Disable TCP and ICMP Timestamps
Disable TCP Timestamps
|TCP timestamps provide protection against wrapped sequence numbers.|
The downside of TCP timestamps is adversaries can remotely calculate the system uptime and boot time of the machine and the host's clock down to millisecond precision. These calculated uptimes and boot times can also help to detect hidden network-enabled operating systems, as well as link spoofed IP and MAC addresses together and more. 
To prevent this information leaking to an adversary, it is recommended to disable TCP timestamps on any operating systems being used. The less information available to attackers, the better the security.
To temporarily disable TCP timestamps for testing purposes (rather than permanently), see the footnote. 
Open a terminal (Konsole).
Add the following line to /etc/sysctl.d/tcp_timestamps.conf.
net.ipv4.tcp_timestamps = 0
To do that, use the following command.
echo "net.ipv4.tcp_timestamps = 0" > /etc/sysctl.d/tcp_timestamps.conf
To apply the sysctl settings without a reboot, run the following command.
Check if the changes have been properly set.
If it worked correctly, the system should provide the following output.
net.ipv4.tcp_timestamps = 0
|This procedure is untested. It should also work for BSD-like operating systems.|
Users must disable rfc1323 which handles TCP timestamps. To check system-set TCP values, run. 
A value of 1 against
net.inet.tcp.rfc1323 indicates it is enabled, while 0 indicates it is disabled.
echo net.inet.tcp.rfc1323=0 > /etc/sysctl.conf
To temporarily disable TCP timestamps (until reboot) for testing purposes, run.
sudo sysctl -w net.inet.tcp.rfc1323=0
TCP timestamps are disabled by default in Qubes R3.1 and above. 
|The user must have administrator privileges.|
To disable TCP timestamps on Windows, run the following root command.
netsh int tcp set global timestamps=disabled
Disable ICMP Timestamps
The Internet Control Message Protocol (ICMP) is used by network devices, including routers, to send operational information and error messages such as whether a service is available or if a host/router cannot be reached. Unlike TCP and UDP, it is a network level, not transport layer protocol. Commonly network utilities are based on ICMP messages, such as traceroute and ping. 
The ICMP protocol includes timestamps for time synchronization, with the originating timestamp being set to the time (in milliseconds since midnight) since the sender last touched the packet. A timestamp reply is also generated, consisting of the originating timestamp (sent by the sender) as well as a "receive timestamp", which captures when the timestamp was received and a reply sent. 
ICMP timestamps need to be blocked with the firewall.  This is distribution dependent and varies widely as does having a firewall enabled on your specific OS. Be aware that some distributions do not turn on the firewall by default.
There are many differing ways to accomplish blocking ICMP timestamps via the command line, therefore users are recommended to consult the specific distribution's documentation.  The most straightforward way is to download a GUI front-end (like gufw) to configure the firewall and have it set to silently drop all incoming connections by default, and allow only outgoing traffic from the machine.
|This is untested.|
MacOS systems should have ICMP timestamps disabled by default. Therefore, if the firewall is enabled and "Stealth Mode" is set, the system should not respond to any ICMP requests. This is how to check the system is properly secured: 
System Preferences ->
Security & Privacy ->
Select the Firewall tab ->
Check Firewall is On ->
Click Firewall Options ->
Enable Stealth Mode ->
The "Block all incoming connections" checkbox should also be enabled for greater security.
The user can also manually change or check the timestamp status of ICMP, since the system variable is
net.inet.icmp.timestamp in the /etc/sysctl.conf file. 
To permanently disable ICMP timestamps. 
sudo sh -c "echo net.inet.icmp.timestamp=0 >> /etc/sysctl.conf"
|This is untested.|
The easiest solution is to configure the firewall to block incoming and outgoing ICMP packets with ICMP types 13 (timestamp request) and 14 (timestamp response). 
0 (it is enabled by default). In a terminal, run.
sysctl -w net.inet.icmp.tstamprepl=0
ICMP timestamps are disabled by default in Qubes R3.1 and above. 
|This is untested. Windows users should explicitly check this setting is disabled.|
The firewall in recent Windows operating systems (Win 10, Win 8/8.1, Win 7) should have disabled ICMP settings by default. 
From the Menu
The status of ICMP timestamps can be manually checked and changed on Windows systems via the Firewall settings. 
Right-click on Start button ->
Select Control Panel ->
Select Windows Firewall ->
Select Advanced Settings tab
The ICMP Settings dialog box should show the ICMP timestamp is disabled:
Allow incoming timestamp request is unchecked. 
From the Command Line
ICMP timestamp responses can be disabled via the netsh command line utility. This is necessary for Vista and earlier Windows versions. 
Open a terminal.
Run as an administrator.
netsh firewall set icmpsetting 13 disable
Outgoing ICMP timestamp responses are now blocked.
Note: Users can skip this temporary option and instead apply the chapter's main instructions if a permanent solution is desired.
To dynamically disable TCP timestamping on Linux (when using Qubes: in the NetVM).
echo 0 > /proc/sys/net/ipv4/tcp_timestamps
- Advanced users can of course use IP tables. For example in Debian:
ipchains -p icmp -s $INTIP/0 13 -i $INTIF -j DENYand
ipchains -p icmp -s 0.0.0.0/0 14 -i $EXTIF -j DENY
- For instance, Debian users could edit the /etc/systcl.conf file manually and add
net.ipv4.icmp_echo_ignore_all = 1
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