Linux Kernel /etc/sysctl.conf Security Hardening nixCraft Updated Tutorials/Posts

How do I set advanced security options of the TCP/IP stack and virtual memory to improve the security and performance of my Linux based system? How do I configure Linux kernel to prevent certain kinds of attacks using /etc/sysctl.conf? How do I set Linux kernel parameters?

sysctl is an interface that allows you to make changes to a running Linux kernel. With /etc/sysctl.conf you can configure various Linux networking and system settings such as:

  1. Limit network-transmitted configuration for IPv4
  2. Limit network-transmitted configuration for IPv6
  3. Turn on execshield protection
  4. Prevent against the common ‘syn flood attack’
  5. Turn on source IP address verification
  6. Prevents a cracker from using a spoofing attack against the IP address of the server.
  7. Logs several types of suspicious packets, such as spoofed packets, source-routed packets, and redirects.

Linux Kernel /etc/sysctl.conf Security Hardening

Linux Kernel /etc/sysctl.conf Security Hardening with sysctl

The sysctl command is used to modify kernel parameters at runtime. /etc/sysctl.conf is a text file containing sysctl values to be read in and set by sysct at boot time. To view current values, enter:
# sysctl -a
# sysctl -A
# sysctl mib
# sysctl net.ipv4.conf.all.rp_filter
# sysctl -a --pattern 'net.ipv4.conf.(eth|wlan)0.arp'

To load settings, enter:
# sysctl -p

Sample /etc/sysctl.conf for Linux server hardening

Edit /etc/sysctl.conf or /etc/sysctl.d/99-custom.conf and update it as follows. The file is documented with comments. However, I recommend reading the official Linux kernel sysctl tuning help file (see below):

# The following is suitable for dedicated web server, mail, ftp server etc. 
# ---------------------------------------
# BOOLEAN Values:
# a) 0 (zero) - disabled / no / false
# b) Non zero - enabled / yes / true
# --------------------------------------
# Controls IP packet forwarding
net.ipv4.ip_forward = 0 # 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 # Controls the use of TCP syncookies
# Turn on SYN-flood protections
net.ipv4.tcp_syncookies = 1
net.ipv4.tcp_synack_retries = 5 ########## IPv4 networking start ##############
# Send redirects, if router, but this is just server
# So no routing allowed 
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0 # Accept packets with SRR option? No
net.ipv4.conf.all.accept_source_route = 0 # Accept Redirects? No, this is not router
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.all.secure_redirects = 0 # Log packets with impossible addresses to kernel log? yes
net.ipv4.conf.all.log_martians = 1
net.ipv4.conf.default.accept_source_route = 0
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.conf.default.secure_redirects = 0 # Ignore all ICMP ECHO and TIMESTAMP requests sent to it via broadcast/multicast
net.ipv4.icmp_echo_ignore_broadcasts = 1 # Prevent against the common 'syn flood attack'
net.ipv4.tcp_syncookies = 1 # Enable source validation by reversed path, as specified in RFC1812
net.ipv4.conf.all.rp_filter = 1 # Controls source route verification
net.ipv4.conf.default.rp_filter = 1 ########## IPv6 networking start ##############
# Number of Router Solicitations to send until assuming no routers are present.
# This is host and not router
net.ipv6.conf.default.router_solicitations = 0 # Accept Router Preference in RA?
net.ipv6.conf.default.accept_ra_rtr_pref = 0 # Learn Prefix Information in Router Advertisement
net.ipv6.conf.default.accept_ra_pinfo = 0 # Setting controls whether the system will accept Hop Limit settings from a router advertisement
net.ipv6.conf.default.accept_ra_defrtr = 0 #router advertisements can cause the system to assign a global unicast address to an interface
net.ipv6.conf.default.autoconf = 0 #how many neighbor solicitations to send out per address?
net.ipv6.conf.default.dad_transmits = 0 # How many global unicast IPv6 addresses can be assigned to each interface?
net.ipv6.conf.default.max_addresses = 1 ########## IPv6 networking ends ############## #Enable ExecShield protection
#Set value to 1 or 2 (recommended) 
#kernel.exec-shield = 2
#kernel.randomize_va_space=2 # TCP and memory optimization 
# increase TCP max buffer size setable using setsockopt()
#net.ipv4.tcp_rmem = 4096 87380 8388608
#net.ipv4.tcp_wmem = 4096 87380 8388608 # increase Linux auto tuning TCP buffer limits
#net.core.rmem_max = 8388608
#net.core.wmem_max = 8388608
#net.core.netdev_max_backlog = 5000
#net.ipv4.tcp_window_scaling = 1 # increase system file descriptor limit 
fs.file-max = 65535 #Allow for more PIDs 
kernel.pid_max = 65536 #Increase system IP port limits
net.ipv4.ip_local_port_range = 2000 65000 # RFC 1337 fix
net.ipv4.tcp_rfc1337=1

Reboot the machine soon after a kernel panic

kernel.panic=10

Addresses of mmap base, heap, stack and VDSO page are randomized

kernel.randomize_va_space=2

Ignore bad ICMP errors

net.ipv4.icmp_ignore_bogus_error_responses=1

Protects against creating or following links under certain conditions

fs.protected_hardlinks=1
fs.protected_symlinks=1

How do I tune Linux VM subsystem?

How do I tune Linux network stack?

Other Linux security tips

References:

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.

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