Documented rationale for sysctl values set.

Signed-off-by: Farid Joubbi <[email protected]>

roles/os_hardening/defaults/main.yml

@@ -137,10 +137,17 @@ sysctl_config:
   # kernel.yama.ptrace_scope = 1
   kernel.yama.ptrace_scope: 1
 
-  # Disable IPv4 traffic forwarding. | sysctl-01
+  # Disable traffic forwarding.
+  # Setting the flag to 0 ensures that a system with multiple interfaces (for example, a hard
+  # proxy), will never be able to forward packets, and therefore, never serve as a router.
+  # | sysctl-01 | sysctl-19
   net.ipv4.ip_forward: 0
+  net.ipv6.conf.all.forwarding: 0
 
-  # Enable RFC-recommended source validation feature. | sysctl-02
+  # Enable RFC-recommended source validation feature.
+  # If the return packet does not go out the same interface that the corresponding
+  # source packet came from, the packet is dropped (and logged if log_martians is set).
+  # | sysctl-02
   net.ipv4.conf.all.rp_filter: 1
   net.ipv4.conf.default.rp_filter: 1
 
@@ -148,7 +155,9 @@ sysctl_config:
   # Make sure to ignore ECHO broadcasts, which are only required in broad network analysis.
   net.ipv4.icmp_echo_ignore_broadcasts: 1
 
-  # There is no reason to accept bogus error responses from ICMP, so ignore them instead. | sysctl-03
+  # Some routers (and some attackers) will send responses that violate RFC-1122 and attempt
+  # to fill up a log file system with many useless error messages.
+  # | sysctl-03
   net.ipv4.icmp_ignore_bogus_error_responses: 1
 
   # Limit the amount of traffic the system uses for ICMP. | sysctl-05
@@ -158,7 +167,7 @@ sysctl_config:
   # source quench, ime exceed, param problem, timestamp reply, information reply | sysctl-06
   net.ipv4.icmp_ratemask: 88089
 
-  # Protect against wrapping sequence numbers at gigabit speeds | sysctl-07
+  # Disable TCP timestamps in order to not reveal system uptime. | sysctl-07
   net.ipv4.tcp_timestamps: 0
 
   # Define restriction level for announcing the local source IP | sysctl-08
@@ -188,16 +197,30 @@ sysctl_config:
   net.ipv4.conf.all.accept_source_route: 0
   net.ipv4.conf.default.accept_source_route: 0
 
-  # For non-routers: don't send redirects, these settings are 0 | sysctl-16
+  # For non-routers: don't send redirects.
+  # An attacker could use a compromised host to send invalid ICMP redirects to other
+  # router devices in an attempt to corrupt routing and have users access a system
+  # set up by the attacker as opposed to a valid system.
+  # | sysctl-16
   net.ipv4.conf.all.send_redirects: 0
   net.ipv4.conf.default.send_redirects: 0
 
-  # log martian packets | sysctl-17
+  # log martian packets
+  # This feature logs packets with un-routable source addresses to the kernel log.
+  # Enabling this feature and logging these packets allows an administrator to investigate
+  # the possibility that an attacker is sending spoofed packets to their system.
+  # | sysctl-17
   net.ipv4.conf.all.log_martians: 1
   net.ipv4.conf.default.log_martians: 1
 
   # Accepting redirects can lead to malicious networking behavior, so disable
-  # it if not needed. | sysctl-13 | sysctl-14 | sysctl-15 | sysctl-20
+  # it if not needed.
+  # Attackers could use bogus ICMP redirect messages to maliciously alter the system
+  # routing tables and get them to send packets to incorrect networks and allow
+  # your system packets to be captured.
+  # Setting net.ipv4.conf.all.secure_redirects to 0 protects the system from
+  # routing table updates by possibly compromised known gateways.
+  # | sysctl-13 | sysctl-14 | sysctl-15 | sysctl-20
   net.ipv4.conf.default.accept_redirects: 0
   net.ipv4.conf.all.accept_redirects: 0
   net.ipv4.conf.all.secure_redirects: 0
@@ -208,9 +231,6 @@ sysctl_config:
   # Disable IPv6 | sysctl-18
   net.ipv6.conf.all.disable_ipv6: 1
 
-  # Disable IPv6 traffic forwarding. | sysctl-19
-  net.ipv6.conf.all.forwarding: 0
-
   # ignore RAs on Ipv6. | sysctl-25
   net.ipv6.conf.all.accept_ra: 0
   net.ipv6.conf.default.accept_ra: 0