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 in configfile with 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.

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, usually /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, usually /etc/blocklistd.conf.

-D dbfile
    The Berkeley DB file where blocklistd stores its state, usually /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 sockspathsfile
    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