X-Git-Url: https://git.exim.org/exim.git/blobdiff_plain/6c1c3d1dbe1a62ffd24ad9b3cd9efdfe275c74c5..exim-4_85_RC4:/doc/doc-txt/experimental-spec.txt diff --git a/doc/doc-txt/experimental-spec.txt b/doc/doc-txt/experimental-spec.txt index 031c5f4c1..4a2a04bb4 100644 --- a/doc/doc-txt/experimental-spec.txt +++ b/doc/doc-txt/experimental-spec.txt @@ -6,97 +6,6 @@ about experimental features, all of which are unstable and liable to incompatible change. -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, -or with GnuTLS 3.1.3 or later, 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. - -Under OpenSSL 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, the smtp transport gains two options: -- "hosts_require_ocsp"; a host-list for which an OCSP Stapling -is requested and required for the connection to proceed. The default -value is empty. -- "hosts_request_ocsp"; a host-list for which (additionally) an OCSP -Stapling is requested (but not necessarily verified). The default -value is "*" meaning that requests are made unless configured -otherwise. - -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). - -Note that the proof only covers the terminal server certificate, -not any of the chain from CA to it. - -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 failure mode seen was the OCSP Signer cert expiring before the end - of validity 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 -------------------------------------------------------------- @@ -850,90 +759,103 @@ b. Configure, somewhere before the DATA ACL, the control option to -Transport post-delivery actions +Event 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. +(Renamed from TPDA, Transport post-delivery actions) + +An arbitrary per-transport string can be expanded upon various transport events. +Additionally a main-section configuration option can be expanded on some +per-message events. 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 +In order to use the feature, you must compile with -EXPERIMENTAL_TPDA=yes +EXPERIMENTAL_EVENT=yes in your Local/Makefile -and define the expandable strings in the runtime config file, to -be executed at end of delivery. +and define one or both of +- the event_action option in the transport +- the event_action main option +to be expanded when the event fires. + +A new variable, $event_name, is set to the event type when the +expansion is done. The current list of events is: -Additionally, there are 6 more variables, available at end of -delivery: + msg:complete after main per message + msg:delivery after transport per recipient + msg:host:defer after transport per attempt + msg:fail:delivery after main per recipient + msg:fail:internal after main per recipient + tcp:connect before transport per connection + tcp:close after transport per connection + tls:cert before both per certificate in verification chain + smtp:connect after transport per connection -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 +The expansion is called for all event types, and should use the $event_name +value to decide when to act. The variable data is a colon-separated +list, describing an event tree. -In case of a deferral caused by a host-error: -tpda_defer_errno Error number -tpda_defer_errstr Error string possibly containing more details +There is an auxilary variable, $event_data, for which the +content is event_dependent: -The $router_name and $transport_name variables are also usable. + msg:delivery smtp confirmation mssage + msg:host:defer error string + tls:cert verification chain depth + smtp:connect smtp banner +The msg:host:defer event populates one extra variable, $event_defer_errno. -To take action after successful deliveries, set the following option -on any transport of interest. +The following variables are likely to be useful depending on the event type: + + router_name, transport_name + local_part, domain + host, host_address, host_port + tls_out_peercert + lookup_dnssec_authenticated, tls_out_dane + sending_ip_address, sending_port + message_exim_id, verify_mode -tpda_delivery_action An example might look like: -tpda_delivery_action = \ -${lookup pgsql {SELECT * FROM record_Delivery( \ +event_action = ${if eq {msg:delivery}{$event_name} \ +{${lookup pgsql {SELECT * FROM record_Delivery( \ '${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. + '${quote_pgsql:$domain}', \ + '${quote_pgsql:${lc:$local_part}}', \ + '${quote_pgsql:$host_address}', \ + '${quote_pgsql:${lc:$host}}', \ + '${quote_pgsql:$message_exim_id}')}} \ +} {}} + +The string is expanded when each of the supported events occur +and any side-effects of the expansion will happen. 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: +The expansion of the event_action option should normally +return an empty string. Should it return anything else the +following will be forced: -tpda_host_defer_action + msg:delivery (ignored) + msg:host:defer (ignored) + msg:fail:delivery (ignored) + tcp:connect do not connect + tcp:close (ignored) + tls:cert refuse verification + smtp:connect close connection -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. +No other use is made of the result string. -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}' \ - }} +Known issues: +- the tls:cert event is only called for the cert chain elements + received over the wire, with GnuTLS. OpenSSL gives the entire + chain including thse loaded locally. Redis Lookup @@ -1108,11 +1030,27 @@ an example, in my connect ACL, I have: logwrite = Internal Server Address: $received_ip_address:$received_port -4. Runtime issues to be aware of: +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: @@ -1131,7 +1069,7 @@ 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 @@ -1205,6 +1143,9 @@ 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 @@ -1212,16 +1153,183 @@ 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". +included to do so for server certificates, and a new smtp transport option +"tls_verify_cert_hostnames" supported which takes a hostlist +which must match the target host for the additional checks must be made. +The option currently defaults to empty, but this may change in +the future. "*" is probably a suitable value. +Whether certificate verification is done at all, and the result of +it failing, is stll under the control of "tls_verify_hosts" nad +"tls_try_verify_hosts". + +The name being checked is that for the host, generally +the result of an MX lookup. 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). +The equivalent check on the server for client certificates is not +implemented. At least one major email provider is using a client +certificate which fails this check. They do not retry either without +the client certificate or in clear. + +It is possible to duplicate the effect of this checking by +creative use of Events. + + + + +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 /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). --------------------------------------------------------------