-Exim now has Experimental "Proxy Protocol" support. It was built on
-specifications from:
-http://haproxy.1wt.eu/download/1.5/doc/proxy-protocol.txt
-Above URL revised May 2014 to change version 2 spec:
-http://git.1wt.eu/web?p=haproxy.git;a=commitdiff;h=afb768340c9d7e50d8e
-
-The purpose of this function is so that an application load balancer,
-such as HAProxy, can sit in front of several Exim servers and Exim
-will log the IP that is connecting to the proxy server instead of
-the IP of the proxy server when it connects to Exim. It resets the
-$sender_address_host and $sender_address_port to the IP:port of the
-connection to the proxy. It also re-queries the DNS information for
-this new IP address so that the original sender's hostname and IP
-get logged in the Exim logfile. There is no logging if a host passes or
-fails Proxy Protocol negotiation, but it can easily be determined and
-recorded in an ACL (example is below).
-
-1. To compile Exim with Proxy Protocol support, put this in
-Local/Makefile:
-
-EXPERIMENTAL_PROXY=yes
-
-2. Global configuration settings:
-
-proxy_required_hosts = HOSTLIST
-
-The proxy_required_hosts option will require any IP in that hostlist
-to use Proxy Protocol. The specification of Proxy Protocol is very
-strict, and if proxy negotiation fails, Exim will not allow any SMTP
-command other than QUIT. (See end of this section for an example.)
-The option is expanded when used, so it can be a hostlist as well as
-string of IP addresses. Since it is expanded, specifying an alternate
-separator is supported for ease of use with IPv6 addresses.
-
-To log the IP of the proxy in the incoming logline, add:
- log_selector = +proxy
-
-A default incoming logline (wrapped for appearance) will look like this:
-
- 2013-11-04 09:25:06 1VdNti-0001OY-1V <= me@example.net
- H=mail.example.net [1.2.3.4] P=esmtp S=433
-
-With the log selector enabled, an email that was proxied through a
-Proxy Protocol server at 192.168.1.2 will look like this:
-
- 2013-11-04 09:25:06 1VdNti-0001OY-1V <= me@example.net
- H=mail.example.net [1.2.3.4] P=esmtp PRX=192.168.1.2 S=433
-
-3. In the ACL's the following expansion variables are available.
-
-proxy_host_address The (internal) src IP of the proxy server
- making the connection to the Exim server.
-proxy_host_port The (internal) src port the proxy server is
- using to connect to the Exim server.
-proxy_target_address The dest (public) IP of the remote host to
- the proxy server.
-proxy_target_port The dest port the remote host is using to
- connect to the proxy server.
-proxy_session Boolean, yes/no, the connected host is required
- to use Proxy Protocol.
-
-There is no expansion for a failed proxy session, however you can detect
-it by checking if $proxy_session is true but $proxy_host is empty. As
-an example, in my connect ACL, I have:
-
- warn condition = ${if and{ {bool{$proxy_session}} \
- {eq{$proxy_host_address}{}} } }
- log_message = Failed required proxy protocol negotiation \
- from $sender_host_name [$sender_host_address]
-
- warn condition = ${if and{ {bool{$proxy_session}} \
- {!eq{$proxy_host_address}{}} } }
- # But don't log health probes from the proxy itself
- condition = ${if eq{$proxy_host_address}{$sender_host_address} \
- {false}{true}}
- log_message = Successfully proxied from $sender_host_name \
- [$sender_host_address] through proxy protocol \
- host $proxy_host_address
-
- # Possibly more clear
- warn logwrite = Remote Source Address: $sender_host_address:$sender_host_port
- logwrite = Proxy Target Address: $proxy_target_address:$proxy_target_port
- logwrite = Proxy Internal Address: $proxy_host_address:$proxy_host_port
- logwrite = Internal Server Address: $received_ip_address:$received_port
-
-
-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.
- - 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:
- "503 Command refused, required Proxy negotiation failed"
- - If the incoming connection is configured in Exim to be a Proxy
- Protocol host, but the proxy is not sending the header, the banner
- does not get sent until the timeout occurs. If the sending host
- sent any input (before the banner), this causes a standard Exim
- synchronization error (i.e. trying to pipeline before PIPELINING
- was advertised).
- - This is not advised, but is mentioned for completeness if you have
- a specific internal configuration that you want this: If the Exim
- server only has an internal IP address and no other machines in your
- organization will connect to it to try to send email, you may
- simply set the hostlist to "*", however, this will prevent local
- mail programs from working because that would require mail from
- localhost to use Proxy Protocol. Again, not advised!
-
-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
-rejected except for QUIT:
-
-# nc mail.example.net 25
-220-mail.example.net, ESMTP Exim 4.82+proxy, Mon, 04 Nov 2013 10:45:59
-220 -0800 RFC's enforced
-EHLO localhost
-503 Command refused, required Proxy negotiation failed
-QUIT
-221 mail.example.net closing connection
-
-
-
-
-DANE
-------------------------------------------------------------
-DNS-based Authentication of Named Entities, as applied
-to SMTP over TLS, provides assurance to a client that
-it is actually talking to the server it wants to rather
-than some attacker operating a Man In The Middle (MITM)
-operation. The latter can terminate the TLS connection
-you make, and make another one to the server (so both
-you and the server still think you have an encrypted
-connection) and, if one of the "well known" set of
-Certificate Authorities has been suborned - something
-which *has* been seen already (2014), a verifiable
-certificate (if you're using normal root CAs, eg. the
-Mozilla set, as your trust anchors).
-
-What DANE does is replace the CAs with the DNS as the
-trust anchor. The assurance is limited to a) the possibility
-that the DNS has been suborned, b) mistakes made by the
-admins of the target server. The attack surface presented
-by (a) is thought to be smaller than that of the set
-of root CAs.
-
-It also allows the server to declare (implicitly) that
-connections to it should use TLS. An MITM could simply
-fail to pass on a server's STARTTLS.
-
-DANE scales better than having to maintain (and
-side-channel communicate) copies of server certificates
-for every possible target server. It also scales
-(slightly) better than having to maintain on an SMTP
-client a copy of the standard CAs bundle. It also
-means not having to pay a CA for certificates.
-
-DANE requires a server operator to do three things:
-1) run DNSSEC. This provides assurance to clients
-that DNS lookups they do for the server have not
-been tampered with. The domain MX record applying
-to this server, its A record, its TLSA record and
-any associated CNAME records must all be covered by
-DNSSEC.
-2) add TLSA DNS records. These say what the server
-certificate for a TLS connection should be.
-3) offer a server certificate, or certificate chain,
-in TLS connections which is traceable to the one
-defined by (one of?) the TSLA records
-
-There are no changes to Exim specific to server-side
-operation of DANE.
-
-The TLSA record for the server may have "certificate
-usage" of DANE-TA(2) or DANE-EE(3). The latter specifies
-the End Entity directly, i.e. the certificate involved
-is that of the server (and should be the sole one transmitted
-during the TLS handshake); this is appropriate for a
-single system, using a self-signed certificate.
- DANE-TA usage is effectively declaring a specific CA
-to be used; this might be a private CA or a public,
-well-known one. A private CA at simplest is just
-a self-signed certificate which is used to sign
-cerver certificates, but running one securely does
-require careful arrangement. If a private CA is used
-then either all clients must be primed with it, or
-(probably simpler) the server TLS handshake must transmit
-the entire certificate chain from CA to server-certificate.
-If a public CA is used then all clients must be primed with it
-(losing one advantage of DANE) - but the attack surface is
-reduced from all public CAs to that single CA.
-DANE-TA is commonly used for several services and/or
-servers, each having a TLSA query-domain CNAME record,
-all of which point to a single TLSA record.
-
-The TLSA record should have a Selector field of SPKI(1)
-and a Matching Type field of SHA2-512(2).
-
-At the time of writing, https://www.huque.com/bin/gen_tlsa
-is useful for quickly generating TLSA records; and commands like
-
- openssl x509 -in -pubkey -noout <certificate.pem \
- | openssl rsa -outform der -pubin 2>/dev/null \
- | openssl sha512 \
- | awk '{print $2}'
-
-are workable for 4th-field hashes.
-
-For use with the DANE-TA model, server certificates
-must have a correct name (SubjectName or SubjectAltName).
-
-The use of OCSP-stapling should be considered, allowing
-for fast revocation of certificates (which would otherwise
-be limited by the DNS TTL on the TLSA records). However,
-this is likely to only be usable with DANE-TA. NOTE: the
-default of requesting OCSP for all hosts is modified iff
-DANE is in use, to:
-
- hosts_request_ocsp = ${if or { {= {0}{$tls_out_tlsa_usage}} \
- {= {4}{$tls_out_tlsa_usage}} } \
- {*}{}}
-
-The (new) variable $tls_out_tlsa_usage is a bitfield with
-numbered bits set for TLSA record usage codes.
-The zero above means DANE was not in use,
-the four means that only DANE-TA usage TLSA records were
-found. If the definition of hosts_request_ocsp includes the
-string "tls_out_tlsa_usage", they are re-expanded in time to
-control the OCSP request.
-
-This modification of hosts_request_ocsp is only done if
-it has the default value of "*". Admins who change it, and
-those who use hosts_require_ocsp, should consider the interaction
-with DANE in their OCSP settings.
-
-
-For client-side DANE there are two new smtp transport options,
-hosts_try_dane and hosts_require_dane. They do the obvious thing.
-[ should they be domain-based rather than host-based? ]
-
-DANE will only be usable if the target host has DNSSEC-secured
-MX, A and TLSA records.
-
-A TLSA lookup will be done if either of the above options match
-and the host-lookup succeded using dnssec.
-If a TLSA lookup is done and succeeds, a DANE-verified TLS connection
-will be required for the host.
-
-(TODO: specify when fallback happens vs. when the host is not used)
-
-If DANE is requested and useable (see above) the following transport
-options are ignored:
- hosts_require_tls
- tls_verify_hosts
- tls_try_verify_hosts
- tls_verify_certificates
- tls_crl
- tls_verify_cert_hostnames
-
-If DANE is not usable, whether requested or not, and CA-anchored
-verification evaluation is wanted, the above variables should be set
-appropriately.
-
-Currently dnssec_request_domains must be active (need to think about that)
-and dnssec_require_domains is ignored.
-
-If verification was successful using DANE then the "CV" item
-in the delivery log line will show as "CV=dane".
-
-There is a new variable $tls_out_dane which will have "yes" if
-verification succeeded using DANE and "no" otherwise (only useful
-in combination with EXPERIMENTAL_EVENT), and a new variable
-$tls_out_tlsa_usage (detailed above).
+
+
+
+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 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. Tickets have limited lifetime.
+
+ 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}_certificate_verified will be set, and verify = certificate
+ will be true, when verify failed but tls_try_verify_hosts allowed the
+ connection (under OpenSSL)
+ $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