-Early pipelining support
-------------------------
-Ref: https://datatracker.ietf.org/doc/draft-harris-early-pipe/
-
-If compiled with EXPERIMENTAL_PIPE_CONNECT support is included for this feature.
-The server advertises the feature in its EHLO response, currently using the name
-"X_PIPE_CONNECT" (this will change, some time in the future).
-A client may cache this information, along with the rest of the EHLO response,
-and use it for later connections. Those later ones can send esmtp commands before
-a banner is received.
-
-Up to 1.5 roundtrip times can be taken out of cleartext connections, 2.5 on
-STARTTLS connections.
-
-In combination with the traditional PIPELINING feature the following example
-sequences are possible (among others):
-
-(client) (server)
-
-EHLO,MAIL,RCPT,DATA ->
- <- banner,EHLO-resp,MAIL-ack,RCPT-ack,DATA-goahead
-message-data ->
-------
-
-EHLO,MAIL,RCPT,BDAT ->
- <- banner,EHLO-resp,MAIL-ack,RCPT-ack
-message-data ->
-------
-
-EHLO,STARTTLS ->
- <- banner,EHLO-resp,TLS-goahead
-TLS1.2-client-hello ->
- <- TLS-server-hello,cert,hello-done
-client-Kex,change-cipher,finished ->
- <- change-cipher,finished
-EHLO,MAIL,RCPT,DATA ->
- <- EHLO-resp,MAIL-ack,RCPT-ack,DATA-goahead
-
-------
-(tls-on-connect)
-TLS1.2-client-hello ->
- <- TLS-server-hello,cert,hello-done
-client-Kex,change-cipher,finished ->
- <- change-cipher,finshed
- <- banner
-EHLO,MAIL,RCPT,DATA ->
- <- EHLO-resp,MAIL-ack,RCPT-ack,DATA-goahead
-
-Where the initial client packet is SMTP, it can combine with the TCP Fast Open
-feature and be sent in the TCP SYN.
-
-
-A main-section option "pipelining_connect_advertise_hosts" (default: *)
-and an smtp transport option "hosts_pipe_connect" (default: unset)
-control the feature.
-
-If the "pipelining" log_selector is enabled, the "L" field in server <=
-log lines has a period appended if the feature was advertised but not used;
-or has an asterisk appended if the feature was used. In client => lines
-the "L" field has an asterisk appended if the feature was used.
-
-The "retry_data_expire" option controls cache invalidation.
-Entries are also rewritten (or cleared) if the adverised features
-change.
-
-
-NOTE: since the EHLO command must be constructed before the connection is
-made it cannot depend on the interface IP address that will be used.
-Transport configurations should be checked for this. An example avoidance:
-
- helo_data = ${if def:sending_ip_address \
- {${lookup dnsdb{>! ptr=$sending_ip_address} \
- {${sg{$value} {^([^!]*).*\$} {\$1}}} fail}} \
- {$primary_hostname}}
-
-
-
-
TLS Session Resumption
----------------------
-TLS Session Resumption for TLS 1.2 and TLS1.3 connections can be used (defined
+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.
+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
Operational cost/benefit:
The extra data being transmitted costs a minor amount, and the client has
-extra costs in storing and retrieving the data.
+ 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.
+ 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. Tickets have limited lifetime.
+ 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
+ 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 bit 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 {}{}}.
+ 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
--------------------------------------------------------------
git://git.exim.org / exim.git / blobdiff