X-Git-Url: https://git.exim.org/exim.git/blobdiff_plain/a3bddaa8058346a11835f0d9947f3f60bb029fef..ac20058fb01a79d2011352107b9b6e4aa3e3e35b:/doc/doc-txt/experimental-spec.txt diff --git a/doc/doc-txt/experimental-spec.txt b/doc/doc-txt/experimental-spec.txt index b80e02b4c..6657f63c7 100644 --- a/doc/doc-txt/experimental-spec.txt +++ b/doc/doc-txt/experimental-spec.txt @@ -6,114 +6,6 @@ about experimental features, all of which are unstable and liable to incompatible change. -PRDR support --------------------------------------------------------------- - -Per-Recipient Data Reponse is an SMTP extension proposed by Eric Hall -in a (now-expired) IETF draft from 2007. It's not hit mainstream -use, but has apparently been implemented in the META1 MTA. - -There is mention at http://mail.aegee.org/intern/sendmail.html -of a patch to sendmail "to make it PRDR capable". - - ref: http://www.eric-a-hall.com/specs/draft-hall-prdr-00.txt - -If Exim is built with EXPERIMENTAL_PRDR there is a new config -boolean "prdr_enable" which controls whether PRDR is advertised -as part of an EHLO response, a new "acl_data_smtp_prdr" ACL -(called for each recipient, after data arrives but before the -data ACL), and a new smtp transport option "hosts_try_prdr". - -PRDR may be used to support per-user content filtering. Without it -one must defer any recipient after the first that has a different -content-filter configuration. With PRDR, the RCPT-time check -for this can be disabled when the MAIL-time $smtp_command included -"PRDR". Any required difference in behaviour of the main DATA-time -ACL should however depend on the PRDR-time ACL having run, as Exim -will avoid doing so in some situations (eg. single-recipient mails). - - - -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, -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. - -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 and OpenSSL, the smtp transport gains -a "hosts_require_ocsp" option; a host-list for which an OCSP Stapling -is requested and required for the connection to proceed. 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). - -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 fail mode seen was the OCSP Signer cert expiring before the end - of vailidity 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 -------------------------------------------------------------- @@ -452,15 +344,21 @@ which the spf condition should succeed. Valid strings are: This means the queried domain has published a SPF record, but wants to allow outside servers to send mail under its domain as well. - o err_perm This indicates a syntax error in the SPF - record of the queried domain. This should be - treated like "none". - o err_temp This indicates a temporary error during all + This should be treated like "none". + o permerror This indicates a syntax error in the SPF + record of the queried domain. You may deny + messages when this occurs. (Changed in 4.83) + o temperror This indicates a temporary error during all processing, including Exim's SPF processing. You may defer messages when this occurs. + (Changed in 4.83) + o err_temp Same as permerror, deprecated in 4.83, will be + removed in a future release. + o err_perm Same as temperror, deprecated in 4.83, will be + removed in a future release. You can prefix each string with an exclamation mark to invert -is meaning, for example "!fail" will match all results but +its meaning, for example "!fail" will match all results but "fail". The string list is evaluated left-to-right, in a short-circuit fashion. When a string matches the outcome of the SPF check, the condition succeeds. If none of the listed @@ -510,8 +408,8 @@ variables. $spf_result This contains the outcome of the SPF check in string form, - one of pass, fail, softfail, none, neutral, err_perm or - err_temp. + one of pass, fail, softfail, none, neutral, permerror or + temperror. $spf_smtp_comment This contains a string that can be used in a SMTP response @@ -773,7 +671,7 @@ fails. Of course, you can also use any other lookup method that Exim supports, including LDAP, Postgres, MySQL, etc, as long as the -result is a list of colon-separated strings; +result is a list of colon-separated strings. Several expansion variables are set before the DATA ACL is processed, and you can use them in this ACL. The following @@ -781,7 +679,10 @@ expansion variables are available: o $dmarc_status This is a one word status indicating what the DMARC library - thinks of the email. + thinks of the email. It is a combination of the results of + DMARC record lookup and the SPF/DKIM/DMARC processing results + (if a DMARC record was found). The actual policy declared + in the DMARC record is in a separate expansion variable. o $dmarc_status_text This is a slightly longer, human readable status. @@ -790,6 +691,11 @@ expansion variables are available: This is the domain which DMARC used to look up the DMARC policy record. + o $dmarc_domain_policy + This is the policy declared in the DMARC record. Valid values + are "none", "reject" and "quarantine". It is blank when there + is any error, including no DMARC record. + o $dmarc_ar_header This is the entire Authentication-Results header which you can add using an add_header modifier. @@ -825,6 +731,9 @@ b. Configure, somewhere before the DATA ACL, the control option to warn !domains = +screwed_up_dmarc_records control = dmarc_enable_forensic + warn condition = (lookup if destined to mailing list) + set acl_m_mailing_list = 1 + (DATA ACL) warn dmarc_status = accept : none : off !authenticated = * @@ -840,6 +749,10 @@ b. Configure, somewhere before the DATA ACL, the control option to set $acl_m_quarantine = 1 # Do something in a transport with this flag variable + deny condition = ${if eq{$dmarc_domain_policy}{reject}} + condition = ${if eq{$acl_m_mailing_list}{1}} + message = Messages from $dmarc_used_domain break mailing lists + deny dmarc_status = reject !authenticated = * message = Message from $domain_used_domain failed sender's DMARC policy, REJECT @@ -1021,6 +934,8 @@ 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 @@ -1066,10 +981,16 @@ Proxy Protocol server at 192.168.1.2 will look like this: 3. In the ACL's the following expansion variables are available. -proxy_host_address The src IP of the proxy server making the connection -proxy_host_port The src port the proxy server is using -proxy_session Boolean, yes/no, the connected host is required to use - Proxy Protocol. +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 $proxy_host is empty. As @@ -1089,11 +1010,34 @@ an example, in my connect ACL, I have: [$sender_host_address] through proxy protocol \ host $proxy_host_address -4. Runtime issues to be aware of: + # 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: @@ -1112,7 +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! -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 @@ -1127,6 +1071,83 @@ 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). + + -------------------------------------------------------------- End of file