[squid-users] Transparent Squid

[Amos Jeffries]

On 29/11/17 02:03, Matus UHLAR - fantomas wrote:
> On 27.11.17 11:24, LINGYUN ZHAO wrote:
>> I need Squid as a real 'transparent' proxy on Fedora without changing 5
>> tuples. Is it possible?
> 
> tuples?
> 

The technical name for an entry in the system NAT table is a 3-tuple or 
4-tuple.

Though what a "5 tuples" means interests me. There are not 5 IP and port 
details in a TCP message. Only two of each.



>> The setup is simple as Client ---------- Fedora --------Server
> 
> is fedora NAT device and also running squid?
> 
>> The Squid version is 3.5.20.The key configuration on Squid as below:
>>
>>   http_port 0.0.0.0:3128 transparent
>>
>>   acl localnet src 10.0.0.0/24
>>
>>   http_access allow localnet
>>
>> And I configured a NAT on Fedora.
>>
>>   iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 80 -j DNAT --to
>> 10.0.0.1:3128
> 
> this only works if "fedora" does the NAT and runs squid.
> (just for sure)
> 
>> When I run curl on Client to server. I found the server receives the
>> traffic with Fedora's IP address and different source port, instead of
>> Client IP address and original source port.
> 
> when you redirect traffic tyo the squid and squid connect to the server,
> it's logical that server sees squid's IP.

Not just logical. That is how NAT works and why "transparent" is such a 
wrong way to describe NAT interception.


> 
> if you want to keep source client's IP, you need tproxy:
> https://wiki.squid-cache.org/Features/Tproxy4
> 

Indeed.

  ... and to answer what appears to be the original question directly:

   No you cannot make Squid use both src-IP *and* src-port from the 
client details. They are already being used by the client TCP connection.


All you can do is pick between one of them or neither.

For "neither" use NAT. NAT interception is *not* transparent - the Squid 
machines IP:port always used on outgoing traffic to servers. Otherwise 
you break the routing system or the NAT system.


For "one" use TPROXY. T(ransparent)PROXY uses only the client src-IP. 
The difference in src-port is the only way to distinguish the packets 
arriving into the Squid machine from server.

Amos