htpdate

Synchronize date and time

$ sudo htpdate [host]

$ htpdate -q [host]

$ sudo htpdate -x [host]

$ sudo htpdate -s [host]

htpdate [OPTION]... URL...

-s, --silent
    Suppress normal output. Only error messages will be displayed.

-q, --query
    Query mode. Print the remote time but do not set the local clock. Useful for checking the time difference without requiring root privileges.

-d, --debug
    Enable debug output for detailed information about the request and response.

-t , --attempts=
    Specify the maximum number of attempts to connect to the server and retrieve the time. Default is 1.

-l, --localtime
    Treat the received time as local time rather than UTC. This is usually not recommended as system clocks are typically UTC.

-i , --interval=
    Run in daemon mode, synchronizing the clock every interval seconds. Requires root privileges.

-p , --proxy=
    Use the specified HTTP proxy server. Format: host:port.

-a , --user-agent=
    Specify a custom User-Agent string for the HTTP request.

-c , --certfile=
    Path to the client certificate file for HTTPS authentication.

-k , --keyfile=
    Path to the client private key file for HTTPS authentication.

-r , --rootcafile=
    Path to a file containing trusted CA certificates (PEM format) for verifying the server's SSL certificate.

-H , --hostheader=
    Specify a custom Host header for the HTTP request, useful for virtual hosting.

-S , --ciphers=
    Specify a list of acceptable SSL ciphers (e.g., 'ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH').

-u , --user=
    Specify username and password for proxy authentication.

-v, --verbose
    Enable verbose output, showing more details about the process.

--help
    Display a help message and exit.

--version
    Display version information and exit.

The htpdate command is a utility designed to synchronize the system's local clock by querying the time from a remote HTTP or HTTPS server. It extracts the time information from the Date: header of an HTTP response, making it a viable alternative to the Network Time Protocol (NTP) in environments where NTP ports are blocked but HTTP/HTTPS traffic is permitted. While generally less accurate than NTP due to reliance on TCP/IP latency and the server's HTTP header generation, htpdate provides a simple and effective method for time synchronization, particularly for clients or embedded systems behind strict firewalls or on networks with limited NTP access. It typically requires root privileges to modify the system clock.

htpdate is generally less accurate than NTP for time synchronization due to inherent TCP/IP latency and the imprecise nature of the HTTP Date header. It relies on the remote web server's clock being accurate and correctly set. Furthermore, it requires root privileges to set the system clock, and its effectiveness can be limited by firewalls or proxies that deeply inspect or modify HTTP headers.

htpdate is ideal for systems in highly firewalled environments where only HTTP/HTTPS traffic is allowed. It's also suitable for devices or virtual machines that require basic time synchronization but lack full NTP client capabilities or where a full NTP daemon is considered overkill.

When using HTTPS (https:// URL), htpdate can verify the server's certificate. However, unlike NTP which includes authentication mechanisms, htpdate relies solely on the integrity of the HTTP Date header from the specified server. If the server is compromised or the Date header is manipulated, an attacker could potentially set an incorrect system time.

Developed as a simple alternative to NTP, htpdate addresses scenarios where NTP might be unavailable or blocked by network firewalls. It provides a lightweight solution for basic clock synchronization by leveraging common HTTP/HTTPS ports (80/443), which are often open. Its niche is particularly relevant for embedded systems or clients in restrictive network environments where precise NTP synchronization isn't critical, but some level of clock accuracy is required.

ntpdate(8), ntpd(8), chronyd(8), timedatectl(1), date(1)