Question

Using IPTABLES to redirect http traffic to a transparent proxy

Asked by: manav08

We have a proxy server which is hosted at 192.168.102.3, running on port 8080.
This proxy is running Squid.
We have another linux server which provides DHCP, DNS, etc. for the 192.168.102.x network. What I want to do is, use IPTABLES on this server to direct all http traffic to our proxy.
Here are the 2 commands I am using.

iptables -t nat -I PREROUTING -i eth0 -p tcp --dport 80 -s ! 192.168.102.3 -j DNAT --to-destination 192.168.102.3:8080
iptables -t nat -I PREROUTING -i eth0 -p tcp --dport 80 -d 192.168.102.1 -RETURN

This does work as it redirects traffic to the proxy but the problem is, that squid doesn;t pick up the URL and comes up with an error.
Example:
http://www.google.com/ - is detected as - /
http://www.google.com/search?hl=en&rls=com.microsoft%3A*&q=test - is detected as - /search?hl=en&rls=com.microsoft%3A*&q=test

Basically it strikes of the main URL completely and hence an error comes up saying INVALID URL. Anyone know how to fix it??

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Asked On: 2009-02-05 at 20:06:15ID24118280
Topics: IP Tables/IP Chains
,; Bourne-Again Shell (bash)
Participating Experts: 1
Points: 200
Comments: 19

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Answers

by: BlazPosted on 2009-02-05 at 23:26:24ID: 23567460

You have to configure squid as a transparent proxy:

http://tldp.org/HOWTO/TransparentProxy.html
http://www.cyberciti.biz/tips/linux-setup-transparent-proxy-squid-howto.html

by: manav08Posted on 2009-02-06 at 05:34:16ID: 23569542

I am using Squid 3.0 and the following are not available.

httpd_accel_host virtual
httpd_accel_port 80
httpd_accel_with_proxy on
httpd_accel_uses_host_header on

Please advise

by: manav08Posted on 2009-02-06 at 05:43:34ID: 23569599

I think the answer to my question might be in

http://kuscsik.blogspot.com/2008/01/transparent-proxy-with-squid-3-on.html

I am unable to access the server now, so I will have to try it on Monday when I get back to work.
Interesting that in SQUID 3.0 they have made it more simple than the previous versions.
Hope this works or I will be back on experts-exchange on Monday.

by: manav08Posted on 2009-02-10 at 16:03:28ID: 23607001

Hi Blaz,

Got the transparent proxy going now. Slight problem now my access.log file shows all traffic coming from 192.168.102.1 whereas earlier it was showing the actual IP addresses. Any ideas?

by: BlazPosted on 2009-02-11 at 00:49:27ID: 23609248

With the DNAT rules above this should not happen.

Do you have any SNAT rules or MASQUERADE rule on the 192.168.102.1 box?

by: manav08Posted on 2009-02-11 at 02:50:20ID: 23609890

No. This is a box that I actually inherited. Is there any way to check??

by: BlazPosted on 2009-02-11 at 03:36:08ID: 23610169

This command will list all NAT rules on the box
/sbin/iptables -t nat -L

by: manav08Posted on 2009-02-11 at 04:08:17ID: 23610457

Here you go -

Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination
DNATP1     all  --  anywhere             some internet IP
DNATP2     all  --  anywhere             some internet IP
DNAT       tcp  --  anywhere             some internet IP
 
 
Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination
ACCEPT     ipv6-crypt--  anywhere             anywhere            MARK match 0x0
 
ACCEPT     ipv6-auth--  anywhere             anywhere            MARK match 0x0
MASQUERADE  all  --  anywhere             anywhere
MASQUERADE  all  --  anywhere             anywhere            ctstate DNAT MARK
match 0xc
MASQUERADE  all  --  anywhere             anywhere            ctstate DNAT MARK
match 0xd
MASQUERADE  all  --  anywhere             anywhere            ctstate DNAT MARK
match 0xe
MASQUERADE  all  --  anywhere             anywhere            ctstate DNAT MARK
match 0x11
MASQUERADE  all  --  anywhere             anywhere            ctstate DNAT MARK
match 0x12
MASQUERADE  all  --  anywhere             anywhere
MASQUERADE  all  --  anywhere             anywhere
 
Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination
 
Chain DNATP1 (1 references)
target     prot opt source               destination
DNAT       tcp  --  anywhere             anywhere            tcp dpt:http to:192.168.11.10
DNAT       tcp  --  anywhere             anywhere            tcp dpt:https to:192.168.11.10
DNAT       tcp  --  anywhere             anywhere            tcp dpt:snpp to:192.168.11.10
 
Chain DNATP2 (1 references)
target     prot opt source               destination
DNAT       udp  --  anywhere             anywhere            udp dpt:datametrics to:192.168.11.17
DNAT       udp  --  anywhere             anywhere            udp dpt:sa-msg-port to:192.168.11.17

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by: BlazPosted on 2009-02-11 at 04:35:37ID: 23610655

In the POSTROUTING chain you can see that all the traffic is MASQUREADEd. You should exclude all the traffic exiting on internal LAN from MASQUERADEing.

You can do this by inserting the rule:
iptables -t nat -I POSTROUTING -o eth0 -j ACCEPT

or by modifying the rule which now probably looks like:
iptables -t nat -A POSTROUTING -j MASQUERADE
to:
iptables -t nat -A POSTROUTING ! -o eth0 -j MASQUERADE

by: manav08Posted on 2009-02-11 at 06:45:57ID: 23611843

Thanks I will try this tomorrow and let you know how I went.

by: manav08Posted on 2009-02-11 at 15:05:54ID: 23617657

Hi Blaz,

This didn't work - iptables -t nat -I POSTROUTING -o eth0 -j ACCEPT
When I did it my proxy stopped working altogether.

2. iptables -t nat -A POSTROUTING ! -o eth0 -j MASQUERADE

This made no difference and the request still seem to be coming from 192.168.102.1

Also did you mean, run this in the following order -

iptables -t nat -I POSTROUTING -o eth0 -j ACCEPT
iptables -t nat -A POSTROUTING ! -o eth0 -j MASQUERADE

If this is what you meant I haven't tried. I juest ran them individually. Please comment.

by: BlazPosted on 2009-02-11 at 23:04:37ID: 23619778

I meant those two rules as alternatives.

Let me explain:
All the traffic from your internal lan (eth0) goes to your firewall (102.1) and when forwarding to internet you have to assign the firewalls public IP to the packets. You acomplish this with the command MASQUERADE (or you could have done it with a SNAT - source NAT). To the internet all the packets from your network seem to come from your firewall (a single machine).

The problem is that currently you are now forwarding traffic to an internal machine (the proxy) and obviously your rules do the MASQUERADING to that packets also (appearing to come from your firewall). This is not neccessary. You should make sure the MASQUERADING rules do not affect the traffic from your LAN to your proxy.

You have many MASQUREADING rules could you post them all? There may be some tricky rules at your site...
Could you also describe the machine - what interfaces does it have, how do you connect to the internet.

You could also change
iptables -t nat -I POSTROUTING -o eth0 -j ACCEPT

to:
iptables -t nat -I POSTROUTING -s 192.168.102.0/24 -o eth0 -d 192.168.102.3 -j ACCEPT
to catch only the traffic from your LAN to your proxy

by: manav08Posted on 2009-02-12 at 19:35:40ID: 23629663

Hi Blaz,

Posted all the MASQUERADING rules for you above.

"You could also change
iptables -t nat -I POSTROUTING -o eth0 -j ACCEPT

to:
iptables -t nat -I POSTROUTING -s 192.168.102.0/24 -o eth0 -d 192.168.102.3 -j ACCEPT
to catch only the traffic from your LAN to your proxy"

This still didn't fix the problem

by: BlazPosted on 2009-02-12 at 23:07:48ID: 23630417

No, that output is a bit short. I would be interested in the original rules in a script. This could be script /etc/sysconfig/iptables or rules listed in /etc/rc.local or a script called from there (maybe there is a call to iptables-restore).

At least post againt the output of:
/sbin/iptables -t nat -L -v -x

Additionally the output of the command would be usefull:
/sbin/ip rule list

There are several MASQURADE rules that MARK packets. Usually this is to change routing preferences.

by: manav08Posted on 2009-02-13 at 20:14:02ID: 23639180

Your request 1:
/sbin/iptables -t nat -L -v -x

Note I have reverted all the changes to original since what you suggested didn't work for me. At this point in time even these 2 commands are not active

iptables -t nat -I PREROUTING -i eth0 -p tcp --dport 80 -s ! 192.168.102.3 -j DNAT --to-destination 192.168.102.3:8080
iptables -t nat -I PREROUTING -i eth0 -p tcp --dport 80 -d 192.168.102.1 -RETURN

Chain PREROUTING (policy ACCEPT 12751 packets, 1307348 bytes)
    pkts      bytes target     prot opt in     out     source               dest
ination
     403    24051 DNATP1     all  --  any    any     anywhere              some internet IP
      57     7199 DNATP2     all  --  any    any     anywhere              some internet IP
      15      712 DNAT       tcp  --  any    any     anywhere              some internet IP  tcp dpt:smtp to:192.168.11.30
 
Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
    pkts      bytes target     prot opt in     out     source               dest
ination
       0        0 ACCEPT     ipv6-crypt--  any    any     anywhere             anywhere            MARK match 0x0
       0        0 ACCEPT     ipv6-auth--  any    any     anywhere             anywhere            MARK match 0x0
    1500    99691 MASQUERADE  all  --  any    any     anywhere             anywhere
       0        0 MASQUERADE  all  --  any    eth2  anywhere             anywhere            ctstate DNAT MARK match 0xc
       0        0 MASQUERADE  all  --  any    eth3  anywhere             anywhere            ctstate DNAT MARK match 0xd
       0        0 MASQUERADE  all  --  any    eth0  anywhere             anywhere            ctstate DNAT MARK match 0xe
       0        0 MASQUERADE  all  --  any    ppp1    anywhere             anywhere            ctstate DNAT MARK match 0x11
       0        0 MASQUERADE  all  --  any    ppp0    anywhere             anywhere
       0        0 MASQUERADE  all  --  any    eth1    anywhere             anywhere
 
Chain OUTPUT (policy ACCEPT 316 packets, 20674 bytes)
    pkts      bytes target     prot opt in     out     source               destination
 
Chain DNATP1 (1 references)
    pkts      bytes target     prot opt in     out     source               destination
     227    11140 DNAT       tcp  --  any    any     anywhere             anywhere            tcp dpt:http to:192.168.11.30
       0        0 DNAT       tcp  --  any    any     anywhere             anywhere            tcp dpt:snpp to:192.168.11.30
     119     5712 DNAT       tcp  --  any    any     anywhere             anywhere            tcp dpt:https to:192.168.11.30
 
Chain DNATP2 (1 references)
    pkts      bytes target     prot opt in     out     source               destination
       0        0 DNAT       udp  --  any    any     anywhere             anywhere            udp dpt:datametrics to:192.168.11.17
       0        0 DNAT       udp  --  any    any     anywhere             anywhere            udp dpt:sa-msg-port to:192.168.11.17

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by: manav08Posted on 2009-02-13 at 20:16:39ID: 23639186

Your request 2:
/sbin/ip rule list

Note I have reverted all the changes to original since what you suggested didn't work for me. At this point in time even these 2 commands are not active

iptables -t nat -I PREROUTING -i eth0 -p tcp --dport 80 -s ! 192.168.102.3 -j DNAT --to-destination 192.168.102.3:8080
iptables -t nat -I PREROUTING -i eth0 -p tcp --dport 80 -d 192.168.102.1 -RETURN

0:      from all lookup local
1024:   from all fwmark        7 lookup 1
1025:   from all fwmark       12 lookup 2
2048:   from OURINTERNETIP_1 lookup 1
2049:   from OURINTERNETIP_2 lookup 2
32766:  from all lookup main
32767:  from all lookup default

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by: BlazPosted on 2009-02-16 at 06:25:09ID: 23649963

Could you describe your connection to the internet - you have two xDSLs with two public IPs or something like that?
What are the interfaces on the machine (eth0, eth2, ...)

The output of the following commands:
/sbin/iptables -t mangle -L -v -x
/sbin/ip rule list 1
/sbin/ip rule list 2
/sbin/ip rule list default

by: manav08Posted on 2009-02-17 at 03:24:51ID: 23658228

Hi Blaz,

I wont have time to check this all out now as the project has been deferred for the moment. I will reul. helpfopen the question once I get around to it. For now I will award the points to you. You were extremely helpful so I have raised the points to 200.

Regards

Manav

by: BlazPosted on 2009-02-17 at 04:41:22ID: 23658747

I must admit that the simptoms you describe confuse me somewhat.

You have some complicated firewall (and you don't know how it was set up) - marking of packets, routing depending on marking etc.

For example there are several MASQUERADING rules but it seems that only the first one is used (all traffic -> masquerade). The traffic you forward to your proxy should not be masqueraded and you will get the correct source IP on the proxy. Why it didn't work when implementing my suggested rules I don't know but they are in the right direction. With some iptables logging I believe you could figure it out.

I hope that my questions helped you understand your firewall a bit more.

Good luck

20120131-EE-VQP-002

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Using IPTABLES to redirect http traffic to a transparent proxy

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IP Tables/IP Chains

Bourne-Again Shell (bash)

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