sshguard

Source : http://howtounix.info/howto/sshguard-freebsd

sshguard is a security tool protecting networked hosts from brute force attacks against ssh servers. It detects these attacks and blocks attacker's address creating a firewall rule.

sshguard with pf

Installing sshguard from ports:

cd /usr/ports/security/sshguard-pf
make install clean

Add to /etc/syslog.conf:

auth.info;authpriv.info |exec /usr/local/sbin/sshguard -a 15 -p 9200 -s 12000

Edit pf.conf:

table <sshguard> persist
block in quick on re0 proto tcp from <sshguard> to any port ssh label "ssh brute"

change re0 to your external interface name.

This command will display addresses in the table:

pfctl -t sshguard -T show

To delete an address from the table (for example 192.168.1.5) use:

pfctl -t sshguard -T delete 192.168.1.5

sshguard with ipfw

It's easy to install and configure sshguard with ipfw.sshguard:

cd /usr/ports/security/sshguard-ipfw/
make install clean

There are no build options so you can get straight to configuring.

Configuring sshguard

Add to /etc/syslog.conf:

auth.info;authpriv.info |/usr/local/sbin/sshguard -w 192.168.1.10

-w command-line option is used for whitelisting. This option can add explicit addresses, host names and address blocks. Specify the address directly, like:

-w 192.168.1.10

or in multiple occurrences:

-w 192.168.1.10 -w 192.168.1.11 -w 192.168.1.12

sshguard detects 4 attack attempts as brute force by default. You can change this number using -a option. When brute force is detected sshguard adds new rule to firewall rules: deny ip from brute.ip.add.ress to me

This rule will be deleted in 420 seconds. If the attack from this address happens again block time doubles.

-b option is used for creating blacklists. -b 10:/var/db/sshguard/blacklist.db means that after 10 firewall blocks the address will be put on the blacklist and blocked forever. Blocking rules are in 55000-55050 IDs range.

source : http://www.sshguard.net/docs/man/sshguard/1_5/

Depending on the way chosen for giving logs to sshguard, and depending on the chosen blocking system, some setup may be needed. Instructions are documented at http://www.sshguard.net/docs/setup/.

sshguard does not make use of any configuration file. Instead, a combination of optional arguments can be passed to its process on the command line, for modifying its default behaviour:

-b [num:]filename

enable blacklisting: blacklist after num (or 3) blocked abuses, and hold the permanent blacklist in filename. SeeTOUCHINESS & BLACKLISTING below.

-v

print summary information on sshguard and exit.

-l source

enable the Log Sucker, and add source to the list of log sources to monitor. source is a filename, a FIFO name, or the magic symbol "-" to identify sshguard's standard input. sshguard handles autonomously file-like sources disappearing, reappearing, or "rotating". This option can be used multiple times. When omitted, source defaults to standard input. Otherwise, standard input is ignored unless explicitly added.

-a sAfety_thresh

block an attacker after it incurred a total dangerousness exceeding sAfety_thresh. Most attacks incur dangerousness 10. See http://www.sshguard.net/docs/reference/attack-signatures/ for details. (Default: 40)

-p secs

release a blocked address not sooner than secs seconds after being blocked. sshguard will release the address between X and 3/2 * X seconds. (Default: 7*60)

-s secs

forget about an address after secs seconds. If host A issues one attack every this many seconds, it will never be blocked. (Default: 20*60)

-w addr/host/block/file

see the WHITELISTING section.

-f servicecode:pidfile

see the LOG MESSAGE AUTHENTICATION section.

When sshguard is signalled with SIGTSTP, it suspends activity. When sshguard is signalled with SIGCONT, it resumes monitoring. During suspension, log entries are discarded without being analyzed

When sshguard senses the SSHGUARD_DEBUG environment variable, it enables debugging mode: logging is directed to standard error instead of syslog, and includes comprehensive details of the activity and parsing process. Debugging mode can help investigating patterns: once enabled, a pattern can be directly pasted into the tool from the console, and the behavior is immediately and minutely shown beneath.

WHITELISTING

sshguard supports address whitelisting. Whitelisted addresses are not blocked even if they appear to generate attacks. This is useful for protecting lame LAN users (or external friendly users) from being incidentally blocked.

Whitelist addresses are controlled through the -w command-line option. This option can add explicit addresses, host names and address blocks:

addresses

specify the numeric IPv4 or IPv6 address directly, like:

-w 192.168.1.10

or in multiple occurrences:

-w 192.168.1.10 -w 2001:0db8:85a3:0000:0000:8a2e:0370:7334

host names

specify the host name directly, like:

-w friendhost.enterprise.com

or in multiple occurrences:

-w friendhost.enterprise.com -w friend2.enterprise.com

All IPv4 and IPv6 addresses that the host resolves to are whitelisted. Hosts are resolved to addresses once, when sshguard starts up.

address blocks

specify the IPv4 or IPv6 address block in the usual CIDR notation:

-w 2002:836b:4179::836b:0000/126

or in multiple occurrences:

-w 192.168.0.0/24 -w 1.2.3.128/26

file

When longer lists are needed for whitelisting, they can be wrapped into a plain text file, one address/hostname/block per line, with the same syntax given above.
sshguard can take whitelists from files when the -w option argument begins with a `.' (dot) or `/' (slash). This is a sample whitelist file (say /etc/friends):

# comment line (a '#' as very first character)
#   a single IPv4 and IPv6 address
1.2.3.4
2001:0db8:85a3:08d3:1319:8a2e:0370:7344
#   address blocks in CIDR notation
127.0.0.0/8
10.11.128.0/17
192.168.0.0/24
2002:836b:4179::836b:0000/126
#   hostnames
rome-fw.enterprise.com
hosts.friends.com

And this is how sshguard is told to make a whitelist up from the /etc/friends file:

sshguard -w /etc/friends

The -w option can be used only once for files. For addresses, host names and address blocks it can be used with any multiplicity, even with mixes of them.

LOG MESSAGE AUTHENTICATION

Syslog and syslog-ng typically insert a PID of the generating process in every log line. This can be checked for authenticating the source of the message and avoid false attacks to be detected because malicious local users inject crafted log lines. This way sshguard can be safely used even on hosts where this assumption does not hold.

Log message authentication is only needed when sshguard is fed log messages from syslog or from syslog-ng. When a process logs directly to a raw file and sshguard is configured for polling logs directly from it, you only need to adjust the log file permissions so that only root can write on it.

For enabling log message authentication on a given service the -f option is used as follows:

-f 100:/var/run/sshd.pid

which associates the given pidfile to the ssh service (code 100). A list of well-known service codes is available athttp://www.sshguard.net/docs/reference/service-codes/.

The -f option can be used multiple times for associating different services with their pidfile:

sshguard -f 100:/var/run/sshd.pid -f 123:/var/run/mydaemon.pid

Services that are not configured for log message authentication follow a default-allow policy (all of their log messages are accepted by default).

PIDs are checked with the following policy:

1. the logging service is searched in the list of services configured for authentication. If not found, the entry is accepted.
2. the logged PID is compared with the pidfile. If it matches, the entry is accepted
3. the PID is checked for being a direct child of the authoritative process. If it is, the entry is accepted.
4. the entry is ignored.

Low I/O load is committed to the operating system because of an internal caching mechanism. Changes in the pidfile value are handled transparently.

TOUCHINESS & BLACKLISTING

In many cases, attacks against services are performed in bulk in an automated form. For example, the attacker goes trough a dictionary of 150 username/password pairs and sequentially tries to violate the SSH service with any of them, continuing blindly while blocked, and re-appearing once the block expires.

To counteract these cases, sshguard by default behaves with touchiness. Besides observing abuses from the log activity, it monitors also the overall behavior of attackers. The decision on when and how to block is thus made respective to the entire history of the attacker as well. For example, if address A attacks repeatedly and the base blocking time is 420 seconds, A will be blocked for 420 seconds (7 mins) at the first abuse, 2*420 (14 mins) the second, 2*2*420 (28 mins) the third ... and 2^(n-1)*420 the n-th time.

Touchiness has two major benefits: to legitimate users, it grants forgiving blockings on failed logins; to real attackers, it effectively renders large scale attacks infeasible, because the time to perform it explodes with the number of attempts.

Touchiness can be augmented with blacklisting (-b). With this option, after a number of abuses, the address is added to a list of attackers to be blocked permanently. The list is intended to be loaded at each startup, and maintained/extended with new entries during operation. sshguard inserts a new address after it exceeded a threshold of abuses. This threshold is configurable within the -b option argument. Blacklisted addresses are never scheduled for releasing.

The -b command line option enables blacklisting and requires the filename to use for permanent storage of the blacklist. Optionally, a custom blacklist threshold can be prefixed to this path, separated by ':'. For example,

-b 5:/var/db/sshguard/blacklist.db

requires to blacklist addresses after the 5th abuse, and store the blacklist in file /var/db/sshguard/blacklist.db. Although the blacklist file is not meant to be in human-readable format, the strings(1) command can be used to peek in it for listing the blacklisted addresses.

EXTENSIONS

sshguard operates firewalls through a general interface, which enables easy extension, and allows back-ends to be non-local (e.g. remote appliances), and non-blocking (e.g. report tools). Additions can be suggested athttp://www.sshguard.net/feedback/firewall/submit/.

Extending attack signatures needs some expertise with context-free parsers; users are welcome to submit samples of the desired log messages to http://www.sshguard.net/support/attacks/submit/.

SEE ALSO

syslog(1), syslog.conf(5)

sshguard website at: http://www.sshguard.net/