Command Center: RHCE "Cheat Sheet"

Friday, April 9, 2010

RHCE "Cheat Sheet"

Category: CentOS/RedHat, Cheatsheets — SkyHi @ Friday, April 09, 2010

This document attempts to provide answers to all study points on the RHCE and RHCT Exam Preparation Guide in a single-page (and thus, printable) format. This is not a “brain dump” or an attempt to cheat the RH302 exam in any way. These are just my self-study notes. Use them at your own risk.

:!: Note: Study points last updated on 2009-08-11. This list may become out of date without notice (especially after I pass the test ;-)).
Testing Environment with Sun VirtualBox

install guest additions:

yum install gcc kernel-devel
sh /media/VBOXADDITIONS*/VBoxLinuxAdditions-x86.run
reboot

Prerequisite skills for RHCT and RHCE

Candidates should possess the following skills, as they may be necessary in order to fulfill requirements of the RHCT and RHCE exams:
use standard command line tools (e.g., ls, cp, mv, rm, tail, cat, etc.) to create, remove, view, and investigate files and directories
use grep, sed, and awk to process text streams and files
use a terminal-based text editor, such as vim or nano, to modify text files
use input/output redirection
operator description
> redirect STDOUT to a file
2> redirect STDERR to a file
&> redirect all output to a file
2>&1 redirect all output to a pipe

*
use » to append instead of overwrite

understand basic principles of TCP/IP networking, including IP addresses, netmasks, and gateways for IPv4 and IPv6
use su to switch user accounts

su -

use passwd to set passwords

passwd

use tar, gzip, and bzip2

# compress (tar/gzip)
tar cvzf .tgz

# extract (tar/gzip)
tar xvzf .tgz

# compress (tar/bzip)
tar cvjf .tbz

# extract (tar/bzip)
tar xvjf .tbz

configure an email client on Red Hat Enterprise Linux

echo "message" | mail -s "subject"
mail -s "subject" <

use text and/or graphical browser to access HTTP/HTTPS URLs

*
elinks
*
lynx

use lftp to access FTP URLs
RHCT skills
Troubleshooting and System Maintenance

RHCTs should be able to:
boot systems into different run levels for troubleshooting and system maintenance

append the desired runlevel to grub's kernel line:

*
1-5 runs appropriate rc and init scripts
*
single only runs rc.sysinit
*
emergency skips all rc and init scripts

diagnose and correct misconfigured networking

1.
check /etc/sysconfig/network
2.
check /etc/sysconfig/network-scripts/ifcfg-
3.
service network restart
4.
chkconfig network on
5.
ifconfig
6.
ping
7.
netstat -r
8.
ping
9.
ping 4.2.2.2

redhat network config tool:

system-config-network

diagnose and correct hostname resolution problems

1.
check /etc/nsswitch.conf
2.
check /etc/resolv.conf
3.
check /etc/hosts
4.
dig @ google.com

redhat network config tool:

system-config-network

configure the X Window System and a desktop environment

install x:

yum groupinstall "x window system"

*
init respawns /etc/X11/prefdm -nodaemon to keep x running in runlevel 5
*
startx to start manually

xfs is supposedly required for x windows (even though i can run x fine without it…):

service xfs on
chkconfig xfs on

x environment config:

*
/etc/sysconfig/desktop
*
/etc/X11/xinit/xinitrc
*
/etc/X11/xinit/Xclients
*
~/.xinitrc
*
~./Xclients

redhat display config tool:

system-config-display [--reconfig]

install gnome desktop:

yum groupinstall "gnome desktop environment"

switchdesk allows you to change your desktop environment:

yum install switchdesk
switchdesk

if switchdesk is not available, edit /etc/sysconfig/desktop:

DISPLAYMANAGER=
DESKTOP=

add new partitions, filesystems, and swap to existing systems
partitions

manage partitions:

fdisk
partprobe

filesystems

make filesystems:

mkfs.

label filesystems:

e2label
blkid

manage filesystem settings:

tune2fs
dumpe2fs

swap

note that it's possible to create a swap file instead of a partition:

dd if=/dev/zero of= bs=1024 count=

format the file/partition:

mkswap
nano -w /etc/fstab
swapon -va
cat /proc/swaps

use standard command-line tools to analyze problems and configure system

*
check for full filesystems, quotas

Installation and Configuration

RHCTs must be able to:
perform network OS installation

at boot prompt:

linux askmethod

implement a custom partitioning scheme
configure printing

printing support is provided by cups:

service cups start
chkconfig cups on

redhat printer config tool:

system-config-printer

web config tool:

http://localhost:631

printing via command line:

# print
lpr
# view print queue
lpq
# remove print job
lprm

configure the scheduling of tasks using cron and at
cron

make sure vixie cron is installed and running:

yum install vixie-cron
service crond start
chkconfig crond on

1.
if /etc/cron.allow exists, only these users are allowed (/etc/cron.deny is ignored)
2.
if /etc/cron.allow does not exist, everyone allowed except users in /etc/cron.deny
3.
if neither exists, only root allowed
4.
empty /etc/cron.deny means all users allowed (default)

edit your cron jobs:

crontab -e

crontab format:

:!: /etc/crontab has additional user field before command.
at/batch

make sure at is installed and running:

yum install at
service atd start
chkconfig atd on

1.
if /etc/at.allow exists, only these users are allowed (/etc/at.deny is ignored)
2.
if /etc/at.allow does not exist, everyone allowed except users in /etc/at.deny
3.
if neither exists, only root allowed
4.
empty /etc/at.deny means all users allowed (default)

# add jobs
at now + 1 hour
at>

at 09:00 2009-07-23
at>

batch
at>

# list jobs
atq

remove jobs
atrm

attach system to a network directory service, such as NIS or LDAP

redhat config tools:

system-config-authentication
authconfig-tui

required packages for nis:

yum install ypbind portmap

required packages for ldap:

yum install nss-ldap openldap

configure autofs

make sure the autofs service is running:

service autofs start
chkconfig autofs on

ensure the following line in /etc/nsswitch.conf:

automount: files nis

define an autofs-controlled mountpoint called test by adding the following to /etc/auto.master:

/test /etc/auto.test

create /etc/auto.test:

blah example.com:/pub/something
* example:/home/&

1.
local /test/blah ⇒ remote example.com:/pub/something
2.
local /test/user ⇒ remote example:/home/user (:!: this method can be used to automount home directories)

test automounting:

ls /test/blah
ls /test/user

# redhat defaults
ls /net/
ls /misc/cd

add and manage users, groups, quotas, and File Access Control Lists

redhat user/group config tool:

system-config-users

users

/etc/passwd file format:

username:password:uid:gid:gecos:homedir:shell

/etc/shadow file format:

username:password:lastpwchange:minpwchange:maxpwage:pwchangewarn:inactive:expire

command line user management:

useradd
usermod
chage
userdel
pwck

*
default account expiration settings in /etc/login.defs

groups

/etc/group file format:

groupname:password:gid:members

command line group management:

groups
groupadd
groupmod
groupdel
grpck

quotas

install quota package

yum install quota

add fs options to /etc/fstab:

usrquota,grpquota

remount device

mount -o remount

init quota database:

quotacheck -cugm

enable/disable quotas

quotaon
quotaoff

edit quotas

edquota -u
edquota -g

edit grace time

edquota -ut
edquota -gt

check/report quotas

quota
repquota -aug

Access Control Lists

install acl package

yum install acl

add fs options to /etc/fstab:

acl

remount device:

mount -o remount

manage acls:

# set acls
setfacl -m [d:]u::
setfacl -m [d:]g::

# get acls
getfacl

# remove acls
setfacl -x u:
setfacl -x g:
setfacl --remove-all
setfacl --remove-default

configure filesystem permissions for collaboration

1.
create new group
2.
add users to group
3.
chown folder to root.
4.
chmod folder to 2770 (g+s)

install and update packages using rpm

# install
rpm -ivh .rpm

# update
rpm -Uvh .rpm

# freshen
rpm -Fvh .rpm

# remove
rpm -e

# query by file name
rpm -qf

# verify a file
rpm -Vf >

# verify status of all packages
rpm -Va > /tmp/rpmverify

:!: while inside the rescue environment, use the –root option to specify the real location of your root file system (e.g. –root=/mnt/sysimage).
properly update the kernel package

1.
always do an install (i.e. rpm -ivh ) rather than an update
2.
check /boot/grub/grub.conf for proper configuration

configure the system to update/install packages from remote repositories using yum or pup

yum config goes in /etc/yum.repos.d/

[id]
name=my repo
baseurl=http://example.com/centos/
enabled=1

modify the system bootloader

*
production config is in /boot/grub/grub.conf
*
see examples in /usr/share/doc/grub-*/menu.lst

implement software RAID at install-time and run-time

to start, we need at least two devices/partitions of type “linux raid autodetect” (use fdisk to set partition type to “fd”)

create raid device:

mdadm --create /dev/md0 --level=<0|1|4|5|6|10> --raid-devices=

fail disk in array:

mdadm /dev/md0 -f

remove disk from array:

mdadm /dev/md0 -r

add disk to array:

mdadm /dev/md0 -a

stop array:

mdadm --stop /dev/md0

check raid status:

mdadm --detail /dev/md0

cat /proc/mdstat

format works as usual:

mkfs.ext3 /dev/md0

:!: don't forget to configure /etc/fstab appropriately.
use /proc/sys and sysctl to modify and set kernel run-time parameters

config is in /etc/sysctl.conf

# search through parameters
sysctl -a | grep
# apply changes from config file immediately
sysctl -p

use scripting to automate system maintenance tasks
configure NTP for time synchronization with a higher-stratum server

redhat config tool:

system-config-date

*
config is in /etc/ntp.conf

synchronization configuration example:

server 0.pool.ntp.org
server 1.pool.ntp.org
server 2.pool.ntp.org

apply changes:

service ntpd restart
chkconfig ntpd on

verify changes:

ntpq -p

RHCE skills
Troubleshooting and System Maintenance

RHCEs must demonstrate the RHCT skills listed above, and should be able to:
use the rescue environment provided by first installation CD

linux rescue

*
when working in non-chrooted rescue mode:
o
mount /dev/hdc /mnt/source (to access install files on the cd/dvd)
o
rpm commands should use the –root=/mnt/sysimage option

manually make /dev and /proc available in chrooted mode:

mount -o bind /dev /mnt/sysimage/dev
mount -o bind /proc /mnt/sysimage/proc

diagnose and correct boot failures arising from bootloader, module, and filesystem errors

check in order:

1.
mbr
2.
/boot/grub/grub.conf
3.
/etc/fstab
4.
/etc/inittab
5.
/etc/rc.d/rc.sysinit
6.
/etc/rc.d/rc*.d
7.
/etc/rc.d/init.d/*
8.
/etc/rc.d/rc.local

grub errors

*
in general, use the last line before the error message to see where grub error'd out
*
to find correct value for root option, type find /grub/stage1 at the grub command line (:!: remember that all file names in grub.conf are relative to the root option)
*
check for missing files in kernel and/or initrd lines

kernel errors

*
missing/corrupt initrd file results in: kernel panic - not syncing: vfs: unable to mount root fs on unknown-block
*
invalid root parameter for kernel results in: setuproot: error mounting /proc: No such file or directory

reinstall grub to mbr:

grub-install

recreate initrd:

mkinitrd

fix corrupt filesystem:

fsck

if fsck is unable to locate a superblock, you can specify an alternative one:

dumpe2fs
fsck -b

diagnose and correct problems with network services (see Installation and Configuration below for a list of these services)

see what's listening on what port:

netstat -ntaupe

add, remove, and resize logical volumes

redhat lvm config tool:

yum install system-config-lvm
system-config-lvm

create physical volume:

pvcreate

create volume group:

vgcreate [pv device]

extend volume group:

vgextend

create logical volume:

lvcreate --size M --name

extend logical volume:

lvextend --size M
resize2fs

shrink logical volume:

resize2fs M
lvreduce --size M

remove logical volume:

lvremove

diagnose and correct networking services problems where SELinux contexts are interfering with proper operation.

enable/disable selinux in /etc/sysconfig/selinux:

SELINUX=enforcing
SELINUXTYPE=targeted

install selinux troubleshooter:

yum install setroubleshoot
service setroubleshoot start
chkconfig setroubleshoot on

install selinux management tool:

yum install policycoreutils-gui

list selinux errors:

sealert -a /var/log/audit/audit.log | less

launch gui browser:

sealert -b

list selinux booleans:

getsebool -a

set selinux boolean:

setsebool -P = <0|1>

list security contexts:

ls -Z

change security contexts:

# using reference (copy contexts from existing known-good file)
chcon -R --reference

# manual
chcon -R -u
chcon -R -t

Installation and Configuration

RHCEs must demonstrate the RHCT-level skills listed above, and they must be capable of configuring the following network services. For each of these services, RHCEs must be able to:

*
install the packages needed to provide the service
*
configure SELinux to support the service
*
configure the service to start when the system is booted
*
configure the service for basic operation
*
Configure host-based and user-based security for the service

HTTP/HTTPS
install

yum install httpd mod_ssl

selinux

make new DocumentRoot match default DocumentRoot (:!: this applies to any directory that apache will serve files from):

chcon -R --reference /var/www /www

start at boot

chkconfig httpd on

basic config

*
requirements for ~user/ directories:
o
UserDir directive
o
chmod 701 the user's home directory
o
change security context on the user's UserDir

*
requirements for .htaccess file usage:
o
AllowOverride All directive

*
requirements for name-based virtual hosts:
o
NameVirtualHost *:80 and NameVirtualHost *:443 directives
o
each virtual host requires appropriate ServerName and ServerAlias directives
o
:!: a single virtual host cannot span multiple ports (i.e. 80 and 443). two separate VirtualHost *: sections are needed to do this.

self-signed ssl cert:

cd /etc/pki/tls/certs
rm localhost.crt
make testcert

check virtual host config:

httpd -D DUMP_VHOSTS

host-based security

firewall config:
protocol ports
tcp 80, 443

hosts are allowed by default and must be explicitly denied:

Order deny,allow
Deny from 192.168.0.0/255.255.255.0
Deny from badguys.example.com

hosts are denied by default and must be explicitly allowed:

Order allow,deny
Allow from 192.168.0.0/255.255.255.0
Allow from goodguys.example.com

user-based security

create web password file:

htpasswd -c /etc/httpd/webusers testuser1
htpasswd /etc/httpd/webusers testuser2

create web group file (/etc/httpd/webgroups):

testgroup: testuser1 testuser2

allow access by group:

AuthType Basic
AuthName "top secret area"
AuthUserFile /etc/httpd/webusers
AuthGroupFile /etc/httpd/webgroups
Require group testgroup

verify service functionality

test http/https:

elinks :///[path]

SMB
install

yum install samba samba-client

selinux

allow samba to share home directories:

setsebool -P samba_enable_home_dirs=1

mark a directory as sharable with samba:

chcon -R -T samba_share_t

start at boot

chkconfig smb on

basic config

redhat samba config tool:

yum install system-config-samba
system-config-samba

set workgroup/domain:

workgroup =

security modes:

# connections check local pwdb (default)
security = user

# member server on a domain, uses pwdb on a dc
security = domain
workgroup = EXAMPLE

# member server on an ad domain using kerberos, uses pwdb on a dc
security = ads
realm = EXAMPLE.COM
password server = kerberos.example.com

# used when samba was not capable of being a domain member server (DO NOT USE)
security = server
encrypt passwords = yes
password server =

# each share requires a password (DO NOT USE)
security = share

share options:

[]
# path for share
path =

# share is visible
browseable =

# rw enabled
writeable =

# this is a shared printer
printable =

# all users connecting to this share use as their primary group
group =

join domain:

net rpc join -U root

fstab example:

/// cifs user=,pass= 0 0

:!: mount.cifs and umount.cifs need to be chmod'ed u+s in order to be used by non-root users
host-based security

firewall config:
protocol ports
tcp 139, 445
udp 137, 138

hosts allow/deny can be used per-server or per-share:

hosts allow = 127.0.0.1 192.168.2.0/24 192.168.3.0/24
hosts deny = 0.0.0.0/0

user-based security

account maintenance:

# add account (local linux account must exist first, or be translated via /etc/samba/smbusers):
smbpasswd -a

# enable/disable account:
smbpasswd -e
smbpasswd -d

# remove account:
smbpasswd -x

:!: service smb reload may be needed after account changes

share access:

valid users = @

*
share access is also controlled by unix file permissions

verify service functionality

list shares:

smbclient -L -U

browse shares:

smbclient /// -U

test allow/deny statements for a host:

testparm /etc/samba/smb.conf

NFS
install

yum install portmap nfs-utils

start at boot

chkconfig portmap on
chkconfig nfs on
chkconfig nfslock on
chkconfig netfs on

basic config

redhat config tool:

yum install system-config-nfs
system-config-nfs

format of /etc/exports:

() [() ...]

activate new exports:

/etc/init.d/nfs restart

host-based security

:!: edit /etc/sysconfig/nfs and restart nfs to set static ports

firewall config:

# see ports
rpcinfo -p

host based security is intrinsic to the format of the exports file
user-based security

use standard file permissions
verify service functionality

list exports:

showmount -e

FTP
install

yum install vsftpd

selinux

allow local users to log in and cd into home directories:

setsebool -P ftp_home_dir=1

start at boot

chkconfig vsftpd on

basic config
host-based security

*
use ipchains with -[!]s option

firewall config:
protocol ports
tcp 21

:!: ftp data transfers will not work unless ip_conntrack_ftp is added to IPTABLES_MODULES in /etc/sysconfig/iptables-config

tcp_wrappers example:

vsftpd : 192.168.0.

user-based security

*
allow/deny controlled via /etc/vsftpd/user_list (:!: users in /etc/vsftpd/ftpusers are always denied via pam)
*
default allow/deny is configured by userlist_deny statement in vsftpd.conf

verify service functionality

test ftp:

ftp

Web proxy
install

yum install squid

selinux

allow squid to connect to the network (this is recommended, but was not needed in my testing):

setsebool -P squid_connect_any=1

start at boot

chkconfig squid on

host-based security

firewall config:
protocol ports
tcp 3128

allow access from local networks:

acl our_networks src 192.168.1.0/24 192.168.2.0/23
http_access allow our_networks

user-based security

FIXME
verify service functionality

test proxy:

HTTP_PROXY=:3128 elinks

SMTP
install

yum install postfix
alternatives --config mta
service sendmail stop

start at boot

chkconfig postfix on

basic config

listen on public interfaces:

inet_interfaces = all

specify all destination hostnames/domains:

mydestination = , , ...

specify origin domain:

myorigin = $mydomain

local aliases in /etc/aliases (:!: dont forget to run newaliases to apply changes):

: [, user2]

virtual aliases in /etc/postfix/virtual (:!: dont forget to run postmap /etc/postfix/virtual to apply changes):

:

enable virtual aliases:

virtual_alias_maps = hash:/etc/postfix/virtual

outbound address rewriting in /etc/postfix/generic (:!: dont forget to run postmap /etc/postfix/generic to apply changes):

:

enable outbound aliases:

smtp_generic_maps = hash:/etc/postfix/generic

host-based security

*
use ipchains with -[!]s option

firewall config:
protocol ports
tcp 25
user-based security

FIXME use smtp auth?
verify service functionality

test smtp:

telnet 25

IMAP, IMAPS, and POP3
install

yum install dovecot

start at boot

chkconfig dovecot on

basic config

enable protocols:

protocols =

create custom ssl cert:

nano -w /etc/pki/dovecot/dovecot-openssl.cnf
/usr/share/doc/dovecot-*/examples/mkcert.sh
service dovecot restart

host-based security

use ipchains with -[!]s option
protocol ports
tcp 143, 110, 995, 993
user-based security

use pam_listfile in /etc/pam.d/dovecot
verify service functionality

test mailbox acess:

mutt -f ://@

SSH
install

yum install openssh-server

start at boot

chkconfig sshd on

user-based security

allow/deny user access:

AllowUsers user1 user2 user3@example.com
DenyUsers user4 user5 user6@example.com

host-based security

*
use ipchains with -[!]s option

firewall config:
protocol ports
tcp 22

tcp_wrappers example:

sshd : 192.168.0.

verify service functionality

test logging in:

ssh @

DNS (caching name server, slave name server)
install

yum install bind-chroot caching-nameserver

start at boot

chkconfig named on

basic config

copy sample config:

cp -a /var/named/chroot/etc/named.caching-nameserver.conf /var/named/chroot/etc/named.conf

caching-only nameserver:

*
edit listen-on directives (comment out to listen on all interfaces)
*
edit allow-query directives (comment out allow queries from everyone)
*
edit match-clients and match-destinations directives to allow recursive queries from other hosts

slave nameserver:

*
get slave example from /usr/share/doc/bind-*/sample/etc/named.conf

host-based security

firewall config:
protocol ports
tcp 53
udp 53

allow-query example:

allow-query { 192.168.0.0/16; localnets; };

user-based security

N/A
verify service functionality

test query:

dig @

test zone transfer:

dig @ axfr

NTP
install

yum install ntp

start at boot

chkconfig ntpd on

host-based security

firewall config:
protocol ports
udp 123

allow other servers to sync with us:

restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap

user-based security

N/A
verify service functionality

show peers:

ntpq -p

RHCEs must also be able to:
configure hands-free installation using Kickstart

yum install system-config-kickstart

1.
make installation tree available
2.
create kickstart file (use system-config-kickstart to create ks.cfg) and validate (using ksvalidator)
3.
validate kickstart file
4.
make kickstart file available
*
bootable diskette (place in top level directory)
*
bootable cdrom (place in top level directory)
*
network (http, ftp, nfs)
5.
use bootable media and supply appropriate kernel parameter

ks=floppy:/ks.cfg
ks=cdrom:/ks.cfg
ks=http://example.com/ks.cfg
ks=nfs:example.com:/ks.cfg

implement logical volumes at install-time
use iptables to implement packet filtering and/or NAT

:!: do not use system-config-securitylevel, as it will overwrite your custom iptables rules. the following method seems to be the best way to go:

1.
make changes in /etc/sysconfig/iptables
2.
run /etc/init.d/iptables restart to apply changes

packet filtering

packet filtering example:

-A -p -m [-s[!] ] --dport -j ACCEPT

NAT

enable ip forwarding in /etc/sysctl.conf:

net.ipv4.ip_forward = 1

to test from another machine:

ip route replace default via

inbound dnat:

iptables -t nat -A PREROUTING -p --dport -j DNAT --to-dest :

outbound dnat:

iptables -t nat -A OUTPUT -p --dport -j DNAT --to-dest :

masquerading:

iptables -t nat -A POSTROUTING -o -j MASQUERADE

snat:

iptables -t nat -A POSTROUTING -j SNAT --to-source :

FIXME
use PAM to implement user-level restrictions
module documentation

*
/usr/share/doc/pam-*/txts

module configuration

*
/etc/pam.d
*
/etc/security

module interface description
auth user authentication (e.g. verifies password, set group membership or kerberos tickets, etc.)
account verifies that access is allowed (e.g. expired account?, check group membership, etc.)
password handles password changes
session manages user sessions (e.g. mount home dir, create mailbox, logging, etc.)
control flag description
required must pass, continue testing on failure
requisite must pass, stop testing on failure
sufficient failure is ignored, but if passing so far, return success at this point
optional pass or failure is irrelevant
include include another file
pam_listfile.so example

allow/deny users if listed in /etc/special:

auth required pam_listfile.so onerr=success item=user sense= file=/etc/special

Additional Notes
tcp_wrappers

file format:

: [except ] [:

Command Center

Friday, April 9, 2010

RHCE "Cheat Sheet"

Labels

Blog Archive

My Blog List