Posts Tagged ‘syslog’

How to build Linux logging bash shell script write_log, logging with Named Pipe buffer, Simple Linux common log files logging with logger command

Monday, August 26th, 2019

how-to-build-bash-script-for-logging-buffer-named-pipes-basic-common-files-logging-with-logger-command

Logging into file in GNU / Linux and FreeBSD is as simple as simply redirecting the output, e.g.:
 

echo "$(date) Whatever" >> /home/hipo/log/output_file_log.txt


or with pyping to tee command

 

echo "$(date) Service has Crashed" | tee -a /home/hipo/log/output_file_log.txt


But what if you need to create a full featured logging bash robust shell script function that will run as a daemon continusly as a background process and will output
all content from itself to an external log file?
In below article, I've given example logging script in bash, as well as small example on how a specially crafted Named Pipe buffer can be used that will later store to a file of choice.
Finally I found it interesting to mention few words about logger command which can be used to log anything to many of the common / general Linux log files stored under /var/log/ – i.e. /var/log/syslog /var/log/user /var/log/daemon /var/log/mail etc.
 

1. Bash script function for logging write_log();


Perhaps the simplest method is just to use a small function routine in your shell script like this:
 

write_log()
LOG_FILE='/root/log.txt';
{
  while read text
  do
      LOGTIME=`date "+%Y-%m-%d %H:%M:%S"`
      # If log file is not defined, just echo the output
      if [ “$LOG_FILE” == “” ]; then
    echo $LOGTIME": $text";
      else
        LOG=$LOG_FILE.`date +%Y%m%d`
    touch $LOG
        if [ ! -f $LOG ]; then echo "ERROR!! Cannot create log file $LOG. Exiting."; exit 1; fi
    echo $LOGTIME": $text" | tee -a $LOG;
      fi
  done
}

 

  •  Using the script from within itself or from external to write out to defined log file

 

echo "Skipping to next copy" | write_log

 

2. Use Unix named pipes to pass data – Small intro on what is Unix Named Pipe.


Named Pipe –  a named pipe (also known as a FIFO (First In First Out) for its behavior) is an extension to the traditional pipe concept on Unix and Unix-like systems, and is one of the methods of inter-process communication (IPC). The concept is also found in OS/2 and Microsoft Windows, although the semantics differ substantially. A traditional pipe is "unnamed" and lasts only as long as the process. A named pipe, however, can last as long as the system is up, beyond the life of the process. It can be deleted if no longer used.
Usually a named pipe appears as a file, and generally processes attach to it for IPC.

 

Once named pipes were shortly explained for those who hear it for a first time, its time to say named pipe in unix / linux is created with mkfifo command, syntax is straight foward:
 

mkfifo /tmp/name-of-named-pipe


Some older Linux-es with older bash and older bash shell scripts were using mknod.
So idea behind logging script is to use a simple named pipe read input and use date command to log the exact time the command was executed, here is the script.

 

#!/bin/bash
named_pipe='/tmp/output-named-pipe';
output_named_log='
/tmp/output-named-log.txt ';

if [ -p $named_pipe ]; then
rm -f $named_pipe
fi
mkfifo $named_pipe

while true; do
read LINE <$named_pipe
echo $(date): "$LINE" >>/tmp/output-named-log.txt
done


To write out any other script output and get logged now, any of your output with a nice current date command generated output write out any output content to the loggin buffer like so:

 

echo 'Using Named pipes is so cool' > /tmp/output-named-pipe
echo 'Disk is full on a trigger' > /tmp/output-named-pipe

  • Getting the output with the date timestamp

# cat /tmp/output-named-log.txt
Mon Aug 26 15:21:29 EEST 2019: Using Named pipes is so cool
Mon Aug 26 15:21:54 EEST 2019: Disk is full on a trigger


If you wonder why it is better to use Named pipes for logging, they perform better (are generally quicker) than Unix sockets.

 

3. Logging files to system log files with logger

 

If you need to do a one time quick way to log any message of your choice with a standard Logging timestamp, take a look at logger (a part of bsdutils Linux package), and is a command which is used to enter messages into the system log, to use it simply invoke it with a message and it will log your specified output by default to /var/log/syslog common logfile

 

root@linux:/root# logger 'Here we go, logging'
root@linux:/root # tail -n 3 /var/log/syslog
Aug 26 15:41:01 localhost CRON[24490]: (root) CMD (chown qscand:qscand -R /var/run/clamav/ 2>&1 >/dev/null)
Aug 26 15:42:01 localhost CRON[24547]: (root) CMD (chown qscand:qscand -R /var/run/clamav/ 2>&1 >/dev/null)
Aug 26 15:42:20 localhost hipo: Here we go, logging

 

If you have took some time to read any of the init.d scripts on Debian / Fedora / RHEL / CentOS Linux etc. you will notice the logger logging facility is heavily used.

With logger you can print out message with different priorities (e.g. if you want to write an error message to mail.* logs), you can do so with:
 

 logger -i -p mail.err "Output of mail processing script"


To log a normal non-error (priority message) with logger to /var/log/mail.log system log.

 

 logger -i -p mail.notice "Output of mail processing script"


A whole list of supported facility named priority valid levels by logger (as taken of its current Linux manual) are as so:

 

FACILITIES AND LEVELS
       Valid facility names are:

              auth
              authpriv   for security information of a sensitive nature
              cron
              daemon
              ftp
              kern       cannot be generated from userspace process, automatically converted to user
              lpr
              mail
              news
              syslog
              user
              uucp
              local0
                to
              local7
              security   deprecated synonym for auth

       Valid level names are:

              emerg
              alert
              crit
              err
              warning
              notice
              info
              debug
              panic     deprecated synonym for emerg
              error     deprecated synonym for err
              warn      deprecated synonym for warning

       For the priority order and intended purposes of these facilities and levels, see syslog(3).

 


If you just want to log to Linux main log file (be it /var/log/syslog or /var/log/messages), depending on the Linux distribution, just type', even without any shell quoting:

 

logger 'The reason to reboot the server Currently was a System security Update

 

So what others is logger useful for?

 In addition to being a good diagnostic tool, you can use logger to test if all basic system logs with its respective priorities work as expected, this is especially
useful as I've seen on a Cloud Holsted OpenXEN based servers as a SAP consultant, that sometimes logging to basic log files stops to log for months or even years due to
syslog and syslog-ng problems hungs by other thirt party scripts and programs.
To test test all basic logging and priority on system logs as expected use the following logger-test-all-basic-log-logging-facilities.sh shell script.

 

#!/bin/bash
for i in {auth,auth-priv,cron,daemon,kern, \
lpr,mail,mark,news,syslog,user,uucp,local0 \
,local1,local2,local3,local4,local5,local6,local7}

do        
# (this is all one line!)

 

for k in {debug,info,notice,warning,err,crit,alert,emerg}
do

logger -p $i.$k "Test daemon message, facility $i priority $k"

done

done

Note that on different Linux distribution verions, the facility and priority names might differ so, if you get

logger: unknown facility name: {auth,auth-priv,cron,daemon,kern,lpr,mail,mark,news, \
syslog,user,uucp,local0,local1,local2,local3,local4, \
local5,local6,local7}

check and set the proper naming as described in logger man page.

 

4. Using a file descriptor that will output to a pre-set log file


Another way is to add the following code to the beginning of the script

#!/bin/bash
exec 3>&1 4>&2
trap 'exec 2>&4 1>&3' 0 1 2 3
exec 1>log.out 2>&1
# Everything below will go to the file 'log.out':

The code Explaned

  •     Saves file descriptors so they can be restored to whatever they were before redirection or used themselves to output to whatever they were before the following redirect.
    trap 'exec 2>&4 1>&3' 0 1 2 3
  •     Restore file descriptors for particular signals. Not generally necessary since they should be restored when the sub-shell exits.

          exec 1>log.out 2>&1

  •     Redirect stdout to file log.out then redirect stderr to stdout. Note that the order is important when you want them going to the same file. stdout must be redirected before stderr is redirected to stdout.

From then on, to see output on the console (maybe), you can simply redirect to &3. For example
,

echo "$(date) : Do print whatever you want logging to &3 file handler" >&3


I've initially found out about this very nice bash code from serverfault.com's post how can I fully log all bash script actions (but unfortunately on latest Debian 10 Buster Linux  that is prebundled with bash shell 5.0.3(1)-release the code doesn't behave exactly, well but still on older bash versions it works fine.

Sum it up


To shortlysummarize there is plenty of ways to do logging from a shell script logger command but using a function or a named pipe is the most classic. Sometimes if a script is supposed to write user or other script output to a a common file such as syslog, logger command can be used as it is present across most modern Linux distros.
If you have a better ways, please drop a common and I'll add it to this article.

 

Disabling PHP system(); and exec(); functions to raise up Apache security on Debian GNU / Linux

Wednesday, July 18th, 2012

Disabling PHP system(); and exec(); functions to raise up Apache security on Debian Gnu / Linux

At security critical hosts running Apache + PHP based sites it is recommended functions like:

system();
exec();shell_exec();.....

to be disabled. The reason is to mainly harden against script kiddies who might exploit your site/s and upload some shitty SK tool like PHP WebShell, PHP Shell and the probably thousands of “hacker” variations that exist nowdays.

In latest Debian stable Squeeze, suhosinadvanced protection module for php5 is being installed and enabled in Apache (by default).
Simply disabling a number of functions using suhosin, could prevent multiple of future headaches and hours of pondering on who 0wn3d your server ….

Disabling the basic PHP system(); and other similar functions which allows shell spawn is not always possible, since some websites or CMS platforms depends on them for proper runnig, anyways whether it is possible disabling ’em is a must.
There are two ways to disable system(); functions; One is through using /etc/php5/apache2/conf.d/suhosin.ini and 2nd by adding a list of functions that has to be disabled directly in Website Virtualhost file or in apache2.conf (/etc/apache2/apache2.conf;
For people hosting multiple virtualhost websites on the same server using the custom domain Virtualhost method is probably better, since on a global scale the functions could be enabled if some of the websites hosted on the server requires exec(); to work OK. In any case using /etc/php5/apache2/conf.d/suhosin.ini to disable system(); functions in PHP is less messy …

1. Disabling PHP system(); fuctions through /etc/apache2/apache2.conf and custom site Vhosts

Place somewhere (I prefer near the end of config);;;


php_admin_flag safe_mode on
php_admin_value disable_functions "system, exec, shell_exec, passthru , ini_alter, dl, pfsockopen, openlog, syslog, readlink, symlink, link, leak, fsockopen, popen, escapeshellcmd, apache_child_terminate apache_get_modules, apache_get_version, apache_getenv, apache_note,apache_setenv,virtual"

Disabling it for custom virtualhost is done by simply adding above Apache directvies (before the closing tag in /etc/apache2/sites-enabled/custom-vhost.com

2. Disabling PHP system();, exec(); shell spawn with suhosin.ini

In /etc/php5/apache2/conf.d/suhosin.ini add;;

suhosin.executor.func.blacklist =system, exec, shell_exec, passthru, ini_alter, dl,
pfsockopen, openlog, syslog, readlink, symlink, link, leak, fsockopen, popen,
escapeshellcmd, apache_child_terminate apache_get_modules, apache_get_version,
apache_getenv, apache_note,apache_setenv,virtual

To do it directly via shell issue;;;

server: conf.d/# cd /etc/php5/apache2/conf.d/
server: conf.d# echo 'suhosin.executor.func.blacklist =system, exec, shell_exec, passthru, ini_alter, dl,' >> suhosin.ini
server: conf.d# echo 'pfsockopen, openlog, syslog, readlink, symlink, link, leak, fsockopen, popen,' >> suhosin.ini
server: conf.d# echo escapeshellcmd, apache_child_terminate apache_get_modules, apache_get_version,' >> suhosin.ini
server: conf.d# echo 'apache_getenv, apache_note,apache_setenv,virtual' >> suhosin.ini

Then to re-load the memory loaded Apache libphp library an Apache restart is necessary;

server: conf.d# /etc/init.d/apache2 restart
Restarting web server: apache2 ... waiting .
server: conf.d#

Tadam, this should be quite a good security against annoying automated script attacks. Cheers 😉

Resolving “nf_conntrack: table full, dropping packet.” flood message in dmesg Linux kernel log

Wednesday, March 28th, 2012

nf_conntrack_table_full_dropping_packet
On many busy servers, you might encounter in /var/log/syslog or dmesg kernel log messages like

nf_conntrack: table full, dropping packet

to appear repeatingly:

[1737157.057528] nf_conntrack: table full, dropping packet.
[1737157.160357] nf_conntrack: table full, dropping packet.
[1737157.260534] nf_conntrack: table full, dropping packet.
[1737157.361837] nf_conntrack: table full, dropping packet.
[1737157.462305] nf_conntrack: table full, dropping packet.
[1737157.564270] nf_conntrack: table full, dropping packet.
[1737157.666836] nf_conntrack: table full, dropping packet.
[1737157.767348] nf_conntrack: table full, dropping packet.
[1737157.868338] nf_conntrack: table full, dropping packet.
[1737157.969828] nf_conntrack: table full, dropping packet.
[1737157.969928] nf_conntrack: table full, dropping packet
[1737157.989828] nf_conntrack: table full, dropping packet
[1737162.214084] __ratelimit: 83 callbacks suppressed

There are two type of servers, I've encountered this message on:

1. Xen OpenVZ / VPS (Virtual Private Servers)
2. ISPs – Internet Providers with heavy traffic NAT network routers
 

I. What is the meaning of nf_conntrack: table full dropping packet error message

In short, this message is received because the nf_conntrack kernel maximum number assigned value gets reached.
The common reason for that is a heavy traffic passing by the server or very often a DoS or DDoS (Distributed Denial of Service) attack. Sometimes encountering the err is a result of a bad server planning (incorrect data about expected traffic load by a company/companeis) or simply a sys admin error…

– Checking the current maximum nf_conntrack value assigned on host:

linux:~# cat /proc/sys/net/ipv4/netfilter/ip_conntrack_max
65536

– Alternative way to check the current kernel values for nf_conntrack is through:

linux:~# /sbin/sysctl -a|grep -i nf_conntrack_max
error: permission denied on key 'net.ipv4.route.flush'
net.netfilter.nf_conntrack_max = 65536
error: permission denied on key 'net.ipv6.route.flush'
net.nf_conntrack_max = 65536

– Check the current sysctl nf_conntrack active connections

To check present connection tracking opened on a system:

:

linux:~# /sbin/sysctl net.netfilter.nf_conntrack_count
net.netfilter.nf_conntrack_count = 12742

The shown connections are assigned dynamicly on each new succesful TCP / IP NAT-ted connection. Btw, on a systems that work normally without the dmesg log being flooded with the message, the output of lsmod is:

linux:~# /sbin/lsmod | egrep 'ip_tables|conntrack'
ip_tables 9899 1 iptable_filter
x_tables 14175 1 ip_tables

On servers which are encountering nf_conntrack: table full, dropping packet error, you can see, when issuing lsmod, extra modules related to nf_conntrack are shown as loaded:

linux:~# /sbin/lsmod | egrep 'ip_tables|conntrack'
nf_conntrack_ipv4 10346 3 iptable_nat,nf_nat
nf_conntrack 60975 4 ipt_MASQUERADE,iptable_nat,nf_nat,nf_conntrack_ipv4
nf_defrag_ipv4 1073 1 nf_conntrack_ipv4
ip_tables 9899 2 iptable_nat,iptable_filter
x_tables 14175 3 ipt_MASQUERADE,iptable_nat,ip_tables

 

II. Remove completely nf_conntrack support if it is not really necessery

It is a good practice to limit or try to omit completely use of any iptables NAT rules to prevent yourself from ending with flooding your kernel log with the messages and respectively stop your system from dropping connections.

Another option is to completely remove any modules related to nf_conntrack, iptables_nat and nf_nat.
To remove nf_conntrack support from the Linux kernel, if for instance the system is not used for Network Address Translation use:

/sbin/rmmod iptable_nat
/sbin/rmmod ipt_MASQUERADE
/sbin/rmmod rmmod nf_nat
/sbin/rmmod rmmod nf_conntrack_ipv4
/sbin/rmmod nf_conntrack
/sbin/rmmod nf_defrag_ipv4

Once the modules are removed, be sure to not use iptables -t nat .. rules. Even attempt to list, if there are any NAT related rules with iptables -t nat -L -n will force the kernel to load the nf_conntrack modules again.

Btw nf_conntrack: table full, dropping packet. message is observable across all GNU / Linux distributions, so this is not some kind of local distribution bug or Linux kernel (distro) customization.
 

III. Fixing the nf_conntrack … dropping packets error

– One temporary, fix if you need to keep your iptables NAT rules is:

linux:~# sysctl -w net.netfilter.nf_conntrack_max=131072

I say temporary, because raising the nf_conntrack_max doesn't guarantee, things will get smoothly from now on.
However on many not so heavily traffic loaded servers just raising the net.netfilter.nf_conntrack_max=131072 to a high enough value will be enough to resolve the hassle.

– Increasing the size of nf_conntrack hash-table

The Hash table hashsize value, which stores lists of conntrack-entries should be increased propertionally, whenever net.netfilter.nf_conntrack_max is raised.

linux:~# echo 32768 > /sys/module/nf_conntrack/parameters/hashsize
The rule to calculate the right value to set is:
hashsize = nf_conntrack_max / 4

– To permanently store the made changes ;a) put into /etc/sysctl.conf:

linux:~# echo 'net.netfilter.nf_conntrack_count = 131072' >> /etc/sysctl.conf
linux:~# /sbin/sysct -p

b) put in /etc/rc.local (before the exit 0 line):

echo 32768 > /sys/module/nf_conntrack/parameters/hashsize

Note: Be careful with this variable, according to my experience raising it to too high value (especially on XEN patched kernels) could freeze the system.
Also raising the value to a too high number can freeze a regular Linux server running on old hardware.

– For the diagnosis of nf_conntrack stuff there is ;

/proc/sys/net/netfilter kernel memory stored directory. There you can find some values dynamically stored which gives info concerning nf_conntrack operations in "real time":

linux:~# cd /proc/sys/net/netfilter
linux:/proc/sys/net/netfilter# ls -al nf_log/

total 0
dr-xr-xr-x 0 root root 0 Mar 23 23:02 ./
dr-xr-xr-x 0 root root 0 Mar 23 23:02 ../
-rw-r--r-- 1 root root 0 Mar 23 23:02 0
-rw-r--r-- 1 root root 0 Mar 23 23:02 1
-rw-r--r-- 1 root root 0 Mar 23 23:02 10
-rw-r--r-- 1 root root 0 Mar 23 23:02 11
-rw-r--r-- 1 root root 0 Mar 23 23:02 12
-rw-r--r-- 1 root root 0 Mar 23 23:02 2
-rw-r--r-- 1 root root 0 Mar 23 23:02 3
-rw-r--r-- 1 root root 0 Mar 23 23:02 4
-rw-r--r-- 1 root root 0 Mar 23 23:02 5
-rw-r--r-- 1 root root 0 Mar 23 23:02 6
-rw-r--r-- 1 root root 0 Mar 23 23:02 7
-rw-r--r-- 1 root root 0 Mar 23 23:02 8
-rw-r--r-- 1 root root 0 Mar 23 23:02 9

 

IV. Decreasing other nf_conntrack NAT time-out values to prevent server against DoS attacks

Generally, the default value for nf_conntrack_* time-outs are (unnecessery) large.
Therefore, for large flows of traffic even if you increase nf_conntrack_max, still shorty you can get a nf_conntrack overflow table resulting in dropping server connections. To make this not happen, check and decrease the other nf_conntrack timeout connection tracking values:

linux:~# sysctl -a | grep conntrack | grep timeout
net.netfilter.nf_conntrack_generic_timeout = 600
net.netfilter.nf_conntrack_tcp_timeout_syn_sent = 120
net.netfilter.nf_conntrack_tcp_timeout_syn_recv = 60
net.netfilter.nf_conntrack_tcp_timeout_established = 432000
net.netfilter.nf_conntrack_tcp_timeout_fin_wait = 120
net.netfilter.nf_conntrack_tcp_timeout_close_wait = 60
net.netfilter.nf_conntrack_tcp_timeout_last_ack = 30
net.netfilter.nf_conntrack_tcp_timeout_time_wait = 120
net.netfilter.nf_conntrack_tcp_timeout_close = 10
net.netfilter.nf_conntrack_tcp_timeout_max_retrans = 300
net.netfilter.nf_conntrack_tcp_timeout_unacknowledged = 300
net.netfilter.nf_conntrack_udp_timeout = 30
net.netfilter.nf_conntrack_udp_timeout_stream = 180
net.netfilter.nf_conntrack_icmp_timeout = 30
net.netfilter.nf_conntrack_events_retry_timeout = 15
net.ipv4.netfilter.ip_conntrack_generic_timeout = 600
net.ipv4.netfilter.ip_conntrack_tcp_timeout_syn_sent = 120
net.ipv4.netfilter.ip_conntrack_tcp_timeout_syn_sent2 = 120
net.ipv4.netfilter.ip_conntrack_tcp_timeout_syn_recv = 60
net.ipv4.netfilter.ip_conntrack_tcp_timeout_established = 432000
net.ipv4.netfilter.ip_conntrack_tcp_timeout_fin_wait = 120
net.ipv4.netfilter.ip_conntrack_tcp_timeout_close_wait = 60
net.ipv4.netfilter.ip_conntrack_tcp_timeout_last_ack = 30
net.ipv4.netfilter.ip_conntrack_tcp_timeout_time_wait = 120
net.ipv4.netfilter.ip_conntrack_tcp_timeout_close = 10
net.ipv4.netfilter.ip_conntrack_tcp_timeout_max_retrans = 300
net.ipv4.netfilter.ip_conntrack_udp_timeout = 30
net.ipv4.netfilter.ip_conntrack_udp_timeout_stream = 180
net.ipv4.netfilter.ip_conntrack_icmp_timeout = 30

All the timeouts are in seconds. net.netfilter.nf_conntrack_generic_timeout as you see is quite high – 600 secs = (10 minutes).
This kind of value means any NAT-ted connection not responding can stay hanging for 10 minutes!

The value net.netfilter.nf_conntrack_tcp_timeout_established = 432000 is quite high too (5 days!)
If this values, are not lowered the server will be an easy target for anyone who would like to flood it with excessive connections, once this happens the server will quick reach even the raised up value for net.nf_conntrack_max and the initial connection dropping will re-occur again …

With all said, to prevent the server from malicious users, situated behind the NAT plaguing you with Denial of Service attacks:

Lower net.ipv4.netfilter.ip_conntrack_generic_timeout to 60 – 120 seconds and net.ipv4.netfilter.ip_conntrack_tcp_timeout_established to stmh. like 54000

linux:~# sysctl -w net.ipv4.netfilter.ip_conntrack_generic_timeout = 120
linux:~# sysctl -w net.ipv4.netfilter.ip_conntrack_tcp_timeout_established = 54000

This timeout should work fine on the router without creating interruptions for regular NAT users. After changing the values and monitoring for at least few days make the changes permanent by adding them to /etc/sysctl.conf

linux:~# echo 'net.ipv4.netfilter.ip_conntrack_generic_timeout = 120' >> /etc/sysctl.conf
linux:~# echo 'net.ipv4.netfilter.ip_conntrack_tcp_timeout_established = 54000' >> /etc/sysctl.conf