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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Christos Zoulas.
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.Dd April 21, 2020
.Dt BLOCKLISTD 8
.Os
.Sh NAME
.Nm blocklistd
.Nd block and release ports on demand to avoid DoS abuse
.Sh SYNOPSIS
.Nm
.Op Fl dfrv
.Op Fl C Ar controlprog
.Op Fl c Ar configfile
.Op Fl D Ar dbfile
.Op Fl P Ar sockpathsfile
.Op Fl R Ar rulename
.Op Fl s Ar sockpath
.Op Fl t Ar timeout
.Sh DESCRIPTION
.Nm
is a daemon similar to
.Xr syslogd 8
that listens to sockets at paths specified in the
.Ar 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
.Ar sockpath
or if that is not specified to
.Pa /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
.Ar configfile
with syntax specified in
.Xr 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.
.Pp
The way
.Nm
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.
.Pp
The file descriptor is used to retrieve information (address and port)
about the remote side with
.Xr getpeername 2
and the local side with
.Xr getsockname 2 .
.Pp
By examining the port of the local side,
.Nm
can determine if the client program
.Dq 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.
.Pp
Finally
.Nm
can examine the socket credentials to match the user in the configuration file.
.Pp
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.
.Pp
Once an entry is matched
.Nm
can perform various actions.
If the action is
.Dq add
and the number of tries limit is reached, then a
control script
.Ar controlprog
is invoked with arguments:
.Bd -literal -offset indent
control add <rulename> <proto> <address> <mask> <port>
.Ed
.Pp
and should invoke a packet filter command to block the connection
specified by the arguments.
The
.Ar rulename
argument can be set from the command line (default
.Dv 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.
.Pp
If the action is
.Dq rem
Then the same control script is invoked as:
.Bd -literal -offset indent
control rem <rulename> <proto> <address> <mask> <port> <id>
.Ed
.Pp
where
.Ar id
is the number returned from the
.Dq add
action.
.Pp
.Nm
maintains a database of known connections in
.Ar dbfile .
On startup it reads entries from that file, and updates its internal state.
.Pp
.Nm
checks the list of active entries every
.Ar timeout
seconds (default
.Dv 15 )
and removes entries and block rules using the control program as necessary.
.Pp
The following options are available:
.Bl -tag -width indent
.It Fl C Ar controlprog
Use
.Ar controlprog
to communicate with the packet filter, usually
.Pa /libexec/blocklistd-helper .
The following arguments are passed to the control program:
.Bl -tag -width protocol
.It action
The action to perform:
.Dv add ,
.Dv rem ,
or
.Dv flush
to add, remove or flush a firewall rule.
.It name
The rule name.
.It protocol
The optional protocol name (can be empty):
.Dv tcp ,
.Dv tcp6 ,
.Dv udp ,
.Dv udp6 .
.It address
The IPv4 or IPv6 numeric address to be blocked or released.
.It mask
The numeric mask to be applied to the blocked or released address
.It port
The optional numeric port to be blocked (can be empty).
.It 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.
.El
.It Fl c Ar configuration
The name of the configuration file to read, usually
.Pa /etc/blocklistd.conf .
.It Fl D Ar dbfile
The Berkeley DB file where
.Nm
stores its state, usually
.Pa /var/db/blocklistd.db .
.It Fl d
Normally,
.Nm
disassociates itself from the terminal unless the
.Fl d
flag is specified, in which case it stays in the foreground.
.It Fl f
Truncate the state database and flush all the rules named
.Ar rulename
are deleted by invoking the control script as:
.Bd -literal -offset indent
control flush <rulename>
.Ed
.It Fl P Ar sockspathsfile
A file containing a list of pathnames, one per line that
.Nm
will create sockets to listen to.
This is useful for chrooted environments.
.It Fl R Ar rulename
Specify the default rule name for the packet filter rules, usually
.Dv blocklistd .
.It Fl 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.
.It Fl s Ar sockpath
Add
.Ar sockpath
to the list of Unix sockets
.Nm
listens to.
.It Fl t Ar timeout
The interval in seconds
.Nm
polls the state file to update the rules.
.It Fl v
Cause
.Nm
to print
diagnostic messages to
.Dv stdout
instead of
.Xr syslogd 8 .
.El
.Sh SIGNAL HANDLING
.Nm
deals with the following signals:
.Bl -tag -width "USR2"
.It Dv HUP
Receipt of this signal causes
.Nm
to re-read the configuration file.
.It Dv INT , Dv TERM & Dv QUIT
These signals tell
.Nm
to exit in an orderly fashion.
.It Dv USR1
This signal tells
.Nm
to increase the internal debugging level by 1.
.It Dv USR2
This signal tells
.Nm
to decrease the internal debugging level by 1.
.El
.Sh FILES
.Bl -tag -width /libexec/blocklistd-helper -compact
.It Pa /libexec/blocklistd-helper
Shell script invoked to interface with the packet filter.
.It Pa /etc/blocklistd.conf
Configuration file.
.It Pa /var/db/blocklistd.db
Database of current connection entries.
.It Pa /var/run/blocklistd.sock
Socket to receive connection notifications.
.El
.Sh SEE ALSO
.Xr blocklistd.conf 5 ,
.Xr blocklistctl 8 ,
.Xr npfctl 8 ,
.Xr syslogd 8
.Sh HISTORY
.Nm
first appeared in
.Nx 7 .
.Fx
support for
.Nm
was implemented in
.Fx 11 .
.Sh AUTHORS
.An Christos Zoulas