BLOCKLISTD(8) | System Manager's Manual | BLOCKLISTD(8)

# NAME

`blocklistd` — block and release ports on demand to avoid DoS abuse

# SYNOPSIS

`blocklistd` | [`-dfrv`] [`-C` controlprog] [`-c` configfile] [`-D` dbfile] [`-P` sockpathsfile] [`-R` rulename] [`-s` sockpath] [`-t` timeout]

# DESCRIPTION

`blocklistd` is a daemon similar to syslogd(8) that listens to sockets at paths specified in the sockpathsfile for notifications from other daemons about successful or failed connection attempts. If no such file is specified, then it only listens to the socket path specified by sockpath or if that is not specified to /var/run/blocklistd.sock. Each notification contains an (action, port, protocol, address, owner) tuple that identifies the remote connection and the action. This tuple is consulted against entries from the configfile, with the syntax specified in blocklistd.conf(5). If an entry is matched, a state entry is created for that tuple. Each entry contains a number of tries limit and a duration.

If configfile is a directory, or a directory exists with the same name as configfile with ".d" appended to it, each file in the directory will be read as configuration file. If configfile exists as a file it will be processed before the contents of the configfile.d directory if that also exists.

The way `blocklistd` does configuration entry matching is by having the client side pass the file descriptor associated with the connection the client wants to blocklist as well as passing socket credentials.

The file descriptor is used to retrieve information (address and port) about the remote side with getpeername(2) and the local side with getsockname(2).

By examining the port of the local side, `blocklistd` can determine if the client program “owns” the port. By examining the optional address portion on the local side, it can match interfaces. By examining the remote address, it can match specific allow or deny rules.

Finally `blocklistd` can examine the socket credentials to match the user in the configuration file.

While this works well for TCP sockets, it cannot be relied on for unbound UDP sockets. It is also less meaningful when it comes to connections using non-privileged ports. On the other hand, if we receive a request that has a local endpoint indicating a UDP privileged port, we can presume that the client was privileged to be able to acquire that port.

Once an entry is matched `blocklistd` can perform various actions. If the action is “add” and the number of tries limit is reached, then a control script controlprog is invoked with arguments:

    control add <rulename> <proto> <address> <mask> <port>

and should invoke a packet filter command to block the connection specified by the arguments. The rulename argument can be set from the command line (default `blocklistd`). The script could print a numerical id to stdout as a handle for the rule that can be used later to remove that connection, but that is not required as all information to remove the rule is kept.

If the action is “rem” Then the same control script is invoked as:

    control rem <rulename> <proto> <address> <mask> <port> <id>

where id is the number returned from the “add” action.

`blocklistd` maintains a database of known connections in dbfile. On startup it reads entries from that file, and updates its internal state.

`blocklistd` checks the list of active entries every timeout seconds (default `15`) and removes entries and block rules using the control program as necessary.

The following options are available:

`-C` controlprog
  Use controlprog to communicate with the packet filter, instead of the default, which is /libexec/blocklistd-helper. The following arguments are passed to the control program:
  action
    The action to perform: `add`, `rem`, or `flush`; to add, remove or flush a firewall rule.
  name
    The rule name.
  protocol
    The optional protocol name (can be empty): `tcp`, `tcp6`, `udp`, `udp6`.
  address
    The IPv4 or IPv6 numeric address to be blocked or released.
  mask
    The numeric mask to be applied to the blocked or released address
  port
    The optional numeric port to be blocked (can be empty).
  id
    For packet filters that support removal of rules by rule identifier, the identifier of the rule to be removed. The add command is expected to return the rule identifier string to stdout.
`-c` configuration
  The name of the configuration file to read. The default when `-c` is not given is /etc/blocklistd.conf.
`-D` dbfile
  The Berkeley DB file where `blocklistd` stores its state. It defaults to /var/db/blocklistd.db.
`-d`
  Normally, `blocklistd` disassociates itself from the terminal unless the `-d` flag is specified, in which case it stays in the foreground.
`-f`
  Truncate the state database and flush all the rules named rulename are deleted by invoking the control script as:

      control flush <rulename>

`-P` sockpathsfile
  A file containing a list of pathnames, one per line that `blocklistd` will create sockets to listen to. This is useful for chrooted environments.
`-R` rulename
  Specify the default rule name for the packet filter rules, usually `blocklistd`.
`-r`
  Re-read the firewall rules from the internal database, then remove and re-add them. This helps for packet filters that do not retain state across reboots.
`-s` sockpath
  Add sockpath to the list of Unix sockets `blocklistd` listens to.
`-t` timeout
  The interval in seconds `blocklistd` polls the state file to update the rules.
`-v`
  Cause `blocklistd` to print diagnostic messages to `stdout` instead of syslogd(8).

# SIGNAL HANDLING

`blocklistd` deals with the following signals:

`HUP`
  Receipt of this signal causes `blocklistd` to re-read the configuration file.
`INT`, `TERM &` `QUIT`
  These signals tell `blocklistd` to exit in an orderly fashion.
`USR1`
  This signal tells `blocklistd` to increase the internal debugging level by 1.
`USR2`
  This signal tells `blocklistd` to decrease the internal debugging level by 1.

# FILES

/libexec/blocklistd-helper
  Shell script invoked to interface with the packet filter.
/etc/blocklistd.conf
  Configuration file.
/var/db/blocklistd.db
  Database of current connection entries.
/var/run/blocklistd.sock
  Socket to receive connection notifications.

# SEE ALSO

blocklistd.conf(5), blocklistctl(8), npfctl(8), syslogd(8)

# HISTORY

`blocklistd` first appeared in NetBSD 7. FreeBSD support for `blocklistd` was implemented in FreeBSD 11.

# AUTHORS

Christos Zoulas

February 25, 2025 | NetBSD 11.99