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### acl/30_exim4-config_check_rcpt ################################# # define macros to be used below in this file to check recipient # local parts for strange characters. Documentation below. # This blocks local parts that begin with a dot or contain a quite # broad range of non-alphanumeric characters. .ifndef CHECK_RCPT_LOCAL_LOCALPARTS CHECK_RCPT_LOCAL_LOCALPARTS = ^[.] : ^.*[@%!/|`#&?] .endif .ifndef CHECK_RCPT_REMOTE_LOCALPARTS CHECK_RCPT_REMOTE_LOCALPARTS = ^[./|] : ^.*[@%!`#&?] : ^.*/\\.\\./ .endif # This access control list is used for every RCPT command in an incoming # SMTP message. The tests are run in order until the address is either # accepted or denied. # acl_check_rcpt: # Accept if the source is local SMTP (i.e. not over TCP/IP). We do this by # testing for an empty sending host field. accept hosts = : control = dkim_disable_verify # Do not try to verify DKIM signatures of incoming mail if DC_minimaldns # or DISABLE_DKIM_VERIFY are set. .ifdef DC_minimaldns warn control = dkim_disable_verify .else .ifdef DISABLE_DKIM_VERIFY warn control = dkim_disable_verify .endif .endif # The following section of the ACL is concerned with local parts that contain # certain non-alphanumeric characters. Dots in unusual places are # handled by this ACL as well. # # Non-alphanumeric characters other than dots are rarely found in genuine # local parts, but are often tried by people looking to circumvent # relaying restrictions. Therefore, although they are valid in local # parts, these rules disallow certain non-alphanumeric characters, as # a precaution. # # Empty components (two dots in a row) are not valid in RFC 2822, but Exim # allows them because they have been encountered. (Consider local parts # constructed as "firstinitial.secondinitial.familyname" when applied to # a name without a second initial.) However, a local part starting # with a dot or containing /../ can cause trouble if it is used as part of a # file name (e.g. for a mailing list). This is also true for local parts that # contain slashes. A pipe symbol can also be troublesome if the local part is # incorporated unthinkingly into a shell command line. # # These ACL components will block recipient addresses that are valid # from an RFC5322 point of view. We chose to have them blocked by # default for security reasons. # # If you feel that your site should have less strict recipient # checking, please feel free to change the default values of the macros # defined in main/01_exim4-config_listmacrosdefs or override them from a # local configuration file. # # Two different rules are used. The first one has a quite strict # default, and is applied to messages that are addressed to one of the # local domains handled by this host. # The default value of CHECK_RCPT_LOCAL_LOCALPARTS is defined # at the top of this file. .ifdef CHECK_RCPT_LOCAL_LOCALPARTS deny domains = +local_domains local_parts = CHECK_RCPT_LOCAL_LOCALPARTS message = restricted characters in address .endif # The second rule applies to all other domains, and its default is # considerably less strict. # The default value of CHECK_RCPT_REMOTE_LOCALPARTS is defined in # main/01_exim4-config_listmacrosdefs: # CHECK_RCPT_REMOTE_LOCALPARTS = ^[./|] : ^.*[@%!`#&?] : ^.*/\\.\\./ # It allows local users to send outgoing messages to sites # that use slashes and vertical bars in their local parts. It blocks # local parts that begin with a dot, slash, or vertical bar, but allows # these characters within the local part. However, the sequence /../ is # barred. The use of some other non-alphanumeric characters is blocked. # Single quotes might probably be dangerous as well, but they're # allowed by the default regexps to avoid rejecting mails to Ireland. # The motivation here is to prevent local users (or local users' malware) # from mounting certain kinds of attack on remote sites. .ifdef CHECK_RCPT_REMOTE_LOCALPARTS deny domains = !+local_domains local_parts = CHECK_RCPT_REMOTE_LOCALPARTS message = restricted characters in address .endif # Accept mail to postmaster in any local domain, regardless of the source, # and without verifying the sender. # accept .ifndef CHECK_RCPT_POSTMASTER local_parts = postmaster .else local_parts = CHECK_RCPT_POSTMASTER .endif domains = +local_domains : +relay_to_domains # Deny unless the sender address can be verified. # # This is disabled by default so that DNSless systems don't break. If # your system can do DNS lookups without delay or cost, you might want # to enable this feature. # # This feature does not work in smarthost and satellite setups as # with these setups all domains pass verification. See spec.txt section # "Access control lists" subsection "Address verification" with the added # information that a smarthost/satellite setup routes all non-local e-mail # to the smarthost. .ifdef CHECK_RCPT_VERIFY_SENDER deny !acl = acl_local_deny_exceptions !verify = sender message = Sender verification failed .endif # Verify senders listed in local_sender_callout with a callout. # # In smarthost and satellite setups, this causes the callout to be # done to the smarthost. Verification will thus only be reliable if the # smarthost does reject illegal addresses in the SMTP dialog. deny !acl = acl_local_deny_exceptions senders = ${if exists{CONFDIR/local_sender_callout}\ {CONFDIR/local_sender_callout}\ {}} !verify = sender/callout .ifndef CHECK_RCPT_NO_FAIL_TOO_MANY_BAD_RCPT # Reject all RCPT commands after too many bad recipients # This is partly a defense against spam abuse and partly attacker abuse. # Real senders should manage, by the time they get to 10 RCPT directives, # to have had at least half of them be real addresses. # # This is a lightweight check and can protect you against repeated # invocations of more heavy-weight checks which would come after it. deny condition = ${if and {\ {>{$rcpt_count}{10}}\ {<{$recipients_count}{${eval:$rcpt_count/2}}} }} message = Rejected for too many bad recipients logwrite = REJECT [$sender_host_address]: bad recipient count high [${eval:$rcpt_count-$recipients_count}] .endif # Accept if the message comes from one of the hosts for which we are an # outgoing relay. It is assumed that such hosts are most likely to be MUAs, # so we set control=submission to make Exim treat the message as a # submission. It will fix up various errors in the message, for example, the # lack of a Date: header line. If you are actually relaying out out from # MTAs, you may want to disable this. If you are handling both relaying from # MTAs and submissions from MUAs you should probably split them into two # lists, and handle them differently. # Recipient verification is omitted here, because in many cases the clients # are dumb MUAs that don't cope well with SMTP error responses. If you are # actually relaying out from MTAs, you should probably add recipient # verification here. # Note that, by putting this test before any DNS black list checks, you will # always accept from these hosts, even if they end up on a black list. The # assumption is that they are your friends, and if they get onto black # list, it is a mistake. accept hosts = +relay_from_hosts control = submission/sender_retain control = dkim_disable_verify # Accept if the message arrived over an authenticated connection, from # any host. Again, these messages are usually from MUAs, so recipient # verification is omitted, and submission mode is set. And again, we do this # check before any black list tests. accept authenticated = * control = submission/sender_retain control = dkim_disable_verify # Insist that any other recipient address that we accept is either in one of # our local domains, or is in a domain for which we explicitly allow # relaying. Any other domain is rejected as being unacceptable for relaying. require message = relay not permitted domains = +local_domains : +relay_to_domains # We also require all accepted addresses to be verifiable. This check will # do local part verification for local domains, but only check the domain # for remote domains. require verify = recipient # Verify recipients listed in local_rcpt_callout with a callout. # This is especially handy for forwarding MX hosts (secondary MX or # mail hubs) of domains that receive a lot of spam to non-existent # addresses. The only way to check local parts for remote relay # domains is to use a callout (add /callout), but please read the # documentation about callouts before doing this. deny !acl = acl_local_deny_exceptions recipients = ${if exists{CONFDIR/local_rcpt_callout}\ {CONFDIR/local_rcpt_callout}\ {}} !verify = recipient/callout # CONFDIR/local_sender_blacklist holds a list of envelope senders that # should have their access denied to the local host. Incoming messages # with one of these senders are rejected at RCPT time. # # The explicit white lists are honored as well as negative items in # the black list. See exim4-config_files(5) for details. deny !acl = acl_local_deny_exceptions senders = ${if exists{CONFDIR/local_sender_blacklist}\ {CONFDIR/local_sender_blacklist}\ {}} message = sender envelope address $sender_address is locally blacklisted here. If you think this is wrong, get in touch with postmaster log_message = sender envelope address is locally blacklisted. # deny bad sites (IP address) # CONFDIR/local_host_blacklist holds a list of host names, IP addresses # and networks (CIDR notation) that should have their access denied to # The local host. Messages coming in from a listed host will have all # RCPT statements rejected. # # The explicit white lists are honored as well as negative items in # the black list. See exim4-config_files(5) for details. deny !acl = acl_local_deny_exceptions hosts = ${if exists{CONFDIR/local_host_blacklist}\ {CONFDIR/local_host_blacklist}\ {}} message = sender IP address $sender_host_address is locally blacklisted here. If you think this is wrong, get in touch with postmaster log_message = sender IP address is locally blacklisted. # Warn if the sender host does not have valid reverse DNS. # # If your system can do DNS lookups without delay or cost, you might want # to enable this. # If sender_host_address is defined, it's a remote call. If # sender_host_name is not defined, then reverse lookup failed. Use # this instead of !verify = reverse_host_lookup to catch deferrals # as well as outright failures. .ifdef CHECK_RCPT_REVERSE_DNS warn condition = ${if and{{def:sender_host_address}{!def:sender_host_name}}\ {yes}{no}} add_header = X-Host-Lookup-Failed: Reverse DNS lookup failed for $sender_host_address (${if eq{$host_lookup_failed}{1}{failed}{deferred}}) .endif .ifdef CHECK_RCPT_SPF .ifdef _HAVE_SPF deny !acl = acl_local_deny_exceptions spf = fail message = [SPF] $sender_host_address is not allowed to send mail from \ ${if def:sender_address_domain {$sender_address_domain}{$sender_helo_name}}. log_message = SPF check failed. defer !acl = acl_local_deny_exceptions spf = temperror message = Temporary DNS error while checking SPF record. Try again later. warn spf = pass:softfail:neutral:permerror add_header = :at_start:$spf_received .endif .endif # Check against classic DNS "black" lists (DNSBLs) which list # sender IP addresses .ifdef CHECK_RCPT_IP_DNSBLS warn dnslists = CHECK_RCPT_IP_DNSBLS add_header = X-Warning: $sender_host_address is listed at $dnslist_domain ($dnslist_value: $dnslist_text) log_message = $sender_host_address is listed at $dnslist_domain ($dnslist_value: $dnslist_text) .endif # Check against DNSBLs which list sender domains, with an option to locally # whitelist certain domains that might be blacklisted. # # Note: If you define CHECK_RCPT_DOMAIN_DNSBLS, you must append # "/$sender_address_domain" after each domain. For example: # CHECK_RCPT_DOMAIN_DNSBLS = rhsbl.foo.org/$sender_address_domain \ # : rhsbl.bar.org/$sender_address_domain .ifdef CHECK_RCPT_DOMAIN_DNSBLS warn !senders = ${if exists{CONFDIR/local_domain_dnsbl_whitelist}\ {CONFDIR/local_domain_dnsbl_whitelist}\ {}} dnslists = CHECK_RCPT_DOMAIN_DNSBLS add_header = X-Warning: $sender_address_domain is listed at $dnslist_domain ($dnslist_value: $dnslist_text) log_message = $sender_address_domain is listed at $dnslist_domain ($dnslist_value: $dnslist_text) .endif # This hook allows you to hook in your own ACLs without having to # modify this file. If you do it like we suggest, you'll end up with # a small performance penalty since there is an additional file being # accessed. This doesn't happen if you leave the macro unset. .ifdef CHECK_RCPT_LOCAL_ACL_FILE .include CHECK_RCPT_LOCAL_ACL_FILE .endif ############################################################################# # This check is commented out because it is recognized that not every # sysadmin will want to do it. If you enable it, the check performs # Client SMTP Authorization (csa) checks on the sending host. These checks # do DNS lookups for SRV records. The CSA proposal is currently (May 2005) # an Internet draft. You can, of course, add additional conditions to this # ACL statement to restrict the CSA checks to certain hosts only. # # require verify = csa ############################################################################# # Accept if the address is in a domain for which we are an incoming relay, # but again, only if the recipient can be verified. accept domains = +relay_to_domains endpass verify = recipient # At this point, the address has passed all the checks that have been # configured, so we accept it unconditionally. accept