rblsmtpd − block emails from RBL-listed sites and unwanted sources
rblsmtpd opts prog
rblsmtpd works with any SMTP server that can run under tcpserver(1). It accepts a series of getopt-style options opts while prog consists of one or more arguments.
Normally rblsmtpd runs prog. prog is expected to carry out an SMTP conversation to receive incoming emails.
However, rblsmtpd does not invoke prog if it is told to block emails from this client. Instead it carries out its own limited SMTP conversation, temporarily rejecting all attempts to send an email. Meanwhile it prints one line on descriptor 2 to log its activity.
rblsmtpd drops the limited SMTP conversation after 60 seconds, even if the client has not quit by then.
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−t n |
Change the timeout to n seconds. |
Blocked clients
If the RBLSMTPD environment variable is set and is nonempty, rblsmtpd blocks emails. It uses RBLSMTPD as an error message for the client. Normally rblsmtpd runs under tcpserver(1); you can use tcprules(1) to set RBLSMTPD for selected clients.
If RBLSMTPD is set and is empty, rblsmtpd does not block emails.
If
RBLSMTPD is not set, rblsmtpd looks up
TCPREMOTEIP in the RBL, and blocks emails if
TCPREMOTEIP is listed. tcpserver sets up
TCPREMOTEIP as the IP address of the remote host.
−r base
Use base as an RBL source. An IPv4 address a.b.c.d is listed by that source if d.c.b.a.base has a TXT record. An IPv6 address is expanded in it’s inverse nibble format. rblsmtpd uses the contents of the TXT record as an error message for the client.
−a base
Use base as an anti-RBL source. An IPv4 address a.b.c.d is anti-listed by that source if d.c.b.a.base has an A record. In this case rblsmtpd does not block emails. The same holds for IPv6 addresses (see below) and a corresponding AAAA record.
You may supply any number of −r and −a options. rblsmtpd tries each source in turn until it finds one that lists or anti-lists TCPREMOTEIP.
If you want to run your own RBL source or anti-RBL source for rblsmtpd, you can use rbldns from the DNScache (djbdns) package.
Normally, if RBLSMTPD is set, rblsmtpd uses a 451 error code in it’s limited SMTP conversation. This tells legitimate clients to try again later. It gives innocent relay operators a chance to see the problem, prohibit relaying, get off the RBL, and get the email delivered.
However, if RBLSMTPD begins with a hyphen (’-’), rblsmtpd removes the hyphen and uses a 553 error code together with the message. This tells legitimate clients to bounce the email immediately.
There are several error-handling options for RBL lookups:
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−B |
(Default.) Use a 451 error code for IP addresses listed in the RBL. | ||
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−b |
Use a 553 error code for IP addresses listed in the RBL. | ||
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−C |
(Default.) Handle RBL lookups in a ‘‘fail-open’’ mode. If an RBL lookup fails temporarily, assume that the address is not listed; if an anti-RBL lookup fails temporarily, assume that the address is anti-listed. Unfortunately, a knowledgeable attacker can force an RBL lookup or an anti-RBL lookup to fail temporarily, so that his email is not blocked. | ||
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−c |
Handle RBL lookups in a ‘‘fail-closed’’ mode. If an RBL lookup fails temporarily, assume that the address is listed (but use a 451 error code even with -b). If an anti-RBL lookup fails temporarily, assume that the address is not anti-listed (but use a 451 error code even if a subsequent RBL lookup succeeds with -b). Unfortunately, this sometimes delays legitimate emails. |
rblsmtpd may be used to only query RBLs and to present the results to qmail-smtpd in an interrogation mode.
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−i |
interrogation mode; the RBL information is written on descriptor 2 and available via the environment variable RBLSMTPD for further decisions. |
Introduce a certain delay in the SMTP connection; either before or after the RBL, the anti-RBL lookups respectively, depending whether this option is defined before or after the lookups.
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−w n |
provides a delay of ’n’ seconds for all connections. | ||
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−W |
evaluate the environment variable GREETDELAY and potentially delay the call to qmail-smtpd. |
rblsmtpd constructs from the incoming IP address TCPREMOTEIP by means of it’s inverse nibble format the resulting inverse domain name. If PROTO=TCP the standard inverse IPv4 address scheme is used, whereas in case of PROTO=TCP6 the IPv6 inverse domain name is computed based on the expanded IPv6 address:
fe80::1 => 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.e.f.(tld)
rblsmtpd recognizes IPv4-mapped IPv6 addresses and strips the IPv6 prefix piror to build the IPv4 inverse address for the RBL lookup.
tcpserver(1), tcprules(1), tcprulescheck(1), fixcrio(1), recordio(1), rblsmtpd(1), tcpclient(1), who@(1), date@(1), finger@(1), http@(1), tcpcat(1), mconnect(1), tcp-environ(5)