Securing RHEL - CentOS - Fedora

We’ve had a prompting to investigate our OS security at my work. As a result, I’ve been given a pretty open mandate to investigate and deliver some simple changes that help lock down our systems and make measurable changes to security and incident analysis.

First, I used some common sense. Second, I did my research. Third, I used tools to help look at things that I would otherwise have missed.

The best tool I used was certainly OpenSCAP. Very simple to use, and gives some really basic recommendations that just make sense. Some of it’s answers I took with a grain of salt. For example, account lockout modules in pam aren’t needed, as we handle this via our directory services. But it can highlight areas you may have missed.

To run a scap scan:

yum install scap-security-guide openscap openscap-scanner
FEDORA
oscap xccdf eval --profile xccdf_org.ssgproject.content_profile_common --results /tmp/`hostname`-ssg-results.xml \
--report /tmp/`hostname`-ssg-results.html /usr/share/xml/scap/ssg/content/ssg-fedora-ds.xml
RHEL / CENTOS
oscap xccdf eval --profile xccdf_org.ssgproject.content_profile_common --results /tmp/`hostname`-ssg-results.xml \
--report /tmp/`hostname`-ssg-results.html /usr/share/xml/scap/ssg/content/ssg-rhel7-ds.xml

Then view the output in a web browser.

Here is what I came up with.

– Partitioning –

Sadly, you need to reinstall for these, but worth rolling out for “future builds”. Here is my partition section from ks.conf. Especially important is putting audit on its own partition.

# Partition clearing information
bootloader --location=mbr
clearpart --initlabel --all
# Disk partitioning information
part /boot --fstype=ext4 --size=512 --asprimary --fsoptions=x-systemd.automount,nodev,nosuid,defaults
# LVM
part pv.2 --size=16384 --grow --asprimary
volgroup vg00 pv.2
logvol swap --fstype=swap --size=2048 --name=swap_lv --vgname=vg00
logvol / --fstype=xfs --size=512 --name=root_lv --vgname=vg00 --fsoptions=defaults
logvol /usr --fstype=xfs --size=3072 --name=usr_lv --vgname=vg00 --fsoptions=nodev,defaults
logvol /home --fstype="xfs" --size=512 --name=home_lv --vgname=vg00 --fsoptions=nodev,nosuid,defaults
logvol /var  --fstype=xfs --size=3072 --name=var_lv --vgname=vg00 --fsoptions=nodev,nosuid,noexec,defaults
logvol /var/log --fstype="xfs" --size=1536 --name=var_log_lv --vgname=vg00 --fsoptions=nodev,nosuid,noexec,defaults
logvol /var/log/audit --fstype="xfs" --size=512 --name=var_log_audit_lv --vgname=vg00 --fsoptions=nodev,nosuid,noexec,defaults
logvol /srv --fstype="xfs" --size=512 --name=srv_lv --vgname=vg00 --fsoptions=nodev,nosuid,defaults
logvol /opt --fstype="xfs" --size=512 --name=opt_lv --vgname=vg00 --fsoptions=nodev,nosuid,defaults

With /tmp, if you mount this, and run redhat satellite, you need to be careful. Satellite expects to be able to execute out of /tmp, so don’t set noexec on that partition!

– SSH keys –

It’s just good practice to use these. It saves typing in a password to a prompt which helps to limit credential exposure. We are enabling LDAP backed SSH keys now to make this easier in our workplace.

– SELinux –

SELinux isn’t perfect by any means, but it helps a lot. It can make the work of an attacker more complex, and it can help prevent data leakage via the network. Consider that by default httpd_t cannot make outgoing network connections. This is awesome to prevent data being leaked back to attackers. Well worth the time to setup these policies correctly.

If you have to set permissive to make an application work, do it on a per-domain basis with:

semanage permissive -a httpd_t

This way the protections on all other processes are not removed.

On some of my systems I even run confined staff users to help prevent mistakes / malware from users. I manage this via FreeIPA.

– Auditing –

This allows us to see who / what is altering things on our system. We extended the core auditing rules to include a few extras.

/etc/audit/rules.d/audit.rules

# This file contains the auditctl rules that are loaded
# whenever the audit daemon is started via the initscripts.
# The rules are simply the parameters that would be passed
# to auditctl.

# First rule - delete all
-D

# Increase the buffers to survive stress events.
# Make this bigger for busy systems
-b 8192

#
# Feel free to add below this line. See auditctl man page
-w /etc/ -p wa -k etc_modification

# Detect login log tampering
-w /var/log/faillog -p wa -k logins
-w /var/log/lastlog -p wa -k logins
-w /var/run/utmp -p wa -k session
-w /var/log/btmp -p wa -k session
-w /var/log/wtmp -p wa -k session

# audit_time_rules
## REMOVE STIME ON RHEL
#-a always,exit -F arch=b32 -S stime -S adjtimex -S settimeofday -S clock_settime -k audit_time_rules
#-a always,exit -F arch=b64 -S stime -S adjtimex -S settimeofday -S clock_settime -k audit_time_rules

# audit_rules_networkconfig_modification
-a always,exit -F arch=b32 -S sethostname -S setdomainname -k audit_rules_networkconfig_modification
-a always,exit -F arch=b64 -S sethostname -S setdomainname -k audit_rules_networkconfig_modification

# Audit kernel module manipulation
-a always,exit -F arch=b32 -S init_module -S delete_module -k modules
-a always,exit -F arch=b64 -S init_module -S delete_module -k modules

################################################################################
# These are super paranoid rules at this point. Only use if you are willing to take
# a 3% to 10% perf degredation.

# Perhaps remove the uid limits on some of these actions? We often get attacked via services, not users. These rules are more for workstations...

#-a always,exit -F arch=b32 -S chmod -S chown -S fchmod -S fchmodat -S fchown -S fchownat -S fremovexattr -S fsetxattr -S lchown -S lremovexattr -S lsetxattr -S removexattr -S setxattr -F auid>=500 -F auid!=4294967295 -k perm_mod
#-a always,exit -F arch=b32 -S creat -S open -S openat -S open_by_handle_at -S truncate -S ftruncate -F exit=-EACCES -F auid>=500 -F auid!=4294967295 -k access
#-a always,exit -F arch=b32 -S creat -S open -S openat -S open_by_handle_at -S truncate -S ftruncate -F exit=-EPERM -F auid>=500 -F auid!=4294967295 -k access
#-a always,exit -F arch=b32 -S rmdir -S unlink -S unlinkat -S rename -S renameat -F auid>=500 -F auid!=4294967295 -k delete
# This rule is more useful on a workstation with automount ...
#-a always,exit -F arch=b32 -S mount -F auid>=500 -F auid!=4294967295 -k export

#-a always,exit -F arch=b64 -S chmod -S chown -S fchmod -S fchmodat -S fchown -S fchownat -S fremovexattr -S fsetxattr -S lchown -S lremovexattr -S lsetxattr -S removexattr -S setxattr -F auid>=500 -F auid!=4294967295 -k perm_mod
#-a always,exit -F arch=b64 -S creat -S open -S openat -S open_by_handle_at -S truncate -S ftruncate -F exit=-EACCES -F auid>=500 -F auid!=4294967295 -k access
#-a always,exit -F arch=b64 -S creat -S open -S openat -S open_by_handle_at -S truncate -S ftruncate -F exit=-EPERM -F auid>=500 -F auid!=4294967295 -k access
#-a always,exit -F arch=b64 -S rmdir -S unlink -S unlinkat -S rename -S renameat -F auid>=500 -F auid!=4294967295 -k delete
# This rule is more useful on a workstation with automount ...
#-a always,exit -F arch=b64 -S mount -F auid>=500 -F auid!=4294967295 -k export

# This setting means you need a reboot to changed audit rules.
#  probably worth doing ....
#-e 2

To handle all the extra events I increased my audit logging sizes

/etc/audit/auditd.conf

log_file = /var/log/audit/audit.log
log_format = RAW
log_group = root
priority_boost = 4
flush = INCREMENTAL
freq = 20
num_logs = 5
disp_qos = lossy
dispatcher = /sbin/audispd
name_format = NONE
max_log_file = 20
max_log_file_action = ROTATE
space_left = 100
space_left_action = EMAIL
action_mail_acct = root
admin_space_left = 75
admin_space_left_action = SUSPEND
admin_space_left_action = email
disk_full_action = SUSPEND
disk_error_action = SUSPEND
tcp_listen_queue = 5
tcp_max_per_addr = 1
tcp_client_max_idle = 0
enable_krb5 = no
krb5_principal = auditd

– PAM and null passwords –

Scap noticed that the default config of password-auth-ac contained nullok on some lines. Remove this:

BEFORE
auth        sufficient    pam_unix.so nullok try_first_pass
AFTER
auth        sufficient    pam_unix.so try_first_pass

– Firewall (Backups, SMH, NRPE) –

Backup clients (Amanda, netbackup, commvault) tend to have very high privilege, no SELinux, and are security swiss cheese. Similar is true for vendor systems like HP system management homepage, and NRPE (nagios). It’s well worth locking these down. Before we had blanket “port open” rules, now these are tighter.

In iptables, you should use the “-s” to specify a source range these are allowed to connect from. The smaller the range, the better.

In firewalld, you need to use the rich language. Which is a bit more verbose, and finicky than iptables. My rules end up as:

rule family="ipv4" source address="10.0.0.0/24" port port="2381" protocol="tcp" accept

For example. Use the firewalld-cmd with the –add-rich-rule, or use ansibles rich_rule options.

– AIDE (HIDS) –

Aide is a fantastic and simple file integrity checker. I have an ansible role that I can tack onto the end of all my playbooks to automatically update the AIDE database so that it stays consistent with changes, but will allow us to see out of band changes.

The default AIDE config often picks up files that change frequently. I have an aide.conf that still provides function, but without triggering false alarms. I include aide-local.conf so that other teams / staff can add application specific aide monitoring that doesn’t conflict with my work.

# Example configuration file for AIDE.

@@define DBDIR /var/lib/aide
@@define LOGDIR /var/log/aide

# The location of the database to be read.
database=file:@@{DBDIR}/aide.db.gz

# The location of the database to be written.
#database_out=sql:host:port:database:login_name:passwd:table
#database_out=file:aide.db.new
database_out=file:@@{DBDIR}/aide.db.new.gz

# Whether to gzip the output to database
gzip_dbout=yes

# Default.
verbose=5

#report_url=file:@@{LOGDIR}/aide.log
report_url=stdout
#report_url=stderr
#NOT IMPLEMENTED report_url=mailto:root@foo.com
report_url=syslog:LOG_AUTH

# These are the default rules.
#
#p:      permissions
#i:      inode:
#n:      number of links
#u:      user
#g:      group
#s:      size
#b:      block count
#m:      mtime
#a:      atime
#c:      ctime
#S:      check for growing size
#acl:           Access Control Lists
#selinux        SELinux security context
#xattrs:        Extended file attributes
#md5:    md5 checksum
#sha1:   sha1 checksum
#sha256:        sha256 checksum
#sha512:        sha512 checksum
#rmd160: rmd160 checksum
#tiger:  tiger checksum

#haval:  haval checksum (MHASH only)
#gost:   gost checksum (MHASH only)
#crc32:  crc32 checksum (MHASH only)
#whirlpool:     whirlpool checksum (MHASH only)

FIPSR = p+i+n+u+g+s+m+c+acl+selinux+xattrs+sha256

# Fips without time because of some database/sqlite issues
FIPSRMT = p+i+n+u+g+s+acl+selinux+xattrs+sha256

#R:             p+i+n+u+g+s+m+c+acl+selinux+xattrs+md5
#L:             p+i+n+u+g+acl+selinux+xattrs
#E:             Empty group
#>:             Growing logfile p+u+g+i+n+S+acl+selinux+xattrs

# You can create custom rules like this.
# With MHASH...
# ALLXTRAHASHES = sha1+rmd160+sha256+sha512+whirlpool+tiger+haval+gost+crc32
ALLXTRAHASHES = sha1+rmd160+sha256+sha512+tiger
# Everything but access time (Ie. all changes)
EVERYTHING = R+ALLXTRAHASHES

# Sane, with multiple hashes
# NORMAL = R+rmd160+sha256+whirlpool
NORMAL = FIPSR+sha512

# For directories, don't bother doing hashes
DIR = p+i+n+u+g+acl+selinux+xattrs

# Access control only
PERMS = p+i+u+g+acl+selinux+xattrs

# Logfile are special, in that they often change
LOG = >

# Just do sha256 and sha512 hashes
LSPP = FIPSR+sha512
LSPPMT = FIPSRMT+sha512

# Some files get updated automatically, so the inode/ctime/mtime change
# but we want to know when the data inside them changes
DATAONLY =  p+n+u+g+s+acl+selinux+xattrs+sha256

# Next decide what directories/files you want in the database.

/boot   NORMAL
/bin    NORMAL
/sbin   NORMAL
/usr/bin NORMAL
/usr/sbin NORMAL
/lib    NORMAL
/lib64  NORMAL
# These may be too variable
/opt    NORMAL
/srv    NORMAL
# These are too volatile
# We can check USR if we want, but it doesn't net us much.
#/usr    NORMAL
!/usr/src
!/usr/tmp

# Check only permissions, inode, user and group for /etc, but
# cover some important files closely.
/etc    PERMS
!/etc/mtab
# Ignore backup files
!/etc/.*~
/etc/exports  NORMAL
/etc/fstab    NORMAL
/etc/passwd   NORMAL
/etc/group    NORMAL
/etc/gshadow  NORMAL
/etc/shadow   NORMAL
/etc/security/opasswd   NORMAL

/etc/hosts.allow   NORMAL
/etc/hosts.deny    NORMAL

/etc/sudoers NORMAL
/etc/sudoers.d NORMAL
/etc/skel NORMAL

/etc/logrotate.d NORMAL

/etc/resolv.conf DATAONLY

/etc/nscd.conf NORMAL
/etc/securetty NORMAL

# Shell/X starting files
/etc/profile NORMAL
/etc/bashrc NORMAL
/etc/bash_completion.d/ NORMAL
/etc/login.defs NORMAL
/etc/zprofile NORMAL
/etc/zshrc NORMAL
/etc/zlogin NORMAL
/etc/zlogout NORMAL
/etc/profile.d/ NORMAL
/etc/X11/ NORMAL

# Pkg manager
/etc/yum.conf NORMAL
/etc/yumex.conf NORMAL
/etc/yumex.profiles.conf NORMAL
/etc/yum/ NORMAL
/etc/yum.repos.d/ NORMAL

# Ignore lvm files that change regularly
!/etc/lvm/archive
!/etc/lvm/backup
!/etc/lvm/cache

# Don't scan log by default, because not everything is a "growing log file".
!/var/log   LOG
!/var/run/utmp LOG

# This gets new/removes-old filenames daily
!/var/log/sa
# As we are checking it, we've truncated yesterdays size to zero.
!/var/log/aide.log
!/var/log/journal

# LSPP rules...
# AIDE produces an audit record, so this becomes perpetual motion.
# /var/log/audit/ LSPP
/etc/audit/ LSPP
/etc/audisp/ LSPP
/etc/libaudit.conf LSPP
/usr/sbin/stunnel LSPP
/var/spool/at LSPP
/etc/at.allow LSPP
/etc/at.deny LSPP
/etc/cron.allow LSPP
/etc/cron.deny LSPP
/etc/cron.d/ LSPP
/etc/cron.daily/ LSPP
/etc/cron.hourly/ LSPP
/etc/cron.monthly/ LSPP
/etc/cron.weekly/ LSPP
/etc/crontab LSPP
/var/spool/cron/root LSPP

/etc/login.defs LSPP
/etc/securetty LSPP
/var/log/faillog LSPP
/var/log/lastlog LSPP

/etc/hosts LSPP
/etc/sysconfig LSPP

/etc/inittab LSPP
#/etc/grub/ LSPP
/etc/rc.d LSPP

/etc/ld.so.conf LSPP

/etc/localtime LSPP

/etc/sysctl.conf LSPP

/etc/modprobe.conf LSPP

/etc/pam.d LSPP
/etc/security LSPP
/etc/aliases LSPP
/etc/postfix LSPP

/etc/ssh/sshd_config LSPP
/etc/ssh/ssh_config LSPP

/etc/stunnel LSPP

/etc/vsftpd.ftpusers LSPP
/etc/vsftpd LSPP

/etc/issue LSPP
/etc/issue.net LSPP

/etc/cups LSPP

# Check our key stores for tampering.
/etc/pki LSPPMT
!/etc/pki/nssdb/
/etc/pki/nssdb/cert8.db LSPP
/etc/pki/nssdb/cert9.db LSPP
/etc/pki/nssdb/key3.db LSPP
/etc/pki/nssdb/key4.db LSPP
/etc/pki/nssdb/pkcs11.txt LSPP
/etc/pki/nssdb/secmod.db LSPP

# Check ldap and auth configurations.
/etc/openldap LSPP
/etc/sssd LSPP

# Ignore the prelink cache as it changes.
!/etc/prelink.cache

# With AIDE's default verbosity level of 5, these would give lots of
# warnings upon tree traversal. It might change with future version.
#
#=/lost\+found    DIR
#=/home           DIR

# Ditto /var/log/sa reason...
!/var/log/and-httpd

#/root   NORMAL
# Admins dot files constantly change, just check PERMS
#/root/\..* PERMS
# Check root sensitive files
/root/.ssh/ NORMAL
/root/.bash_profile NORMAL
/root/.bashrc NORMAL
/root/.cshrc NORMAL
/root/.tcshrc NORMAL
/root/.zshrc NORMAL


@@include /etc/aide-local.conf

– Time –

Make sure you run an NTP client. I’m a fan of chrony these days, as it’s syncs quickly and reliably.

– Collect core dumps and abrt –

Install and run kdump and abrtd so you can analyse why something crashed, to determine if it was malicious or not.

yum install kexec-tools abrt abrt-cli
systemctl enable abrtd

At the same time, you need to alter kdump.conf to dump correctly

xfs /dev/os_vg/var_lv
path /crash
core_collector makedumpfile -l --message-level 7 -d 23,31
default reboot

Finally, append crashkernel=auto to your grub commandline.

– Sysctl –

These are an evolved set of sysctls and improvements to our base install that help tune some basic network and other areas to strengthen the network stack and base OS.

# Ensure ASLR
kernel.randomize_va_space = 2
# limit access to dmesg
## does this affect ansible facts
kernel.dmesg_restrict = 1

# Prevent suid binaries core dumping. Helps to prevent memory / data leaks
fs.suid_dumpable = 0

# https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt
# Controls IP packet forwarding
net.ipv4.ip_forward = 0

# Controls source route verification
net.ipv4.conf.default.rp_filter = 1

# Do not accept source routing
net.ipv4.conf.default.accept_source_route = 0

# Controls the System Request debugging functionality of the kernel
kernel.sysrq = 0

# Controls whether core dumps will append the PID to the core filename.
# Useful for debugging multi-threaded applications.
kernel.core_uses_pid = 1
# Decrease the time default value for tcp_fin_timeout connection
net.ipv4.tcp_fin_timeout = 35
# Decrease the time default value for tcp_keepalive_time connection
net.ipv4.tcp_keepalive_time = 600
# Provide more ports and timewait buckets to increase connectivity
net.ipv4.ip_local_port_range = 8192 61000
net.ipv4.tcp_max_tw_buckets = 1000000

## Network Hardening ##
net.ipv4.tcp_max_syn_backlog = 4096
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.all.secure_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.conf.default.secure_redirects = 0
net.ipv4.icmp_echo_ignore_broadcasts = 1
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0
net.ipv4.icmp_ignore_bogus_error_responses = 1

net.nf_conntrack_max = 262144