# SURBL module

This module performs scanning of URL’s found in messages against a list of known DNS lists. It can add different symbols depending on the DNS replies from a specific DNS URL list. It is also possible to resolve domains of URLs and then check the IP addresses against the normal RBL style list.

## Module configuration

The default configuration defines several public URL lists. However, their terms of usage normally disallows commercial or very extensive usage without purchasing a specific sort of license.

Nonetheless, they can be used by personal services or low volume requests free of charge.

# local.d/surbl.conf
# List of domains that are not checked by surbl
whitelist = "file://$CONFDIR/surbl-whitelist.inc"; # Additional exceptions for TLD rules exceptions = "file://$CONFDIR/2tld.inc";

rules {
"SURBL_MULTI" {
# DNS suffix for this rule
suffix = "multi.surbl.org";
bits {
# List of bits ORed when reply is given
JP_SURBL_MULTI = 64;
AB_SURBL_MULTI = 32;
MW_SURBL_MULTI = 16;
PH_SURBL_MULTI = 8;
WS_SURBL_MULTI = 4;
SC_SURBL_MULTI = 2;
}
}
"URIBL_MULTI" {
suffix = "multi.uribl.com";
bits {
URIBL_BLACK = 2;
URIBL_GREY = 4;
URIBL_RED = 8;
}
}
"RAMBLER_URIBL" {
suffix = "uribl.rambler.ru";
# Also check images
images = true;
}
"DBL" {
suffix = "dbl.spamhaus.org";
# Do not check numeric URL's
noip = true;
}
"SEM_URIBL_UNKNOWN" {
suffix = "uribl.spameatingmonkey.net";
bits {
SEM_URIBL = 2;
}
noip = true;
}
"SEM_URIBL_FRESH15_UNKNOWN" {
suffix = "fresh15.spameatingmonkey.net";
bits {
SEM_URIBL_FRESH15 = 2;
}
noip = true;
}
}


In general, the configuration of surbl module is definition of DNS lists. Each list must have suffix that defines the list itself and optionally for some lists it is possible to specify either bit or ips sections.

Since some URL lists do not accept IP addresses, it is also possible to disable sending of URLs with IP address in the host to such lists. That could be done by specifying noip = true option:

"DBL" {
suffix = "dbl.spamhaus.org";
# Do not check numeric URL's
noip = true;
}


It is also possible to check HTML images URLs using URL blacklists. Just specify images = true for such list and you are done:

"RAMBLER_URIBL" {
suffix = "uribl.rambler.ru";
# Also check images
images = true;
}


By default, Rspamd checks each SURBL sanity by queriyng of facebook.com domain. URL black list must NOT reply by some positive result (other than NXDOMAIN) to such a query. However, sometimes you might need to change that to another domain (e.g. to INVALID), so you can use monitored_domain option from Rspamd 1.6:

"HOSTKARMA_URIBL" {
suffix = "hostkarma.junkemailfilter.com";
noip = true;
enabled = false;
ips = {
URIBL_HOSTKARMA_WHITE = "127.0.0.1";
URIBL_HOSTKARMA_BLACK = "127.0.0.2";
URIBL_HOSTKARMA_YELLOW = "127.0.0.3";
URIBL_HOSTKARMA_BROWN = "127.0.0.4";
URIBL_HOSTKARMA_NOBLACK = "127.0.0.5";
URIBL_HOSTKARMA_24_48H = "127.0.2.1";
URIBL_HOSTKARMA_LAST_10D = "127.0.2.2";
URIBL_HOSTKARMA_OLDER_10D = "127.0.2.3";
}
monitored_domain = "INVALID";
}


## Principles of operation

In this section, we define how surbl module performs its checks.

### TLD composition

By default, we want to check some top level domain, however, many domains contain two components while others can have 3 or even more components to check against the list. By default, rspamd takes top level domain as defined in the public suffixes. Then one more component is prepended, for example:

sub.example.com -> [.com] -> example.com
sub.co.uk -> [.co.uk] -> sub.co.uk


However, sometimes even more levels of domain components are required. In this case, the exceptions map can be used. For example, if we want to check all subdomains of example.com and example.co.uk, then we can define the following list:

example.com
example.co.uk


Here are new composition rules:

sub.example.com -> [.example.com] -> sub.example.com
sub1.sub2.example.co.uk -> [.example.co.uk] -> sub2.example.co.uk


### DNS composition

SURBL module composes the DNS request of two parts:

• TLD component as defined in the previous section;
• DNS list suffix

For example, to form a request to multi.surbl.org, the following applied:

example.com -> example.com.multi.surbl.com


### Results parsing

Normally, DNS blacklists encode reply in A record from some private network (namely, 127.0.0.0/8). Encoding varies from one service to another. Some lists use bits encoding, where a single DNS list or error message is encoded as a bit in the least significant octet of the IP address. For example, if bit 1 encodes LISTA and bit 2 encodes LISTB, then we need to perform bitwise OR for each specific bit to decode reply:

 127.0.0.3 -> LISTA | LISTB -> both bit symbols are added
127.0.0.2 -> LISTB only
127.0.0.1 -> LISTA only


This encoding can save DNS requests to query multiple lists one at a time.

Some other lists use direct encoding of lists by some specific addresses. In this case you should define results decoding principle in ips section not bits since bitwise rules are not applicable to these lists. In ips section you explicitly match the ip returned by a list and its meaning.

## IP lists

From rspamd 1.1 it is also possible to do two step checks:

1. Resolve IP addresses of each URL
2. Check each IP resolved against SURBL list

In general this procedure could be represented as following:

• Check A or AAAA records for example.com
• For each ip address resolve it using reverse octets composition: so if IP address of example.com is 1.2.3.4, then checks would be for 4.3.2.1.uribl.tld

For example, SBL list of spamhaus project provides such functions using ZEN multi list. This is included in rspamd default configuration:

    rule {
suffix = "zen.spamhaus.org";
symbol = "ZEN_URIBL";
resolve_ip = true;
ips {
URIBL_SBL = "127.0.0.2";
}
}


## Disabling SURBLs

Rules can be disabled by setting the enabled setting to false. This allows for easily disabling SURBLs without overriding the full default configuration. The example below could be added to /etc/rspamd/local.d/surbl.conf to disable the RAMBLER_URIBL URIBL.

rules {
"RAMBLER_URIBL" {
enabled = false;
}
}


## Use of URL redirectors

SURBL module is designed to work with url_redirector module which can help to resolve some known redirectors and extract the real URL to check with this module. Please refer to the module’s documentation about how to work with it. SURBL module will automatically use that results.