SSHGUARD(8) - Linux man page online | Administration and privileged commands

Block brute-force attacks by aggregating system logs.

October 11, 2016


sshguard - block brute-force attacks by aggregating system logs


sshguard [-v] [-a thresh] [-b thresh:file] [-f service:pidfile] [-i pidfile] [-l source] [-p interval] [-s interval] [-w address | file]


sshguard protects hosts from brute-force attacks against SSH and other services. It aggre‐ gates system logs and blocks repeat offenders using one of several firewall backends, including iptables, ipfw, and pf. sshguard can read log messages from standard input (suitable for piping from syslog) or monitor one or more log files. Log messages are parsed, line-by-line, for recognized pat‐ terns. If an attack, such as several login failures within a few seconds, is detected, the offending IP is blocked. Offenders are unblocked after a set interval, but can be semi-permanently banned using the blacklist option. See for setup instructions. Other features, attack signatures, and additional documentation can be found at


-a thresh (default 30) Block an attacker when its dangerousness exceeds thresh. Each attack pattern that is matched contributes a fixed dangerousness of 10. -b thresh:file Blacklist an attacker when its dangerousness exceeds thresh. Blacklisted addresses are added to file so they can be read at the next startup. Blacklisted addresses are never automatically unblocked, but it is good practice to periodically clean out stale blacklist entries. -f service:pidfile Deprecated. See LOG VALIDATION below. -i pidfile Write the PID of sshguard to pidfile. -l source Monitor source for log messages. By default, sshguard reads log messages from stan‐ dard input. Give this option once for every source to monitor instead. sshguard transparently handles log rotations. When using this option, standard input is ignored, but can be re-added by giving '-l -'. -p interval (default 120 secs, or 2 minutes) Wait at least interval seconds before releasing a blocked address. Repeat attack‐ ers are blocked for 1.5 times longer after each attack. Because sshguard unblocks attackers only at infrequent intervals, this parameter is inexact (actual blocks will be longer). -s interval (default 1800 secs, or 30 minutes) Forget about an attacker interval seconds after its last attempt. Its dangerousness will be reset to zero. -w address | file Whitelist the given address, hostname, or address block. Alternatively, read whitelist entires from file. This option can be given multiple times. See WHITELISTING below for details. -v Print version information and exit.


SSHGUARD_DEBUG Enable additional debugging information.


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 or in multiple occurrences: -w -w 2001:0db8:85a3:0000:0000:8a2e:0370:7334 host names specify the host name directly, like: -w or in multiple occurrences: -w -w 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 -w 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 2001:0db8:85a3:08d3:1319:8a2e:0370:7344 # address blocks in CIDR notation 2002:836b:4179::836b:0000/126 # hostnames 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.


Syslog and syslog-ng typically insert a PID of the generating process in every log mes‐ sage. 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 sys‐ log-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: -f 100:/var/run/ which associates the given pidfile to the ssh service (code 100). A list of well-known service codes is available at The -f option can be used multiple times for associating different services with their pidfile: sshguard -f 100:/var/run/ -f 123:/var/run/ 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 mecha‐ nism. Changes in the pidfile value are handled transparently.


syslog(1), syslog.conf(5), hosts_access(5) Glossary: Website:


Michele Mazzucchi <>, Kevin Zheng <>
1.7.1 October 11, 2016 SSHGUARD(8)
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