Last update: Fri Jul 26 08:23:20 UTC 2019 by @luckylittle
# Persisting journal:
mkdir /var/log/journal
chown root:systemd-journal /var/log/journal
chmod 2755 /var/log/journal
killall -USR1 systemd-journal # or reboot# journalctl examples:
journalctl -ef # end & follow
journalctl _SYSTEMD_UNIT=sshd.service # generated by sshd service
journalctl -u sshd.service # generated and about sshd service
journalctl -p emerg..err # priority between emergency and error
journalctl -b -1 # only from the last boot
journalctl --since "2019-01-01 20:30:00" --until "2019-02-02 12:00:00"
journalctl -o verbose # show all fields# Using RH resources:
yum -y install sos
sosreport --help | less
sosreport -l | less # view currently enabled/disabled plugins and plugin options
sosreport -o <PLUGIN(S)> # enable these plugins only (it will only run these plugins)
sosreport -n <PLUGIN(S)> # skip these plugins (it will run all of the plugins, except for these)
sosreport -e <PLUGIN(S)> # enable previously disabled plugins
sosreport -k xfs.logprint # xfs module and logprint option enabled
redhat-support-tool# Insights:
yum -y install redhat-access-insights
redhat-access-insights --register# Cockpit:
yum -y install cockpit
systemctl start cockpit
firewall-cmd -add-service=cockpit --permanent
firewall-cmd --reload # http://localhost:9090
# Co-pilot:
yum -y install pcp # performance co-pilot
systemctl start pmcd # performance metrics collector daemon
systemctl enable pmcd
pmstat -s 5 # 5 samples
pmatop # machine stats and data
pminfo # obtain list of metrics
pminfo -dt proc.nprocs # understand specific metric
pmval -s 5 proc.nprocs # gather sample data about the metric 5x times
pmval -T 1minute kernel.percpu.cpu.idle # per-CPU idle time for one minute
# Historical data:
systemctl start pmlogger # ability to store metrics data to logs (-a <ARCHIVE>)
systemctl enable pmlogger
pcp | grep 'primary logger' # location of the archive log that pmlogger is writing to
ls /var/log/pcp/pmlogger/<HOSTNAME> # collects data every second to this location
pmval -a <ARCHIVE.xz> -f 3 <METRIC> # performance metrics value dump from archive with 3 digits precision
pmval -a /var/log/pcp/pmlogger/serverX.example.com/20190101.00.10.0 kernel.all.load
pmval -a /var/log/pcp/pmlogger/serverX.example.com/20190101.00.10.0 kernel.all.load \
-S '@ Tue Feb 01 12:00:00 2019' -T '@ Tue Feb 01 13:00:00 2019'# Configure central log host:
systemctl is-active rsyslog
systemctl is-enabled rsyslog
man rsyslog.conf # man pages
vim /etc/rsyslog.conf
$ModLoad imudp.so # for UDP
$UDPServerRun 514
$ModLoad imtcp.so # for TCP
$InputTCPServerRun 514
$template DynamicFile,"/var/log/loghost/%HOSTNAME%/cron.log"
cron.* ?DynamicFile # 'DyamicFile' here is arbitrary template name
$template DynamicFile,"/var/log/loghost/%HOSTNAME%/%syslogfacility-text%.log"
*.* -?DynamicFile # minus is turn off syncing of the log file after each write
systemctl restart rsyslog
firewall-cmd --add-port=514/udp --permanent
firewall-cmd --add-port=514/tcp --permanent
firewall-cmd --reload
# Enable log rotation:
vim /etc/logrotate.d/syslog
/var/log/loghost/*/*.log # must not be at the end, but before curly braces {...}# Redirecting logging to central host:
vim /etc/rsyslog.conf
*.info @loghost.example.com[:PORT] # all info messages sent to loghost.example.com via UDP
*.* @@loghost.example.com # all messages sent to loghost.example.com via TCP# Monitor changes:
yum -y install aide # intrusion detection
vim /etc/aide.conf
# Configuration lines:
PERMS = p+i+u+g+acl+selinux # file (p)ermissions, (i)node, (u)ser/(g)roup ownership, acl, selinux
# Selection lines:
/dir1 PERMS # group check on dir1 and all files and dirs below it
=/dir2 PERMS # group check in dir2, but not recursively
!/dir3 # excludes dir3 and all files below it from any checks
# Macro lines:
@@define VAR value # @@{VAR} is reference to the macro defined previously
aide --init # creates /var/lib/aide/aide.db.new.gz every time
mv -v /var/lib/aide/aide.db.new.gz /var/lib/aide/aide.db.gz
aide --check# File system auditing:
man audit.rules # manpages
auditctl -w /etc/passwd -p wa -k <KEY> # watch rule for write and attribute changes with custom key
auditctl -w /etc/sysconfig -p rwa -k <KEY> # recursive watch of all files and dirs in sysconfig & key
auditctl -w /bin -p x # all executions in bin
auditctl -W <PATH> # remove watch rule(s) at path
auditctl -d <RULE> # remove previous -a or -A rule(s)
auditctl -D # remove all rules (or they get removed by reboot)
vim /etc/audit/rules.d/audit.rules # persistent rules, without auditctl at the beginning
-w /etc -p w -k etc_content # w=path, p=permission((r)ead,(w)rite,e(x)ecute,(a)ttribute)
-w /etc -p a -k etc_attribute
# System calls auditing:
auditctl -a always,exit -F arch=b64 -S open\ # list names:task,exit,user,exclude actions:never,always
-F success=0 # system call open() that failed
cat /var/log/audit/audit.log
ausearch -i --raw -a <EVENT-ID> --file <FILENAME> -k <KEY> --start <START-TIME> --end <END-TIME>
ausearch -k etc_content # audit search for specific key/tag
ausearch -k etc_attribute
# Some fields (-F):
auid=The original ID the user logged in with. Its an abbreviation of audit uid. Sometimes its referred to as loginuid.
egid=Effective Group ID.
euid=Effective User ID.
sgid=Saved Group ID.
suid=Saved User ID.
uid=User ID.# Configuring Grub2:
vim /etc/default/grub
GRUB_TIMEOUT = seconds the menu is displayed
GRUB_DEFAULT = starts counting from 0, what is the default entry
GRUB_CMDLINE_LINUX = list of extra kernel params, e.g "rhgb quiet"
grub2-mkconfig -o /boot/grub2/grub.cfg # this is *.cfg and NOT *.conf# An example of the 'linux16' menu entry line:
linux16 /vmlinuz-3.8.0-0.40.el7.x86_64 root=/dev/mapper/rhel-root ro rd.md=0 rd.dm=0 rd.lvm.lv=rhel/swap crashkernel=auto rd.luks=0 vconsole.keymap=us rd.lvm.lv=rhel/root rhgb quiet# Reinstalling Grub2 into the MBR, must reboot into rescue environment (e.g. anaconda):
chroot /mnt/sysimage
ls -l /boot
grub2-install /dev/vda # rewrite the boot loader sections of the MBR# Reinstalling Grub2 into the UEFI:
yum reinstall grub2-efi shim # if the files /boot/efi have been removed
grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg # if the cfg file has been removedyum -y install efibootmgr
efibootmgr # manage list of available UEFI boot targets
efibootmgr -b 1E -B # delete entry 1E from UEFI boot targets completely
# Selecting a temporary boot target:
efibootmgr -n 2c # override normal boot ordering for a next single boot
# Add an entry (/dev/sda2 as the ESP with the application /EFI/yippie.efi on the ESP):
efibootmgr -c -d /dev/sda -p 2 -L "Yippie" -l "\EFI\yippie.efi" # (c)reate,(d)evice,(p)artition,(L)abel# SystemD and failing services:
/etc/systemd/system/<UNITNAME>.d/ # symlinks to /usr/lib/systemd/system/
/etc/systemd/system/<UNITNAME>.requires/<SYMLINK>
/etc/systemd/system/<UNITNAME>.wants/<SYMLINK>
Requires=
stopping listed unit will also stop this unit as well
RequiresOverridable=
failed requirements will not cause the unit to fail when explicitly started
Requisite=,RequisiteOverridable=
the unit will fail if required unit is not already running
After=
listed units have to have finished starting before this unit can be started
Before=
listed units will be delayed
Wants=
when wante unit fails to start, this unit itself will still start
Conflicts=
starting this unit will stop the conflicting units
systemctl daemon-reload # this is required after each change
systemctl list-dependencies <UNITNAME>
systemctl list-unit-files
systemctl status # shows tree of services and corresponding PIDs
# To obtain a root shell during startup (/dev/tty9):
systemctl enable debug-shell.service # do not leave this enabled after finished with debugging!
systemctl list-jobs # troubleshoot startup tasks
# Resetting a root password:
1. Interrupt countdown
2. Press ["e"] on the highlighted entry # changes made like this in the Grub2 menu screen are only temporary
3. Find kernel arguments line (starts with "linux16" or "linuxefi")
4. Add "rd.break" at the end of this line by pressing ["End"]
5. ["Ctrl"]+["X"]
6. mount -oremount,rw /sysroot
7. chroot /sysroot
8. echo <PASSWORD> | passwd --stdin root
9. touch ./autorelabel (or "load_policy -i; restorecon -Rv /etc")
10. ["Ctrl"]+["D"]# Basic commands:
lscpu # identifying processor
cat /proc/cpuinfo # identifying what flags CPU supports
dmidecode -t memory # identifying memory
lsscsi -v # identifying disks
hdparm -I /dev/sda # more information about individual disks
lspci # identifying PCI hardware
lsusb # identifying USB hardware# Memory failures:
# Older:
yum -y install mcelog # framework for catching and logging exceptions
# Newer:
yum -y install rasdaemon # modern replacement for mcelog
systemctl enable rasdaemon
systemctl start rasdaemon
ras-mc-ctl --status # what does subsystem know about memory
ras-mc-ctl --errors# Test memory:
yum -y install memtest86+
memtest-setup # this adds new template to Grub2 (/etc/grub.d/)
grub2-mkconfig -o /boot/grub2/grub.cfg # update Grub2 config# Managing Kernel modules:
ls /lib/modules/<KERNEL_VERSION>/.. # all possible drivers
lsmod # view currently loaded Kernel modules (same as /sys/module/..)
modprobe -v <MODULE> # load the module manually
modprobe -r <MODULE> # unload the module manually
modinfo -p <MODULE> # list of supported options for the module
cat /sys/module/<MODULE>/parameters/<PARAMETER> # what is the active value of the module option
modprobe -v st buffer_kbs=64 # set option buffer_kbs for the st module to 64 when loaded
vim /etc/modprobe.d/00-st.conf # automatically set every time, parsed alphabetically
options st buffer_kbs=64 max_sg_segs=512 # permanent, needs unload & reload to take effect# Hardware virtualization support:
modprobe -v kvm-intel # or 'kvm-amd'
virsh capabilities # usually on the host, not guest
# Overcommitting resources:
virsh nodecpustats
virsh nodememstats
virsh dommemstats <DOMAIN>
# Libvirt XML config:
virsh define <FILENAME.xml> # attempt to create VM
xmllint --noout <FILENAME.xml> # verify XML syntax
virt-xml-validate <FILENAME.xml> # verify if it matches libvirt XML schema
/etc/libvirt/qemu/networks # network definitions# See the device mappings:
dmsetup ls # list top level devices (e.g. VolGroup00-LogVol01 [253:1])
ls -l /dev/mapper/<LOGICAL_VOL> --> /dm-0 # symlink to device mapper#0 also in /dev/<VG_NAME>
dmsetup table /dev/mapper/<LOGICAL_VOL> # shows block device's minor/major number
ls -l /dev/vdb* # or lsblk -r | awk '{ print $1, $2 }', disks & partitions
yum -y install device-mapper-multipath
multipath -v 2 <DEVICE> # device mapper target autoconfig# What I/O scheduler is used with device:
cat /sys/block/<DEVICE>/queue/scheduler # e.g. noop deadline [cfq]yum -y install e2fsprogs
# e2fsck options: # 'df -TH' shows filesystems
e2fsck -b <LOCATION> # use alternative superblock
e2fsck -p # automatically repair, only prompt un-safe problems
e2fsck -v # verbose
e2fsck -y # non-interactive mode, answer yes to all# Recovering from FS corruption:
# ext3/ext4:
umount /dev/<DEVICE>
e2fsck -n /dev/<DEVICE> # dry-run (read-only + answer no to everything)
# If corrupt supeblock (bad magic number): # magic number = where superblock starts
dumpe2fs /dev/<DEVICE> | grep 'Backup superblock'
e2fsck [-n] /dev/<DEVICE> -b <NUMBER> # alternative superblock to use from the previous cmd
# XFS:
yum -y install xfsprogs
umount /dev/<DEVICE> # re-mount on systems where journal corruption suspected
xfs_repair -n /dev/<DEVICE> # perform only check
xfs_repair [-o force_geometry] /dev/<DEVICE> # perform all corrective actions, shows invalid inodes
mount /dev/<DEVICE> /mountpoint
ls /mountpoint/lost+found # unreferenced files
find /mountpoint -inum <NUMBER> # locate directory with the inode number
diff -s /file/from/backup /mountpoint/lost+found/<NUMBER>
# If corrupt journal log:
xfs_repair -L /dev/<DEVICE> # zeros out the journal log, potentially dangerous# Recovering LVM:
# Config file:
vim /etc/lvm/lvm.conf
dir # scan for physical volumes (/dev)
obtain_device_list_from_udev # shoud udev be used (1)
preferred_names # which path name to display for block device
filter # which devices to scan for presence of PV signature
backup # save text-based metadata before each disk change (1)
backup_dir # where the backup of VG metadata should be stored
archive # should old configurations be also archived (1)
archive_dir # where the archives will be stored
retain_min # minimum number of archives to store
retain_days # minimum number of days for archive to be kept
# Reverting LVM changes:
ls /etc/lvm/backup/<VG_NAME>
cat /etc/lvm/archive/<VG_NAME>_timestamp.vg | grep 'description ='
vgcfgrestore -l <VG_NAME> # list descriptions of each archives of the volume group
umount ALL FS CREATED ON THE LOGICAL VOLUME
vgcfgrestore -f /etc/lvm/archive/<VG_NAME>_timestamp.vg
lvchange -an /dev/<VG_NAME>/<LV_NAME> # activate no (deactivate)
lvchange -ay /dev/<VG_NAME>/<LV_NAME> # activate yes (reactivate)
xfs_growfs /dev/<VG_NAME>/<LV_NAME> # eventually grow the filesystem if needed
mount ALL FS CREATED ON THE LOGICAL VOLUME# Dealing with LUKS:
dmsetup ls --target crypt # e.g. 'luks-0123456789-abcde-987654321-fghij (253,0)'
cat /etc/crypttab # may contain UUID instead of ENCDEVICE
<NAME> /dev/<ENCDEVICE> /path/keyfile or none # if none, you will be asked for password on boot
/dev/mapper/<NAME> # decrypted device mapper location
cryptsetup luksDump /dev/<ENCDEVICE> # display LUKS header information for encrypted device
# LUKS header backup:
cryptsetup luksHeaderBackup /dev/<ENCDEVICE> --header-backup-file /path/to/backup_file
# Trial decryption with header backup file: # use 'cryptsetup luksClose' when you make a typo
cryptsetup luksOpen /dev/<ENCDEVICE> <NAME> [--header /path/to/backup_file]
cryptsetup luksOpen <FILE.img> <NAME> --key-file <EXISTING_KEY_FILE.key>
# Add key to the key slot:
cryptsetup luksAddKey /dev/<ENCDEVICE> --key-file <EXISTING_KEY_FILE.key> --key-slot <ID> [<key file with new key or pswd>]
# Restore header:
cryptsetup luksHeaderRestore /dev/<ENCDEVICE> --header-backup-file /path/to/backup_file# iSCSI initiator/client:
yum -y install iscsi-initiator-utils # 'systemctl enable iscsi --now'
iscsiadm -m node # see already discovered targets/node records
iscsiadm -m session [-P 3] # validate sessions or connections, P=print level
vim /etc/iscsi/iscsid.conf # restart iscsi/iscsid every time you change this file
discovery.sendtargets.auth.<authmetod|username|password|username_in|password_in>
node.session.auth.<authmetod|username|password|username_in|password_in>
vim /etc/iscsi/initiatorname.iscsi # this needs iscsid restart
InitiatorName=iqn.2016-01.com.example.lab:servera
systemctl restart iscsid
iscsiadm -m discovery -t st -p <TARGET>:<PORT> # discovery & sendtargets for portal -> /var/lib/iscsi/nodes
iscsiadm -m node -T iqn.2016-01.com.example.lab:iscsistorage --login [-d8] # -d8=debug
# Disable CHAP authentication:
iscsiadm -m node -T iqn.2016-01.com.example.lab:iscsistorage -o update -n node.session.auth.authmethod \
-v None [-p <TARGET>:<PORT>] # o=overwrite previous config,n=name,v=value
# Purge all node information from cache, recommended when server's setting change:
iscsiadm -m node -T iqn.2016-01.com.example.lab:iscsistorage -o delete [-p <TARGET>:<PORT>]
# Purge all know nodes from cache:
iscsiadm -m node -o delete [-p <TARGET>:<PORT>] # default port 3260/tcp
lsblk --scsi# Display package dependencies:
yum deplist <PACKAGE> # same as rpm -q -R <PACKAGE>
# Resolving package dependencies:
yum downgrade <PACKAGE> # same as rpm -U --oldpackage <PACKAGE>
rpm -U --force <PACKAGE> # same as --oldpackage --replacepkgs --replacefiles
# Using YUM to lock package versions: # 'yum list available yum-plugin*'
yum -y install yum-plugin-versionlock
yum versionlock list # display list of locked package versions
yum versionlock add <PATTERN> # lock current versions of packages matched by wildcard
yum versionlock delete <PATTERN> # delete locks matched by wildcard
yum versionlock clear # clear all package version locks
yum list --showduplicates <PACKAGE>\* # find all available versions of a package# Repair corrupt RPM database:
lsof | grep /var/lib/rpm
rm /var/lib/rpm/__db* # remove database indexes
tar cjvf rpmdb-$(date +%Y%m%d-%H%M).tar.bz2 /var/lib/rpm
cd /var/lib/rpm
/usr/lib/rpm/rpmdb_verify Packages # verify RPM database integrity
mv Packages Packages.bad
/usr/lib/rpm/rpmdb_dump Packages.bad | /usr/lib/rpm/rpmdb_load Packages
/usr/lib/rpm/rpmdb_verify Packages
rpm -v --rebuilddb # rebuild database indexes, 'rpm -qa > /dev/null' shouldn't show anything# Verifying changed files with RPM:
rpm -qf <PATH> # what package does the file belong to
rpm -ql <PACKAGE> # list files in package
tail /var/log/yum.log # see what was installed recently
rpm -V <PACKAGE(S)> # verify the package (S,M,5,L,U,G,T), shows file types (c,d,l,r) for some
rpm -Va # verify files of all installed packages
# Verifying changes with YUM:
yum -y install yum-plugin-verify # works like 'rpm -V'
yum verify <PACKAGE> # does not show configuration files changes
yum verify-rpm <PACKAGE> # includes configuration files diff from original
# Recovering changed files:
rpm --setperms <PACKAGE> # resets the permissions of files in a package
rpm --setguids <PACKAGE> # resets the user/group ownership of files
yum reinstall <PACKAGE> # repair installed packageping -c 1 -W 3 <IPv4> # send single echo request and wait 3s for reply
ping6 [-I <INTERFACE>] <IPv6> # -I is not needed when routable IPv6 is used
# Scanning the network:
yum -y install nmap
nmap -n <IPv4>/<SUBNET> # -n means don't use DNS, discover all ports on all hosts
nmap -n -sn <IPv4>/<SUBNET> # -sn means disable port scanning, only discover hosts
nmap -n -sU <IPv4> # perform UDP scan on the host
nmap <HOSTNAME> # perform IPv4 port scan on hostname
nmap -6 <HOSTNAME> # scan ports of the IPv6 address
# Read/write data across networks:
yum -y install nmap-ncat
nc <HOSTNAME> <PORT> # client/connect mode
nc -6 <HOSTNAME> <PORT> # connect to port of hostname using IPv6
nc -l [-k] <PORT> # server mode, -k means keep listening for >1 connections
nc -l <PORT> -e <COMMAND> # pass incoming traffic to the command
# Monitoring using IPTraf:
yum -y install iptraf-ng
iptraf-ng# Network devices naming: # view /usr/share/doc/initscripts-*/sysconfig.txt
cat /etc/udev/rules.d/80-net-name-slot.rules # udev rules with persistent naming
vim /etc/udev/rules.d/70-persistent-net.rules # these custom rules overwrite defaults
cat /etc/sysconfig/network-scripts/ifcfg-eth1 # filename should match device name
DEVICE=<NAME>
HWADDR=<MAC_ADDRESS>
BOOTPROTO=STATIC # static/none (needs IPADDR0,PREFIX0) or dhcp/bootp
ONBOOT=yes
TYPE=Ethernet
USERCTL=yes
PEERDNS=no # define entries in /etc/resolv.conf (needs DNS1,DNS2)
IPV6INIT=no # use IPv6 (needs IPV6ADDR/MASK,IPV6_AUTOCONF)
IPADDR=<IPv4>
NETMASK=<MASK>
# Troubleshooting:
ip addr show dev <DEVICE_NAME>
ip route
nmcli con # display connection information
nmcli dev # display device information
nmcli conn show '<CONNECTION_NAME>' | grep ipv # all config settings
ipv4.method # auto=dhcp, manual=static (needs addresses,gateway)
ipv6.method
ncmli conn mod '<CONNECTION_NAME>' ipv4.dns '<IPv4>' # good alternative: 'nmtui', restart affected services
nmcli conn reload # after you manually edit network-scripts
nmcli conn down '<CONNECTION_NAME>' ; nmcli \ # changes are not applied to already active interface...
conn up '<CONNECTION_NAME>' # ...also updates /etc/resolv.conf
firewall-cmd --list-all-zones [--permanent] # comparing active and permanent can identify problems
firewall-cmd --runtime-to-permanent # quick convert of runtime rules to permanent
host -v -t aaaa <HOSTNAME> <DNS> # query DNS for hostname's IPv6# Inspecting network traffic:
yum -y install wireshark-gnome
wireshark &
wireshark -r <FILE> & # analyze captured packets previously saved in file
# Capturing packets: # yum -y install tcpdump
tcpdump -c <NUMBER> -w <FILE.pcap> # capture number of packets to the file
tcpdump -r <FILE.pcap> # read from a capture file
tcpdump 'host <HOSTNAME>' # coming to/from host
tcpdump 'src <HOSTNAME>' # from host
tcpdump 'port <NUMBER>'
tcpdump 'icmp and host <IPv4>' # icmp to/from host
tcpdump 'ip host <HOSTNAME1> and not <HOSTNAME2>'
tcpdump -x # display packet header and hexadecimal values
tcpdump -X # display data as hexadecimal and ASCII values
tcpdump -X -r <FILE.pcap> 'host <HOSTNAME>' | grep -i 'pass' # display plaintext passwords# Linking against shared libraries (.so=shared object):
objdump -p /usr/lib64/<LIBRARY>-<VER>.so | grep SONAME
ls -l /usr/lib64/<LIBRARY>* # shared library has symbolic link to DT_SONAME field
ls -l /lib64/ld-linux-x86-64.so* # default 64-bit runtime linker on RHEL7
ls -l /lib/ld-linux.so* # default 32-bit runtime linker on RHEL7
ldconfig [-v] # updates the runtime linker cache
ldconfig -p # list of libraries in /etc/ld.so.cache
ldd <PATH/TO/EXECUTABLE> # required shared libraries by executable (| grep 'not found')
yum whatprovides '*lib/<LIBRARY>.so.0' # identify package that provides shared library
rpm -q --requires -p <FILE>.rpm # required runtime libraries are stored in RPM metadata# Diagnosing memory leaks:
yum -y install valgrind
valgrind --tool=memcheck --leak-check=full <PROGRAM>
watch -d -n1 'free -mh; grep -i commit /proc/meminfo'# Displaying system calls:
strace [-o <FILE> -e <SYSCALLS>] <EXECUTABLE> # launch executable with strace, -o=to file,-e=show only specific events
strace -f <EXECUTABLE> # also follow the execution of forks (child processes)
strace -p <PID> # trace a process already executed
# Displaying library calls:
ltrace -S <EXECUTABLE> # strace + ltrace (needs at least read access to executable)
ltrace -p <PID> # trace a process already executed# SELinux logging (or /var/log/audit/audit.log):
ausearch -m avc -ts recent # display Access Vector Control messages, last 10mins
yum -y install setools-console # provides "seinfo", "sesearch"
sesearch -D # full list of all active "dontaudit" rules
sesearch --allow -b <BOOLEAN> # view allow rules enabled by the boolean
semanage dontaudit off # disable "dontaudit" rules until it is turned 'dontaudit on' - blocks events, but does not log them
seinfo -t httpd_sys_content_t # or -b to show booleans
seinfo --portcon=443 --protocol=tcp# An example of USER_AUTH entry in the audit log:
type=USER_AUTH msg=audit(1564120484.274:12873): pid=22300 uid=0 auid=1000 ses=2 subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 msg='op=PAM:authentication grantors=pam_rootok acct="root" exe="/usr/bin/su" hostname=localhost.localdomain addr=? terminal=pts/0 res=success'# Troubleshooting SELinux:
yum -y install setroubleshoot-server # provides 'sealert', 'sedispatch' plugin for auditd
service auditd restart # auditd's sedispatch plugin requires restart, don't use systemd
sealert -l <UUID_OF_DENIAL>
sealert -a /var/log/audit/audit.log # parse all denial messages out of file
# Common issues:
semanage fcontext -a -t <TYPE> '<PATH>(/.*)?' # add path to the list of standard file contexts
restorecon -Rv <PATH> # apply the new file contexts
touch /.autorelabel # perform automatic relabel of all files after disabled->enforcing, which causes all files to be 'unlabeled_t'
semanage boolean --list # current/default state + description of all SELinux toggles
setsebool -P <BOOLEAN=ON/OFF> # boolean values updated permanently, without -P only in memory
semanage port -a -t <TYPE> -p tcp <PORT> # label an unlabeled port (e.g. http_port_t on port 8001 etc.)
# vsftpd public directory, where anonymous are allowed to write:
semanage fcontext -a -t public_content_rw_t '/var/ftp/pub(/.*)?'
setsebool -P allow_ftpd_anon_write=1# Configuring PAM (Pluggable Authentication Modules):
vim /etc/pam.d/<SERVICE> # if none is found, /etc/pam.d/other will block all access
type control module-path [module-arguments] # shouldn't be configured by hand, but by "authconfig" tools
# Troubleshooting PAM:
tail -f /var/log/secure
journalctl -u <PROBLEMATIC_SERVICE> # good start for logging issues: 'journalctl _COMM=login'
rpm -V <PROBLEMATIC_SERVICE> # did the files belonging to service change, especially PAM config?
mv /etc/pam.d/<PROBLEMATIC_FILE>{,.broken} # rename broken PAM config, otherwise reinstall will not touch it
yum reinstall <PROBLEMATIC_SERVICE>
diff -u /etc/pam.d/<PROBLEMATIC_FILE>{,.broken} # compare good and bad PAM config of problematic service
authconfig
authconfig-tui
authconfig-gtk
authconfig --updateall # recreate all configuration files and re-apply the configuration as stored in /etc/sysconfig/authconfig
yum -y install pam_krb5 # when the module for Kerberos is missing# Solving LDAP issues:
yum -y install openldap-clients # set of tools
cat /etc/openldap/ldap.conf # LDAP defaults (BASE,URI etc.), usually port TCP/389 with STARTTLS
mv <CRT> /etc/openldap/cacerts;cacertdir_rehash # when CAs mismatches are happening, this needs to be done
ldapsearch -x -ZZ -LL '(uid=ldapsuer)' \
cn homeDirectory # -x simple auth, -ZZ enforce TLS, -LL disable comments in output, only return canonical name and home directory
getent passwd <LDAP_USER> # uses nsswitch.conf to query backend password systems
# Solving Kerberos issues:
kinit <USER> # obtain TGT (ticket granting ticket), time must match everywhere!
cat /etc/krb5.conf | grep -A 1 domain_realm # is the [domain_realm] section correct in Kerberos5 config?
yum -y install sssd-common
man sssd-krb5 # when System Security Services Daemon is used (krb5_server,krb5_realm)
/etc/sssd/sssd.conf # this cache may contain KRB5 as well. Change needs restart of sssd
yum -y install krb5-workstation
klist # check if the user received TGT
klist -ek /etc/krb5.keytab # inspect keytabs, KVNO shows version of the password stored. When you overwrite Keytab file with a new one, dependant services (e.g. NFS) must be restarted
cat /etc/exports.d/* /etc/auto.guests /etc/fstab # sec=krb5i vs. sec=krb5p = they must match everywhere, needs autofs restart
ssh -o PreferredAuthentications=keyboard-interactive,password ldapuser@server # when testing LDAP instead of SSH key auth# kdump & kexec:
yum -y install kexec-tools system-config-kdump # provides graphical configuration tool for kdump
cat /etc/default/grub | grep GRUB_CMDLINE_LINUX
... crashkernel=auto ... # after adding this, run "grub2-mkconfig -o ..."
cat /etc/kdump.conf | grep ^path # by default crash dumps go to /var/crash/<IP>-<DATE> (raw,nfs,ssh is possible)
cat /etc/kdump.conf | grep ^core_collector # by default collection is done by "makedumpfile" utility
vim /etc/kdump.conf
core_collector scp # collection of crash dumps using SSH dump targets (needs ssh,ssh_key)
makedumpfile -l --message-level 1 -d 31 # lzo compression, only progress indicator, exclude some pages (-d=dump level)
man 8 makedumpfile # -c=zlib, -l=lzo, -p=snappy, message level 1=Only include progress indicator, 4=Only include error messages, 31=Include all messages
systemctl enable --now kdump # enables and starts, must be restarted when config file is changed
kdumpctl status
kdumpctl showmem # how much memory is reserved for crash kernel
kdumpctl propagate # simplify setup of SSH key (sshkey in kdump.conf) authentication# Kernel crash dump triggers: # 'sysctl -a | grep -e kernel -e vm' is helpful if you don't remember them
echo "vm.panic_on_oom=1" >> /etc/sysctl.conf # panic on OOM-killer events permanently
echo "kernel.hung_task_panic=1" >> /etc/sysctl.conf # panic on hung process perm
cat /proc/sys/kernel/hung_task_timeout_secs # hung task timeout
echo "kernel.softlockup_panic=1" >> /etc/sysctl.conf # soft lockups (kernel loops in kernel mode) perm
echo "kernel.panic_on_io_nmi=1" >> /etc/sysctl.conf # nonrecoverable HW failure (NMI) perm
echo "kernel.sysrq=1" >> /etc/sysctl.conf # enable all magic sysrq (key sequence in case of unresponsive system) perm
echo "c" > /proc/sysrq-trigger # initiate a system crash (other sysrq keys: m,t,p,c,s,u,b,9,f,w)
sysctl -p # load in Kernel parameters# Analyzing kernel crash dumps:
yum -y install kernel-debuginfo
strings vmcore | head # same info as vmcore-dnesg.txt
crash /usr/lib/debug/modules/<KERNEL_VER>/vmlinux \
/var/crash/<IP_ADDRESS-DATE-TIME> # it needs debug version of the kernel image and crash dump# Kernel debugging with SystemTap:
# Install software needed to compile SystemTap modules:
subscription-manager repos --enable rhel-7-server-debug-rpms
yum -y install kernel-debuginfo kernel-devel systemtap
stap-prep # checks current kernel and install matching devel & debuginfo
ls /usr/share/doc/systemtap-client-*/examples # useful *.stp scripts from systemtap package
stap -v /usr/share/doc/systemtap-client-*/examples/process/syscalls_by_proc.stp
# Compile a kernel stap module to a specific dir:
stap -p 4 -v -m syscalls_by_proc \ # generates *.ko in the current dir, '-p 4'=only first 4 steps, -m=filename
/usr/share/doc/systemtap-client-*/examples/process/syscalls_by_proc.stp
# Make module available for users in "stapusr" group:
mkdir /lib/modules/$(uname -r)/systemtap # must be owned by root and not be world writable
ls -ld /lib/modules/$(uname -r)/systemtap
cp /root/syscalls_by_proc.ko /lib/modules/$(uname -r)/systemtap
staprun syscalls_by_proc # run the module, doesn't need to specify extension here
# For only the SystemTap runtime environment you need a single package:
yum -y install systemtap-runtime
man -P 'less +/PERMISSIONS' stap # see the PERMISSIONS section of the stap manpage for all the details
# User in "stapdev" & "stapusr" groups can run the module from anywhere (do this on the destination machine):
usermod -aG stapusr <USER> # can run SystemTap modules, but only if they exist in the /lib/modules/$(uname -r)/systemtap dir
usermod -aG stapdev <USER> # may compile their own SystemTap instrumentation kernel modules using stap, if they are also in 'stapusr', they may use staprun to load a module, even if it does not reside in the /lib/modules/$(uname -r)/systemtap directoryNote: To generate beautiful PDF file, install latex and pandoc:
sudo yum install pandoc pandoc-citeproc texlive
And then use pandoc v1.12.3.1 to output Github Markdown to the PDF:
pandoc -f markdown_github -t latex -V geometry:margin=0.3in -o RH342.pdf RH342.md
For better result (pandoc text-wrap code blocks), you may want to try my listings-setup.tex:
pandoc -f markdown_github --listings -H listings-setup.tex -V geometry:margin=0.3in -o RH342.pdf RH342.md
Thu Jun 20 12:58:03 AEST 2019: Finished four chapters.