[vox-tech] Clock drift bad, HULK SMASH!

[Jeff Newmiller]

On Sat, 23 Feb 2002, Bill Broadley wrote:

> On Wed, Feb 20, 2002 at 05:18:44PM -0800, Ryan wrote:
> > I can't get NTP at school, and at home I'm running Mandrake's firewall 
> > package, which I can't figure out how to get ntp through.
> 
> I'm not really familar with mandrak, but usually something like:
> ipchains -A inet_in -s remote.ntp.host 123 -d 0.0.0.0/0 123 -p UDP -j ACCEPT
> 
> Will work.

Something like that, but there are gotchas.

1) the -A option _appends_ to the chain.  If the chain terminates with
default denys (to enable logging of unspecified connection attempts, for
example) then this rule would never get invoked.  Generally if this is the
case, you should be editing a configuration file to get the rule into the
body of the ruleset and re-load the entire fireall ruleset.

2) the "inet_in" chain is a convention thing... I don't know which
firewall he started with, and different people use different chain names,
so this chain may not be active in the current firewall.

3) this rule only handles inbound packets... would need another rule to
handle outbound packets, or use the "-b" option as shorthand for
"bidirectional".

4) it is a good idea to disallow inbound connection initiation packets
with the "! -y" option on the inbound rule.  this precludes the use of the
"-b" option, since you would not want this restriction on outbound
packets.

5) if you are running kernel 2.4 you could be using iptables, in which
stateful connection management can apply more detailed validation than the
simple "no inbound SYN" described in (4), but I don't know any details
since I am not running 2.4.

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