[CentOS] ASTERISK BOX behind a filewall

gjgowey at tmo.blackberry.net gjgowey at tmo.blackberry.net
Thu Sep 13 00:34:33 UTC 2007

What nat box are you running?  Cable/DSL modem, Cisco router or firewall, or just a plain old home gateway?


Sent from my BlackBerry wireless handheld.

-----Original Message-----
From: "Ross S. W. Walker" <rwalker at medallion.com>

Date: Wed, 12 Sep 2007 20:26:05 
To:"CentOS mailing list" <centos at centos.org>
Subject: RE: [CentOS] ASTERISK BOX behind a filewall

Feizhou wrote:
> Ross S. W. Walker wrote:
> > Feizhou wrote:
> >>>> asterisk <-> nat <-> nat <-> sip client = big pain in the neck.
> >>>>
> >>>> I have never managed to get this to work. Getting the below
> >>>> was trouble
> >>>> enough. Forget about trying to get an asterisk box behind a
> >>>> nat to work
> >>>> with clients outside.
> >>>>
> >>>> asterisk <-> nat <-> sip client.
> >>> Yes, you will need a specific SIP iptables filter for this to
> >>> work from behind a firewall.
> >> Getting it to work with a firewall is not a problem...it is
> >> getting the
> >> thing to work with a natting firewall that is the problem. If
> >> one end is
> >> natted, you can still do some tricks to get it to work but if
> >> both ends
> >> are natted, forget it.
> >
> > Well that was the idea behind the ipfilter stuff. It will change
> > the IPs in the protocol stream to compensate for the NAT.
> It looks like there is a netfilter sip conntrack module.
> >
> > I face the same problem trying to do H.323 behind a NAT'd firewall.
> Man, I stopped playing with netmeeting and gnomemeeting quite
> some time
> ago while waiting for ekiga to be available to support my
> video...only
> that you cannot compile the thing on Centos 4 without some
> major surgery.

Well, no it isn't for Netmeeting or Gnomemeeting, but for gatewaying
our internal Polycom conferencing system to our outside bridging
service. When it comes to video conferencing SIP is still in it's

> >
> >>> I know of an H.323 filter, but haven't explored SIP as we aren't
> >>> running any SIP application here yet.
> >>>
> >>> Another possibility would be a SIP proxy installed on the
> >>> firewall, but it is not as secure as a filter.
> >> asterisk IS a sip proxy.
> >
> > Yes, well what I was hinting at was a dumbed-down install of
> > asterisk installed ON the firewall that would be responsible
> > for handing off calls coming in to and out of the network
> > from/to another larger asterisk system.
> You still have to setup the sip configuration to handle that.
> Not much
> dumb downing on that aspect.

Well yes it's going to need some config, it won't need to know the
full config because it is just going to do a full hand-off to the
internal asterisk server for DID (does sip use DIDs?) routing.

> >
> > That is the setup I had to do with GNU gatekeeper and H.323 since
> > at the time I wasn't able to get the ipfilter h.323 filter to
> > work properly with my Polycom system.
> >
> Ugh. Is that good luck with the sip conntrack module then?

Well, no actually you will probably have better luck then me
because the module was probably written for asterisk behind
a firewall. I was trying to get a proprietary Polycom system
to work which is a little different.


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