NAME¶

sshguard — monitors daemon activity

SYNOPSIS¶

sshguard

[-b thr:filename] [-v] [-l source] [-a sAfety_thresh] [-p pardon_min_interval] [-s preScribe_interval] [-w addr/host/block/file] [-f srv:pidfile]

DESCRIPTION¶

sshguard monitors logging activity and reacts to attacks by blocking their source addresses. sshguard was born for protecting SSH servers from the today's widespread brute force attacks, and evolved to an extensible log supervisor for blocking attacks to applications in real-time. sshguard can monitor a number of log sources proactively, or receive log messages in its standard input. By means of a parser, it decides whether an entry is normal activity or attack; in the latter case, it remarks the attacker's address. When sshguard determines that an attacker committed enough danger to the system to discern an abuse, it blocks the attacker's address with the firewall. The attacker becomes offender, and is released after an adequate period of time. sshguard supports the following firewalls:

AIX native firewall

for IBM AIX operating systems

netfilter/iptables

for Linux-based operating systems

Packet Filter (PF)

for BSD operating systems (Open, Free, Net, DragonFly-BSD)

IPFirewall (IPFW)

for FreeBSD and Mac OS X

IP Filter (IPFILTER)

for FreeBSD, NetBSD and Solaris

tcpd's hosts_access (/etc/hosts.allow)

portable across UNIX

null

portable do-nothing backend for running detection without prevention

Some terms in this manual have a special meaning in the context. sshguard refers to them consistently throughout documentation, source code, and log messages. See http://www.sshguard.net/docs/terminology/ to get acquainted with them.

USAGE¶

sshguard reads log entries to analyze by monitoring a number of log sources. It can interpret logs with all of the following formats:

• syslog
• syslog-ng
• metalog
• multilog
• raw messages

sshguard can interface with the following blocking systems to block attackers:

• IBM AIX firewall
• PF
• netfilter/iptables
• IPFW
• IP Filter
• /etc/hosts.allow
• null firewall

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:

-v

print summary information on sshguard and exit.

-i pidfile

store my PID in file pidfile at startup (for control scripts).

-b [thresh:]filename

enable blacklisting: blacklist after thresh (or 40) dangerousness committed, and hold the permanent blacklist in filename. See TOUCHINESS & BLACKLISTING below.

-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 no sooner than secs seconds after being blocked for the first time. sshguard will release the address between X and 3/2 * X seconds after blocking it. (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 VALIDATION 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 attack signatures: once enabled, a log message 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 VALIDATION¶

Syslog and syslog-ng typically insert a PID of the generating process in every log message. 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 messages. This way sshguard can be safely used even on hosts where this assumption does not hold. Log validation 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 validation on a given service the -f option is used as follows: which associates the given pidfile to the ssh service (code 100). A list of well-known service codes is available at http://www.sshguard.net/docs/reference/service-codes/. The -f option can be used multiple times for associating different services with their pidfile: Services that are not configured for log validation 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 validation. 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 1500 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 also monitors the overall behavior of attackers. The decision on when and how to block is thus made respective to the entire history of the offender 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 one explodes with the number of attempts. Touchiness can be augmented with blacklisting (-b). With this option, after a certain total danger committed, the address is added to a list of offenders 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 danger committed over recorded history. 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, requires to blacklist addresses after having committed attacks for danger 50 (default per-attack danger is 10), 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 at http://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/