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17. Interception Caching/Proxying

How can I make my users' browsers use my cache without configuring the browsers for proxying?

First, it is critical to read the full comments in the squid.conf file! That is the only authoritative source for configuration information. However, the following instructions are correct as of this writing (July 1999.)

Getting interception caching to work requires four distinct steps:

  1. Compile and run a version of Squid which accepts connections for other addresses. For some operating systems, you need to have configured and built a version of Squid which can recognize the hijacked connections and discern the destination addresses. For Linux this seems to work automatically. For *BSD-based systems, you probably have to configure squid with the --enable-ipf-transparent option. (Do a make clean if you previously configured without that option, or the correct settings may not be present.)
  2. Configure Squid to accept and process the connections. You have to change the Squid configuration settings to recognize the hijacked connections and discern the destination addresses. Here are the important settings in squid.conf:
    http_port 8080
    httpd_accel_host virtual
    httpd_accel_port 80
    httpd_accel_with_proxy  on
    httpd_accel_uses_host_header on
  3. Get your cache server to accept the packets. You have to configure your cache host to accept the redirected packets - any IP address, on port 80 - and deliver them to your cache application. This is typically done with IP filtering/forwarding features built into the kernel. On linux they call this iptables (kernel 2.4.x), ipchains (2.2.x) or ipfwadm (2.0.x). On FreeBSD its called ipfw. Other BSD systems may use ip filter or ipnat. On most systems, it may require rebuilding the kernel or adding a new loadable kernel module.
  4. Get the packets to your cache server. There are several ways to do this. First, if your proxy machine is already in the path of the packets (i.e. it is routing between your proxy users and the Internet) then you don't have to worry about this step. This would be true if you install Squid on a firewall machine, or on a UNIX-based router. If the cache is not in the natural path of the connections, then you have to divert the packets from the normal path to your cache host using a router or switch. You may be able to do this with a Cisco router using their "route maps" feature, depending on your IOS version. You might also use a so-called layer-4 switch, such as the Alteon ACE-director or the Foundry Networks ServerIron. Finally, you might be able to use a stand-alone router/load-balancer type product, or routing capabilities of an access server.


17.1 Interception caching for Solaris, SunOS, and BSD systems

NOTE: You don't need to use IP Filter on FreeBSD. Use the built-in ipfw feature instead. See the FreeBSD subsection below.

Install IP Filter

First, get and install the IP Filter package.

Configure ipnat

Put these lines in /etc/ipnat.rules:

        # Redirect direct web traffic to local web server.
        rdr de0 port 80 -> port 80 tcp

        # Redirect everything else to squid on port 8080
        rdr de0 port 80 -> port 8080 tcp

Modify your startup scripts to enable ipnat. For example, on FreeBSD it looks something like this:

        /sbin/modload /lkm/if_ipl.o
        /sbin/ipnat -f /etc/ipnat.rules
        chgrp nobody /dev/ipnat
        chmod 644 /dev/ipnat

Configure Squid


Squid-2 (after version beta25) has IP filter support built in. Simple enable it when you run configure:

        ./configure --enable-ipf-transparent
Add these lines to your squid.conf file:
        http_port 8080
        httpd_accel_host virtual
        httpd_accel_port 80
        httpd_accel_with_proxy on
        httpd_accel_uses_host_header on
Note, you don't have to use port 8080, but it must match whatever you used in the /etc/ipnat.rules file.


Patches for Squid-1.X are available from Quinton Dolan's Squid page. Add these lines to squid.conf:

        http_port 8080
        httpd_accel virtual 80
        httpd_accel_with_proxy on
        httpd_accel_uses_host_header on

Thanks to Quinton Dolan.

17.2 Interception caching with Linux 2.0 and ipfwadm

by Rodney van den Oever

Note: Interception proxying does NOT work with Linux 2.0.30! Linux 2.0.29 is known to work well. If you're using a more recent kernel, like 2.2.X, then you should probably use an ipchains configuration, as described below.

Warning: this technique has some shortcomings.

  1. This method only supports the HTTP protocol, not gopher or FTP
  2. Since the browser wasn't set up to use a proxy server, it uses the FTP protocol (with destination port 21) and not the required HTTP protocol. You can't setup a redirection-rule to the proxy server since the browser is speaking the wrong protocol. A similar problem occurs with gopher. Normally all proxy requests are translated by the client into the HTTP protocol, but since the client isn't aware of the redirection, this never happens.

If you can live with the side-effects, go ahead and compile your kernel with firewalling and redirection support. Here are the important parameters from /usr/src/linux/.config:

        # Code maturity level options
        # Networking options
        # CONFIG_NET_ALIAS is not set
        # CONFIG_IP_MULTICAST is not set
        # CONFIG_IP_FIREWALL_VERBOSE is not set
        # CONFIG_IP_ACCT is not set

You may also need to enable IP Forwarding. One way to do it is to add this line to your startup scripts:

        echo 1 > /proc/sys/net/ipv4/ip_forward

Go to the Linux IP Firewall and Accounting page, obtain the source distribution to ipfwadm and install it. Older versions of ipfwadm may not work. You might need at least version 2.3.0. You'll use ipfwadm to setup the redirection rules. I added this rule to the script that runs from /etc/rc.d/rc.inet1 (Slackware) which sets up the interfaces at boot-time. The redirection should be done before any other Input-accept rule. To really make sure it worked I disabled the forwarding (masquerading) I normally do.


        # rc.firewall   Linux kernel firewalling rules

        # Flush rules, for testing purposes
        for i in I O F # A      # If we enabled accounting too
                ${FW} -$i -f

        # Default policies:
        ${FW} -I -p rej         # Incoming policy: reject (quick error)
        ${FW} -O -p acc         # Output policy: accept
        ${FW} -F -p den         # Forwarding policy: deny

        # Input Rules:

        # Loopback-interface (local access, eg, to local nameserver):
        ${FW} -I -a acc -S localhost/32 -D localhost/32

        # Local Ethernet-interface:

        # Redirect to Squid proxy server:
        ${FW} -I -a acc -P tcp -D default/0 80 -r 8080

        # Accept packets from local network:
        ${FW} -I -a acc -P all -S localnet/8 -D default/0 -W eth0

        # Only required for other types of traffic (FTP, Telnet):

        # Forward localnet with masquerading (udp and tcp, no icmp!):
        ${FW} -F -a m -P tcp -S localnet/8 -D default/0
        ${FW} -F -a m -P udp -S localnet/8 -D default/0

Here all traffic from the local LAN with any destination gets redirected to the local port 8080. Rules can be viewed like this:

        IP firewall input rules, default policy: reject
        type  prot source               destination          ports
        acc   all              n/a
        acc/r tcp             * -> 80 => 8080
        acc   all             n/a
        acc   tcp              * -> *

I did some testing on Windows 95 with both Microsoft Internet Explorer 3.01 and Netscape Communicator pre-release and it worked with both browsers with the proxy-settings disabled.

At one time squid seemed to get in a loop when I pointed the browser to the local port 80. But this could be avoided by adding a reject rule for client to this address:

        ${FW} -I -a rej -P tcp -S localnet/8 -D hostname/32 80

        IP firewall input rules, default policy: reject
        type  prot source               destination          ports
        acc   all              n/a
        rej   tcp              * -> 80
        acc/r tcp             * -> 80 => 8080
        acc   all             n/a
        acc   tcp              * -> *

NOTE on resolving names: Instead of just passing the URLs to the proxy server, the browser itself has to resolve the URLs. Make sure the workstations are setup to query a local nameserver, to minimize outgoing traffic.

If you're already running a nameserver at the firewall or proxy server (which is a good idea anyway IMHO) let the workstations use this nameserver.

Additional notes from Richard Ayres

I'm using such a setup. The only issues so far have been that:

  1. It's fairly useless to use my service providers parent caches (cache-?.www.demon.net) because by proxying squid only sees IP addresses, not host names and demon aren't generally asked for IP addresses by other users;
  2. Linux kernel 2.0.30 is a no-no as interception proxying is broken (I use 2.0.29);
  3. Client browsers must do host name lookups themselves, as they don't know they're using a proxy;
  4. The Microsoft Network won't authorize its users through a proxy, so I have to specifically *not* redirect those packets (my company is a MSN content provider).

Aside from this, I get a 30-40% hit rate on a 50MB cache for 30-40 users and am quite pleased with the results.

See also Daniel Kiracofe's page.

17.3 Interception caching with Linux 2.2 and ipchains

by Martin Lyons

You need to configure your kernel for ipchains. Configuring Linux kernels is beyond the scope of this FAQ. One way to do it is:

        # cd /usr/src/linux
        # make menuconfig

The following shows important kernel features to include:

        [*] Network firewalls
        [ ] Socket Filtering
        [*] Unix domain sockets
        [*] TCP/IP networking
        [ ] IP: multicasting
        [ ] IP: advanced router
        [ ] IP: kernel level autoconfiguration
        [*] IP: firewalling
        [ ] IP: firewall packet netlink device
        [*] IP: always defragment (required for masquerading)
        [*] IP: transparent proxy support

You must include the IP: always defragment, otherwise it prevents you from using the REDIRECT chain.

You can use this script as a template for your own rc.firewall to configure ipchains:

        # rc.firewall   Linux kernel firewalling rules
        # Leon Brooks (leon at brooks dot fdns dot net)
        ADD="$FW -A"

        # Flush rules, for testing purposes
        for i in I O F # A      # If we enabled accounting too
                ${FW} -F $i

        # Default policies:
        ${FW} -P input REJECT   # Incoming policy: reject (quick error)
        ${FW} -P output ACCEPT  # Output policy: accept
        ${FW} -P forward DENY   # Forwarding policy: deny

        # Input Rules:

        # Loopback-interface (local access, eg, to local nameserver):
        ${ADD} input -j ACCEPT -s localhost/32 -d localhost/32

        # Local Ethernet-interface:

        # Redirect to Squid proxy server:
        ${ADD} input -p tcp -d 0/0 80 -j REDIRECT 8080

        # Accept packets from local network:
        ${ADD} input -j ACCEPT -s localnet/8 -d 0/0 -i eth0

        # Only required for other types of traffic (FTP, Telnet):

        # Forward localnet with masquerading (udp and tcp, no icmp!):
        ${ADD} forward -j MASQ -p tcp -s localnet/8 -d 0/0
        ${ADD} forward -j MASQ -P udp -s localnet/8 -d 0/0

Also, Andrew Shipton notes that with 2.0.x kernels you don't need to enable packet forwarding, but with the 2.1.x and 2.2.x kernels using ipchains you do. Packet forwarding is enabled with the following command:

        echo 1 > /proc/sys/net/ipv4/ip_forward

17.4 Interception caching with Linux 2.4 and netfilter

NOTE: this information comes from Daniel Kiracofe's Transparent Proxy with Squid mini-HOWTO.

To support netfilter transparent interception on Linux 2.4 Squid must be compiled with the --enable-linux-netfilter option.

To enable netwfilter support you may need to build a new kernel. Be sure to enable all of these options:

You must say NO to ``Fast switching''

After building the kernel, install it and reboot.

You may need to enable packet forwarding (e.g. in your startup scripts):

echo 1 > /proc/sys/net/ipv4/ip_forward

Use the iptables command to make your kernel intercept HTTP connections and send them to Squid:

iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 3128 

17.5 Interception caching with Cisco routers

by John Saunders

This works with at least IOS 11.1 and later I guess. Possibly earlier, as I'm no CISCO expert I can't say for sure. If your router is doing anything more complicated that shuffling packets between an ethernet interface and either a serial port or BRI port, then you should work through if this will work for you.

First define a route map with a name of proxy-redirect (name doesn't matter) and specify the next hop to be the machine Squid runs on.

        route-map proxy-redirect permit 10
         match ip address 110
         set ip next-hop
Define an access list to trap HTTP requests. The second line allows the Squid host direct access so an routing loop is not formed. By carefully writing your access list as show below, common cases are found quickly and this can greatly reduce the load on your router's processor.
        access-list 110 deny   tcp any any neq www
        access-list 110 deny   tcp host any
        access-list 110 permit tcp any any
Apply the route map to the ethernet interface.
        interface Ethernet0
         ip policy route-map proxy-redirect

possible bugs

Bruce Morgan notes that there is a Cisco bug relating to interception proxying using IP policy route maps, that causes NFS and other applications to break. Apparently there are two bug reports raised in Cisco, but they are not available for public dissemination.

The problem occurs with o/s packets with more than 1472 data bytes. If you try to ping a host with more than 1472 data bytes across a Cisco interface with the access-lists and ip policy route map, the icmp request will fail. The packet will be fragmented, and the first fragment is checked against the access-list and rejected - it goes the "normal path" as it is an icmp packet - however when the second fragment is checked against the access-list it is accepted (it isn't regarded as an icmp packet), and goes to the action determined by the policy route map!

John notes that you may be able to get around this bug by carefully writing your access lists. If the last/default rule is to permit then this bug would be a problem, but if the last/default rule was to deny then it won't be a problem. I guess fragments, other than the first, don't have the information available to properly policy route them. Normally TCP packets should not be fragmented, at least my network runs an MTU of 1500 everywhere to avoid fragmentation. So this would affect UDP and ICMP traffic only.

Basically, you will have to pick between living with the bug or better performance. This set has better performance, but suffers from the bug:

        access-list 110 deny   tcp any any neq www
        access-list 110 deny   tcp host any
        access-list 110 permit tcp any any
Conversely, this set has worse performance, but works for all protocols:
        access-list 110 deny   tcp host any
        access-list 110 permit tcp any any eq www
        access-list 110 deny   tcp any any

17.6 Interception caching with LINUX 2.0.29 and CISCO IOS 11.1

Just for kicks, here's an email message posted to squid-users on how to make interception proxying work with a Cisco router and Squid running on Linux.

by Brian Feeny

Here is how I have Interception proxying working for me, in an environment where my router is a Cisco 2501 running IOS 11.1, and Squid machine is running Linux 2.0.33.

Many thanks to the following individuals and the squid-users list for helping me get redirection and interception proxying working on my Cisco/Linux box.

First, here is what I added to my Cisco, which is running IOS 11.1. In IOS 11.1 the route-map command is "process switched" as opposed to the faster "fast-switched" route-map which is found in IOS 11.2 and later. You may wish to be running IOS 11.2. I am running 11.1, and have had no problems with my current load of about 150 simultaneous connections to squid.:

        interface Ethernet0
         description To Office Ethernet
         ip address
         no ip directed-broadcast
         no ip mroute-cache
         ip policy route-map proxy-redir
        access-list 110 deny   tcp host any eq www
        access-list 110 permit tcp any any eq www
        route-map proxy-redir permit 10
         match ip address 110
         set ip next-hop

So basically from above you can see I added the "route-map" declaration, and an access-list, and then turned the route-map on under int e0 "ip policy route-map proxy-redir"

ok, so the Cisco is taken care of at this point. The host above:, is the ip number of my squid host.

My squid box runs Linux, so I had to do the following on it:

my kernel (2.0.33) config looks like this:

        # Networking options
        # CONFIG_NET_ALIAS is not set
        # CONFIG_RST_COOKIES is not set
        # CONFIG_IP_FIREWALL_VERBOSE is not set
        # CONFIG_IP_ACCT is not set

You will need Firewalling and Transparent Proxy turned on at a minimum.

Then some ipfwadm stuff:

        # Accept all on loopback
        ipfwadm -I -a accept -W lo
        # Accept my own IP, to prevent loops (repeat for each interface/alias)
        ipfwadm -I -a accept -P tcp -D 80
        # Send all traffic destined to port 80 to Squid on port 3128
        ipfwadm -I -a accept -P tcp -D 0/0 80 -r 3128

it accepts packets on port 80 (redirected from the Cisco), and redirects them to 3128 which is the port my squid process is sitting on. I put all this in /etc/rc.d/rc.local

I am using v1.1.20 of Squid with Henrik's patch installed. You will want to install this patch if using a setup similar to mine.

17.7 Interception caching with FreeBSD

by Duane Wessels

I set out yesterday to make interception caching work with Squid and FreeBSD. It was, uh, fun.

It was relatively easy to configure a cisco to divert port 80 packets to my FreeBSD box. Configuration goes something like this:

access-list 110 deny   tcp host any eq www
access-list 110 permit tcp any any eq www
route-map proxy-redirect permit 10
 match ip address 110
 set ip next-hop
int eth2/0
 ip policy route-map proxy-redirect
Here, is the IP address of the FreeBSD cache machine.

Once I have packets going to the FreeBSD box, I need to get the kernel to deliver them to Squid. I started on FreeBSD-2.2.7, and then downloaded IPFilter. This was a dead end for me. The IPFilter distribution includes patches to the FreeBSD kernel sources, but many of these had conflicts. Then I noticed that the IPFilter page says ``It comes as a part of [FreeBSD-2.2 and later].'' Fair enough. Unfortunately, you can't hijack connections with the FreeBSD-2.2.X IPFIREWALL code (ipfw), and you can't (or at least I couldn't) do it with natd either.

FreeBSD-3.0 has much better support for connection hijacking, so I suggest you start with that. You need to build a kernel with the following options:

        options         IPFIREWALL
        options         IPFIREWALL_FORWARD

Next, its time to configure the IP firewall rules with ipfw. By default, there are no "allow" rules and all packets are denied. I added these commands to /etc/rc.local just to be able to use the machine on my network:

        ipfw add 60000 allow all from any to any
But we're still not hijacking connections. To accomplish that, add these rules:
        ipfw add 49  allow tcp from to any
        ipfw add 50  fwd tcp from any to any 80
The second line (rule 50) is the one which hijacks the connection. The first line makes sure we never hit rule 50 for traffic originated by the local machine. This prevents forwarding loops.

Note that I am not changing the port number here. That is, port 80 packets are simply diverted to Squid on port 80. My Squid configuration is:

        http_port 80
        httpd_accel_host virtual
        httpd_accel_port 80
        httpd_accel_with_proxy on
        httpd_accel_uses_host_header on

If you don't want Squid to listen on port 80 (because that requires root privileges) then you can use another port. In that case your ipfw redirect rule looks like:

        ipfw add 50 fwd,3128 tcp from any to any 80
and the squid.conf lines are:
        http_port 3128
        httpd_accel_host virtual
        httpd_accel_port 80
        httpd_accel_with_proxy on
        httpd_accel_uses_host_header on

17.8 Interception caching with ACC Tigris digital access server

by John Saunders

This is to do with configuring interception proxy for an ACC Tigris digital access server (like a CISCO 5200/5300 or an Ascend MAX 4000). I've found that doing this in the NAS reduces traffic on the LAN and reduces processing load on the CISCO. The Tigris has ample CPU for filtering.

Step 1 is to create filters that allow local traffic to pass. Add as many as needed for all of your address ranges.


Step 2 is to create a filter to trap port 80 traffic.


Step 3 is to set the "APPLICATION_ID" on port 80 traffic to 80. This causes all packets matching this filter to have ID 80 instead of the default ID of 0.


Step 4 is to create a special route that is used for packets with "APPLICATION_ID" set to 80. The routing engine uses the ID to select which routes to use.


Step 5 is to bind everything to a filter ID called transproxy. List all local filters first and the http one last.

        ADD PROFILE ENTRY transproxy local1 local2 http

With this in place use your RADIUS server to send back the ``Framed-Filter-Id = transproxy'' key/value pair to the NAS.

You can check if the filter is being assigned to logins with the following command:

        display profile port table

17.9 Interception caching with Foundry L4 switches

by Brian Feeny.

First, configure Squid for interception caching as detailed at the beginning of this section.

Next, configure the Foundry layer 4 switch to redirect traffic to your Squid box or boxes. By default, the Foundry redirects to port 80 of your squid box. This can be changed to a different port if needed, but won't be covered here.

In addition, the switch does a "health check" of the port to make sure your squid is answering. If you squid does not answer, the switch defaults to sending traffic directly thru instead of redirecting it. When the Squid comes back up, it begins redirecting once again.

This example assumes you have two squid caches:


We will assume you have various workstations, customers, etc, plugged into the switch for which you want them to be intercepted and sent to Squid. The squid caches themselves should be plugged into the switch as well. Only the interface that the router is connected to is important. Where you put the squid caches or other connections does not matter.

This example assumes your router is plugged into interface 17 of the switch. If not, adjust the following commands accordingly.

  1. Enter configuration mode:
    telnet@ServerIron#conf t
  2. Configure each squid on the Foundry:
    telnet@ServerIron(config)# server cache-name squid1
    telnet@ServerIron(config)# server cache-name squid2
  3. Add the squids to a cache-group:
    telnet@ServerIron(config)#server cache-group 1
    telnet@ServerIron(config-tc-1)#cache-name squid1
    telnet@ServerIron(config-tc-1)#cache-name squid2
  4. Create a policy for caching http on a local port
    telnet@ServerIron(config)# ip policy 1 cache tcp http local
  5. Enable that policy on the port connected to your router
    telnet@ServerIron(config)#int e 17
    telnet@ServerIron(config-if-17)# ip-policy 1 

Since all outbound traffic to the Internet goes out interface 17 (the router), and interface 17 has the caching policy applied to it, HTTP traffic is going to be intercepted and redirected to the caches you have configured.

The default port to redirect to can be changed. The load balancing algorithm used can be changed (Least Used, Round Robin, etc). Ports can be exempted from caching if needed. Access Lists can be applied so that only certain source IP Addresses are redirected, etc. This information was left out of this document since this was just a quick howto that would apply for most people, not meant to be a comprehensive manual of how to configure a Foundry switch. I can however revise this with any information necessary if people feel it should be included.

17.10 Interception caching with Cabletron/Entrasys products

By Dave Wintrip, dave at purevanity dot net, June 3, 2004.

I have verified this configuration as working on a Cabletron SmartSwitchRouter 2000, and it should work on any layer-4 aware Cabletron or Entrasys product.

You must first configure Squid to enable interception caching, outlined earlier.

Next, make sure that you have connectivity from the layer-4 device to your squid box, and that squid is correctly configured to intercept port 80 requests thrown it's way.

I generally create two sets of redirect ACLs, one for cache, and one for bypassing the cache. This method of interception is very similar to Cisco's route-map.

Log into the device, and enter enable mode, as well as configure mode.

        ssr> en

        ssr# conf

I generally create two sets of redirect ACLs, one for specifying who to cache, and one for destination addresses that need to bypass the cache. This method of interception is very similar to Cisco's route-map in this way. The ACL cache-skip is a list of destination addresses that we do not want to transparently redirect to squid.

        ssr(conf)# acl cache-skip permit tcp any any http

The ACL cache-allow is a list of source addresses that will be redirected to Squid.

        ssr(conf)# acl cache-allow permit tcp any any http

Save your new ACLs to the running configuration.

        ssr(conf)# save a

Next, we need to create the ip-policies that will work to perform the redirection. Please note that is my Squid server, and that is my standard default next hop. By pushing the cache-skip ACL to the default gateway, the web request is sent out as if the squid box was not present. This could just as easily be done using the squid configuration, but I would rather Squid not touch the data if it has no reason to.

        ssr(conf)# ip-policy cache-allow permit acl cache-allow next-hop-list action policy-only
        ssr(conf)# ip-policy cache-skip permit acl cache-skip next-hop-list action policy-only

Apply these new policies into the active configuration.

        ssr(conf)# save a

We now need to apply the ip-policies to interfaces we want to cache requests from. Assuming that localnet-gw is the interface name to the network we want to cache requests from, we first apply the cache-skip ACL to intercept requests on our do-not-cache list, and forward them out the default gateway. We then apply the cache-allow ACL to the same interface to redirect all other requests to the cache server.

        ssr(conf)# ip-policy cache-skip apply interface localnet-gw
        ssr(conf)# ip-policy cache-allow apply interface localnet-gw

We now need to apply, and permanently save our changes. Nothing we have done before this point would effect anything without adding the ip-policy applications into the active configuration, so lets try it.

        ssr(conf)# save a
        ssr(conf)# save s

Provided your Squid box is correct configured, you should now be able to surf, and be transparently cached if you are using the localnet-gw address as your gateway.

Some Cabletron/Entrasys products include another method of applying a web cache, but details on configuring that is not covered in this document, however is it fairly straight forward.

Also note, that if your Squid box is plugged directly into a port on your layer-4 switch, and that port is part of its own VLAN, and its own subnet, if that port were to change states to down, or the address becomes uncontactable, then the switch will automatically bypass the ip-policies and forward your web request though the normal means. This is handy, might I add.

17.11 The cache is trying to connect to itself...

by Henrik Nordstrom

I think almost everyone who have tried to build a interception proxy setup have been bitten by this one.

Measures you can take:

17.12 ``Connection reset by peer'' and Cisco policy routing

Fyodor has tracked down the cause of unusual ``connection reset by peer'' messages when using Cisco policy routing to hijack HTTP requests.

When the network link between router and the cache goes down for just a moment, the packets that are supposed to be redirected are instead sent out the default route. If this happens, a TCP ACK from the client host may be sent to the origin server, instead of being diverted to the cache. The origin server, upon receiving an unexpected ACK packet, sends a TCP RESET back to the client, which aborts the client's request.

To work around this problem, you can install a static route to the null0 interface for the cache address with a higher metric (lower precedence), such as 250. Then, when the link goes down, packets from the client just get dropped instead of sent out the default route. For example, if is the IP address of your Squid cache, you may add:

        ip route Null0 250
This appears to cause the correct behaviour.

17.13 WCCP - Web Cache Coordination Protocol

Contributors: Glenn Chisholm, Lincoln Dale and Reuben Farrelly.

Does Squid support WCCP?

CISCO's Web Cache Coordination Protocol V1.0 is supported in squid 2.3 and later. support WCCP V2.0. Now that WCCP V2 is an open protocol, Squid may be able to support it in the future.

Configuring your Router

There are two different methods of configuring WCCP on CISCO routers. The first method is for routers that only support V1.0 of the protocol. The second is for routers that support both.

IOS Version 11.x

It is possible that later versions of IOS 11.x will support V2.0 of the protocol. If that is the case follow the 12.x instructions. Several people have reported that the squid implimentation of WCCP does not work with their 11.x routers. If you experience this please mail the debug output from your router to squid-bugs.

        conf t

        wccp enable
        interface [Interface carrying Outgoing Traffic]x/x
        ip wccp web-cache redirect
        CTRL Z
        write mem

IOS Version 12.x

Some of the early versions of 12.x do not have the 'ip wccp version' command. You will need to upgrade your IOS version to use V1.0.

You will need to be running at least IOS Software Release 12.0(5)T if you're running the 12.0 T-train. IOS Software Releases 12.0(3)T and 12.0(4)T do not have WCCPv1, but 12.0(5)T does.

        conf t

        ip wccp version 1
        ip wccp web-cache redirect-list 150
        interface [Interface carrying Outgoing/Incoming Traffic]x/x
        ip wccp web-cache redirect out|in
        CTRL Z
        write mem

Replace 150 with an access list number (either standard or extended) which lists IP addresses which you do not wish to be transparently redirected to your cache. Otherwise simply user the word 'redirect' on it's own to redirect traffic from all sources to all destinations.

IOS 12.x problems

Some people report problems with WCCP and IOS 12.x. They see truncated or fragmented GRE packets arriving at the cache. Apparently it works if you disable Cisco Express Forwarding for the interface:

conf t
ip cef          # some systems may already have 'ip cef global'
int Ethernet 0/0      (or int FastEthernet 0/0 or other internal interface)
no ip route-cache cef

This may well be fixed in later releases of IOS.

Configuring FreeBSD

FreeBSD first needs to be configured to receive and strip the GRE encapsulation from the packets from the router. To do this you will need to patch and recompile your kernel. The steps depend on your kernel version.


  1. Apply the patch for FreeBSD-3.x kernels:
    # cd /usr/src
    # patch -s < /tmp/gre.patch
  2. Download gre.c for FreeBSD-3.x. Save this file as /usr/src/sys/netinet/gre.c.
  3. Add "options GRE" to your kernel config file and rebuild your kernel. Note, the opt_gre.h file is created when you run config. Once your kernel is installed you will need to configure FreeBSD for interception proxying.

FreeBSD-4.0 through 4.7

The procedure is nearly identical to the above for 3.x, but the source files are a little different.

  1. Apply the most appropriate patch file from the list of patches for 4.x kernels.
  2. Download gre.c for FreeBSD-3.x. Save this file as /usr/src/sys/netinet/gre.c.
  3. Add "options GRE" to your kernel config file and rebuild your kernel. Note, the opt_gre.h file is created when you run config. Once your kernel is installed you will need to configure FreeBSD for interception proxying.

FreeBSD-4.8 and later

The operating system now comes standard with some GRE support. You need to make a kernel with the GRE code enabled:

pseudo-device   gre

And then configure the tunnel so that the router's GRE packets are accepted:

# ifconfig gre0 create
# ifconfig gre0 $squid_ip $router_ip netmask up
# ifconfig gre0 tunnel $squid_ip $router_ip
# route delete $router_ip

Alternatively, you can try it like this:

ifconfig gre0 create
ifconfig gre0 $squid_ip netmask link1 tunnel $squid_ip $router_ip up

Since the WCCP/GRE tunnel is one-way, Squid never sends any packets to and that particular address doesn't matter.

Configuring Linux 2.2

Al Blake has written a Cookbook for setting up transparent WCCP using Squid on RedHat Linux and a cisco access server.

There are currently two methods for supporting WCCP with Linux 2.2. A specific purpose module. Or the standard Linux GRE tunneling driver. People have reported difficulty with the standard GRE tunneling driver, however it does allow GRE functionality other than WCCP. You should choose the method that suits your enviroment.

Standard Linux GRE Tunnel

Linux 2.2 kernels already support GRE, as long as the GRE module is compiled into the kernel. However, WCCP uses a slightly non-standard GRE encapsualtion format and Linux versions earlier than 2.6.9 may need to be patched to support WCCP.

Ensure that the GRE code is either built as static or as a module by chosing the appropriate option in your kernel config. Then rebuild your kernel. If it is a module you will need to:

        modprobe ip_gre

The next step is to tell Linux to establish an IP tunnel between the router and your host. Daniele Orlandi reports that you have to give the gre1 interface an address, but any old address seems to work.

        iptunnel add gre1 mode gre remote <Router-IP> local <Host-IP> dev <interface>
        ifconfig gre1 up
<Router-IP> is the IP address of your router that is intercepting the HTTP packets. <Host-IP> is the IP address of your cache, and <interface> is the network interface that receives those packets (probably eth0).

Note that WCCP is incompatible with the rp_filter function in Linux and you must disable this if enabled (default off). If enabled any packets redirected by WCCP and intercepted by Netfilter/iptables will be silendly discarded by the TCP/IP stack due to their "unexpected" origin from the gre interface.

Joe Cooper's Patch

Joe Cooper has a patch for Linux 2.2.18 kernel on his Squid page.

WCCP Specific Module

This module is not part of the standard Linux distributon. It needs to be compiled as a module and loaded on your system to function. Do not attempt to build this in as a static part of your kernel.

Download the Linux WCCP module and compile it as you would any Linux network module.

Copy the module to /lib/modules/kernel-version/ipv4/ip_wccp.o. Edit /lib/modules/kernel-version/modules.dep and add:


or run

        depmod -a

Finally you will need to load the module:

        modprobe ip_wccp

Common Steps

The machine should now be striping the GRE encapsulation from any packets recieved and requeuing them. The system will also need to be configured for interception proxying, either with ipfwadm or with ipchains.

Configuring Others

If you have managed to configuring your operating system to support WCCP with Squid please contact us with the details so we may share them with others.

17.14 Can someone tell me what version of cisco IOS WCCP is added in?

IOS releases:

17.15 What about WCCPv2?

Cisco has published WCCPv2 as an Internet Draft (expired Jan 2001). There is a ongoing project at the Squid development projects website aiming to add support for WCCPv2 and at the time of writing this patch provides at least the same functionality as WCCPv1.

17.16 Can I use proxy_auth with interception?

No, you cannot. With interception proxying, the client thinks it is talking to an origin server and would never send the Proxy-authorization request header.

17.17 Interception on Linux with Squid and the Browser on the same box

by Joshua N Pritikin


iptables -t nat -F  # clear table

# normal transparent proxy
iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 80 -j REDIRECT --to-port 8080

# handle connections on the same box ( is a loopback instance)
gid=`id -g proxy`
iptables -t nat -A OUTPUT -p tcp --dport 80 -m owner --gid-owner $gid -j ACCEPT
iptables -t nat -A OUTPUT -p tcp --dport 80 -j DNAT --to-destination

17.18 Interception caching with an Alcatel OmnySwitch 7700

by Pedro A M Vazquez

On the switch define a network group to be intercepted:

 policy network group MyGroup mask

Define the tcp services to be intercepted:

 policy service web80 destination tcp port 80 
 policy service web8080 destination tcp port 8080

Define a group of services using the services above:

 policy service group WebPorts web80 web8080

And use these to create an intercept condition:

 policy condition WebFlow source network group MyGroup service group WebPorts

Now, define an action to redirect the traffic to the host running squid:

 policy action Redir alternate gateway ip 

Finally, create a rule using this condition and the corresponding action:

 policy rule Intercept  condition WebFlow action Redir

Apply the rules to the QoS system to make them effective

 qos apply

Don't forget that you still need to configure Squid and Squid's operating system to handle the intercepted connections. See above for Squid and OS-specific details.

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