDB distribution our your favourite language bindings.
-
-3. Add the single key lookups to your exim.conf file, example lookups
-are below.
-
-${lookup{$sender_address_domain}lmdb{/var/lib/baruwa/data/db/relaydomains.mdb}{$value}}
-${lookup{$sender_address_domain}lmdb{/var/lib/baruwa/data/db/relaydomains.mdb}{$value}fail}
-${lookup{$sender_address_domain}lmdb{/var/lib/baruwa/data/db/relaydomains.mdb}}
-
-
Queuefile transport
-------------------
Queuefile is a pseudo transport which does not perform final delivery.
-
-TLS Session Resumption
-----------------------
-TLS Session Resumption for TLS 1.2 and TLS 1.3 connections can be used (defined
-in RFC 5077 for 1.2). The support for this can be included by building with
-EXPERIMENTAL_TLS_RESUME defined. This requires GnuTLS 3.6.3 or OpenSSL 1.1.1
-(or later).
-
-Session resumption (this is the "stateless" variant) involves the server sending
-a "session ticket" to the client on one connection, which can be stored by the
-client and used for a later session. The ticket contains sufficient state for
-the server to reconstruct the TLS session, avoiding some expensive crypto
-calculation and one full packet roundtrip time.
-
-Operational cost/benefit:
- The extra data being transmitted costs a minor amount, and the client has
- extra costs in storing and retrieving the data.
-
- In the Exim/Gnutls implementation the extra cost on an initial connection
- which is TLS1.2 over a loopback path is about 6ms on 2017-laptop class hardware.
- The saved cost on a subsequent connection is about 4ms; three or more
- connections become a net win. On longer network paths, two or more
- connections will have an average lower startup time thanks to the one
- saved packet roundtrip. TLS1.3 will save the crypto cpu costs but not any
- packet roundtrips.
-
- Since a new hints DB is used, the hints DB maintenance should be updated
- to additionally handle "tls".
-
-Security aspects:
- The session ticket is encrypted, but is obviously an additional security
- vulnarability surface. An attacker able to decrypt it would have access
- all connections using the resumed session.
- The session ticket encryption key is not committed to storage by the server
- and is rotated regularly (OpenSSL: 1hr, and one previous key is used for
- overlap; GnuTLS 6hr but does not specify any overlap).
- Tickets have limited lifetime (2hr, and new ones issued after 1hr under
- OpenSSL. GnuTLS 2hr, appears to not do overlap).
-
- There is a question-mark over the security of the Diffie-Helman parameters
- used for session negotiation. TBD. q-value; cf bug 1895
-
-Observability:
- New log_selector "tls_resumption", appends an asterisk to the tls_cipher "X="
- element.
-
- Variables $tls_{in,out}_resumption have bits 0-4 indicating respectively
- support built, client requested ticket, client offered session,
- server issued ticket, resume used. A suitable decode list is provided
- in the builtin macro _RESUME_DECODE for ${listextract {}{}}.
-
-Issues:
- In a resumed session:
- $tls_{in,out}_cipher will have values different to the original (under GnuTLS)
- $tls_{in,out}_ocsp will be "not requested" or "no response", and
- hosts_require_ocsp will fail
-
-
-
Dovecot authenticator via inet socket
-------------------------------------
+--------------------------------------------------------------
If Dovecot is configured similar to :-
service auth {
inet_listener {
name = exim
port = 12345
- ssl = yes
}
...
}
+
+Logging protocol unusual states
+---------------------------------------------------------------
+An extra log_selector, "protocol_detail" has been added in the default build.
+The name may change in future, hence the Experimenal status.
+
+Currrently the only effect is to enable logging, under TLS,
+of a TCP RST received directly after a QUIT (in server mode).
+
+Outlook is consistently doing this; not waiting for the SMTP response
+to its QUIT, not properly closing the TLS session and not properly closing
+the TCP connection. Previously this resulted is an error from SSL_write
+being logged.
+
+
+
+Limits ESMTP extension
+---------------------------------------------------------------
+Per https://datatracker.ietf.org/doc/html/draft-freed-smtp-limits-01
+
+If compiled with EXPERIMENTAL_ESMTP_LIMITS=yes :-
+
+As a server, Exim will advertise, in the EHLO response, the limit for RCPT
+commands set by the recipients_max main-section config option (if it is set),
+and the limit for MAIL commands set by the smtp_accept_max_per_connection
+option.
+
+Note that as of writing, smtp_accept_max_per_connection is expanded but
+recipients_max is not.
+
+A new main-section option "limits_advertise_hosts" controls whether
+the limits are advertised; the default for the option is "*".
+
+As a client, Exim will:
+
+ - note an advertised MAILMAX; the lower of the value given and the
+ value from the transport connection_max_messages option is used.
+
+ - note an advertised RCPTMAX; the lower of the
+ value given and the value from the transport max_rcpt option is used.
+ Parallisation of transactions is not done if due to a RCPTMAX, unlike
+ max_rcpt.
+
+ - note an advertised RCPTDOMAINMAX, and behave as if the transport
+ multi_domains option was set to false. The value advertised is ignored.
+
+Values advertised are only noted for TLS connections and ones for which
+the server does not advertise TLS support.
+
+
--------------------------------------------------------------
End of file
--------------------------------------------------------------
git://git.exim.org / exim.git / blobdiff