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CISS.debian.installer/func/cdi_4400_hardening/4420_hardening_fail2ban.sh
Marc S. Weidner 77856e9436
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V8.00.000.2025.06.17 
Signed-off-by: Marc S. Weidner <msw@coresecret.dev>
2025-10-14 10:32:08 +01:00

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#!/bin/bash
# SPDX-Version: 3.0
# SPDX-CreationInfo: 2025-06-17; WEIDNER, Marc S.; <msw@coresecret.dev>
# SPDX-ExternalRef: GIT https://git.coresecret.dev/msw/CISS.debian.installer.git
# SPDX-FileContributor: WEIDNER, Marc S.; Centurion Intelligence Consulting Agency
# SPDX-FileCopyrightText: 2024-2025; WEIDNER, Marc S.; <msw@coresecret.dev>
# SPDX-FileType: SOURCE
# SPDX-License-Identifier: EUPL-1.2 OR LicenseRef-CCLA-1.0
# SPDX-LicenseComment: This file is part of the CISS.debian.installer.secure framework.
# SPDX-PackageName: CISS.debian.installer
# SPDX-Security-Contact: security@coresecret.eu
guard_sourcing
#######################################
# Hardening 'fail2ban'.
# Globals:
# ARY_ALLOW_IPV4
# ARY_ALLOW_IPV6
# TARGET
# VAR_FINAL_FQDN
# VAR_FINAL_IPV4
# VAR_FINAL_IPV6
# VAR_LINK_IPV6
# VAR_PROVIDER
# VAR_SSH_PORT
# Arguments:
# None
# Returns:
# 0: on success
#######################################
hardening_fail2ban() {
### Declare Arrays, HashMaps, and Variables.
declare -r var_logfile="/root/.ciss/cdi/log/4420_hardening_fail2ban.log"
chroot_logger "${TARGET}${var_logfile}"
mkdir -p "${TARGET}/root/.ciss/cdi/backup/etc/fail2ban/jail.d"
cp "${TARGET}/etc/fail2ban/fail2ban.conf" "${TARGET}/root/.ciss/cdi/backup/etc/fail2ban/fail2ban.conf.bak"
mv "${TARGET}/etc/fail2ban/jail.d/defaults-debian.conf" "${TARGET}/root/.ciss/cdi/backup/etc/fail2ban/jail.d/defaults-debian.conf.bak"
# https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1024305
insert_header "${TARGET}/etc/fail2ban/fail2ban.local"
insert_comments "${TARGET}/etc/fail2ban/fail2ban.local"
cat << 'EOF' >> "${TARGET}/etc/fail2ban/fail2ban.local"
[DEFAULT]
allowipv6 = auto
EOF
insert_header "${TARGET}/etc/fail2ban/jail.d/ciss-default.conf"
insert_comments "${TARGET}/etc/fail2ban/jail.d/ciss-default.conf"
if [[ "${#ARY_ALLOW_IPV4[@]}" -gt 0 ]]; then
### fail2ban ufw aggressive mode, one attempt for jumphost configuration.
cat << EOF >> "${TARGET}/etc/fail2ban/jail.d/ciss-default.conf"
[DEFAULT]
usedns = yes
ignoreip = 127.0.0.0/8 ::1
# ${VAR_FINAL_FQDN}
${VAR_FINAL_IPV4}
EOF
if [[ "${VAR_LINK_IPV6}" == "true" ]]; then
cat << EOF >> "${TARGET}/etc/fail2ban/jail.d/ciss-default.conf"
${VAR_FINAL_IPV6}/64
EOF
fi
cat << EOF >> "${TARGET}/etc/fail2ban/jail.d/ciss-default.conf"
# Jumphost
${ARY_ALLOW_IPV4[*]}
EOF
if [[ "${VAR_LINK_IPV6}" == "true" ]]; then
cat << EOF >> "${TARGET}/etc/fail2ban/jail.d/ciss-default.conf"
${ARY_ALLOW_IPV6[*]}
EOF
fi
cat << EOF >> "${TARGET}/etc/fail2ban/jail.d/ciss-default.conf"
maxretry = 8
findtime = 24h
bantime = 24h
[sshd]
enabled = true
backend = systemd
filter = sshd
mode = normal
port = ${VAR_SSH_PORT}
protocol = tcp
logpath = /var/log/auth.log
maxretry = 3
findtime = 24h
bantime = 24h
#
# ufw aggressive approach:
# Any valid client communicating with our server should be going directly to the service ports opened in ufw (ssh, 80, ...).
# Any client touching other ports is treated as malicious and therefore should be blocked access to ALL ports after 1 attempt.
#
[ufw]
enabled = true
filter = ciss.ufw
action = iptables-allports
logpath = /var/log/ufw.log
maxretry = 1
findtime = 24h
bantime = 24h
protocol = tcp,udp
# vim: number et ts=2 sw=2 sts=2 ai tw=128 ft=conf
EOF
else
### fail2ban ufw aggressive mode, 32 attempts for NO jumphost configuration.
cat << EOF >> "${TARGET}/etc/fail2ban/jail.d/ciss-default.conf"
[DEFAULT]
usedns = yes
ignoreip = 127.0.0.0/8 ::1
# ${VAR_FINAL_FQDN}
${VAR_FINAL_IPV4}
EOF
if [[ "${VAR_LINK_IPV6}" == "true" ]]; then
cat << EOF >> "${TARGET}/etc/fail2ban/jail.d/ciss-default.conf"
${VAR_FINAL_IPV6}/64
EOF
fi
cat << EOF >> "${TARGET}/etc/fail2ban/jail.d/ciss-default.conf"
maxretry = 8
findtime = 24h
bantime = 24h
[sshd]
enabled = true
backend = systemd
filter = sshd
mode = normal
port = ${VAR_SSH_PORT}
protocol = tcp
logpath = /var/log/auth.log
maxretry = 3
findtime = 24h
bantime = 24h
#
# ufw aggressive approach:
# Any valid client communicating with our server should be going directly to the service ports opened in ufw (ssh, 80, ...).
# Any client touching other ports is treated as malicious and therefore should be blocked access to ALL ports after 8 attempts.
#
[ufw]
enabled = true
filter = ciss.ufw
action = iptables-allports
logpath = /var/log/ufw.log
maxretry = 8
findtime = 24h
bantime = 24h
protocol = tcp,udp
# vim: number et ts=2 sw=2 sts=2 ai tw=128 ft=conf
EOF
fi
### Provider Hetzner needs special ignoreip rules.
if [[ "${VAR_PROVIDER}" == "hetzner" ]]; then
sed -i '0,/^maxretry/{s/^maxretry/# Hetzner Intern\n 172.31.1.1\/16\n&/}' "${TARGET}/etc/fail2ban/jail.d/ciss-default.conf"
fi
insert_header "${TARGET}/etc/fail2ban/filter.d/ciss.ufw.conf"
insert_comments "${TARGET}/etc/fail2ban/filter.d/ciss.ufw.conf"
cat << EOF >> "${TARGET}/etc/fail2ban/filter.d/ciss.ufw.conf"
[Definition]
failregex = ^.*UFW BLOCK.* SRC=<HOST> .*DPT=\d+ .*
ignoreregex =
EOF
# Hardening of fail2ban systemd: https://wiki.archlinux.org/title/fail2ban#Service_hardening
# 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 a command-line shell interface.
# By using 'ProtectSystem=strict' the filesystem hierarchy will only be read-only; 'ReadWritePaths' allows Fail2ban to have
# write access on required paths.
mkdir -p "${TARGET}/etc/systemd/system/fail2ban.service.d"
mkdir -p "${TARGET}/var/log/fail2ban"
insert_header "${TARGET}/etc/systemd/system/fail2ban.service.d/override.conf"
insert_comments "${TARGET}/etc/systemd/system/fail2ban.service.d/override.conf"
cat << EOF >> "${TARGET}/etc/systemd/system/fail2ban.service.d/override.conf"
[Service]
PrivateDevices=yes
PrivateTmp=yes
ProtectHome=read-only
ProtectSystem=strict
ReadWritePaths=-/var/run/fail2ban
ReadWritePaths=-/var/lib/fail2ban
ReadWritePaths=-/var/log/fail2ban
ReadWritePaths=-/var/spool/postfix/maildrop
ReadWritePaths=-/run/xtables.lock
CapabilityBoundingSet=CAP_AUDIT_READ CAP_DAC_READ_SEARCH CAP_NET_ADMIN CAP_NET_RAW
ProtectClock=true
ProtectHostname=true
EOF
cat << 'EOF' >> "${TARGET}/etc/fail2ban/fail2ban.local"
[Definition]
logtarget = /var/log/fail2ban/fail2ban.log
# vim: number et ts=2 sw=2 sts=2 ai tw=128 ft=conf
EOF
### Logrotate must be updated too.
mkdir -p "${TARGET}/root/.ciss/cdi/backup/etc/logrotate.d"
cp "${TARGET}/etc/logrotate.d/fail2ban" "${TARGET}/root/.ciss/cdi/backup/etc/logrotate.d/fail2ban.bak"
sed -i 's/\/var\/log\/fail2ban.log/\/var\/log\/fail2ban\/fail2ban.log/1' "${TARGET}/etc/logrotate.d/fail2ban"
touch "${TARGET}/var/log/fail2ban/fail2ban.log"
chmod 640 "${TARGET}/var/log/fail2ban/fail2ban.log"
if [[ ! -f "${TARGET}/var/log/ufw.log" ]]; then
install -d -m 0755 "${TARGET}/var/log"
: >| "${TARGET}/var/log/ufw.log"
chmod 0640 "${TARGET}/var/log/ufw.log"
fi
### Merge / Dump-Parse via 'fail2ban-client -d'. All '*.conf', '*.local', and 'jail.*'-files are read, inherited, and merged.
### Syntax, path, and key errors result in a non-zero exit.
chroot_script "${TARGET}" "
fail2ban-client -d >> ${var_logfile} && echo "OK: config parsed" >> ${var_logfile} || echo "ERROR: config invalid" >> ${var_logfile}
"
guard_dir && return 0
}
### Prevents accidental 'unset -f'.
# shellcheck disable=SC2034
readonly -f hardening_fail2ban
# vim: number et ts=2 sw=2 sts=2 ai tw=128 ft=sh
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