On Thursday 08 September 2016 at 10:44:12, John Sayce wrote: > After I wrote this I realised it should be changing the mac not the ip, > which is not what’s happeneing. I think it's my firewall configuration > that's wrong. In that case your firewall is doing NAT instead of policy routing. Regards, Antony. > -----Original Message----- > From: squid-users [mailto:squid-users-bounces@xxxxxxxxxxxxxxxxxxxxx] On > Behalf Of Antony Stone Sent: 08 September 2016 09:36 > To: squid-users@xxxxxxxxxxxxxxxxxxxxx > Subject: Re: Transparent Proxy > > On Thursday 08 September 2016 at 10:12:48, John Sayce wrote: > > For testing purposes I've reduced it to the following: > > > > http_port 3128 intercept > > #dns_v4_first on > > dns_nameservers 10.8.2.3 194.168.4.100 10.8.2.2 8.8.8.8 acl wifi src > > 10.8.14.0/24 acl all src all http_access allow all maximum_object_size > > 1 GB minimum_object_size 0 KB maximum_object_size_in_memory 4 MB > > cache_mem 1700 MB cache_dir aufs /var/cache/squid 40000 32 512 > > coredump_dir /var/cache/squid access_log /var/log/squid/access.log > > squid cache_log /var/log/squid/cache.log > > refresh_pattern ^ftp: 1440 20% 10080 > > refresh_pattern ^gopher: 1440 0% 1440 > > refresh_pattern -i (/cgi-bin/|\?) 0 0% 0 > > refresh_pattern . 0 20% 4320 > > cache_effective_user asd > > cache_effective_group asd > > cache_mgr jsayce@xxxxxxxxxxxxxxx > > forwarded_for off > > > > The version is 3.5.12 > > > > Okay. Sorry, to clarify with a specific example. > > Don't apologise - specific examples are good, because it makes sure we're > both talking about the same thing (and sometimes, as below, reveals little > details about the network arrangement which weren't previously clear). > > > Lets say I'm contacting http://1.1.1.1/ then the ack packet starts off > > with the client with ip address 10.8.14.9 > > So, source IP = 10.8.14.9 : destination IP = 1.1.11 > > > in subnet 10.8.14.9/24 with default gateway 10.8.14.1. > > It's routed through my core switch to my my firewall with ip 10.8.1.1. > > So that's a router, not just a switch? It has one interface 10.8.14.1 on > subnet 10.8.14.0/24 and another interface on (presumably) 10.8.1.0/24 > pointing at 10.8.1.1 as the next-hop route towards 1.1.1.1 > > > My firewall recognises that the packet has a destination port 80 and > > is in subnet 10.8.14.0/24 > > The source address is in that subnet, yes. > > > and changes the destination address to be that of my proxy server > > 10.8.2.11. > > No - see below. > > > So now the ack packet has source 10.8.14.9 and destination 10.8.2.11. > > No, it doesn't. When a packet goes via a router, its destination IP > address is not changed to the address of the next-hop router (otherwise > things would never work across the Internet). > > It's only the destination MAC address in the encapsulating ethernet frame > which gets changed to that of the next-hop router. The source and > destination IP addresses inside are not touched. > > > How does iptables know to reply to my client 10.8.14.9 with source > > address 1.1.1.1? Does iptables know to read the header? > > TCP header, yes. > > HTTP header, no. > > Just think about the very first link between the client and its default > gateway: > > Packet with source address = 10.8.14.9, destinatoin address = 1.1.1.1 > > How does that packet get to the default router 10.8.14.1? Its destination > IP is 1.1.1.1, so that doesn't help. > > It's because the destination MAC address in the ethernet frame containing > that IP packet is the MAC address of 10.8.14.1. > > A few minutes playing around with wireshark on your network could be quite > enlightening :) > > > > Regards, > > > Antony. -- "Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns - the ones we don't know we don't know." - Donald Rumsfeld, US Secretary of Defence Please reply to the list; please *don't* CC me. _______________________________________________ squid-users mailing list squid-users@xxxxxxxxxxxxxxxxxxxxx http://lists.squid-cache.org/listinfo/squid-users
