Hardening Red Hat Enterprise Linux 5
Steve Grubb, Red Hat (Updated 12 August 2010)

Hardening RHEL5

Learn a little about some threats Go over some often missed configuration items Show how to make the system security better

System Update

Keep your system updated! If we know there is a problem, you should seriously consider taking the update Some vulnerabilities can be mitigated by configuration Some cannot
How Do We Find Vulnerabilities?

March 2005 March 2007

Setting a severity rating
Based on a technical assessment of the flaw, not the threat Unique to each Red Hat Enterprise Linux distribution Sets the priority through Engineering and QA Trend tracking (source, reported, public)

Severity Rating

Critical A vulnerability whose exploitation could allow the propagation of an Internet worm without user action.
Important easily compromise the Confidentiality, Integrity or Availability of resources
Moderate harder or more unlikely to be exploitable
Low unlikely circumstances. or where a successful exploit would lead to minimal consequences

Release Policy

For critical vulnerabilities
Will be pushed immediately as embargo is lifted, or when passed QE Will be pushed at any time or day May be held until reasonable time or day May be held until other issues come up in the same package, or the next Update release
For important vulnerabilities
For moderate or low vulnerabilities
secalert @redhat.com - Address used for internal and external customers to ask security vulnerability related questions
Reporting new vulnerabilities Asking how we addressed various vulnerabilities

Partitioning

Keep directories that users can write to on their own partition
Prevents hard linking to setuid programs Allows precise control over mount options
$ ls -li test 13697075 -rwsr-x--- 1 root root 8666 2008-02-15 14:20 test $ ln./test test2 $ ls -li test-rwsr-x--- 2 root root 8666 2008-02-15 14:20 test2 $ make gcc -g -W -Wall -Wundef test.c -o test $ ls -li test 13697055 -rwsr-x--- 1 root root 8948 2008-02-17 15:53 test $ ls -li test-rwsr-x--- 1 root root 8666 2008-02-15 14:20 test2
Allow minimal privileges via mount options Noexec on everything possible Nodev everywhere except / and chroot partitions Nosetuid everywhere except / Consider making /var/tmp link to /tmp, or maybe mount bind option A reasonable /etc/fstab:
LABEL=/ LABEL=/tmp LABEL=/var/log/audit LABEL=/home LABEL=/var LABEL=/boot /tmp tmpfs devpts sysfs proc LABEL=SWAP-sda6 / /tmp /var/log/audit /home /var /boot /var/tmp /dev/shm /dev/pts /sys /proc swap ext3 ext3 ext3 ext3 ext3 ext3 ext3 tmpfs devpts sysfs proc swap defaults defaults,nosuid,noexec,nodev defaults,nosuid,noexec,nodev defaults,nosuid,nodev defaults,nosuid defaults,nosuid,noexec,nodev defaults,bind,nosuid,noexec,nodev defaults,nosuid,noexec,nodev gid=5,mode=0 defaults defaults defaults 0 0

Network Configuration

Strategy Minimize protocols being used Minimize addresses being listened to Minimize ports being listened on Tools that help ifconfig look at device and address mappings netstat look at processes and their socket states route look at the routing table nmap scan the system from outside the firewall
On by default There are daemons that are IPv6 aware: sshd, apache, bind, xinetd, etc Ip6tables has to be specifically setup Could have service unexpectedly open to attack ifconfig | grep inet6 inet6 addr: fe80::21d:7eff:fe00:af5d/64 Scope:Link inet6 addr: ::1/128 Scope:Host Create a file /etc/modprobe.d/ipv6 Add this line inside: install ipv6 /bin/true

Detection

Disabling
Zeroconf On by default Used by avahi for local service discovery Requires a hole in firewall to allow access Advertises services to others Detection route | grep link-local link-local * 255.255.0.0 U 0 eth2 Disabling Edit /etc/sysconfig/network Add NOZEROCONF=yes Then remove the avahi package and its dependencies

Review Listening Daemons Default install is tuned for general use Probably a few things that are unnecessary Detection netstat -tanp | grep LISTEN Typical output:
[root ~]# netstat -tanp | grep LISTEN tcp 127.0.0.1:8000 0.0.0.0:* tcp 127.0.0.1:3306 0.0.0.0:* tcp 127.0.0.1:4690 0.0.0.0:* tcp 127.0.0.1:631 0.0.0.0:* tcp 127.0.0.1:25 0.0.0.0:* tcp :::22 :::* LISTEN LISTEN LISTEN LISTEN LISTEN LISTEN 2256/nasd 2166/mysqld 2376/prelude-manage 2057/cupsd 2244/master 2068/sshd
Disabling Listening Daemons Locate the pid in the netstat command cat /proc/<pid>/cmdline If not full path, run which or locate to find utility rpm -qf full-path-of-daemon rpm -e package If difficult to remove due to dependencies: chkconfig <service> off
/etc/sysctl.conf settings
# Don't reply to broadcasts. Prevents joining a smurf attack net.ipv4.icmp_echo_ignore_broadcasts = 1 # Enable protection for bad icmp error messages net.ipv4.icmp_ignore_bogus_error_responses = 1 # Enable syncookies for SYN flood attack protection net.ipv4.tcp_syncookies = 1 # Log spoofed, source routed, and redirect packets net.ipv4.conf.all.log_martians = 1 net.ipv4.conf.default.log_martians = 1
# Don't allow source routed packets net.ipv4.conf.all.accept_source_route = 0 net.ipv4.conf.default.accept_source_route = 0 # Turn on reverse path filtering net.ipv4.conf.all.rp_filter = 1 net.ipv4.conf.default.rp_filter = 1 # Don't allow outsiders to alter the routing tables net.ipv4.conf.all.accept_redirects = 0 net.ipv4.conf.default.accept_redirects = 0 net.ipv4.conf.all.secure_redirects = 0 net.ipv4.conf.default.secure_redirects = 0 # Don't pass traffic between networks or act as a router net.ipv4.ip_forward = 0 net.ipv4.conf.all.send_redirects = 0 net.ipv4.conf.default.send_redirects = 0
Iptables Default should be pretty good To see rules: service iptables status Use a GUI tool if not familiar with iptables rule syntax Use nmap from another machine to check effectiveness
tcp_wrappers Even if iptables is in use, configure this just in case Set /etc/hosts.deny to ALL: ALL Many daemons compiled with support Find by using: egrep libwrap /usr/bin/* /usr/sbin/* | sort For each program found, use its base name to set expected access rights (if there are any) Example: smbd: 192.168.1.

Unused Daemon Removal

Remove all daemons (and packages) not being used This reduces attack footprint and improves performance Many daemons listen on the network and could be accessible Viewing chkconfig list Disabling rpm -qf /etc/rc.d/init.d/name rpm -e package-name OR chkconfig <service> off
Notes Leave cpuspeed for speedshifting cpu and irqbalance for multicore CPU Disable readahead, mcstransd, firstboot, (and NetworkManager for machines without wireless networking) since they are not needed.

System Time

Keep system time in sync You may need to correlate the time of disparate events across several machines to determine a chain of events Near impossible without common time base Use ntp in cron job Create a file /etc/cron.daily/ntpdate containing the following crontab: #!/bin/sh /usr/sbin/ntpdate ntp-server where ntp-server is the hostname or IP address of the site NTP server
Configure Remaining Daemons
At & cron Only allow root and people with verified need to run cron jobs Setup cron.allow and cron.deny Setup equivalents if you have 'at' installed Sshd
Enable only ssh2 protocol (this is default in RHEL5) If multi-homed, consider if it needs to listen on all addresses or just one Do not allow root logins Consider adding group permission for logins, AllowGroups wheel
MySQL If database is used internally to machine, make it listen on localhost Change passwords
Bind Use chroot package Use ACLs Consider who the DNS server is used for (internal/external) and only serve DNS for those. Do not do both in one server instance. Do not allow zone transfers Do not do recursion Apache Remove all unneeded modules Use mod_security to weed out injection attacks Set correct SE Linux Booleans to maintain functionality and protection
Disable interactive boot by editing /etc/sysconfig/init Make PROMPT=no to disable Also add password to single user mode. Edit /etc/inittab Add the following ~~:S:wait:/sbin/sulogin

SE Linux

Leave enabled and in enforcing mode Does not affect daemons it doesn't know about - unless they are started in a confined domain, apache cgi-bin programs for example Provides a behavioral model that known applications should be following Can stop attacks before they become complete system breaches Use targeted policy Strict and MLS should be used only if you need that kind of protection Do boolean lockdown Review all booleans and set appropriately getsebool -a Generally, to secure the machine, look at things that are set to 'on' and change to 'off' if they do not apply

SE Linux Boolean Lockdown
[root ~]# getsebool -a | grep ' on' allow_daemons_dump_core --> on allow_daemons_use_tty --> on allow_execmem --> on allow_execstack --> on allow_gadmin_exec_content --> on allow_gssd_read_tmp --> on allow_kerberos --> on allow_mounton_anydir --> on allow_postfix_local_write_mail_spool --> on allow_staff_exec_content --> on allow_sysadm_exec_content --> on allow_unconfined_exec_content --> on allow_unlabeled_packets --> on allow_user_exec_content --> on allow_xserver_execmem --> on allow_zebra_write_config --> on browser_confine_xguest --> on httpd_builtin_scripting --> on httpd_enable_cgi --> on httpd_enable_homedirs --> on httpd_tty_comm --> on httpd_unified --> on nfs_export_all_ro --> on nfs_export_all_rw --> on read_default_t --> on samba_run_unconfined --> on spamd_enable_home_dirs --> on use_nfs_home_dirs --> on user_ping --> on

Enable

Install auditd chkconfig auditd on Audit daemon will turn on kernel auditing at boot and load rules
Setup correctly Add audit=1 to grub.conf kernel config line Have /var/log/audit on its own partition Edit /etc/audit/auditd.conf flush parameter should be set to sync or data max_log_file and num_logs need to be adjusted so that you get complete use of your partition space_left should be set to a number that gives the admin enough time to react to any alert message and perform some maintenance to free up disk space disk_full_action is triggered when no more room exists on the partition. All access should be terminated since no more audit capability exists.

Auditd

Set some defaults Place watches on critical files Edit /etc/audit/audit.rules -w /etc/shadow -p wa -k shadow Monitor important syscalls -a exit,always -S open -S openat -F exit=-EPERM Auditd package has CAPP, LSPP, and NISPOM rules for samples Syscall rules are evaluated for every syscall of every program! Use judiciously Review aureport output regularly Aureport gives system security summary report

Aureport system summary

Summary Report ====================== Range of time in logs: 07/22/2006 08:29:01.394 - 05/07/2007 16:12:29.832 Selected time for report: 05/01/2007 00:00:01 - 05/07/2007 16:12:29.832 Number of changes in configuration: 85 Number of changes to accounts, groups, or roles: 2 Number of logins: 25 Number of failed logins: 1 Number of authentications: 29 Number of failed authentications: 1 Number of users: 2 Number of terminals: 11 Number of host names: 3 Number of executables: 59 Number of files: 3 Number of AVC denials: 46 Number of MAC events: 21 Number of failed syscalls: 16 Number of anomaly events: 33 Number of responses to anomaly events: 0 Number of crypto events: 0 Number of process IDs: 4087 Number of events: 5885

Access Control

Do not allow root logins This messes up the audit system since root is a shared account Sshd and gdm have settings to disallow root login pam_tally2 This is used to lockout an account for consecutive failed login attempts pam_access Used to forbid logins from certain locations, consoles, and accounts /etc/security/access.conf controls its config pam_time Used to forbid logins during non-business hours /etc/security/time.conf controls its config
pam_limits Used to limit maximum concurrent sessions and other user restrictions /etc/security/limits.conf controls its config pam_loginuid Used for all entry point daemons to set the task's loginuid and session identifier Loginuid and session ID are inherited by all processes at fork Stored inside the task struct in the kernel Using require-auditd module option will forbid login if auditd is not running Limit access to su command Edit /etc/pam.d/su Uncomment the line saying require wheel to allow uid change auth required pam_wheel.so use_uid

Disable Unused Devices

USB Mass Storage This can be used to transfer files in and out of the system Best to disable when possible by editing a file /etc/modprobe.d/no-usb Add this line inside: install usb-storage /bin/true Wireless Disable in BIOS rm -rf /lib/modules/2.6.18*/kernel/drivers/net/wireless/* Must be run after each upgrade working on something better Firewire Check for /etc/modprobe.d/blacklist-firewire If not there, disable when possible by creating a file /etc/modprobe.d/no-firewire Add this line inside: install firewire_ohci /bin/true

Secure Physical Machine

Disable boot to anything except hard drive Do not allow booting from CD/DVD or USB devices Disable any hardware unused Protects against device driver flaws should any ever be found Lock BIOS After making sure to disallow USB booting, you don't want anyone to undo it Set grub password

Integrity Checking

Abstract Machine Test utility Memory, network, disk, cpu security tests Can be run as cron job to repeatedly assure basic security assumptions Results sent to audit system File Integrity testing utility Configured by /etc/aide.conf --init snapshots the disksystem to /var/lib/aide/aide.db.new.gz Copy snapshot to immutable or safe location Rename snapshot to /var/lib/aide/aide.db.gz before doing comparison --check will compare current with snapshot for differences Summary sent to audit system
New Security Features since RHEL5 GA
NULL Pointer Dereference Protection MAP_FIXED flag to mmap syscall can be used to map page 0. vm.mmap_min_addr sysctl defaults to 64k SE Linux policy arbitrates access and CAP_SYS_RAWIO for DAC SHA256 Password hashes Previously only md5 and des, now sha256 and sha512 have been added authconfig --passalgo=sha256 --update Rsyslog Regex file splitting Execute commands TCP connection Database backend TCG/TPM Tech preview in 5.2