4 Dec
2003
4 Dec
'03
6:43 p.m.
Hi!
-----Original Message-----
From: Martin Anderberg [mailto:martin@namnupplysaren.net]
Sent: Thursday, December 04, 2003 5:26 PM
To: serusers(a)lists.iptel.org
Subject: [Serusers] symmetric nat/ broadband routers
Howdy!
I'm having some trouble with the nathelper module and certain
types of
broadband routers (ie d-link 604 & d-link 624). I'll try to
explain the
situation below and hope that someone is willing to help me
out, because
i'm stuck.
In short, the setup is a ata-186 box (which is symmetric) behind a
d-link 604 (which isn't symmetric at all times).
The nathelper module included in the distribution (both 0.8.11 and
0.8.12) has a function called fix_nated_contact().
fix_nated_contact()
rewrites the contact-header with the source-ip & source-port
of the packet.
However, in some cases (ie non-symmetric d-link router between the
ata-box and the internet) this is a problem since the d-link router
sometimes rewrites the source-port which is then used as a
location in
ser. When the session has timed out on the d-link (doesn't
really seem
to help with the natping) the location-information in ser is
no longer
valid.
Is there any reason why the nathelper rewrites the port in the
contact-header? If the client is symmetric the source-port
and the port
in the contact-header shouldn't differ anyway? I trust there is a
reason, i just dont see it ;)
Yes, the ports at the client are identical, but the NAT router uses
other ports at the public interface, e.g. if the client behind the NAT
uses port 5060 for SIP (send&receive), the NATs publice interface may
user for example port 50000. Therefore, the client listen on port 5060,
but the packets have to be sent to port 50000 of the public IP address
and then the NAT router rewrites the port back to 5060. Hence, the
nathelper modules rewrites the IP address and the port in the contact
header before saving them in the location database.
If the session in the NAT router times out although using natping, thats
a pit. Maybe it helps to ping the proxy from the client, e.g. the
budgetone phones support keep alive pinging.
Klaus
On a side-note, when glancing at nathelper.c it looks
as if
the int len
is calculated with the original values of the header, then filled
through snprintf with values which are not 100% positively the same
length (msg->rcv.src_port). Isn't it for example possible
that the port
in the header is 5060 but the source_port is 22444 (which is one
character longer than the length of len is calculated to).
I hope someone can shed some light over the matter.
/Martin
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