30.6. Blacklistd

Blacklistd is a daemon listening to sockets to receive notifications from other daemons about connection attempts that failed or were successful. It is most widely used in blocking too many connection attempts on open ports. A prime example is SSH running on the internet getting a lot of requests from bots or scripts trying to guess passwords and gain access. Using blacklistd, the daemon can notify the firewall to create a filter rule to block excessive connection attempts from a single source after a number of tries. Blacklistd was first developed on NetBSD and appeared there in version 7. FreeBSD 11 imported blacklistd from NetBSD.

This chapter describes how to set up blacklistd, configure it, and provides examples on how to use it. Readers should be familiar with basic firewall concepts like rules. For details, refer to the firewall chapter. PF is used in the examples, but other firewalls available on FreeBSD should be able to work with blacklistd, too.

30.6.1. Enabling Blacklistd

The main configuration for blacklistd is stored in blacklistd.conf(5). Various command line options are also available to change blacklistd's run-time behavior. Persistent configuration across reboots should be stored in /etc/blacklistd.conf. To enable the daemon during system boot, add a blacklistd_enable line to /etc/rc.conf like this:

# sysrc blacklistd_enable=yes

To start the service manually, run this command:

# service blacklistd start

30.6.2. Creating a Blacklistd Ruleset

Rules for blacklistd are configured in blacklistd.conf(5) with one entry per line. Each rule contains a tuple separated by spaces or tabs. Rules either belong to a local or a remote, which applies to the machine where blacklistd is running or an outside source, respectively.

30.6.2.1. Local Rules

An example blacklistd.conf entry for a local rule looks like this:

[local]
ssh             stream  *       *               *       3       24h

All rules that follow the [local] section are treated as local rules (which is the default), applying to the local machine. When a [remote] section is encountered, all rules that follow it are handled as remote machine rules.

Seven fields define a rule separated by either tabs or spaces. The first four fields identify the traffic that should be blacklisted. The three fields that follow define backlistd's behavior. Wildcards are denoted as asterisks (*), matching anything in this field. The first field defines the location. In local rules, these are the network ports. The syntax for the location field is as follows:

[address|interface][/mask][:port]

Adressses can be specified as IPv4 in numeric format or IPv6 in square brackets. An interface name like em0 can also be used.

The socket type is defined by the second field. TCP sockets are of type stream, whereas UDP is denoted as dgram. The example above uses TCP, since SSH is using that protocol.

A protocol can be used in the third field of a blacklistd rule. The following protocols can be used: tcp, udp, tcp6, udp6, or numeric. A wildcard, like in the example, is typically used to match all protocols unless there is a reason to distinguish traffic by a certain protocol.

In the fourth field, the effective user or owner of the daemon process that is reporting the event is defined. The username or UID can be used here, as well as a wildcard (see example rule above).

The packet filter rule name is declared by the fifth field, which starts the behavior part of the rule. By default, blacklistd puts all blocks under a pf anchor called blacklistd in pf.conf like this:

anchor "blacklistd/*" in on $ext_if
block in
pass out

For separate blacklists, an anchor name can be used in this field. In other cases, the wildcard will suffice. When a name starts with a hyphen (-) it means that an anchor with the default rule name prepended should be used. A modified example from the above using the hyphen would look like this:

ssh             stream  *       *               -ssh       3       24h

With such a rule, any new blacklist rules are added to an anchor called blacklistd-ssh.

To block whole subnets for a single rule violation, a / in the rule name can be used. This causes the remaining portion of the name to be interpreted as the mask to be applied to the address specified in the rule. For example, this rule would block every address adjoining /24.

22              stream  tcp       *               */24    3       24h

Note:

It is important to specify the proper protocol here. IPv4 and IPv6 treat /24 differently, that is the reason why * cannot be used in the third field for this rule.

This rule defines that if any one host in that network is misbehaving, everything else on that network will be blocked, too.

The sixth field, called nfail, sets the number of login failures required to blacklist the remote IP in question. When a wildcard is used at this position, it means that blocks will never happen. In the example rule above, a limit of three is defined meaning that after three attempts to log into SSH on one connection, the IP is blocked.

The last field in a blacklistd rule definition specifies how long a host is blacklisted. The default unit is seconds, but suffixes like m, h, and d can also be specified for minutes, hours, and days, respectively.

The example rule in its entirety means that after three times authenticating to SSH will result in a new PF block rule for that host. Rule matches are performed by first checking local rules one after another, from most specific to least specific. When a match occurs, the remote rules are applied and the name, nfail, and disable fields are changed by the remote rule that matched.

30.6.2.2. Remote Rules

Remote rules are used to specify how blacklistd changes its behavior depending on the remote host currently being evaluated. Each field in a remote rule is the same as in a local rule. The only difference is in the way blacklistd is using them. To explain it, this example rule is used:

[remote]
203.0.113.128/25 *      *       *               =/25    =       48h

The address field can be an IP address (either v4 or v6), a port or both. This allows setting special rules for a specific remote address range like in this example. The fields for type, protocol and owner are identically interpreted as in the local rule.

The name fields is different though: the equal sign (=) in a remote rule tells blacklistd to use the value from the matching local rule. It means that the firewall rule entry is taken and the /25 prefix (a netmask of 255.255.255.128) is added. When a connection from that address range is blacklisted, the entire subnet is affected. A PF anchor name can also be used here, in which case blacklistd will add rules for this address block to the anchor of that name. The default table is used when a wildcard is specified.

A custom number of failures in the nfail column can be defined for an address. This is useful for exceptions to a specific rule, to maybe allow someone a less strict application of rules or a bit more leniency in login tries. Blocking is disabled when an asterisk is used in this sixth field.

Remote rules allow a stricter enforcement of limits on attempts to log in compared to attempts coming from a local network like an office.

30.6.3. Blacklistd Client Configuration

There are a few software packages in FreeBSD that can utilize blacklistd's functionality. The two most prominent ones are ftpd(8) and sshd(8) to block excessive connection attempts. To activate blacklistd in the SSH daemon, add the following line to /etc/ssh/sshd_config:

UseBlacklist yes

Restart sshd afterwards to make these changes take effect.

Blacklisting for ftpd(8) is enabled using -B, either in /etc/inetd.conf or as a flag in /etc/rc.conf like this:

ftpd_flags="-B"

That is all that is needed to make these programs talk to blacklistd.

30.6.4. Blacklistd Management

Blacklistd provides the user with a management utility called blacklistctl(8). It displays blocked addresses and networks that are blacklisted by the rules defined in blacklistd.conf(5). To see the list of currently blocked hosts, use dump combined with -b like this.

# blacklistctl dump -b
      address/ma:port id      nfail   last access
213.0.123.128/25:22   OK      6/3     2019/06/08 14:30:19

This example shows that there were 6 out of three permitted attempts on port 22 coming from the address range 213.0.123.128/25. There are more attempts listed than are allowed because SSH allows a client to try multiple logins on a single TCP connection. A connection that is currently going on is not stopped by blacklistd. The last connection attempt is listed in the last access column of the output.

To see the remaining time that this host will be on the blacklist, add -r to the previous command.

# blacklistctl dump -br
      address/ma:port id      nfail   remaining time
213.0.123.128/25:22   OK      6/3     36s

In this example, there are 36s seconds left until this host will not be blocked any more.

30.6.5. Removing Hosts from the Block List

Sometimes it is necessary to remove a host from the block list before the remaining time expires. Unfortunately, there is no functionality in blacklistd to do that. However, it is possible to remove the address from the PF table using pfctl. For each blocked port, there is a child anchor inside the blacklistd anchor defined in /etc/pf.conf. For example, if there is a child anchor for blocking port 22 it is called blacklistd/22. There is a table inside that child anchor that contains the blocked addresses. This table is called port followed by the port number. In this example, it would be called port22. With that information at hand, it is now possible to use pfctl(8) to display all addresses listed like this:

# pfctl -a blacklistd/22 -t port22 -T show
...
213.0.123.128/25
...

After identifying the address to be unblocked from the list, the following command removes it from the list:

# pfctl -a blacklistd/22 -T delete 213.0.123.128/25

The address is now removed from PF, but will still show up in the blacklistctl list, since it does not know about any changes made in PF. The entry in blacklistd's database will eventually expire and be removed from its output eventually. The entry will be added again if the host is matching one of the block rules in blacklistd again.

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