[zorp] tproxy problem

[Tito Flagella]

We are experiencing some problems using:

- cttproxy, version cttproxy-2.4.18-10, applied without any apparent 
problem on a Linux 2.4.9-e.10 kernel, derived from the Redhat "Advanced 
Server" kernel.
- zorp 2

We use an udp Plug proxy, defined as follows:

def udp():
        Service("udp", PlugProxy)
        Receiver(SockAddrInet("10.0.0.191", 20001), "udp")
        Receiver(SockAddrInet("192.168.1.1", 20001), "udp")
        Receiver(SockAddrInet("192.168.7.1", 20001), "udp")

and used through iptables rules, like:

[root@pif root]# iptables --list -t tproxy
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination        
TPROXY     udp  --  dmz.sede-pisa.link.it  gondor.sede-pisa.link.it udp 
spts:1024:65535 dpt:domain TPROXY redirect 0.0.0.0:20001
TPROXY     udp  --  dmz.sede-pisa.link.it  gondor.sede-pisa.link.it udp 
spt:domain dpt:domain TPROXY redirect 0.0.0.0:20001
...

With tcpdump, we observe that DNS queries coming from the internal 
networks (both from a DNS server from the 53 port and from DNS clients 
on non privileged ports) are sometimes redirected to the 20001 port, 
sometimes not. Obviously, when not redirected, zorp is not activated and 
the query doesn't work.

Do you have any idea of what's wrong in our environment?

Thanks a lot,

tito.