ipconfig: Write NTP server IPs to /proc/net/ipconfig/ntp_servers

Distributed filesystems are most effective when the server and client
clocks are synchronised. Embedded devices often use NFS for their
root filesystem but typically do not contain an RTC, so the clocks of
the NFS server and the embedded device will be out-of-sync when the root
filesystem is mounted (and may not be synchronised until late in the
boot process).

Extend ipconfig with the ability to export IP addresses of NTP servers
it discovers to /proc/net/ipconfig/ntp_servers. They can be supplied as
follows:

 - If ipconfig is configured manually via the "ip=" or "nfsaddrs="
   kernel command line parameters, one NTP server can be specified in
   the new "<ntp0-ip>" parameter.
 - If ipconfig is autoconfigured via DHCP, request DHCP option 42 in
   the DHCPDISCOVER message, and record the IP addresses of up to three
   NTP servers sent by the responding DHCP server in the subsequent
   DHCPOFFER message.

ipconfig will only write the NTP server IP addresses it discovers to
/proc/net/ipconfig/ntp_servers, one per line (in the order received from
the DHCP server, if DHCP autoconfiguration is used); making use of these
NTP servers is the responsibility of a user space process (e.g. an
initrd/initram script that invokes an NTP client before mounting an NFS
root filesystem).

Signed-off-by: Chris Novakovic <chris@chrisn.me.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 29 insertions, 6 deletions

diff --git a/Documentation/filesystems/nfs/nfsroot.txt b/Documentation/filesystems/nfs/nfsroot.txt
index a1030bea60d3..d2963123eb1c 100644
--- a/Documentation/filesystems/nfs/nfsroot.txt
+++ b/Documentation/filesystems/nfs/nfsroot.txt
@@ -5,6 +5,7 @@ Written 1996 by Gero Kuhlmann <gero@gkminix.han.de>
 Updated 1997 by Martin Mares <mj@atrey.karlin.mff.cuni.cz>
 Updated 2006 by Nico Schottelius <nico-kernel-nfsroot@schottelius.org>
 Updated 2006 by Horms <horms@verge.net.au>
+Updated 2018 by Chris Novakovic <chris@chrisn.me.uk>
 
 
 
@@ -79,7 +80,7 @@ nfsroot=[<server-ip>:]<root-dir>[,<nfs-options>]
 
 
 ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>:
-   <dns0-ip>:<dns1-ip>
+   <dns0-ip>:<dns1-ip>:<ntp0-ip>
 
   This parameter tells the kernel how to configure IP addresses of devices
   and also how to set up the IP routing table. It was originally called
@@ -178,9 +179,18 @@ ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>:
   <dns1-ip>	IP address of secondary nameserver.
 		See <dns0-ip>.
 
-  After configuration (whether manual or automatic) is complete, a file is
-  created at /proc/net/pnp in the following format; lines are omitted if
-  their respective value is empty following configuration.
+  <ntp0-ip>	IP address of a Network Time Protocol (NTP) server.
+		Value is exported to /proc/net/ipconfig/ntp_servers, but is
+		otherwise unused (see below).
+
+		Default: None if not using autoconfiguration; determined
+		automatically if using autoconfiguration.
+
+  After configuration (whether manual or automatic) is complete, two files
+  are created in the following format; lines are omitted if their respective
+  value is empty following configuration:
+
+  - /proc/net/pnp:
 
 	#PROTO: <DHCP|BOOTP|RARP|MANUAL>	(depending on configuration method)
 	domain <dns-domain>			(if autoconfigured, the DNS domain)
@@ -189,13 +199,26 @@ ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>:
 	nameserver <dns2-ip>			(tertiary name server IP)
 	bootserver <server-ip>			(NFS server IP)
 
-  <dns-domain> and <dns2-ip> are requested during autoconfiguration; they
-  cannot be specified as part of the "ip=" kernel command line parameter.
+  - /proc/net/ipconfig/ntp_servers:
+
+	<ntp0-ip>				(NTP server IP)
+	<ntp1-ip>				(NTP server IP)
+	<ntp2-ip>				(NTP server IP)
+
+  <dns-domain> and <dns2-ip> (in /proc/net/pnp) and <ntp1-ip> and <ntp2-ip>
+  (in /proc/net/ipconfig/ntp_servers) are requested during autoconfiguration;
+  they cannot be specified as part of the "ip=" kernel command line parameter.
 
   Because the "domain" and "nameserver" options are recognised by DNS
   resolvers, /etc/resolv.conf is often linked to /proc/net/pnp on systems
   that use an NFS root filesystem.
 
+  Note that the kernel will not synchronise the system time with any NTP
+  servers it discovers; this is the responsibility of a user space process
+  (e.g. an initrd/initramfs script that passes the IP addresses listed in
+  /proc/net/ipconfig/ntp_servers to an NTP client before mounting the real
+  root filesystem if it is on NFS).
+
 
 nfsrootdebug