X-Git-Url: https://vcs.fsf.org/?a=blobdiff_plain;f=doc%2Fdoc-txt%2Fexperimental-spec.txt;h=81acfda5ef0bdb8dcf355be1ea1e3991751f5702;hb=f2f2c91b11a6a740b5683ea15ef7e6240b37f086;hp=6eeb5092c935874389ef975e69912d28ba2dc152;hpb=7a31d6431bfe3bce4da48ac8774e3b0e3c038fc7;p=exim.git diff --git a/doc/doc-txt/experimental-spec.txt b/doc/doc-txt/experimental-spec.txt index 6eeb5092c..81acfda5e 100644 --- a/doc/doc-txt/experimental-spec.txt +++ b/doc/doc-txt/experimental-spec.txt @@ -448,6 +448,17 @@ spf_guess = v=spf1 a/16 mx/16 ptr ?all would relax host matching rules to a broader network range. +A lookup expansion is also available. It takes an email +address as the key and an IP address as the database: + + $lookup (username@domain} spf {ip.ip.ip.ip}} + +The lookup will return the same result strings as they can appear in +$spf_result (pass,fail,softfail,neutral,none,err_perm,err_temp). +Currently, only IPv4 addresses are supported. + + + SRS (Sender Rewriting Scheme) Support -------------------------------------------------------------- @@ -469,6 +480,7 @@ in your Local/Makefile. DCC Support -------------------------------------------------------------- +Distributed Checksum Clearinghouse; http://www.rhyolite.com/dcc/ *) Building exim @@ -623,10 +635,10 @@ exim will send these forensic emails. It's also advised that you configure a dmarc_forensic_sender because the default sender address construction might be inadequate. - control = dmarc_forensic_enable + control = dmarc_enable_forensic (AGAIN: You can choose not to send these forensic reports by simply -not putting the dmarc_forensic_enable control line at any point in +not putting the dmarc_enable_forensic control line at any point in your exim config. If you don't tell it to send them, it will not send them.) @@ -755,94 +767,8 @@ b. Configure, somewhere before the DATA ACL, the control option to deny dmarc_status = reject !authenticated = * - message = Message from $domain_used_domain failed sender's DMARC policy, REJECT - - - -Transport post-delivery actions --------------------------------------------------------------- - -An arbitrary per-transport string can be expanded on successful delivery, -and (for SMTP transports) a second string on deferrals caused by a host error. -This feature may be used, for example, to write exim internal log information -(not available otherwise) into a database. - -In order to use the feature, you must set - -EXPERIMENTAL_TPDA=yes - -in your Local/Makefile - -and define the expandable strings in the runtime config file, to -be executed at end of delivery. - -Additionally, there are 6 more variables, available at end of -delivery: - -tpda_delivery_ip IP of host, which has accepted delivery -tpda_delivery_port Port of remote host which has accepted delivery -tpda_delivery_fqdn FQDN of host, which has accepted delivery -tpda_delivery_local_part local part of address being delivered -tpda_delivery_domain domain part of address being delivered -tpda_delivery_confirmation SMTP confirmation message - -In case of a deferral caused by a host-error: -tpda_defer_errno Error number -tpda_defer_errstr Error string possibly containing more details - -The $router_name and $transport_name variables are also usable. - - -To take action after successful deliveries, set the following option -on any transport of interest. - -tpda_delivery_action + message = Message from $dmarc_used_domain failed sender's DMARC policy, REJECT -An example might look like: - -tpda_delivery_action = \ -${lookup pgsql {SELECT * FROM record_Delivery( \ - '${quote_pgsql:$sender_address_domain}',\ - '${quote_pgsql:${lc:$sender_address_local_part}}', \ - '${quote_pgsql:$tpda_delivery_domain}', \ - '${quote_pgsql:${lc:$tpda_delivery_local_part}}', \ - '${quote_pgsql:$tpda_delivery_ip}', \ - '${quote_pgsql:${lc:$tpda_delivery_fqdn}}', \ - '${quote_pgsql:$message_exim_id}')}} - -The string is expanded after the delivery completes and any -side-effects will happen. The result is then discarded. -Note that for complex operations an ACL expansion can be used. - - -In order to log host deferrals, add the following option to an SMTP -transport: - -tpda_host_defer_action - -This is a private option of the SMTP transport. It is intended to -log failures of remote hosts. It is executed only when exim has -attempted to deliver a message to a remote host and failed due to -an error which doesn't seem to be related to the individual -message, sender, or recipient address. -See section 47.2 of the exim documentation for more details on how -this is determined. - -Example: - -tpda_host_defer_action = \ -${lookup mysql {insert into delivlog set \ - msgid = '${quote_mysql:$message_exim_id}', \ - senderlp = '${quote_mysql:${lc:$sender_address_local_part}}', \ - senderdom = '${quote_mysql:$sender_address_domain}', \ - delivlp = '${quote_mysql:${lc:$tpda_delivery_local_part}}', \ - delivdom = '${quote_mysql:$tpda_delivery_domain}', \ - delivip = '${quote_mysql:$tpda_delivery_ip}', \ - delivport = '${quote_mysql:$tpda_delivery_port}', \ - delivfqdn = '${quote_mysql:$tpda_delivery_fqdn}', \ - deliverrno = '${quote_mysql:$tpda_defer_errno}', \ - deliverrstr = '${quote_mysql:$tpda_defer_errstr}' \ - }} Redis Lookup @@ -928,227 +854,6 @@ Where SPAMMER_SET is a macro and it is defined as set acl_c_spam_host = ${lookup redis{GET...}} -Proxy Protocol Support --------------------------------------------------------------- - -Exim now has Experimental "Proxy Protocol" support. It was built on -specifications from: -http://haproxy.1wt.eu/download/1.5/doc/proxy-protocol.txt -Above URL revised May 2014 to change version 2 spec: -http://git.1wt.eu/web?p=haproxy.git;a=commitdiff;h=afb768340c9d7e50d8e - -The purpose of this function is so that an application load balancer, -such as HAProxy, can sit in front of several Exim servers and Exim -will log the IP that is connecting to the proxy server instead of -the IP of the proxy server when it connects to Exim. It resets the -$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 - - -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 --------------------------------------------------------------- -The X509 certificates used for TLS are supposed be verified -that they are owned by the expected host. The coding of TLS -support to date has not made these checks. - -If built with EXPERIMENTAL_CERTNAMES defined, code is -included to do so, and a new smtp transport option -"tls_verify_cert_hostname" supported which takes a list of -names for which the checks must be made. The host must -also be in "tls_verify_hosts". - -Both Subject and Subject-Alternate-Name certificate fields -are supported, as are wildcard certificates (limited to -a single wildcard being the initial component of a 3-or-more -component FQDN). - - DANE ------------------------------------------------------------ DNS-based Authentication of Named Entities, as applied @@ -1171,6 +876,10 @@ 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 @@ -1181,7 +890,10 @@ 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. +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, @@ -1192,12 +904,12 @@ 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 +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 + 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 @@ -1209,19 +921,49 @@ 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 +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 fiels of SHA2-512(2). +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 /dev/null \ + | openssl sha512 \ + | awk '{print $2}' -For use with the DANE_TA model, server certificates +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). +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, @@ -1231,20 +973,82 @@ hosts_try_dane and hosts_require_dane. They do the obvious thing. 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 in use the following transport options are ignored: +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 - hosts_require_ocsp (might rethink those two) - hosts_request_ocsp + +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). + + + +DSN extra information +--------------------- +If compiled with EXPERIMENTAL_DSN_INFO extra information will be added +to DSN fail messages ("bounces"), when available. The intent is to aid +tracing of specific failing messages, when presented with a "bounce" +complaint and needing to search logs. + + +The remote MTA IP address, with port number if nonstandard. +Example: + Remote-MTA: X-ip; [127.0.0.1]:587 +Rationale: + Several addresses may correspond to the (already available) + dns name for the remote MTA. + +The remote MTA connect-time greeting. +Example: + X-Remote-MTA-smtp-greeting: X-str; 220 the.local.host.name ESMTP Exim x.yz Tue, 2 Mar 1999 09:44:33 +0000 +Rationale: + This string sometimes presents the remote MTA's idea of its + own name, and sometimes identifies the MTA software. + +The remote MTA response to HELO or EHLO. +Example: + X-Remote-MTA-helo-response: X-str; 250-the.local.host.name Hello localhost [127.0.0.1] +Limitations: + Only the first line of a multiline response is recorded. +Rationale: + This string sometimes presents the remote MTA's view of + the peer IP connecting to it. + +The reporting MTA detailed diagnostic. +Example: + X-Exim-Diagnostic: X-str; SMTP error from remote mail server after RCPT TO:: 550 hard error +Rationale: + This string somtimes give extra information over the + existing (already available) Diagnostic-Code field. + + +Note that non-RFC-documented field names and data types are used. + + + -------------------------------------------------------------- End of file