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Charles Atkinson, 19/11/2020 07:56
Fixed missing bolding. Added MAC address regex snippet

Logcheck¶

Introduction¶

This page documents logcheck and how to extend it with local filter files.

How does logcheck work?¶

This description applies to a default installation on Debian. The default configuration may be different for other distros. The configuration may have been changed since installation.

Scheduled job and disabling¶

Logcheck is run via /etc/cron.d/logcheck. /etc/cron.d/logcheck runs logcheck at reboot and at 2 minutes past every hour.

Logcheck's scheduled job can be disabled, for example by:

mv /etc/cron.d/logcheck{,.disabled}

Selecting messages to mail¶

Logcheck reads each new line (message) from /var/log/syslog and /var/log/auth.log. For each message:

If the message matches a regular expression in one of the files in /etc/logcheck/cracking.d, it is selected for the ATTACK ALERT section of the email and processing continues with the next message.
If the message matches a regular expression in one of the files in /etc/logcheck/violations.d then:
1. If it also matches a regular expression in one of the files in /etc/logcheck/violations.ignore.d it is ignored and processing continues with the next message.
2. Otherwise the message is selected for the SECURITY EVENTS section of the email and processing continues with the next message
If the message matches a regular expression in one of the files in /etc/logcheck/ignore.d.server then:
1. The message is ignored and processing continues with the next message.
2. Otherwise the message is selected for the SYSTEM EVENTS section of the email.

Note: in the first two steps, a match selects the message. In the last step a match discards the message.

If logcheck doesn't select any messages for the email, the email is not sent.

Mailing¶

The subject of the mail is prefixed with "Reboot: " when logcheck is run at boot.

Mail is sent to logcheck.

The "from" address is "logcheck system account".

Terminology and naming¶

filters, patterns and rules The logcheck documentation uses "filter", "pattern" and "rule" interchangeably, applying them to directories, files and individual regular expressions. On this WIKI page, only "filter" is used. It is used only to mean a single filter (a line in a file). On this page, a file of filters is called a "filter file".
server and workstation are filtering levels, not computer roles. From README.logcheck:
"ignore.d.server; as the name implies, this is intended to cut out the routine messages ..."
"ignore.d.workstation. "... is only appropriate for relatively sheltered, non-critical machines"
Filter file names The filter file for generic messages is called "logcheck". The rest of the filter files are named after a Debian package and contain filters for messages generated by software in the package.

Emails sent by logcheck¶

Unwanted messages under ATTACK ALERT¶

If the messages should be filtered out:

Enable /etc/logcheck/cracking.ignore.d by setting SUPPORT_CRACKING_IGNORE to 1 in logcheck.conf
Develop one or more filters for the messages
Put the filter(s) in a filter file in /etc/logcheck/cracking.ignore.d

Note: the messages are discarded and do not appear anywhere in the email.

Unwanted messages under SECURITY EVENTS¶

If the messages should be filtered out:

Develop one or more filters for the messages
Put the filter(s) in a filter file in /etc/logcheck/violations.ignore.d

Note: the messages are discarded and do not appear anywhere in the email.

Unwanted messages under SYSTEM EVENTS¶

This section applies in the most common case when unwanted messages appear in the SYSTEM EVENTS section of the logcheck emails.

The first task is to decide whether the unwanted messages should be filtered out:

If a message shows there's an issue to be resolved, filtering the message out would hide essential information.
If a message is generated rarely, it is not worth the work of filtering it out. Server boot messages are a good example.
Often it is not clear from the message itself whether it shows there's an issue to be be resolved or not. Research work is required.

If the messages should be filtered out, the next task is to develop one or more regular expressions (filters) that match the messages.

For a single filter, when it works and assuming logcheck has the default configuration, the next step is to put it in a file in /etc/logcheck/ignore.d.server – either in an existing file or a new one.

Unwanted boot messages¶

Typically logcheck sends messages associated with booting the first time it runs after reboot.

This can be fixed by modifying /etc/cron.d/logcheck, inserting a sleep that delays logcheck until the boot sequence has finished:

@reboot logcheck if [ -x /usr/sbin/logcheck ]; then sleep 10; nice -n10 /usr/sbin/logcheck -R; fi

Explanation: during boot, logcheck runs when the cron daemon is started. This is normally part way through the boot sequence. Any messages already in the logs are sent in the mail with subject "Reboot: ...". This is done because it is not practicable to have logcheck filters for all the messages generated during boot. Any later messages are sent in the next mail, subject to the usual filtering.

Missing messages¶

This section applies when messages appear in the logs but do not appear in the emails.

Is the log with the message a log that logcheck is searching? logcheck searches the logs configured in /etc/logcheck/logcheck.logfiles (default /var/log/syslog and /var/log/auth.log) or whichever file is nominated by logcheck's -L option.

Is the message matched by a filter in an ATTACK ALERT or SECURITY EVENT ignore filter file?

If the log is being searched and the message is not matched by an ignore filter file, the message must be matched by a SYSTEM EVENTS filter. In a default installation these are in filter files in the /etc/logcheck/server.d directory, otherwise they are in the /etc/logcheck/<report level>.d directory where <report level> is the value of REPORTLEVEL in /etc/logcheck.conf.

Disabling all logcheck emails¶

Disable the cron job. For example:

mv /etc/cron.d/logcheck{,.disabled}

Custom filters¶

"Custom filters" is used here to mean ones not installed by Debian packages. They might come from third parties or later versions of logcheck. They might be developed locally.

Filters from third parties¶

Sometimes software includes filter files, for example snoopy, provides filters here.

There are some published filter file libraries to extend the logcheck standard filters, for example:

Ties de Kock's at https://github.com/ties/logcheck-extrarules/tree/master/ignore.d.server
sdeziel.info's at https://sdeziel.info/local-logcheck/ignore.d.server/
Blue Light's available by running git clone git://git.bluelightav.org/logcheck

When considering third party filters, be aware that they may be designed to suit particular local conditions so not as widely suitable as filters from logcheck and other packages. For example, filter files may have been designed to suit low reliability Internet connections; in other locations, they would filter out messages indicating an unusual event requiring attention.

Backports¶

Sometimes required filters can be found in filter files in packages from the backports reopsitories.

Developing a filter¶

Find a filter for a similar message¶

For example, suppose logcheck was mailing something like

May 23 10:08:33 LS1 nmbd[1199]: *****

The part in green is usually the name of a daemon or command. In some cases it is more. For example ovpn-client uses ovpn-client.<configuration file name>.

Find filter files for similar messages by, for example

cd /etc/logcheck/ignore.d.server && grep --files-with-matches ovpn-client *

Choose one of the filters in the file(s) listed as the basis for developing a new one. If no files are listed, choose any filter; almost all messages match the message format above up to the last ":" except for the part in green in the example.

Structure of a typical filter¶

A filter is an egrep regular expression, for example:

^\w{3} [ :[:digit:]]{11} [._[:alnum:]-]+ (openvpn|ovpn-[._[:alnum:]-]+)\[[[:digit:]]+\]:( ([-_.@[:alnum:]]+/)?[.[:digit:]]{7,15}:[[:digit:]]{2,5})? Replay-window backtrack occurred \[[[:digit:]]+\]$

Notes:

To ensure a complete match, filters begin with ^ and end with $

Match from start of the line through the time stamp and server name:
```
^\w{3} [ :[:digit:]]{11} [._[:alnum:]-]+
```

Match the the daemon or command name:
```
(openvpn|ovpn-[._[:alnum:]-]+)
```
Match the PID:
```
\[[[:digit:]]+\]:
```
The remaining regular expression, up to but not including the $ is specific to the specific message(s) to be matched.

Design considerations¶

When developing a filter, you can usually use parts 1 to 4 above from a similar filter and develop part 5. It should be designed to match the messages you want to filter in or out – and only those messages.

If your regex is too general it may filter messages that show there's an issue to be resolved. For example ...

^\w{3} [ :[:digit:]]{11} [._[:alnum:]-]+ (openvpn|ovpn-[._[:alnum:]-]+)\[[[:digit:]]+\]:.*$

... would filter out all openvpn log messages.

If your regex is too specific you may need several filters to address closely related messages. For example ... [TODO: add example]

If you are not familiar with egrep regular expressions, it may be worth studying some existing logcheck filters alongside samples of the messages they filter to see how they work.

Regex snippets¶

Day of week (three letter, English) (Mon|Tue|Wed|Thu|Fri|Sat|Sun)
Disk device name (with or without /dev/ and partition number) (/dev/)?(sd[[:lower:]]+[[:digit:]]*)|(dm-[[:digit:]]+)
Domain name [A-Za-z0-9.-]+\.[A-Za-z]{2,4}
Email address [A-Za-z0-9._%+-]+[A-Za-z0-9.-]+\.[A-Za-z]{2,4}@
Hostname (with or without domain name) [-[:alnum:]]+(\.[-[:alnum:]]+)*
Rationale: adequate and simpler/faster than the accurate regex for a hostname with domain name ([a-zA-Z0-9]([a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?\.)+[a-zA-Z]{2,6}
hh:mm:ss [:[:digit:]]{8}
Rationale: adequate and simpler/faster than the accurate regex
IPv4 address [.0-9]{7,15}
The above is conventional.
More precise: [[:digit:]]+(.[[:digit:]]+){3}
Precise: \b(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\b
IPv4 port number [[:digit:]]{1,5}
Rationale: adequate and simpler/faster than matching 0 to 65535.
IPv6 address [[:xdigit:]]*:*[:[:xdigit:]]*:+[[:xdigit:]]+
Rationale: matches both full IPV6 address and the minimal loopback address ::1. The flexibility of valid IPV6 address representations makes a more precise regex impossible.
MAC address [[:xdigit:]:]+
Month of year (three letter, English) (Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)
Network interface name (eth|lo|tap|tun)[[:digit:].]+
Persistent network interface name e(n(p[[:digit:]]+)?(o|s)[:digit:]]+)|x[[:xdigit:]]+
TODO: extend for virtual and VLAN interfaces
Network interface chip name (not complete, simplistic) (ATL1C|ATL1E|atl1c|atl2|bcm5700|bcmemac|bnx2|bnc2x|e100|e1000|e1000e|netxen_nic|uli526x)
User name [^ ]+
Rationale: although there are conventions for user names, they are not universally enforced and a wide variety of names is possible.
Time zone abbreviations (three letter) [[:upper:]]{3}
Rationale: adequate and simpler, faster and easier to maintain than matching valid time zone abbreviations.

Testing and debugging¶

Filters are tested by seeing if they match sample message(s).

Finding sample messages¶

The first step is to find which log the sample message(s) are in. In a default logcheck installation this will be either /var/log/syslog or /var/log/auth.log. /var/log/syslog is assumed in the examples below.

If /var/log/syslog has been rotated since logcheck found the message(s), they will be in syslog.1 or syslog.*.gz. They can all be searched using:

zgrep <your string> /var/log/syslog*

For the log file names only:

zgrep --files-with-matches <your string> /var/log/syslog*

Running tests¶

Having the filter(s) in a file in an editor is useful. Then it's easy to change, redo and undo between tests. Assuming the filter(s) to be tested are in /tmp/my_filter and the messages are in /var/log/syslog, they can be tested by:

sed -e 's/[[:space:]]*$//' /var/log/syslog | egrep --file /tmp/my_filter

Note: the sed -e 's/[[:space:]]*$//' emulates logcheck internals which strip trailing spaces from messages.

When the messages are in a compressed log:

zcat /var/log/syslog.3.gz | sed -e 's/[[:space:]]*$//' | egrep --file /tmp/my_filter

When the messages are in compressed and uncompressed logs:

    zgrep --no-filename . <list of log files> | sed -e 's/[[:space:]]*$//' \
        | egrep --file /tmp/my_filter

If the filter does not work these can help:

Progressively right-truncate the regex until it does match. Then the last part discarded contains the error.
Use egrep's --only-matching (-o) option to see what the regex is actually matching.

Installing filters in a filter file¶

The directory for the filter file is as listed in "Unwanted messages under ATTACK ALERT in the emails", "Unwanted messages under SECURITY EVENTS in the emails" or "Unwanted messages under SYSTEM EVENTS in the emails" above.

Filter file names must be made of upper and lower case letters, digits, underscores, and hyphens. This is because logcheck uses run-parts --list to select the filter files.

Choosing the file name¶

Custom filters can most easily be kept in local-<something> filter files. The alternatives are harder to administer:

Changing the files installed by the logcheck package means those files will not be upgraded when logcheck is upgraded.
A single file for all local filters does not allow installing a local filter file at the same time as a package is installed or upgraded (the format of the log messages may change with the upgrade).
Having multiple local filter files is helpful when a filter has accidentally been added which matches too many messages; individual files can be disabled until the file causing the problem is identified (a powerful technique when the first step is to to disable half the files, and so on in a "binary search").
If the filters are for messages generated by software not installed by a package (such as locally developed software or software installed from source), it is convenient to have a local-<something> file to install with the software in the same way that it might have files for /etc/cron.daily or /etc/logrotate.d. The local- prefix ensures such a file will not be clobbered by a package that happens to have the same name.
If the local filter extends the filters in an as-installed <package name> filter file, the relationship can be made clear by putting it in local-<same package name>.

If the package name is not obvious from the message, it can usually be found by finding which file corresponds to the message and which package the file comes from. For example, for message Jul 12 09:19:55 LS1 named[1329]: listening on IPv4 interface tun0, 10.8.0.1#53 the package is bind9:

# type named
named is /usr/sbin/named
# dpkg -S /usr/sbin/named
bind9: /usr/sbin/named

In case a standard set of local-* filter files are installed on multiple computers:

The processing load can be reduced by installing them with extension .disabled and only removing it when needed.
Host-specific filter files can have prefix local-local-

Filter file backups¶

If updating an existing file, the original can be backed up with a name which is not entirely of upper and lower case letters, digits, underscores, and hyphens; for example local-openvpn.bak. These files will not be used by logcheck.

Filter file contents¶

Filter files may have comment lines (beginning with #) and empty lines (containing only none or more spaces and tabs). These are ignored by logcheck.

Filter files can be sorted to help identify duplicate and similar filters. If doing so, using sort's --version-sort (-V) option may avoid surprises.

Example filter development session¶

The message was Sep 16 17:33:31 rose gnome-keyring-prompt: could not grab keyboard: already grabbed

Found the package:

# dpkg -L gnome-keyring | grep gnome-keyring-prompt
/usr/share/applications/gnome-keyring-prompt.desktop
/usr/lib/gnome-keyring/gnome-keyring-prompt-3
/usr/lib/gnome-keyring/gnome-keyring-prompt

Found no existing filters:

# grep gnome-keyring-prompt /etc/logcheck/ignore.d.server/*
[no output]

Found the log with the message:

# grep 'could not grab keyboard' /var/log/syslog
# grep 'could not grab keyboard' /var/log/syslog.1
# grep 'could not grab keyboard' /var/log/auth.log
Sep 16 17:33:31 rose gnome-keyring-prompt: could not grab keyboard: already grabbed

Developed a filter by copying an existing filter and modifying it. Tested it:

# cd /etc/logcheck/ignore.d.server && egrep  --file /tmp/my_filter /var/log/auth.log
Sep 16 17:33:31 rose gnome-keyring-prompt: could not grab keyboard: already grabbed

Added filter to existing local filter file local-gnome-keyring and sorted the content.

Gotchas¶

If a valid filter in a filter file in /etc/logcheck/ignore.d.server filter does not work:

Does the message match a filter in a filter file in /etc/logcheck/cracking.d or /etc/logcheck/violations.d? Note: if this is the case, the unfiltered message would not be in the SYSTEM EVENTS section of the emails.

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