rc(8) script -- waiting for the network to become usable

Jeremy Chadwick freebsd at jdc.parodius.com
Sun Apr 18 21:37:30 UTC 2010

I'd like to discuss the possibility of introduction of a new script into
/etc/rc.d base system a script, which when enabled, would provide a way
to wait until the IP networking layer (using ping(8)) is up and usable
before continuing with daemon startup.

I've written a script that's in use on all of our RELENG_8 systems (I
have not tested RELENG_7) which works reliably; I'll include that script
at the bottom of my mail, and also a link to it[1].

Let's discuss.  :-)

The situation which brought this debacle to my attention:

I found that on reboot of some of our systems, ntpdate (used to sync the
clock initially before ntpd would be started) wouldn't work.  The daemon
would report that it couldn't resolve any of the FQDNs within ntp.conf,
and would therefore act as a no-op before continuing on.

This failure had dire consequences -- Dovecot (at least with older
versions; newer seems to behave better[2]) would refuse to start up,
citing "time moved backwards".  Dovecot not starting had a trick-down
effect on Postfix (which was compiled to use Dovecot for SMTP AUTH),
where Postfix would start but all inbound mail would fail due to
Dovecot's SMTP AUTH mech not listening on a domain socket.  Ouch.

Since DNS failure was the root issue, I dug around rc.d/named and found
that Doug had introduced a feature to rc.d/named called "named_wait"
which calls "host $named_wait_host" repetitively (sleeping 1 second
between calls), waiting until successful resolution before continuing
onwards.  This worked (e.g. set named_wait_host to "www.google.com" or
something Internet-bound).

However, named itself still complains during startup about "host
unreachable resolving XXX messages" with regards to the root servers.
These errors were visible in logs, etc... and could cause confusion or
unnecessary worry (they did in my case).

The root cause should be fairly obvious: the physical networking layer
hadn't fully come up by the time named had started.  In other cases, the
physical network was available but layer 2 (ARP) hadn't finished.

So I wrote this.

1) Install script as /usr/local/etc/rc.d/waitnetwork
2) chmod 755 /usr/local/etc/rc.d/waitnetwork
3) Set the following in rc.conf:


Note that this does need to be an IP address and *not* an FQDN.  I've
discussed this reasoning with some others and they agree.  Don't pick
something like either (meaning don't be silly).  :-)

Other parameters you can adjust:

waitnetwork_count   -- passed as ping(8) -c flag  (default 5)
waitnetwork_timeout -- passed as ping(8) -t flag  (default 60)

1) This script requires the $waitnetwork_ip box/router/whatever respond
to ICMP ECHO requests.  Please do not bikeshed on this point; we need
something that works, and this requirement shouldn't be that bad to deal
with (firewall/ACL-wise).  For most folks (co-located in particular),
this could be your default gateway, but you can use whatever you want.

2) The needs of some folks may vary depending upon configuration; "we
have two NICs, dual-homed, so what exactly do I put in waitnetwork_ip?"
Yes, I understand the confusion -- hopefully these folks, given their
topologies, can figure out a way to make this work reliably for them.

3) Other stuff I probably haven't thought of.

For those considering arguing that "we should just wait for the NIC to
come up", that won't work -- what's needed is a way to verify layer 3/4
is usable, not layer 1.

I admit there's no universal way to cover every single person's needs,
but providing a simple framework to at least wait until something is
pingable would be a good starting point; it's better than nothing!

The script also contains some XXX comments which should be reviewed by
anyone willing to commit this into the base system.

[1]: http://jdc.parodius.com/freebsd/waitnetwork
[2]: http://wiki.dovecot.org/TimeMovedBackwards

| Jeremy Chadwick                                   jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |

# $FreeBSD: $

# PROVIDE: waitnetwork
# BEFORE: mountcritremote
# KEYWORD: nojail

# XXX - once/if committed to base, it's better to have mountcritremote
# XXX - REQUIRE waitnetwork, rather than use the above BEFORE line.

. /etc/rc.subr



# XXX - once/if committed to base, the following defaults should
# XXX - be placed into src/etc/defaults/rc.conf instead of here

waitnetwork_enable="NO"		# Wait for network availability before
				# continuing with NETWORKING rc scripts
waitnetwork_ip=""		# IP address to ping
waitnetwork_count="5"		# ping count (see ping(8) -c flag)
waitnetwork_timeout="60"	# ping timeout (see ping(8) -t flag)

	local rc

	if [ -z "${waitnetwork_ip}" ]; then
		warn "You must define an IP address in waitnetwork_ip"

	echo "Waiting for ${waitnetwork_ip} to respond to ICMP..."

	if [ -z "${waitnetwork_timeout}" ]; then
		/sbin/ping -c ${waitnetwork_count} ${waitnetwork_ip} >/dev/null 2>&1
		info "Using timeout of ${waitnetwork_timeout} seconds"
		/sbin/ping -t ${waitnetwork_timeout} -c ${waitnetwork_count} ${waitnetwork_ip} >/dev/null 2>&1

	if [ $rc -eq 0 ]; then
		echo "Host reachable; network considered available."
		echo "No response from IP.  Continuing, but be aware you may not"
		echo "have a fully functional networking layer at this point."

load_rc_config $name
run_rc_command "$1"

More information about the freebsd-stable mailing list