# Fail2ban

Warning: Using an IP blacklist will stop trivial attacks but it relies on an additional daemon and successful logging (the partition containing /var can become full, especially if an attacker is pounding on the server). Additionally, if the attacker knows your IP address, they can send packets with a spoofed source header and get you locked out of the server. SSH keys provide an elegant solution to the problem of brute forcing without these problems.

Fail2ban scans various textual log files and bans IP that makes too many password failures by updating firewall rules to reject the IP address, similar to Sshguard.

Warning: For correct function it is essential that the tool parses the IP addresses in the log correctly. You should always test the log filters work as intended per application you want to protect.

## Installation

Install fail2ban from the official repositories.

If you want Fail2ban to send an email when someone has been banned, you have to configure SSMTP (for example).

### systemd

Use the service unit `fail2ban.service`, refer to systemd for instructions.

## Hardening

Currently, fail2ban requires to run as root, therefore you may wish to consider some additional hardening on the process with systemd. Ref:systemd for Administrators, Part XII

### Capabilities

For added security consider limiting fail2ban capabilities by specifying `CapabilityBoundingSet` in the drop-in configuration file for the provided `fail2ban.service`:

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

    [Service]
    CapabilityBoundingSet=CAP_DAC_READ_SEARCH CAP_NET_ADMIN CAP_NET_RAW

In the example above, `CAP_DAC_READ_SEARCH` will allow fail2ban full read access, and `CAP_NET_ADMIN` and `CAP_NET_RAW` allow setting of firewall rules with iptables. Additional capabilities may be required, depending on your fail2ban configuration. See `man capabilities` for more info.

### Filesystem Access

Also considering limiting file system read and write access, by using ReadOnlyDirectories and ReadWriteDirectories, again under the `[Service]` section. For example:

    ReadOnlyDirectories=/
    ReadWriteDirectories=/var/run/fail2ban /var/lib/fail2ban /var/spool/postfix/maildrop

In the example above, this limits the file system to read-only, except for `/var/run/fail2ban` for pid and socket files, and `/var/spool/postfix/maildrop` for postfix sendmail. Again, this will be dependent on you system configuration and fail2ban configuration. Note that adding `/var/log` is necessary if you want fail2ban to log its activity.

## SSH jail

Edit `/etc/fail2ban/jail.conf` and modify the ssh-iptables section to enable it and configure the action.

If your firewall is iptables:

    [ssh-iptables]
    enabled  = true
    filter   = sshd
    action   = iptables[name=SSH, port=ssh, protocol=tcp]
               sendmail-whois[name=SSH, dest=your@mail.org, sender=fail2ban@mail.com]
    logpath  = /var/log/auth.log
    maxretry = 5

Fail2Ban from version 0.9 can also read directly from the systemd journal by setting `backend = systemd`.

If your firewall is shorewall:

    [ssh-shorewall]
    enabled  = true
    filter   = sshd
    action   = shorewall
               sendmail-whois[name=SSH, dest=your@mail.org, sender=fail2ban@mail.com]
    logpath  = /var/log/auth.log
    maxretry = 5

Note: You can set `BLACKLISTNEWONLY` to `No` in `/etc/shorewall/shorewall.conf` otherwise the rule added to ban an IP address will affect only new connections.

Also do not forget to add/change:

    LogLevel VERBOSE

in your `/etc/ssh/sshd_config`. Else, password failures are not logged correctly.

## See also

- sshguard

Category:

- Secure Shell