Co to właściwie jest portsentry ? Jest to mała aplikacja która nasłuchuje na wybranych portach i w momencie proby połaczenia na te porty blokuje ruch z adresu IP który probuje połaczyć sie. Program wykrywa skanowania FIN, półotwartych połączenia, ciągów NULL, pakiety niepoprawnie skonstruowane, SYN i ataki w stylu X-MAS. Instalaja programu nie powinna przysporzyc problemów. Siiagamy źródła z http://sourceforge.net/projects/sentrytools/ rozpakowywujemy na serwerze i ustawiamy zmienne w pliku portsentry_config.h
# define CONFIG_FILE „/usr/local/psionic/portsentry/portsentry.conf”
#define WRAPPER_HOST_DENY „/etc/hosts.deny” 
#define SYSLOG_FACILITY LOG_DEAMON
W tym miejscu sie troche zatrzymamy, a to dlatego że w definicji SYSLOG_FACILITY logowanie odbywa sie do pliku syslog, zmienimy zmienną LOG_DEAMON na LOG_LOCAL0, i dopiszemy sobie w pliku /etc/syslog.conf                                     local0.*                -var/log/portsentry.log
będziemy w ten sposób mieli logowanie do osobnego pliku który wykorzystamy do wysylania na poczte informacji o próbach ataku

Następnie make i make install, w ten sposób program mamy zainstalowany
Zabieramy sie za konfiguracje:

{loadposition Adsense}

# PortSentry Configuration
#
# $Id: portsentry.conf,v 1.25 2003/05/23 16:15:39 crowland Exp crowland $
#
# IMPORTANT NOTE: You CAN NOT put spaces between your port arguments.
#
# The default ports will catch a large number of common probes
#
# All entries must be in quotes.

#######################
# Port Configurations #
#######################
# wybieramy jeden z trzech mozliwych trybów pracy, mamy mozliwośc wybrania
# bare-bones – tryb podstawowy, be aware – tryb skanowania stealth oraz really anal – zaawansowany 
# tryb skanowania stealth
# Un-comment these if you are really anal:
#TCP_PORTS=”1,7,9,11,15,70,79,80,109,110,111,119,138,139,143,512,513,514,515,540,635,1080,
1524,2000,2001,4000,4001,5742,6000,6001,6667,12345,12346,20034,27665,30303,32771,32772,
32773,32774,31337,40421,40425,49724,54320″
#UDP_PORTS=”1,7,9,66,67,68,69,111,137,138,161,162,474,513,517,518,635,640,641,666,700,2049
,31335,27444,34555,32770,32771,32772,32773,32774,31337,54321″
#
# Use these if you just want to be aware:
TCP_PORTS=”1,11,15,22,79,111,119,540,635,1080,1524,2000,5742,6667,12345,12346,20034,27665
,31337,3214,32771,32772,32773,32774,40421,49724,54320,55031,55571,55956″
UDP_PORTS=”1,7,9,22,69,161,162,513,635,640,641,700,37444,34555,31335,32770,32771,32772,32773
,32774,31337,54321″
#
# Use these for just bare-bones
#TCP_PORTS=”1,11,15,110,111,143,540,635,1080,1524,2000,12345,12346,20034,32771,32772,32773,
32774,49724,54320″
#UDP_PORTS=”1,7,9,69,161,162,513,640,700,32770,32771,32772,32773,32774,31337,54321″

###########################################
# Advanced Stealth Scan Detection Options #
###########################################
#
# This is the number of ports you want PortSentry to monitor in Advanced mode.
# Any port *below* this number will be monitored. Right now it watches
# everything below 1024.
#
# On many Linux systems you cannot bind above port 61000. This is because
# these ports are used as part of IP masquerading. I don’t recommend you
# bind over this number of ports. Realistically: I DON’T RECOMMEND YOU MONITOR
# OVER 1024 PORTS AS YOUR FALSE ALARM RATE WILL ALMOST CERTAINLY RISE. You’ve been
# warned! Don’t write me if you have have a problem because I’ll only tell
# you to RTFM and don’t run above the first 1024 ports.
# Określamy największy numer portu jaki bedzie monitorowany dla TCP i UDP
#
ADVANCED_PORTS_TCP=”1500″
ADVANCED_PORTS_UDP=”1024″
#
# This field tells PortSentry what ports (besides listening daemons) to
# ignore. This is helpful for services like ident that services such
# as FTP, SMTP, and wrappers look for but you may not run (and probably
# *shouldn’t* IMHO).
#
# By specifying ports here PortSentry will simply not respond to
# incoming requests, in effect PortSentry treats them as if they are
# actual bound daemons. The default ports are ones reported as
# problematic false alarms and should probably be left alone for
# all but the most isolated systems/networks.
#
# Wyłaczamy porty które są wykorzystane na naszym serwerze
ADVANCED_EXCLUDE_TCP=”113,139,445,993,995″
# Default UDP route (RIP), NetBIOS, bootp broadcasts.
ADVANCED_EXCLUDE_UDP=”520,138,137,67,445″

######################
# Configuration Files#
######################
# położenie plików konfiguracyjnych portsentry
# Hosts to ignore
IGNORE_FILE=”/usr/local/psionic/portsentry/portsentry.ignore”
# Hosts that have been denied (running history)
HISTORY_FILE=”/usr/local/psionic/portsentry/portsentry.history”
# Hosts that have been denied this session only (temporary until next restart)
BLOCKED_FILE=”/usr/local/psionic/portsentry/portsentry.blocked”

##############################
# Misc. Configuration Options#
##############################
# DNS Name resolution – Setting this to „1” will turn on DNS lookups
# for attacking hosts. Setting it to „0” (or any other value) will shut
# it off.
RESOLVE_HOST = „1”

###################
# Response Options#
###################
# Options to dispose of attacker. Each is an action that will
# be run if an attack is detected. If you don’t want a particular
# option then comment it out and it will be skipped.
#
# The variable $TARGET$ will be substituted with the target attacking
# host when an attack is detected. The variable $PORT$ will be substituted
# with the port that was scanned.
#
##################
# Ignore Options #
##################
# These options allow you to enable automatic response
# options for UDP/TCP. This is useful if you just want
# warnings for connections, but don’t want to react for 
# a particular protocol (i.e. you want to block TCP, but
# not UDP). To prevent a possible Denial of service attack
# against UDP and stealth scan detection for TCP, you may
# want to disable blocking, but leave the warning enabled.
# I personally would wait for this to become a problem before
# doing though as most attackers really aren’t doing this.
# The third option allows you to run just the external command
# in case of a scan to have a pager script or such execute
# but not drop the route. This may be useful for some admins
# who want to block TCP, but only want pager/e-mail warnings
# on UDP, etc.
#
#
# 0 = Do not block UDP/TCP scans.
# 1 = Block UDP/TCP scans.
# 2 = Run external command only (KILL_RUN_CMD)

BLOCK_UDP=”1″
BLOCK_TCP=”1″

###################
# Dropping Routes:#
###################
# This command is used to drop the route or add the host into
# a local filter table.
#
# The gateway (333.444.555.666) should ideally be a dead host on
# the *local* subnet. On some hosts you can also point this at
# localhost (127.0.0.1) and get the same effect. NOTE THAT
# 333.444.555.66 WILL *NOT* WORK. YOU NEED TO CHANGE IT!!
#
# ALL KILL ROUTE OPTIONS ARE COMMENTED OUT INITIALLY. Make sure you
# uncomment the correct line for your OS. If you OS is not listed
# here and you have a route drop command that works then please
# mail it to me so I can include it. ONLY ONE KILL_ROUTE OPTION
# CAN BE USED AT A TIME SO DON’T UNCOMMENT MULTIPLE LINES.
#
# NOTE: The route commands are the least optimal way of blocking
# and do not provide complete protection against UDP attacks and
# will still generate alarms for both UDP and stealth scans. I
# always recommend you use a packet filter because they are made
# for this purpose.
# Ustawiamy sposób postępowania z adresem IP jaki nas skanuje

# Generic
#KILL_ROUTE=”/sbin/route add $TARGET$ 333.444.555.666″

# Generic Linux
#KILL_ROUTE=”/sbin/route add -host $TARGET$ gw 333.444.555.666″

# Newer versions of Linux support the reject flag now. This
# is cleaner than the above option.
#KILL_ROUTE=”/sbin/route add -host $TARGET$ reject”

# Generic BSD (BSDI, OpenBSD, NetBSD, FreeBSD)
#KILL_ROUTE=”/sbin/route add $TARGET$ 333.444.555.666″

# Generic Sun
#KILL_ROUTE=”/usr/sbin/route add $TARGET$ 333.444.555.666 1″

# NEXTSTEP
#KILL_ROUTE=”/usr/etc/route add $TARGET$ 127.0.0.1 1″

# FreeBSD
#KILL_ROUTE=”route add -net $TARGET$ -netmask 255.255.255.255 127.0.0.1 -blackhole”

# Digital UNIX 4.0D (OSF/1 / Compaq Tru64 UNIX)
#KILL_ROUTE=”/sbin/route add -host -blackhole $TARGET$ 127.0.0.1″

# Generic HP-UX
#KILL_ROUTE=”/usr/sbin/route add net $TARGET$ netmask 255.255.255.0 127.0.0.1″

##
# Using a packet filter is the PREFERRED. The below lines
# work well on many OS’s. Remember, you can only uncomment *one*
# KILL_ROUTE option.
##

# ipfwadm support for Linux
#KILL_ROUTE=”/sbin/ipfwadm -I -i deny -S $TARGET$ -o”
#
# ipfwadm support for Linux (no logging of denied packets)
#KILL_ROUTE=”/sbin/ipfwadm -I -i deny -S $TARGET$”
#
# ipchain support for Linux
#KILL_ROUTE=”/sbin/ipchains -I input -s $TARGET$ -j DENY -l”
#
# ipchain support for Linux (no logging of denied packets)
#KILL_ROUTE=”/sbin/ipchains -I input -s $TARGET$ -j DENY”
#
# iptables support for Linux
KILL_ROUTE=”/usr/sbin/iptables -I INPUT -s $TARGET$ -j DROP”
#
# For those of you running FreeBSD (and compatible) you can
# use their built in firewalling as well.
#
#KILL_ROUTE=”/sbin/ipfw add 1 deny all from $TARGET$:255.255.255.255 to any”
#
#
# For those running ipfilt (OpenBSD, etc.)
# NOTE THAT YOU NEED TO CHANGE external_interface TO A VALID INTERFACE!!
#
#KILL_ROUTE=”/bin/echo 'block in log on external_interface from $TARGET$/32 to any’ | /sbin/ipf -f -„

###############
# TCP Wrappers#
###############
# This text will be dropped into the hosts.deny file for wrappers
# to use. There are two formats for TCP wrappers:
#
# Format One: Old Style – The default when extended host processing
# options are not enabled.
#
#KILL_HOSTS_DENY=”ALL: $TARGET$”

# Format Two: New Style – The format used when extended option
# processing is enabled. You can drop in extended processing
# options, but be sure you escape all '%’ symbols with a backslash
# to prevent problems writing out (i.e. %c %h )
# Ustawiamy blokowanie adresów IP w pliku hosts.deny
KILL_HOSTS_DENY=”ALL: $TARGET$ : DENY”

###################
# External Command#
###################
# This is a command that is run when a host connects, it can be whatever
# you want it to be (pager, etc.). This command is executed before the
# route is dropped or after depending on the KILL_RUN_CMD_FIRST option below
#
#
# I NEVER RECOMMEND YOU PUT IN RETALIATORY ACTIONS AGAINST THE HOST SCANNING
# YOU!
#
# TCP/IP is an *unauthenticated protocol* and people can make scans appear out
# of thin air. The only time it is reasonably safe (and I *never* think it is
# reasonable) to run reverse probe scripts is when using the „classic” -tcp mode.
# This mode requires a full connect and is very hard to spoof.
#
# The KILL_RUN_CMD_FIRST value should be set to „1” to force the command
# to run *before* the blocking occurs and should be set to „0” to make the
# command run *after* the blocking has occurred.
#
#KILL_RUN_CMD_FIRST = „0”
#
# Dodatkowa komenda która wysyła info o ataku na maila
KILL_RUN_CMD=”/usr/bin/tail -20 /var/log/portsentry.log |
/usr/bin/nail -A abuse -s 'Raport Portsentry.’ abuse@nasza.domena”  
Co robi ten skrypt ? Nic prostszego z pliku /var/log/portsentry.log wyciąga ostatnie 20 linijek i wysyła je na maila abuse@nasza.domena  w tytule poczty jest Raport Portsentry

#####################
# Scan trigger value#
#####################
# Enter in the number of port connects you will allow before an
# alarm is given. The default is 0 which will react immediately.
# A value of 1 or 2 will reduce false alarms. Anything higher is
# probably not necessary. This value must always be specified, but
# generally can be left at 0.
#
# NOTE: If you are using the advanced detection option you need to
# be careful that you don’t make a hair trigger situation. Because
# Advanced mode will react for *any* host connecting to a non-used
# below your specified range, you have the opportunity to really
# break things. (i.e someone innocently tries to connect to you via
# SSL [TCP port 443] and you immediately block them). Some of you
# may even want this though. Just be careful.
# pozwalamy na jednokrotne skanowanie
SCAN_TRIGGER=”1″

######################
# Port Banner Section#
######################
#
# Enter text in here you want displayed to a person tripping the PortSentry.
# I *don’t* recommend taunting the person as this will aggravate them.
# Leave this commented out to disable the feature
#
# Stealth scan detection modes don’t use this feature
#
#PORT_BANNER=”** UNAUTHORIZED ACCESS PROHIBITED *** YOUR CONNECTION ATTEMPT HAS BEEN LOGGED. GO AWAY.”

# EOF

Dobrze jest przenieśc usługe SSH na inny port  i port 22 wystawić jako tzw. fake-port. Od czasu do czasu trzeba też zagladać do naszego pliku rc.portsentry.fw i pousuwać adresy IP np. z neostrady, niepotrzebnie blokują nam firewala. Plik sh.portsentry.fw dodajemy do /etc/rc.d/rc.local by uruchamiał sie przy starcie systemu