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Squid vs. iptables redirection: exception for certain domains ?

Hi,

I'm currently facing a quite tricky problem. Here goes.

I have setup Squid as a transparent HTTP+HTTPS proxy in my local
network. All web traffic gets handed over to Squid by an iptables script
on the server. Here's the relevant section in /etc/squid/squid.conf:

--8<-------------------------------------------------------------
# Ports du proxy
http_port 3130
http_port 3128 intercept
https_port 3129 intercept ssl-bump \
cert=/etc/squid/ssl_cert/amandine.sandbox.lan.pem \
generate-host-certificates=on dynamic_cert_mem_cache_size=4MB
--8<-------------------------------------------------------------

And here's the corresponding section of my firewall script:

--8<-------------------------------------------------------------
# Commandes
IPT=/usr/sbin/iptables
SYS=/usr/sbin/sysctl
SERVICE=/usr/sbin/service

# Internet
IFACE_INET=enp2s0

# Réseau local
IFACE_LAN=virbr0
IFACE_LAN_IP=192.168.2.0/24

# Serveur
SERVER_IP=192.168.2.1

...

# Squid
$IPT -A INPUT -p tcp -i $IFACE_LAN --dport 3128 -j ACCEPT
$IPT -A INPUT -p udp -i $IFACE_LAN --dport 3128 -j ACCEPT
$IPT -A PREROUTING -t nat -i $IFACE_LAN -p tcp ! -d $SERVER_IP \
--dport 80 -j REDIRECT --to-port 3128
$IPT -A INPUT -p tcp -i $IFACE_LAN --dport 3129 -j ACCEPT
$IPT -A INPUT -p udp -i $IFACE_LAN --dport 3129 -j ACCEPT
$IPT -A PREROUTING -t nat -i $IFACE_LAN -p tcp ! -d $SERVER_IP \
--dport 443 -j REDIRECT --to-port 3129
$IPT -A INPUT -p tcp -i $IFACE_LAN --dport 3130 -j ACCEPT
$IPT -A INPUT -p udp -i $IFACE_LAN --dport 3130 -j ACCEPT
--8<-------------------------------------------------------------

This setup works nicely for the vast majority of web sites.

BUT: a handful of sites has some trouble with my local certificate. For
example, I can't sync my local Github repo anymore. Or my local OwnCloud
client spews back a warning message on every startup.

I asked on the Squid mailing list if there was a possibility to create
an exception for a list of domains, so that these can simply bypass the
proxy. The problem is, according to one of the developers, I have to
tackle that problem earlier in the process, e. g. in the firewall setup.

So here's what I want to do, in plain words:

1. Redirect all HTTP traffic (port 80) to port 3128. So far so good.

2. Redirect all HTTPS traffic (port 443) to port 3129. Equally OK.

AND...

3. DO NOT REDIRECT traffic that goes to certain domains, like:

github.com
credit-cooperatif.coop
cloud.microlinux.fr
squid-cache.org
etc.

Ideally, these domains should be read from a simple text file.

Any idea how I could do that? I don't even know if this is theoretically
possible.

Cheers,

Niki

Comments

Re: Squid vs. iptables redirection: exception for certa

By Niki Kovacs at 03/11/2018 - 05:53

Le 11/03/2018 à 11:01, Nicolas Kovacs a écrit :
I've experimented some more, and I have a partial success. Here, I'm
redirecting all HTTPS traffic *except* the one that goes to my bank:

iptables -A PREROUTING -t nat -i $IFACE_LAN -p tcp ! -d
<a href="http://www.credit-cooperatif.coop" title="www.credit-cooperatif.coop">www.credit-cooperatif.coop</a> --dport 443 -j REDIRECT --to-port 3129

This works because my bank is hosted on a single IP. As soon as I
replace that with a domain that's hosted on multiple IP's, I get this:

iptables -A PREROUTING -t nat -i $IFACE_LAN -p tcp ! -d <a href="http://www.google.com" title="www.google.com">www.google.com</a>
--dport 443 -j REDIRECT --to-port 3129

# firewall.sh
iptables v1.4.21: ! not allowed with multiple source or destination IP
addresses

So my question is: how can I write an iptables rule (or series of rules)
that redirect all traffic to my proxy, *except* the one going to
<list_of_domains> ?

Cheers,

Niki

Re: Squid vs. iptables redirection: exception for certa

By Leon Fauster at 03/11/2018 - 07:09

Am 11.03.2018 um 11:53 schrieb Nicolas Kovacs < ... at microlinux dot fr>:

May I ask, after all it doesn't work with google.com, right?

It is not a good practice to place domain names into iptables rules. Define
a custom table, place this table into your rule list (to stick at the right
place) and feed that table with the resolved domain names. This can be altered
while running in the case of changes (check resolving results periodically).

Re: Squid vs. iptables redirection: exception for certa

By Valeri Galtsev at 03/11/2018 - 10:47

On Sun, March 11, 2018 7:09 am, Leon Fauster wrote:
I would also like to add: it is a bad practice IMHO to give preference to
some particular search engine, unless it is single user personal machine.
Many people prefer different search engines (duckduckgo.com just to
mention one), some specifically avoid google.

Valeri

++++++++++++++++++++++++++++++++++++++++
Valeri Galtsev
Sr System Administrator
Department of Astronomy and Astrophysics
Kavli Institute for Cosmological Physics
University of Chicago
Phone: 773-702-4247
++++++++++++++++++++++++++++++++++++++++

Re: Squid vs. iptables redirection: exception for certa

By Niki Kovacs at 03/11/2018 - 07:18

Le 11/03/2018 à 13:09, Leon Fauster a écrit :
I admit I've never worked with custom tables, so I don't know how to do
this.

In the meantime, I found the following working solution.

# Exceptions
EXCEPTIONS=$(egrep -v '(^\#)|(^\s+$)' /usr/local/sbin/no-proxy.txt)
for EXCEPTION in $EXCEPTIONS; do
$IPT -A PREROUTING -t nat -i $IFACE_LAN -d $EXCEPTION -j ACCEPT
done

# Squid
$IPT -A INPUT -p tcp -i $IFACE_LAN --dport 3128 -j ACCEPT
$IPT -A INPUT -p udp -i $IFACE_LAN --dport 3128 -j ACCEPT
$IPT -A PREROUTING -t nat -i $IFACE_LAN -p tcp ! -d $SERVER_IP \
--dport 80 -j REDIRECT --to-port 3128
$IPT -A INPUT -p tcp -i $IFACE_LAN --dport 3129 -j ACCEPT
$IPT -A INPUT -p udp -i $IFACE_LAN --dport 3129 -j ACCEPT
$IPT -A PREROUTING -t nat -i $IFACE_LAN -p tcp ! -d $SERVER_IP \
--dport 443 -j REDIRECT --to-port 3129
$IPT -A INPUT -p tcp -i $IFACE_LAN --dport 3130 -j ACCEPT
$IPT -A INPUT -p udp -i $IFACE_LAN --dport 3130 -j ACCEPT

And my no-proxy.txt file looks like this:

# Ne pas utiliser le proxy pour les domaines suivants
#
# Crédit Agricole
<a href="http://www.credit-agricole.fr" title="www.credit-agricole.fr">www.credit-agricole.fr</a>
# Crédit Coopératif
<a href="http://www.credit-cooperatif.coop" title="www.credit-cooperatif.coop">www.credit-cooperatif.coop</a>
# Github
github.com
# Microlinux
microlinux.fr
microlinux.eu
# Squid
squid-cache.org
# Thunderbird
start.thunderbird.net

Note that I can put either domain names or IP addresses in this file.

And it's only supposed to keep a list of a handful of URLs that don't
play well with a transparent Squid for HTTPS.

Cheers,

Niki

Re: Squid vs. iptables redirection: exception for certa

By nux at 03/12/2018 - 04:37

Hi,

Another idea - but this gets complicated and with that, prone to faults - use a simple shell script to resolve the desired domains and keep their IPs in an ipset, then use the ipset in your firewall rules, this way you can keep your iptables rules static, your squid config static and simply add or remove IPs from the ipset.

Re: Squid vs. iptables redirection: exception for certa

By Niki Kovacs at 03/12/2018 - 04:42

Le 12/03/2018 à 10:37, Nux! a écrit :
Following a suggestion from Yuri Voinov on the Squid mailing list, I've
found a better solution that works perfectly. I've added it to my blog here:

<a href="https://blog.microlinux.fr/squid-exceptions/#squid" title="https://blog.microlinux.fr/squid-exceptions/#squid">https://blog.microlinux.fr/squid-exceptions/#squid</a>

Cheers,

Niki