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100 lines
4.8 KiB
TeX
100 lines
4.8 KiB
TeX
\chapter{NTP}
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\gls{ntp} is the source of all evil and \gls{sla}. A network wide source of time configuration for all network nodes, servers, clients etc. is necessary.
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\textbf{Configure timezone}\\In this case it\tsq{s} for \gls{metdst}\textbf{:}
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\begin{txt}
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clock timezone MET 1 0
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clock summer-time MET-DST recurring last Sun Mar 2:00 last Sun Oct 3:00
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\end{txt}
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\textbf{Configure used timezone}\\when doing logging and debugging operations\textbf{:}
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\begin{txt}
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service timestamps debug datetime msec localtime show-timezone
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service timestamps log datetime msec localtime show-timezone
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\end{txt}
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A select number of Cisco switches support synchronization with the hardware clock, too. The standard is to only sync the software clock.\\\cliline{ntp update-calendar}
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\fig{ntp/ntp}{ntp}{\gls{ntp}}
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\gls{ntp} servers are a hierarchical tree with stratum 0 servers as the authoritative in the tree. These servers get their time from either \gls{gprs} satellites or atomic clocks {\footnotesize (i.e. an authoritative time \gls{src})}.
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\subsection{Characteristics}
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\begin{itemize}
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\item Uses \gls{udp} port 123 on both \gls{src} and \gls{dst},
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\item polling interval ranging from 64-1024 sec. Length of interval is dependant upon network cond.,
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\item large differences between \gls{ntp} reference time and local client time will result in increased pooling interval.
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\end{itemize}
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\fig{ntp/ntpstratum}{ntpstratum}{Stratum levels}
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\section{The old NTP from \tsq{85}}
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\textbf{Team Cymru} has a nice template for how to enable \gls{ntp} \textbf{with} \textit{access control} on \gls{ios} and \gls{junos}\footnote{\url{https://www.team-cymru.org/secure-ntp-template.html}}. Shown below is a copy of the \gls{ios} example from Cymrus website.
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\begin{cisco}
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! Core NTP configuration
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ntp update-calendar ! update hardware clock (certain hardware only, i.e. 6509s)
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ntp server 192.0.2.1 ! a time server you sync with
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ntp peer 192.0.2.2 ! a time server you sync with and allow to sync to you
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ntp source Loopback0 ! we recommend using a loopback interface for sending NTP messages if possible
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!
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! NTP access control
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ntp access-group query-only 1 ! deny all NTP control queries
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ntp access-group serve 1 ! deny all NTP time and control queries by default
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ntp access-group peer 10 ! permit time sync to configured peer(s)/server(s) only
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ntp access-group serve-only 20 ! permit NTP time sync requests from a select set of clients
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!
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! access control lists (ACLs)
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access-list 1 remark utility ACL to block everything
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access-list 1 deny any
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!
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access-list 10 remark NTP peers/servers we sync to/with
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access-list 10 permit 192.0.2.1
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access-list 10 permit 192.0.2.2
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access-list 10 deny any
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!
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access-list 20 remark Hosts/Networks we allow to get time from us
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access-list 20 permit 192.0.2.0 0.0.0.255
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access-list 20 deny any
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\end{cisco}
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\textbf{Beware} when running a cisco node as \gls{ntp} master and are using access-list to restrict possible clients/peers. You need to allow 127.127.[0-255].1 in the access-list\footnote{The 3rd octet will vary depending on the node.}. This because the master NTP node in the network uses this \gls{ipv4} address as internal master.
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\section{Secure NTP}
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\subsection{Characteristics}
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\begin{itemize}
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\item \gls{ntp} is insecure be default, whích prompted for \gls{sntp} to come along,
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\item Cisco \gls{ios} devices typically only support MD5 encryption\footnote{\url{https://en.wikipedia.org/wiki/MD5}}
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\end{itemize}
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\subsubsection{Configure SNTP}
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\textbf{Team Cymru} has a nice template for how to enable \gls{sntp} on \gls{ios} and \gls{junos}\footnote{\url{https://www.team-cymru.org/secure-ntp-template.html}}. Shown below is a copy of the \gls{ios} example from Cymrus website.
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\begin{cisco}
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ntp authenticate ! enable NTP authentication
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ntp authentication-key [key-id] md5 [hash] ! define a NTP authentication key
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ntp trusted-key [key-id] ! mark a NTP authentication key as trusted
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ntp peer [peer_address] key [key-id] ! form a authenticated session with a peer
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ntp server [server_address] key [key-id] ! form a authenticated session with a server
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\end{cisco}
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\subsection{Versions}
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Generally today \gls{ntp}v3 or v4 is found. The difference to v4 \textit{(amongst other)} is
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\begin{itemize}
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\item support for \gls{ipv6}.
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\item The security in the protocol is upped to with support for X509 certs.
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\item Automatic calculation of time-distribution\footnote{to archive high time accuracy against lowest bandwidth cost} in a network based upon specific multicast groups leveraging v6 site-local multicast addresses.
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\item \cliline{ntp-server ipv6-addr version 4}
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\end{itemize}
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\textbf{Beware} when using \gls{sntp} that \gls{ntp} cannot be used. The same \gls{udp} port is used.
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