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# Fail2ban

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- sshguard
- Security

Fail2ban scans log files (e.g. `/var/log/httpd/error_log`) and bans IPs that show the malicious signs like too many authentication attempts, scanning for vulnerabilities, etc. Generally Fail2ban is then used to update firewall rules to reject the IP addresses for a specified amount of time, although any other arbitrary action (e.g. sending an email) could also be configured.

Warning:

- Using an IP banning software will stop trivial attacks but it relies on an additional daemon and successful logging.
- It usually does not make sense to use fail2ban with sshd when only public key authentication or similar is enabled.
- It is also not a substitute for a VPN. Do not expose your services to the internet unless necessary.
- Additionally, if the attacker knows your IP address, they could send packets with a spoofed source header and get your IP address banned. Make sure to specify your IP in `ignoreip`.

## Installation

Install one of the following packages:

- fail2ban - Latest stable version.
- fail2ban-gitAUR - Latest commit to master.

## Usage

Configure Fail2ban and enable/start `fail2ban.service`.

### fail2ban-client

The fail2ban-client allows monitoring jails (reload, restart, status, etc.), to view all available commands:

    $ fail2ban-client

To view all enabled jails:

    # fail2ban-client status

To check the status of a jail, e.g. for sshd:

    # fail2ban-client status sshd

    Status for the jail: sshd
    |- Filter
    |  |- Currently failed: 1
    |  |- Total failed:     9
    |  `- Journal matches:  _SYSTEMD_UNIT=sshd.service + _COMM=sshd
    `- Actions
       |- Currently banned: 1
       |- Total banned:     1
       `- Banned IP list:   0.0.0.0

For a compact version for all jails, including banned IPs:

    # fail2ban-client banned

    [{'sshd': ['192.168.100.50']}, {'apache-auth': []}]

## Configuration

This article or section needs expansion.

Reason: Add a note about `dbpurgeage`, see [1]. (Discuss in Talk:Fail2ban)

Due to the possibility of Pacnew and Pacsave files being created for `/etc/fail2ban/jail.conf` during an upgrade, jail.conf(5) § CONFIGURATION FILES FORMAT recommends that users create a `/etc/fail2ban/jail.local` file to "ease upgrades".

For example, to change the default ban time to 1 day:

    /etc/fail2ban/jail.local

    [DEFAULT]
    bantime = 1d

Or create separate `name.local` files under the `/etc/fail2ban/jail.d` directory, e.g. `/etc/fail2ban/jail.d/sshd.local`.

Reload `fail2ban.service` to apply the configuration changes.

### Enabling jails

By default all jails are disabled. Append `enabled = true` to the jail you want to use, e.g. to enable the OpenSSH jail:

    /etc/fail2ban/jail.local

    [sshd]
    enabled = true

See #Custom SSH jail.

### Receive an alert e-mail

If you want to receive an e-mail when someone has been banned, you have to configure an SMTP client (e.g. msmtp) and change default action, as given below.

    /etc/fail2ban/jail.local

    [DEFAULT]
    destemail = yourname@example.com
    sender = yourname@example.com

    # to ban & send an e-mail with whois report to the destemail.
    action = %(action_mw)s

    # same as action_mw but also send relevant log lines
    #action = %(action_mwl)s

### Firewall and services

By default, Fail2ban uses iptables. However, configuration of most firewalls and services is straightforward. For example, to use nftables:

    /etc/fail2ban/jail.local

    [DEFAULT]
    banaction = nftables
    banaction_allports = nftables[type=allports]

See `/etc/fail2ban/action.d/` for other examples, e.g. ufw.conf.

## Tips and tricks

### Custom SSH jail

Warning: If the attacker knows your IP address, they can send packets with a spoofed source header and get your IP address locked out of the server. SSH keys provide an elegant solution to the problem of brute forcing without these problems.

Edit `/etc/fail2ban/jail.d/sshd.local`, add this section and update the list of trusted IP addresses in `ignoreip`:

    /etc/fail2ban/jail.d/sshd.local

    [sshd]
    enabled   = true
    filter    = sshd
    banaction = iptables
    backend   = systemd
    maxretry  = 5
    findtime  = 1d
    bantime   = 2w
    ignoreip  = 127.0.0.1/8

Note:

- It may be necessary to set `LogLevel VERBOSE` in `/etc/ssh/sshd_config` to allow full fail2ban monitoring as otherwise password failures may not be logged correctly.
- Fail2ban has IPv6 support since version 0.10. Adapt your firewall accordingly, e.g. start/enable `ip6tables.service`.
- When using journal namespaces (by adding `LogNamespace=something` to a unit file), you can make fail2ban read those logs by setting `backend` like this: `backend = systemd[journalfiles="/var/log/journal/*.something/system.journal"]`.

Tip:

- If using iptables front-ends like ufw, one can use `banaction = ufw` instead of using iptables.
- When using Shorewall, one can use `banaction = shorewall` and also set `BLACKLIST` to `ALL` in `/etc/shorewall/shorewall.conf`, otherwise the rule added to ban an IP address will affect only new connections.

### Systemd backend: journald filtering

When using the systemd backend to improve performance, configure a filter with `journalmatch`. For example, to parse only kernel-level log messages:

    /etc/fail2ban/filter.d/fwdrop.local

    [Definition]
    failregex = ^.*DROP_.*SRC=<ADDR> DST=.*$
    journalmatch = _TRANSPORT=kernel

See also systemd.journal-fields(7).

### Service hardening

Currently, Fail2ban must be run as root. Therefore, you may wish to consider hardening the process with systemd.

Create a drop-in configuration file for `fail2ban.service`:

    /etc/systemd/system/fail2ban.service.d/override.conf

    [Service]
    PrivateDevices=yes
    PrivateTmp=yes
    ProtectHome=read-only
    ProtectSystem=strict
    ReadWritePaths=-/var/run/fail2ban
    ReadWritePaths=-/var/lib/fail2ban
    ReadWritePaths=-/var/log/fail2ban.log
    ReadWritePaths=-/var/spool/postfix/maildrop
    ReadWritePaths=-/run/xtables.lock
    CapabilityBoundingSet=CAP_AUDIT_READ CAP_DAC_READ_SEARCH CAP_NET_ADMIN CAP_NET_RAW

The `CapabilityBoundingSet` parameters `CAP_DAC_READ_SEARCH` will allow Fail2ban full read access to every directory and file. `CAP_NET_ADMIN` and `CAP_NET_RAW` allow Fail2ban to operate on any firewall that has command-line shell interface. See capabilities(7) for more info.

By using `ProtectSystem=strict` the filesystem hierarchy will only be read-only, `ReadWritePaths` allows Fail2ban to have write access on required paths.

Finally, reload systemd daemon to apply the changes of the unit and restart `fail2ban.service`.

## See also

- Using a Fail2Ban Jail to Whitelist a User
- Optimising your Fail2Ban filters
- Fail2Ban and sendmail
- Fail2Ban and iptables
- Fail2Ban 0.8.3 Howto
- Monitoring the fail2ban log

Categories:

- Firewalls
- Secure Shell

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