Linux+ 102

User account information

account:password:UID:GID:Gecos:HomeDir:shell

Example: guru:x:464:464:Test account:/home/guru:/bin/bash

Group account information

group:password:GID:user1,user2,user3,etc.

example: Staff:x:200:bob,joe,sally

• Encrypted passwords are stored in root user read only files. Also controls advanced password parameters such as password aging.
• Structure:
• login name
• encrypted password
• date, in days since epoch, of last change
• number of days until change allowed
• number of days until change required
• number of days prior to expiration to begin warning
• number of days after expiration before account disabled
• date, in days since epoch, that password expires

• New users can be added to the system by the root user. useradd is controlled by:
• command line options
• /etc/login.defs config file
• /etc/default/useradd config file

deleting existing users

bulk import of new users

opens default EDITOR for editing

• /etc/passwd
• /etc/group

• usermod -L
• passwd -l

used to change user's GECOS/finger information

creating new groups

deleting existing groups

append group additions

7th field of /etc/passwd

• /bin/false -- simply exits with a return code 1
• /bin/nologin -- exits with message "This account is currently not available." or the contents of /etc/nologin.txt if it is present.

When useradd is used the contents of this directory are copied of the the new user's home directory.

• like su but adds:
• ability to log commands run
• ability to give more granular access to privileged commands on a user-by-user basis

user must be in the /etc/sudoers file to use the command.

Command provides control over the consumption of resources available to the shell and to processes started by it.

• -a -- All current limits are reported
• -c -- the max size of core files created
• -d -- the max size of a process's data segment
• -f -- the max size of files created by the shell
• -l -- the max size that may be locked into memory
• -m -- the max resident set size
• -n -- the max number of open file descriptors
• -s -- the max stack size
• -t -- the max amount of cpu time in seconds
• -u -- max # of processes available to a user
• -v -- max amount of virtual memory available to shell

system wide sh: configuration files

per user sh: configuration files

• ./script_name
• sh script_name

• source script_name
• . script_name

• when invoked as a login shell:
• /etc/profile then
• one of the three ~/.bash_profile, ~/.bash_login, ~/.profile
• then ~/.bash_logout if it exists

when invoked as a sub shell, ~/.bashrc

route add default gw 192.168.1.1

INSERT INTO table (field, ...) VALUES (values, ...)

• example:
• INSERT INTO restaurant (name, category_id, rating) VALUES ('Macho Taco', 3, 5);

UPDATE table SET field = 'value', ...WHERE condition, ...

• example:
• UPDATE restaurant SET rating = 4 WHERE name = 'Macho Taco';

SELECT field, ...FROM table, ...WHERE condition, ...

• example:
• SELECT * FROM restaurant WHERE category_id = 2;
• output: 1|Macho Taco|2|5

DELETE FROM table WHERE condition, ...;

#vi /etc/aliases --- edit to add aliases

# newaliases --- run to activate additions to aliases file

at or batch

cron

anacron

• at uses queue "a" batch uses queue "b" by default. A-Z are available though.
• Queues with higher letters are run with increased niceness.

• if .deny exists, users that are listed in it can not use at; all others can.
• if .allow exists, only users in that list are allowed to use at.

• if .deny exists, users that are listed in it can not use cron; all others can.
• if .allow exists, only users in that list are allowed to use cron.

list/display the current cron table

edit the current cron table

removes the entire current cron table

• /etc/cron.d -- a more flexible crontab folder, allows specifying username.
• /etc/cron.hourly
• /etc/cron.daily
• /etc/cron.weekly
• /etc/cron.monthly

/etc/anacrontab

• 32 bit
• contain 2 pieces (network,host)

• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16

• connection oriented
• reliable
• 20 byte header

• connection-less
• stateless
• lightweight
• 8 byte header

• /etc/services
• file that helps identify most of the commonly used ports.

FTP-Data

FTP-Control

ssh

telnet

SMTP SSMTP (mail)

DNS

HTTP HTTPS

POP3 POP3S

NNTP (news)

NetBios

IMAP IMAPS (email)

SNMP

• has 128bit addressing
• first appeared in 1996
• Header simpler
• improved end-to-end security
• Stateless auto-configuration
• Greatly expanded IP address space

• lo
• eth
• slip
• ppp
• isdn
• tr
• fddi

• +Enhanced functionality specific to Ethernet hardware
• +Dispay and configure Ethernet inerface settings
• +Display Available Options: ethtool (no parameters)
• +Display Setitngs on Inerface: ethtool eth#
• +Configure basic settings: ethtool -s eth# OPTION VALUE

• + OLD!
• +Access to generic, hardware independent MII capabilities of most networking hardware
• +May work better than ethtool on some old hardware

• /sbin/ifconfig
• Start: ifconfig eth0 up
• Stop: ifconfig eth0 down

/etc/resolv.conf

only allows three nameserver entries, any additional entries are ignored.

/etc/hosts

• traceroute

A specialized traceroute like utility that sends ICMP Echo messages encapsulated in IP packets

mtr can output results in many formats including a real-time updating display (default).

can be used to print network connections, routing tables, interface statistics, and masqueraded connections.

• -rn view the routing table
• -s view protocol statistics
• -ta list all TCP connections and listening ports
• -ua list all UDP connections and listening ports
• -ape list TCP UDP and UNIX socket connections and their associtated state, user, program PID/name

-taupen lists everything. good one to remember.

nmap

/etc/X11/xorg.conf

frints out info about specific window. Info includes:

• +placement
• +Geometry
• +color depth

X-display-info

Display info includes:

• +Geometry
• +color depth

• xdm -- not really inuse anymore
• gdm -- standard
• kdm -- used but not standard

command used to control host-based access

xhost (+,-)server.example.com

• + adds
• - takes away

• token-based X access control
• tokens stored in $XAUTHORITY file ( ~/.Xauthority is the most common file)
• uses the MIT_MAGIC_COOKIE_1

xauth add -- adds stores MIT-cookie

• startX starts the full graphical interface
• xinit starts X and opens a terminal
• X only starts the X server and nothing else

hwclock

date

• ntpdate

• ntpd

/etc/ntp.conf

config file for the syslogd deamon

• +syntax
• +selectors
• +action

/var/log

• logs system messages
• conffigured in /etc/syslog.conf
• supports remote logging (-r option)

receives messages from the kernel and sends them to syslogd

In order:

• emerg
• alert
• crit
• err
• warning
• notice
• info
• debug

• sqlite
• mySQL
• postgreSQL

• Oracle
• IBM DB2
• sybase

newaliases

edit the "~/.forward" file

send an email, reply to an email,

mailq

• Sendmail
• Postfix
• Exim
• Qmail