Core OS Index

2.2.2. Sysctl

Sysctl references Arch TCP/IP stack hardening, Cyberciti Nginx Hardning, Cyberciti Security Hardening, Grsecurity and PaX Configuration.

Since kernels on machine-ports have PaX and grsecurity, /etc/sysctl.conf can have follow values;

#
# /etc/sysctl.conf: configuration for system variables, see sysctl.conf(5)
#

kernel.printk = 7 1 1 4
kernel.randomize_va_space = 2
# Shared Memory
#kernel.shmmax = 500000000
# Total allocated file handlers that can be allocated
# fs.file-nr=
vm.mmap_min_addr=65536
# Allow for more PIDs (to reduce rollover problems); may break some programs 32768
kernel.pid_max = 65536

#
# Memory Protections
#

# If you say Y here, all ioperm and iopl calls will return an error.
# Ioperm and iopl can be used to modify the running kernel.
# Unfortunately, some programs need this access to operate properly,
# the most notable of which are XFree86 and hwclock. hwclock can be
# remedied by having RTC support in the kernel, so real-time
# clock support is enabled if this option is enabled, to ensure
# that hwclock operates correctly.
#
# If you're using XFree86 or a version of Xorg from 2012 or earlier,
# you may not be able to boot into a graphical environment with this
# option enabled. In this case, you should use the RBAC system instead.
kernel.grsecurity.disable_priv_io = 1

# If you say Y here, attempts to bruteforce exploits against forking
# daemons such as apache or sshd, as well as against suid/sgid binaries
# will be deterred. When a child of a forking daemon is killed by PaX
# or crashes due to an illegal instruction or other suspicious signal,
# the parent process will be delayed 30 seconds upon every subsequent
# fork until the administrator is able to assess the situation and
# restart the daemon.
# In the suid/sgid case, the attempt is logged, the user has all their
# existing instances of the suid/sgid binary terminated and will
# be unable to execute any suid/sgid binaries for 15 minutes.
#
# It is recommended that you also enable signal logging in the auditing
# section so that logs are generated when a process triggers a suspicious
# signal.
# If the sysctl option is enabled, a sysctl option with name
# "deter_bruteforce" is created.
kernel.grsecurity.deter_bruteforce = 1

#
# Filesystem Protections
#

# Optimization for port usefor LBs
# Increase system file descriptor limit
fs.file-max = 65535

# If you say Y here, /tmp race exploits will be prevented, since users
# will no longer be able to follow symlinks owned by other users in
# world-writable +t directories (e.g. /tmp), unless the owner of the
# symlink is the owner of the directory. users will also not be
# able to hardlink to files they do not own. If the sysctl option is
# enabled, a sysctl option with name "linking_restrictions" is created.
kernel.grsecurity.linking_restrictions = 1

# Apache's SymlinksIfOwnerMatch option has an inherent race condition
# that prevents it from being used as a security feature. As Apache
# verifies the symlink by performing a stat() against the target of
# the symlink before it is followed, an attacker can setup a symlink
# to point to a same-owned file, then replace the symlink with one
# that targets another user's file just after Apache "validates" the
# symlink -- a classic TOCTOU race. If you say Y here, a complete,
# race-free replacement for Apache's "SymlinksIfOwnerMatch" option
# will be in place for the group you specify. If the sysctl option
# is enabled, a sysctl option with name "enforce_symlinksifowner" is
# created.
kernel.grsecurity.enforce_symlinksifowner = 1
kernel.grsecurity.symlinkown_gid = 15

# if you say Y here, users will not be able to write to FIFOs they don't
# own in world-writable +t directories (e.g. /tmp), unless the owner of
# the FIFO is the same owner of the directory it's held in. If the sysctl
# option is enabled, a sysctl option with name "fifo_restrictions" is
# created.
kernel.grsecurity.fifo_restrictions = 1

# If you say Y here, a sysctl option with name "romount_protect" will
# be created. By setting this option to 1 at runtime, filesystems
# will be protected in the following ways:
# * No new writable mounts will be allowed
# * Existing read-only mounts won't be able to be remounted read/write
# * Write operations will be denied on all block devices
# This option acts independently of grsec_lock: once it is set to 1,
# it cannot be turned off. Therefore, please be mindful of the resulting
# behavior if this option is enabled in an init script on a read-only
# filesystem.
# Also be aware that as with other root-focused features, GRKERNSEC_KMEM
# and GRKERNSEC_IO should be enabled and module loading disabled via
# config or at runtime.
# This feature is mainly intended for secure embedded systems.
#kernel.grsecurity.romount_protect = 1

# if you say Y here, the capabilities on all processes within a
# chroot jail will be lowered to stop module insertion, raw i/o,
# system and net admin tasks, rebooting the system, modifying immutable
# files, modifying IPC owned by another, and changing the system time.
# This is left an option because it can break some apps. Disable this
# if your chrooted apps are having problems performing those kinds of
# tasks. If the sysctl option is enabled, a sysctl option with
# name "chroot_caps" is created.
kernel.grsecurity.chroot_caps = 1

#kernel.grsecurity.chroot_deny_bad_rename = 1

# If you say Y here, processes inside a chroot will not be able to chmod
# or fchmod files to make them have suid or sgid bits. This protects
# against another published method of breaking a chroot. If the sysctl
# option is enabled, a sysctl option with name "chroot_deny_chmod" is
# created.
kernel.grsecurity.chroot_deny_chmod = 1

# If you say Y here, processes inside a chroot will not be able to chroot
# again outside the chroot. This is a widely used method of breaking
# out of a chroot jail and should not be allowed. If the sysctl
# option is enabled, a sysctl option with name
# "chroot_deny_chroot" is created.
kernel.grsecurity.chroot_deny_chroot = 1

# If you say Y here, a well-known method of breaking chroots by fchdir'ing
# to a file descriptor of the chrooting process that points to a directory
# outside the filesystem will be stopped. If the sysctl option
# is enabled, a sysctl option with name "chroot_deny_fchdir" is created.
kernel.grsecurity.chroot_deny_fchdir = 1

# If you say Y here, processes inside a chroot will not be allowed to
# mknod. The problem with using mknod inside a chroot is that it
# would allow an attacker to create a device entry that is the same
# as one on the physical root of your system, which could range from
# anything from the console device to a device for your harddrive (which
# they could then use to wipe the drive or steal data). It is recommended
# that you say Y here, unless you run into software incompatibilities.
# If the sysctl option is enabled, a sysctl option with name
# "chroot_deny_mknod" is created.
kernel.grsecurity.chroot_deny_mknod = 1

# If you say Y here, processes inside a chroot will not be able to
# mount or remount filesystems. If the sysctl option is enabled, a
# sysctl option with name "chroot_deny_mount" is created.
kernel.grsecurity.chroot_deny_mount = 1

# If you say Y here, processes inside a chroot will not be able to use
# a function called pivot_root() that was introduced in Linux 2.3.41. It
# works similar to chroot in that it changes the root filesystem. This
# function could be misused in a chrooted process to attempt to break out
# of the chroot, and therefore should not be allowed. If the sysctl
# option is enabled, a sysctl option with name "chroot_deny_pivot" is
# created.
kernel.grsecurity.chroot_deny_pivot = 1

# If you say Y here, processes inside a chroot will not be able to attach
# to shared memory segments that were created outside of the chroot jail.
# It is recommended that you say Y here. If the sysctl option is enabled,
# a sysctl option with name "chroot_deny_shmat" is created.
kernel.grsecurity.chroot_deny_shmat = 1

# If you say Y here, an attacker in a chroot will not be able to
# write to sysctl entries, either by sysctl(2) or through a /proc
# interface. It is strongly recommended that you say Y here. If the
# sysctl option is enabled, a sysctl option with name
# "chroot_deny_sysctl" is created.
kernel.grsecurity.chroot_deny_sysctl = 1

# If you say Y here, processes inside a chroot will not be able to
# connect to abstract (meaning not belonging to a filesystem) Unix
# domain sockets that were bound outside of a chroot. It is recommended
# that you say Y here. If the sysctl option is enabled, a sysctl option
# with name "chroot_deny_unix" is created.
kernel.grsecurity.chroot_deny_unix = 1

# If you say Y here, the current working directory of all newly-chrooted
# applications will be set to the the root directory of the chroot.
# The man page on chroot(2) states:
# Note that usually chhroot does not change the current working
# directory, so that `.' can be outside the tree rooted at
# `/'. In particular, the super-user can escape from a
# `chroot jail' by doing `mkdir foo; chroot foo; cd ..'.
#
# It is recommended that you say Y here, since it's not known to break
# any software. If the sysctl option is enabled, a sysctl option with
# name "chroot_enforce_chdir" is created.
kernel.grsecurity.chroot_enforce_chdir = 1

# If you say Y here, processes inside a chroot will not be able to
# kill, send signals with fcntl, ptrace, capget, getpgid, setpgid,
# getsid, or view any process outside of the chroot. If the sysctl
# option is enabled, a sysctl option with name "chroot_findtask" is
# created.
kernel.grsecurity.chroot_findtask = 1

# If you say Y here, processes inside a chroot will not be able to raise
# the priority of processes in the chroot, or alter the priority of
# processes outside the chroot. This provides more security than simply
# removing CAP_SYS_NICE from the process' capability set. If the
# sysctl option is enabled, a sysctl option with name "chroot_restrict_nice"
# is created.
kernel.grsecurity.chroot_restrict_nice = 1

#
# Kernel Auditing
#

# If you say Y here, the exec and chdir logging features will only operate
# on a group you specify. This option is recommended if you only want to
# watch certain users instead of having a large amount of logs from the
# entire system. If the sysctl option is enabled, a sysctl option with
# name "audit_group" is created.
kernel.grsecurity.audit_group = 1

# If you say Y here, all execve() calls will be logged (since the
# other exec*() calls are frontends to execve(), all execution
# will be logged). Useful for shell-servers that like to keep track
# of their users. If the sysctl option is enabled, a sysctl option with
# name "exec_logging" is created.
# WARNING: This option when enabled will produce a LOT of logs, especially
# on an active system.
kernel.grsecurity.exec_logging = 0

# If you say Y here, all attempts to overstep resource limits will
# be logged with the resource name, the requested size, and the current
# limit. It is highly recommended that you say Y here. If the sysctl
# option is enabled, a sysctl option with name "resource_logging" is
# created. If the RBAC system is enabled, the sysctl value is ignored.
kernel.grsecurity.resource_logging = 1

# If you say Y here, all executions inside a chroot jail will be logged
# to syslog. This can cause a large amount of logs if certain
# applications (eg. djb's daemontools) are installed on the system, and
# is therefore left as an option. If the sysctl option is enabled, a
# sysctl option with name "chroot_execlog" is created.
kernel.grsecurity.chroot_execlog = 0

# If you say Y here, all attempts to attach to a process via ptrace
# will be logged. If the sysctl option is enabled, a sysctl option
# with name "audit_ptrace" is created.
#kernel.grsecurity.audit_ptrace = 1

# If you say Y here, all attempts to attach to a process via ptrace
# will be logged. If the sysctl option is enabled, a sysctl option
# with name "audit_ptrace" is created.
kernel.grsecurity.audit_chdir = 0

# If you say Y here, all mounts and unmounts will be logged. If the
# sysctl option is enabled, a sysctl option with name "audit_mount" is
# created.
kernel.grsecurity.audit_mount = 1

# If you say Y here, certain important signals will be logged, such as
# SIGSEGV, which will as a result inform you of when a error in a program
# occurred, which in some cases could mean a possible exploit attempt.
# If the sysctl option is enabled, a sysctl option with name
# "signal_logging" is created.
kernel.grsecurity.signal_logging = 1

# If you say Y here, all failed fork() attempts will be logged.
# This could suggest a fork bomb, or someone attempting to overstep
# their process limit. If the sysctl option is enabled, a sysctl option
# with name "forkfail_logging" is created.
kernel.grsecurity.forkfail_logging = 1

# If you say Y here, any changes of the system clock will be logged.
# If the sysctl option is enabled, a sysctl option with name
# "timechange_logging" is created.
kernel.grsecurity.timechange_logging = 1

# if you say Y here, calls to mmap() and mprotect() with explicit
# usage of PROT_WRITE and PROT_EXEC together will be logged when
# denied by the PAX_MPROTECT feature. This feature will also
# log other problematic scenarios that can occur when PAX_MPROTECT
# is enabled on a binary, like textrels and PT_GNU_STACK. If the
# sysctl option is enabled, a sysctl option with name "rwxmap_logging"
# is created.
kernel.grsecurity.rwxmap_logging = 1

#
# Executable Protections
#

# if you say Y here, non-root users will not be able to use dmesg(8)
# to view the contents of the kernel's circular log buffer.
# The kernel's log buffer often contains kernel addresses and other
# identifying information useful to an attacker in fingerprinting a
# system for a targeted exploit.
# If the sysctl option is enabled, a sysctl option with name "dmesg" is
# created.
kernel.grsecurity.dmesg = 1

# Hide symbol addresses in /proc/kallsyms
kernel.kptr_restrict = 2

# If you say Y here, TTY sniffers and other malicious monitoring
# programs implemented through ptrace will be defeated. If you
# have been using the RBAC system, this option has already been
# enabled for several years for all users, with the ability to make
# fine-grained exceptions.
#
# This option only affects the ability of non-root users to ptrace
# processes that are not a descendent of the ptracing process.
# This means that strace ./binary and gdb ./binary will still work,
# but attaching to arbitrary processes will not. If the sysctl
# option is enabled, a sysctl option with name "harden_ptrace" is
# created.
kernel.grsecurity.harden_ptrace = 1

# If you say Y here, unprivileged users will not be able to ptrace unreadable
# binaries. This option is useful in environments that
# remove the read bits (e.g. file mode 4711) from suid binaries to
# prevent infoleaking of their contents. This option adds
# consistency to the use of that file mode, as the binary could normally
# be read out when run without privileges while ptracing.
#
# If the sysctl option is enabled, a sysctl option with name "ptrace_readexec"
# is created.
kernel.grsecurity.ptrace_readexec = 1

# If you say Y here, a change from a root uid to a non-root uid
# in a multithreaded application will cause the resulting uids,
# gids, supplementary groups, and capabilities in that thread
# to be propagated to the other threads of the process. In most
# cases this is unnecessary, as glibc will emulate this behavior
# on behalf of the application. Other libcs do not act in the
# same way, allowing the other threads of the process to continue
# running with root privileges. If the sysctl option is enabled,
# a sysctl option with name "consistent_setxid" is created.
kernel.grsecurity.consistent_setxid = 1

# If you say Y here, access to overly-permissive IPC objects (shared
# memory, message queues, and semaphores) will be denied for processes
# given the following criteria beyond normal permission checks:
# 1) If the IPC object is world-accessible and the euid doesn't match
# that of the creator or current uid for the IPC object
# 2) If the IPC object is group-accessible and the egid doesn't
# match that of the creator or current gid for the IPC object
# It's a common error to grant too much permission to these objects,
# with impact ranging from denial of service and information leaking to
# privilege escalation. This feature was developed in response to
# research by Tim Brown:
# http://labs.portcullis.co.uk/whitepapers/memory-squatting-attacks-on-system-v-shared-memory/
# who found hundreds of such insecure usages. Processes with
# CAP_IPC_OWNER are still permitted to access these IPC objects.
# If the sysctl option is enabled, a sysctl option with name
# "harden_ipc" is created.
kernel.grsecurity.harden_ipc = 1

# If you say Y here, you will be able to choose a gid to add to the
# supplementary groups of users you want to mark as "untrusted."
# These users will not be able to execute any files that are not in
# root-owned directories writable only by root. If the sysctl option
# is enabled, a sysctl option with name "tpe" is created.
kernel.grsecurity.tpe = 1
kernel.grsecurity.tpe_gid = 100

# If you say Y here, the group you specify in the TPE configuration will
# decide what group TPE restrictions will be *disabled* for. This
# option is useful if you want TPE restrictions to be applied to most
# users on the system. If the sysctl option is enabled, a sysctl option
# with name "tpe_invert" is created. Unlike other sysctl options, this
# entry will default to on for backward-compatibility.
kernel.grsecurity.tpe_invert = 1

# If you say Y here, all non-root users will be covered under
# a weaker TPE restriction. This is separate from, and in addition to,
# the main TPE options that you have selected elsewhere. Thus, if a
# "trusted" GID is chosen, this restriction applies to even that GID.
# Under this restriction, all non-root users will only be allowed to
# execute files in directories they own that are not group or
# world-writable, or in directories owned by root and writable only by
# root. If the sysctl option is enabled, a sysctl option with name
# "tpe_restrict_all" is created.
kernel.grsecurity.tpe_restrict_all = 1

kernel.grsecurity.harden_tty = 1

#
# Network Protections
#

# Increase Linux auto tuning TCP buffer limits
# min, default, and max number of bytes to use
# set max to at least 4MB, or higher if you use very high BDP paths
# Tcp Windows etc
net.core.rmem_max = 8388608
net.core.wmem_max = 8388608
net.core.netdev_max_backlog = 5000
net.ipv4.tcp_window_scaling = 1

# Both ports linux-blob and linux-libre don't build with ipv6
# Disable ipv6
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

# Tuen IPv6
#net.ipv6.conf.default.router_solicitations = 0
#net.ipv6.conf.default.accept_ra_rtr_pref = 0
#net.ipv6.conf.default.accept_ra_pinfo = 0
#net.ipv6.conf.default.accept_ra_defrtr = 0
#net.ipv6.conf.default.autoconf = 0
#net.ipv6.conf.default.dad_transmits = 0
#net.ipv6.conf.default.max_addresses = 0

# Avoid a smurf attack, ping scanning
net.ipv4.icmp_echo_ignore_broadcasts = 1

# Turn on protection for bad icmp error messages
net.ipv4.icmp_ignore_bogus_error_responses = 1

# Turn on syncookies for SYN flood attack protection
net.ipv4.tcp_syncookies = 1

## protect against tcp time-wait assassination hazards
## drop RST packets for sockets in the time-wait state
## (not widely supported outside of linux, but conforms to RFC)
net.ipv4.tcp_rfc1337 = 1

## tcp timestamps
## + protect against wrapping sequence numbers (at gigabit speeds)
## + round trip time calculation implemented in TCP
## - causes extra overhead and allows uptime detection by scanners like nmap
## enable @ gigabit speeds
net.ipv4.tcp_timestamps = 0
#net.ipv4.tcp_timestamps = 1

# Turn on and log spoofed, source routed, and redirect packets
net.ipv4.conf.all.log_martians = 1
net.ipv4.conf.default.log_martians = 1

## ignore echo broadcast requests to prevent being part of smurf attacks (default)
net.ipv4.icmp_echo_ignore_broadcasts = 1

## sets the kernels reverse path filtering mechanism to value 1(on)
## will do source validation of the packet's recieved from all the interfaces on the machine
## protects from attackers that are using ip spoofing methods to do harm
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1
#net.ipv6.conf.default.rp_filter = 1
#net.ipv6.conf.all.rp_filter = 1

# Make sure no one can alter the routing tables
# Act as a router, necessary for Access Point
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
# No source routed packets here
# Discard packets with source routes, ip spoofing
net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.default.accept_source_route = 0

net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0

net.ipv4.ip_forward = 0

# Increase system IP port limits
net.ipv4.ip_local_port_range = 2000 65000

# Increase TCP max buffer size setable using setsockopt()
net.ipv4.tcp_rmem = 4096 87380 8388608
net.ipv4.tcp_wmem = 4096 87380 8388608

# Disable proxy_arp
net.ipv4.conf.default.proxy_arp = 0
net.ipv4.conf.all.proxy_arp = 0

# Disable bootp_relay
net.ipv4.conf.default.bootp_relay = 0
net.ipv4.conf.all.bootp_relay = 0

# Decrease TCP fin timeout
net.ipv4.tcp_fin_timeout = 30
# Decrease TCP keep alive time
net.ipv4.tcp_keepalive_time = 1800
# Sen SynAck retries to 3
net.ipv4.tcp_synack_retries = 3

# If you say Y here, neither TCP resets nor ICMP
# destination-unreachable packets will be sent in response to packets
# sent to ports for which no associated listening process exists.
# This feature supports both IPV4 and IPV6 and exempts the
# loopback interface from blackholing. Enabling this feature
# makes a host more resilient to DoS attacks and reduces network
# visibility against scanners.
#
# The blackhole feature as-implemented is equivalent to the FreeBSD
# blackhole feature, as it prevents RST responses to all packets, not
# just SYNs. Under most application behavior this causes no
# problems, but applications (like haproxy) may not close certain
# connections in a way that cleanly terminates them on the remote
# end, leaving the remote host in LAST_ACK state. Because of this
# side-effect and to prevent intentional LAST_ACK DoSes, this
# feature also adds automatic mitigation against such attacks.
# The mitigation drastically reduces the amount of time a socket
# can spend in LAST_ACK state. If you're using haproxy and not
# all servers it connects to have this option enabled, consider
# disabling this feature on the haproxy host.
#
# If the sysctl option is enabled, two sysctl options with names
# "ip_blackhole" and "lastack_retries" will be created.
# While "ip_blackhole" takes the standard zero/non-zero on/off
# toggle, "lastack_retries" uses the same kinds of values as
# "tcp_retries1" and "tcp_retries2". The default value of 4
# prevents a socket from lasting more than 45 seconds in LAST_ACK
# state.
kernel.grsecurity.ip_blackhole = 1
kernel.grsecurity.lastack_retries = 4

# If you say Y here, you will be able to choose a GID of whose users will
# be unable to connect to other hosts from your machine or run server
# applications from your machine. If the sysctl option is enabled, a
# sysctl option with name "socket_all" is created.
kernel.grsecurity.socket_all = 1

# Here you can choose the GID to disable socket access for. Remember to
# add the users you want socket access disabled for to the GID
# specified here. If the sysctl option is enabled, a sysctl option
# with name "socket_all_gid" is created.
kernel.grsecurity.socket_all_gid = 200

# If you say Y here, you will be able to choose a GID of whose users will
# be unable to connect to other hosts from your machine, but will be
# able to run servers. If this option is enabled, all users in the group
# you specify will have to use passive mode when initiating ftp transfers
# from the shell on your machine. If the sysctl option is enabled, a
# sysctl option with name "socket_client" is created.
kernel.grsecurity.socket_client = 1

# Here you can choose the GID to disable client socket access for.
# Remember to add the users you want client socket access disabled for to
# the GID specified here. If the sysctl option is enabled, a sysctl
# option with name "socket_client_gid" is created.
kernel.grsecurity.socket_client_gid = 201

# Here you can choose the GID to disable server socket access for.
# Remember to add the users you want server socket access disabled for to
# the GID specified here. If the sysctl option is enabled, a sysctl
# option with name "socket_server_gid" is created.
kernel.grsecurity.socket_server_gid = 99

#
# Physical Protections
#

# If you say Y here, a new sysctl option with name "deny_new_usb"
# will be created. Setting its value to 1 will prevent any new
# USB devices from being recognized by the OS. Any attempted USB
# device insertion will be logged. This option is intended to be
# used against custom USB devices designed to exploit vulnerabilities
# in various USB device drivers.
#
# For greatest effectiveness, this sysctl should be set after any
# relevant init scripts. This option is safe to enable in distros
# as each user can choose whether or not to toggle the sysctl.
kernel.grsecurity.deny_new_usb = 0

#
# Restrict grsec sysctl changes after this was set
#
kernel.grsecurity.grsec_lock = 0

# End of file

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