[ntp:questions] Is it possible to run ntpd server behind a firewall?

imacat imacat at mail.imacat.idv.tw
Wed Oct 17 09:36:05 UTC 2007


On Wed, 17 Oct 2007 11:14:32 +0800
Shaochun Wang <scwang at ios.ac.cn> wrote:
> Danny Mayer wrote:
> > Shaochun Wang wrote:
> But why the following entry exists in /etc/services file?
> ntp             123/tcp                         # Network Time Protocol

    NTP is "defined" to be using UDP/123.  See the RFC for the protocol
specification:

RFC 958 NTP     http://www.faqs.org/rfcs/rfc958.html
RFC 1305 NTPv3  http://www.faqs.org/rfcs/rfc1305.html
RFC 1769 SNTP   http://www.faqs.org/rfcs/rfc1769.html

    The /etc/services file comes from the IANA port assignment:

http://www.iana.org/assignments/port-numbers

    When IANA officially assigns a port, it always assigns both TCP and
UDP for easier maintainance and fewer confusion.  When you see "ntp
123/tcp" it doesn't necessarily mean that NTP is using TCP/123.  Just to
reserve it in order to prevent future confusion.

    (Am I correct on this? ^^;)

    From the IANA you can see

telnet           23/udp
http             80/udp
https           443/udp
pop3            110/udp

    Telnet/HTTP/HTTPS/POP3 doesn't go UDP, right?  (Oh, you don't know?
^^; Never mind...)

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