Synchronet v3.19b-Win32 (install) has been released (Jan-2022).

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Filter Files

Filter configuration (ctrl/*.cfg) and trash can (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 (\) 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 1)
  • 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) 2)
  • All other patterns are “exact match” string comparisons

There's no effective difference between the patterns “word^” and “word*”.


  • 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


Match strings beginning with the character sequence "[adv]"


Match strings beginning with a space

\ *

Match the string "administrator", exactly (but case-insensitively)


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 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:

This enables the flexibility of filtering ranges of IP addresses based on sub-networks that are not multiples of 8-bits (e.g. “”) 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).

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

IPv6 CIDR notation is not supported at this time.

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 text directory:

Filename / Page Default ContentsRejection Message3)Description
email.can email.canbademail.msgDisallowed (source or destination) e-mail addresses (see also twitlist.cfg)
file.can file.canbadfile.msgDisallowed filenames for upload
host.can host.canbadhost.msgDisallowed hostnames for inbound connections (when hostname lookups are enabled) and content
ip.can ip.canbadip.msgDisallowed IP addresses for inbound connections and content (e.g. messages)
ip-silent.can ip-silent.can Silently-ignored IP addresses for inbound connections
name.can name.canbadname.msgDisallowed user login name/alias (see also block-hackers)
password.can password.canbadpassword.msgDisallowed user passwords
phone.can phone.canbadphone.msgDisallowed phone numbers for new users
subject.can subject.canbadsubject.msgDisallowed 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)

Other Filter Files

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

Filename / Page Default ContentsDescription
spamblock.cfg spamblock.cfgHostnames and IP addresses blocked from sending e-mail to the Mail Server (see also spamblock_exempt.cfg)
twitlist.cfg twitlist.cfgDisallowed (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 ctrl directory:

Filename / Page Default ContentsDescription
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 dnsbl_exempt.cfgHostnames and IP addresses and e-mail address (enclosed in <angle brackets>) which are to be exempt from positive DNS-based Blacklist results in the Mail Server (see also dns_blacklist.cfg)
spamblock_exempt.cfg spamblock_exempt.cfgHostnames and IP addresses which are not to be blocked from sending e-mail to the Mail Server (see also spamblock.cfg)

See Also

The caret is a legacy pattern matching character made obsolete/redundant by the asterisk
Additional (more than one) asterisks in a comparison pattern are not treated specially
Rejection message files are only used/displayed by the terminal server