====== Filter Files ======

Filter configuration (''[[dir:ctrl]]/*.cfg'') and trash can (''[[dir:text]]/*.can'') files allow the sysop to specify words (or any sequence of characters) that will be used to disallow clients, users, or their content.

Each line in a filter file may contain a comparison pattern.  Blank lines and lines beginning with a semicolon character ('';'') are ignored. Lines may contain up to 1000 characters.

A tab (ASCII 9) character or a new-line (CR or CRLF) sequence will terminate (signify the end of) each comparison pattern. All characters between the first tab character and the new-line sequence may be considered metadata for the comparison pattern.

To support the correct auto-addition of filters, it's important that **all comparison patterns end in a new-line sequence** (i.e. a non-empty file must end with an LF character).
===== Comparison Patterns =====
  * Leading white-space characters are ignored
  * Alphabetic character are compared case-insensitively
  * C-style string-literal backslash (''\'') [[wp>C_syntax#Strings|escape sequences]] are supported in patterns (as of v3.17c)
  * Patterns //beginning// with an exclamation mark (''!'') negate the match logic for that pattern
  * Patterns //ending// with a caret (''^'') match only if the preceding (left most) characters are found at the beginning of the comparison string ((The caret is a legacy pattern matching character made obsolete/redundant by the asterisk))
  * Patterns //ending// with a tilde (''~'') match when the preceding string of characters are found //anywhere// within the comparison string
  * Patterns //including// an asterisk (''*'') will match when both the left and right string fragments (on either/both sides of the ''*'') match the comparison string (as of v3.19a) ((Additional (more than one) asterisks in a comparison pattern are not treated specially))
  * All other patterns are "exact match" string comparisons

**Note:**\\
There's no effective difference between the patterns "word^" and "word*".
 
==== Examples ==== 
  * ''sysop'' in the ''name.can'' file would mean new users could not use the name "sysop".
  * ''sysop*'' would mean new users could not use names //beginning// with the word "sysop", like "sysop the" or "sysops".
  * ''sysop~'' would mean new users could not use names that have the word "sysop" //anywhere// in them, like "imthesysop" or "Joe Sysop".

=== Match strings with the character sequence "viagra" anywhere within ===
  viagra~
  
=== Match strings beginning with the character sequence "[adv]" ===
  [adv]*

=== Match strings beginning with a space ===
  \ *
  
=== Match the string "administrator", exactly (but case-insensitively) ===
  administrator  
  
=== Match strings that do not begin with the character sequence: "the " ===
  !the *

==== IPv4 CIDR Notation ====

An additional comparison format was introduced in v3.17 (Feb-9-2017) specifically for partial (ranges of) IPv4 address matching following standard [[wp>Classless_Inter-Domain_Routing]] (CIDR) notation. For example, rather than using the comparison string "192.168.1.*" to match all IP addresses that begin with the first 3 octets of 192, 168, and 1, you could specify the same thing using CIDR notation:
  192.168.1.0/24

This enables the flexibility of filtering ranges of IP addresses based on sub-networks that are not multiples of 8-bits (e.g. "192.168.1.33/30") and the format is compatible with commonly available regional block lists.

All 4 octets of the IPv4 address must be specified in the CIDR comparison string (i.e. "192.168.1/24" is **not** a valid CIDR comparison string).

The CIDR prefix length may be 0 through 32. ''0.0.0.0/0'' matches any IPv4 address (correctly handled as of v3.22a; ''/0'' was previously a no-op).

CIDR comparisons beginning with the negation prefix (''!'') **are** supported (reverses the IP address comparison logic).

==== IPv6 CIDR Notation ====

As of v3.22a (May-2026), IPv6 addresses and CIDR ranges are supported in ''.can'' filter files using the standard textual notation, including zero-compression (''::'') and IPv4-mapped forms. Examples:
  2001:db8::/32
  fe80::/10
  ::ffff:192.0.2.0/120

The CIDR prefix length may be 0 through 128. ''::/0'' matches any IPv6 address.

Bare IPv6 addresses (no ''/<prefix>'') match as exact host comparisons, the same as bare IPv4 addresses. The negation prefix (''!'') is supported for IPv6 patterns just as for IPv4.

Mixed-family lists are fine — IPv6 entries are only matched against IPv6 client addresses, and IPv4 entries only against IPv4. No false cross-family matches occur.

In v3.21 and earlier, IPv6 CIDR entries were silently treated as literal-string patterns and never matched a real client. If you have an ''ip.can'' carried over from such a release (e.g. written by the web server's rate-limit auto-filter when ''RateLimitSubnetPrefix6'' is configured), those entries will start enforcing correctly after upgrade to v3.22a.


===== Trash Can Files =====

The following table lists the supported trash can files, a description of their use, and optional rejection message (''.msg'') files that may be displayed to users that are (or have their content) filtered. Trash can files and their respective optional rejection message files are located in the ''[[dir:text]]'' directory:

^Filename / Page     ^Default Contents^Rejection Message((Rejection message files are only used/displayed by the terminal server))^Description^
|''[[email.can]]''   |[[https://gitlab.synchro.net/sbbs/sbbs/-/raw/master/text/email.can|email.can]]|''[[bademail.msg]]''|Disallowed (source or destination) e-mail addresses (see also ''[[twitlist.cfg]]'')|
|''[[file.can]]''   |[[https://gitlab.synchro.net/sbbs/sbbs/-/raw/master/text/file.can|file.can]]|''[[badfile.msg]]''|Disallowed filenames for upload|
|''[[host.can]]''   |[[https://gitlab.synchro.net/sbbs/sbbs/-/raw/master/text/host.can|host.can]]|''[[badhost.msg]]''|Disallowed hostnames for inbound connections (when hostname lookups are enabled) and content|
|''[[ip.can]]''   |[[https://gitlab.synchro.net/sbbs/sbbs/-/raw/master/text/ip.can|ip.can]]|''[[badip.msg]]''|Disallowed IP addresses for inbound connections and content (e.g. messages)|
|''[[ip-silent.can]]''   |[[https://gitlab.synchro.net/sbbs/sbbs/-/raw/master/text/ip-silent.can|ip-silent.can]]| |Silently-ignored IP addresses for inbound connections|
|''[[name.can]]''   |[[https://gitlab.synchro.net/sbbs/sbbs/-/raw/master/text/name.can|name.can]]|''[[badname.msg]]''|Disallowed user login name/alias (see also [[howto:block-hackers]])|
|''[[password.can]]'' | [[https://gitlab.synchro.net/sbbs/sbbs/-/raw/master/text/password.can|password.can]]|''[[badpassword.msg]]''|Disallowed user passwords|
|''[[phone.can]]''   |[[https://gitlab.synchro.net/sbbs/sbbs/-/raw/master/text/phone.can|phone.can]]|''[[badphone.msg]]''|Disallowed phone numbers for new users|
|''[[subject.can]]''   |[[https://gitlab.synchro.net/sbbs/sbbs/-/raw/master/text/subject.can|subject.can]]|''[[badsubject.msg]]''|Disallowed subjects in posted messages|

**New in Synchronet v3.20:**\\
Comparison patterns in ''.can'' files may contain metadata as tab-delimited ''key=value'' pairs. The keys supported are:
^ Key  ^ Description  ^
|''t'' | Date/time stamp of filter addition (in ISO-8601 format) |
|''e'' | Expiration date/time (in ISO-8601 format) |
|''p'' | Protocol used (informational only) |
|''r'' | Reason for filtering (informational only) |
|''u'' | User name/identification at time of filtering (informational only) |
|''h'' | Host name of client (informational only) |
|''q'' | Quiet filtering (no additional log messages about blocked client) when set to ''true'' or ''1'' (new in v3.21) |

===== Other Filter Files =====

The following table lists other supported filter files and a description of their use. These filter files  are located in the ''[[dir:ctrl]]'' directory and do not have a respective response message file:

^Filename / Page     ^Default Contents^Description^
|''[[spamblock.cfg]]''   |[[https://gitlab.synchro.net/sbbs/sbbs/-/raw/master/ctrl/spamblock.cfg|spamblock.cfg]]|Hostnames and IP addresses blocked from sending e-mail to the [[server:mail|Mail Server]] (see also ''spamblock_exempt.cfg'')|
|''[[twitlist.cfg]]''   |[[https://gitlab.synchro.net/sbbs/sbbs/-/raw/master/ctrl/twitlist.cfg|twitlist.cfg]]|Disallowed (source or destination) e-mail/netmail addresses or names (see also ''[[email.can]]'')|

===== Filter Exemption Files =====
The following table lists supported filter exemption files and a description of their use. These filter exemption files are located in the ''[[dir:ctrl]]'' directory:

^Filename / Page     ^Default Contents^Description^
|''[[ipfilter_exempt.cfg]]'' |[[https://gitlab.synchro.net/sbbs/sbbs/-/raw/master/ctrl/ipfilter_exempt.cfg|ipfilter_exempt.cfg]]| Hostnames and IP addresses that are considered exempt from temporary bans and permanent filtering (added Oct-17-2016)|
|''[[dnsbl_exempt.cfg]]''   |[[https://gitlab.synchro.net/sbbs/sbbs/-/raw/master/ctrl/dnsbl_exempt.cfg|dnsbl_exempt.cfg]]|Hostnames and IP addresses and e-mail address (enclosed in <angle brackets>) which are to be exempt from positive DNS-based Blacklist results in the [[server:mail|Mail Server]] (see also ''dns_blacklist.cfg'')|
|''[[spamblock_exempt.cfg]]''   |[[https://gitlab.synchro.net/sbbs/sbbs/-/raw/master/ctrl/spamblock_exempt.cfg|spamblock_exempt.cfg]]|Hostnames and IP addresses which are not to be blocked from sending e-mail to the [[server:mail|Mail Server]] (see also ''spamblock.cfg'')|


===== See Also =====
  * [[:config:|Configuration]]


{{tag>configuration security abuse spam cfg can}}
