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CISS.debian.installer/func/cdi_4400_hardening/4420_hardening_fail2ban.sh
Marc S. Weidner a09a55e19f
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V8.00.000.2025.06.17 
Signed-off-by: Marc S. Weidner <msw@coresecret.dev>
2025-10-21 23:24:11 +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
# RECOVERY
# TARGET
# VAR_FINAL_FQDN
# VAR_FINAL_IPV4
# VAR_FINAL_IPV6
# VAR_LINK_IPV6
# VAR_PROVIDER
# VAR_RUN_RECOVERY
# 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"
declare var_target="${TARGET}"
### Check for TARGET / RECOVERY.
[[ "${VAR_RUN_RECOVERY}" == "true" ]] && var_target="${RECOVERY}"
chroot_logger "${var_target}${var_logfile}"
mkdir -p "${var_target}/root/.ciss/cdi/backup/etc/fail2ban/jail.d"
cp "${var_target}/etc/fail2ban/fail2ban.conf" "${var_target}/root/.ciss/cdi/backup/etc/fail2ban/fail2ban.conf.bak"
mv "${var_target}/etc/fail2ban/jail.d/defaults-debian.conf" "${var_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 "${var_target}/etc/fail2ban/fail2ban.local"
insert_comments "${var_target}/etc/fail2ban/fail2ban.local"
cat << 'EOF' >> "${var_target}/etc/fail2ban/fail2ban.local"
[DEFAULT]
allowipv6 = auto
EOF
insert_header "${var_target}/etc/fail2ban/jail.d/ciss-default.conf"
insert_comments "${var_target}/etc/fail2ban/jail.d/ciss-default.conf"
if [[ "${#ARY_ALLOW_IPV4[@]}" -gt 0 ]]; then
### fail2ban ufw aggressive mode, one attempt for jumphost configuration.
cat << EOF >> "${var_target}/etc/fail2ban/jail.d/ciss-default.conf"
[DEFAULT]
dbpurgeage = 384d
# 127.0.0.1/8 - IPv4 loopback range (local host)
# ::1/128 - IPv6 loopback
# fe80::/10 - IPv6 link-local (on-link only; NDP/RA/DAD)
# ff00::/8 - IPv6 multicast (not an unicast host)
# ::/128 - IPv6 unspecified (all zeros; never a real peer)
ignoreip = 127.0.0.1/8 ::1/128 fe80::/10 ff00::/8 ::/128
# ${VAR_FINAL_FQDN}
${VAR_FINAL_IPV4}
EOF
if [[ "${VAR_LINK_IPV6}" == "true" ]]; then
cat << EOF >> "${var_target}/etc/fail2ban/jail.d/ciss-default.conf"
${VAR_FINAL_IPV6}/64
EOF
fi
cat << EOF >> "${var_target}/etc/fail2ban/jail.d/ciss-default.conf"
# Jumphost
${ARY_ALLOW_IPV4[*]}
EOF
if [[ "${VAR_LINK_IPV6}" == "true" ]]; then
cat << EOF >> "${var_target}/etc/fail2ban/jail.d/ciss-default.conf"
${ARY_ALLOW_IPV6[*]}
EOF
fi
cat << EOF >> "${var_target}/etc/fail2ban/jail.d/ciss-default.conf"
usedns = yes
[recidive]
enabled = true
banaction = ufw[blocktype=deny]
bantime = 8d
bantime.increment = true
bantime.factor = 1
bantime.maxtime = 128d
bantime.multipliers = 1 2 4 8 16
bantime.overalljails = true
bantime.rndtime = 877s
filter = recidive
findtime = 16d
logpath = /var/log/fail2ban/fail2ban.log*
maxretry = 3
[sshd]
enabled = true
backend = systemd
bantime = 1h
bantime.increment = true
bantime.factor = 1
bantime.maxtime = 16d
bantime.multipliers = 1 2 4 8 16 32 64 128 256 384
bantime.overalljails = true
bantime.rndtime = 877s
filter = sshd
findtime = 16m
mode = aggressive
port = ${VAR_SSH_PORT}
protocol = tcp
maxretry = 4
#
# CISS aggressive approach:
# Any valid client communicating with our server should be going directly to the service ports opened in ufw (ssh, 80, ...).
# Any client touching other ports is treated as malicious and therefore should be blocked access to ALL ports after 1 attempt.
# There is no necessity to ping our servers excessively. Any client pinging us more than 1 times will be blocked.
#
[icmp]
enabled = true
banaction = ufw[blocktype=deny]
bantime = 1h
bantime.increment = true
bantime.factor = 1
bantime.maxtime = 16d
bantime.multipliers = 1 2 4 8 16 32 64 128 256 384
bantime.overalljails = true
bantime.rndtime = 877s
filter = ciss-icmp
findtime = 16m
logpath = /var/log/ufw.log
maxretry = 1
[ufw]
enabled = true
banaction = ufw[blocktype=deny]
bantime = 1h
bantime.increment = true
bantime.factor = 1
bantime.maxtime = 16d
bantime.multipliers = 1 2 4 8 16 32 64 128 256 384
bantime.overalljails = true
bantime.rndtime = 877s
filter = ciss-ufw
findtime = 16m
logpath = /var/log/ufw.log
maxretry = 1
# vim: number et ts=2 sw=2 sts=2 ai tw=128 ft=conf
EOF
else
### fail2ban ufw aggressive mode, 32 attempts for NO jumphost configuration.
cat << EOF >> "${var_target}/etc/fail2ban/jail.d/ciss-default.conf"
[DEFAULT]
dbpurgeage = 384d
# 127.0.0.1/8 - IPv4 loopback range (local host)
# ::1/128 - IPv6 loopback
# fe80::/10 - IPv6 link-local (on-link only; NDP/RA/DAD)
# ff00::/8 - IPv6 multicast (not an unicast host)
# ::/128 - IPv6 unspecified (all zeros; never a real peer)
ignoreip = 127.0.0.1/8 ::1/128 fe80::/10 ff00::/8 ::/128
# ${VAR_FINAL_FQDN}
${VAR_FINAL_IPV4}
EOF
if [[ "${VAR_LINK_IPV6}" == "true" ]]; then
cat << EOF >> "${var_target}/etc/fail2ban/jail.d/ciss-default.conf"
${VAR_FINAL_IPV6}/64
EOF
fi
cat << EOF >> "${var_target}/etc/fail2ban/jail.d/ciss-default.conf"
usedns = yes
[recidive]
enabled = true
banaction = ufw[blocktype=deny]
bantime = 8d
bantime.increment = true
bantime.factor = 1
bantime.maxtime = 128d
bantime.multipliers = 1 2 4 8 16
bantime.overalljails = true
bantime.rndtime = 877s
filter = recidive
findtime = 16d
logpath = /var/log/fail2ban/fail2ban.log*
maxretry = 3
[sshd]
enabled = true
backend = systemd
bantime = 1h
bantime.increment = true
bantime.factor = 1
bantime.maxtime = 16d
bantime.multipliers = 1 2 4 8 16 32 64 128 256 384
bantime.overalljails = true
bantime.rndtime = 877s
filter = sshd
findtime = 16m
mode = normal
port = ${VAR_SSH_PORT}
protocol = tcp
maxretry = 4
#
# CISS aggressive approach:
# Any valid client communicating with our server should be going directly to the service ports opened in ufw (ssh, 80, ...).
# Any client touching other ports is treated as malicious and therefore should be blocked access to ALL ports after 3 attempts.
# There is no necessity to ping our servers excessively. Any client pinging us more than 3 times will be blocked.
#
[icmp]
enabled = true
banaction = ufw[blocktype=deny]
bantime = 1h
bantime.increment = true
bantime.factor = 1
bantime.maxtime = 16d
bantime.multipliers = 1 2 4 8 16 32 64 128 256 384
bantime.overalljails = true
bantime.rndtime = 877s
filter = ciss-icmp
findtime = 16m
logpath = /var/log/ufw.log
maxretry = 3
[ufw]
enabled = true
banaction = ufw[blocktype=deny]
bantime = 1h
bantime.increment = true
bantime.factor = 1
bantime.maxtime = 16d
bantime.multipliers = 1 2 4 8 16 32 64 128 256 384
bantime.overalljails = true
bantime.rndtime = 877s
filter = ciss-ufw
findtime = 16m
logpath = /var/log/ufw.log
maxretry = 3
# vim: number et ts=2 sw=2 sts=2 ai tw=128 ft=conf
EOF
fi
### Provider Hetzner needs special ignoreip rules.
if [[ "${VAR_PROVIDER}" == "hetzner" ]]; then
sed -i '0,/^maxretry/{s/^maxretry/# Hetzner Intern\n 172.31.1.1\/16\n&/}' "${var_target}/etc/fail2ban/jail.d/ciss-default.conf"
fi
insert_header "${var_target}/etc/fail2ban/filter.d/ciss-icmp.conf"
insert_comments "${var_target}/etc/fail2ban/filter.d/ciss-icmp.conf"
cat << EOF >> "${var_target}/etc/fail2ban/filter.d/ciss-icmp.conf"
[Definition]
# Generic ICMP/ICMPv6 blocks
failregex = ^.*UFW (?:BLOCK|REJECT).*?\bSRC=<HOST>\b.*?\bPROTO=ICMP\b.*$
^.*UFW (?:BLOCK|REJECT).*?\bSRC=<HOST>\b.*?\bPROTO=ICMPv6\b.*$
# vim: number et ts=2 sw=2 sts=2 ai tw=128 ft=conf
EOF
insert_header "${var_target}/etc/fail2ban/filter.d/ciss-ufw.conf"
insert_comments "${var_target}/etc/fail2ban/filter.d/ciss-ufw.conf"
cat << EOF >> "${var_target}/etc/fail2ban/filter.d/ciss-ufw.conf"
[Definition]
# Match UFW BLOCK/REJECT with a source IP and *any* port field (SPT or DPT), protocol may be missing.
failregex = ^.*UFW (?:BLOCK|REJECT).*?\bSRC=<HOST>\b.*?(?:\bDPT=\d+\b|\bSPT=\d+\b).*$
ignoreregex =
# vim: number et ts=2 sw=2 sts=2 ai tw=128 ft=conf
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 "${var_target}/etc/systemd/system/fail2ban.service.d"
mkdir -p "${var_target}/var/log/fail2ban"
insert_header "${var_target}/etc/systemd/system/fail2ban.service.d/override.conf"
insert_comments "${var_target}/etc/systemd/system/fail2ban.service.d/override.conf"
cat << EOF >> "${var_target}/etc/systemd/system/fail2ban.service.d/override.conf"
[Service]
PrivateDevices=yes
PrivateTmp=yes
ProtectHome=read-only
ProtectSystem=strict
ReadWritePaths=-/var/run/fail2ban
ReadWritePaths=-/var/lib/fail2ban
ReadWritePaths=-/var/log/fail2ban
ReadWritePaths=-/var/spool/postfix/maildrop
ReadWritePaths=-/run/xtables.lock
CapabilityBoundingSet=CAP_AUDIT_READ CAP_DAC_READ_SEARCH CAP_NET_ADMIN CAP_NET_RAW
ProtectClock=true
ProtectHostname=true
EOF
cat << 'EOF' >> "${var_target}/etc/fail2ban/fail2ban.local"
[Definition]
logtarget = /var/log/fail2ban/fail2ban.log
[Database]
# Keep entries for at least 384 days to cover recidive findtime.
dbpurgeage = 384d
# vim: number et ts=2 sw=2 sts=2 ai tw=128 ft=conf
EOF
### Logrotate must be updated too.
mkdir -p "${var_target}/root/.ciss/cdi/backup/etc/logrotate.d"
cp "${var_target}/etc/logrotate.d/fail2ban" "${var_target}/root/.ciss/cdi/backup/etc/logrotate.d/fail2ban.bak"
cat << EOF >| "${var_target}/etc/logrotate.d/fail2ban"
/var/log/fail2ban/fail2ban.log {
daily
rotate 384
maxage 384
notifempty
dateext
dateyesterday
compress
compresscmd /usr/bin/zstd
compressext .zst
compressoptions -20
uncompresscmd /usr/bin/unzstd
delaycompress
shred
missingok
postrotate
fail2ban-client flushlogs 1>/dev/null
endscript
# If fail2ban runs as non-root it still needs to have write access
# to logfiles.
# create 640 fail2ban adm
create 640 root adm
}
EOF
touch "${var_target}/var/log/fail2ban/fail2ban.log"
chmod 0640 "${var_target}/var/log/fail2ban/fail2ban.log"
if [[ ! -f "${var_target}/var/log/ufw.log" ]]; then
install -d -m 0755 "${var_target}/var/log"
: >| "${var_target}/var/log/ufw.log"
chmod 0640 "${var_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 "${var_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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