Transparenter TOR Proxy

By tobi
10. September 2019

Oberwohl es im Manual von TOR nicht unbedingt empfohlen wird, ist es überraschend einfach ein System so zu konigurieren, dass es ausschliesslich via TOR Netzwerk nach aussen erreichbar ist. Dass es im Manual nicht empfohlen wird, liegt an den massiven Eingriffen in das Routing des Traffics, die nötig sind um das umzusetzen.

Unter der Voraussetzung, dass man TOR auf dem System bereits installiert und konfiguriert hat reicht folgendes Script um die nötigen iptables Regeln zu erstellen. Diese Regeln stellen sicher, dass Traffic welcher das System verlässt durch TOR geht d.h. das System ist nicht mehr von aussen erreichbar, ausser über TOR oder eine IP Adresse aus einem Private LAN.
Zudem müssen in torrc noch folgende Anpassungen gemacht werden

VirtualAddrNetworkIPv4 10.192.0.0/10
AutomapHostsOnResolve 1
TransPort 9040 IsolateClientAddr IsolateClientProtocol IsolateDestAddr IsolateDestPort
DNSPort 5353

#!/bin/sh
#
### Set variables
# The UID that Tor runs as (varies from system to system)
_tor_uid="997" #As per assumption
#_tor_uid=`id -u debian-tor` #Debian/Ubuntu
#_tor_uid=`id -u tor` #ArchLinux/Gentoo

# Tor's TransPort
_trans_port="9040"

# Tor's DNSPort
_dns_port="5353"

# Tor's VirtualAddrNetworkIPv4
_virt_addr="10.192.0.0/10"

# Your outgoing interface
_out_if="eth0"

# LAN destinations that shouldn't be routed through Tor
_non_tor="127.0.0.0/8 10.0.0.0/8 172.16.0.0/12 192.168.0.0/16"

# Other IANA reserved blocks (These are not processed by tor and dropped by default)
_resv_iana="0.0.0.0/8 100.64.0.0/10 169.254.0.0/16 192.0.0.0/24 192.0.2.0/24 192.88.99.0/24 198.18.0.0/15 198.51.100.0/24 203.0.113.0/24 224.0.0.0/4 240.0.0.0/4 255.255.255.255/32"

### Don't lock yourself out after the flush
iptables -P INPUT ACCEPT
iptables -P OUTPUT ACCEPT

### Flush iptables
iptables -F
iptables -t nat -F

### *nat OUTPUT (For local redirection)
# nat .onion addresses
iptables -t nat -A OUTPUT -d $_virt_addr -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -j REDIRECT --to-ports $_trans_port

# nat dns requests to Tor
iptables -t nat -A OUTPUT -d 127.0.0.1/32 -p udp -m udp --dport 53 -j REDIRECT --to-ports $_dns_port

# Don't nat the Tor process, the loopback, or the local network
iptables -t nat -A OUTPUT -m owner --uid-owner $_tor_uid -j RETURN
iptables -t nat -A OUTPUT -o lo -j RETURN

# Allow lan access for hosts in $_non_tor
for _lan in $_non_tor; do
  iptables -t nat -A OUTPUT -d $_lan -j RETURN
done

for _iana in $_resv_iana; do
  iptables -t nat -A OUTPUT -d $_iana -j RETURN
done

# Redirect all other pre-routing and output to Tor's TransPort
iptables -t nat -A OUTPUT -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -j REDIRECT --to-ports $_trans_port

### *filter INPUT
# Don't forget to grant yourself ssh access from remote machines before the DROP.
# iptables -A INPUT -i ethX -s IP.AAD.DRE.SS -p tcp --dport 22 -m state --state NEW -j ACCEPT

iptables -A INPUT -m state --state ESTABLISHED -j ACCEPT
iptables -A INPUT -i lo -j ACCEPT

# Allow INPUT from lan hosts in $_non_tor
# Uncomment these 3 lines to enable.
#for _lan in $_non_tor; do
# iptables -A INPUT -s $_lan -j ACCEPT
#done

# Log & Drop everything else. Uncomment to enable logging
#iptables -A INPUT -j LOG --log-prefix "Dropped INPUT packet: " --log-level 7 --log-uid
iptables -A INPUT -j DROP

### *filter FORWARD
iptables -A FORWARD -j DROP

### *filter OUTPUT
iptables -A OUTPUT -m state --state INVALID -j DROP
iptables -A OUTPUT -m state --state ESTABLISHED -j ACCEPT

# Allow Tor process output
iptables -A OUTPUT -o $_out_if -m owner --uid-owner $_tor_uid -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -m state --state NEW -j ACCEPT

# Allow loopback output
iptables -A OUTPUT -d 127.0.0.1/32 -o lo -j ACCEPT

# Tor transproxy magic
iptables -A OUTPUT -d 127.0.0.1/32 -p tcp -m tcp --dport $_trans_port --tcp-flags FIN,SYN,RST,ACK SYN -j ACCEPT

# Allow OUTPUT to lan hosts in $_non_tor
# Uncomment these 3 lines to enable.
#for _lan in $_non_tor; do
# iptables -A OUTPUT -d $_lan -j ACCEPT
#done

# Log & Drop everything else. Uncomment to enable logging
#iptables -A OUTPUT -j LOG --log-prefix "Dropped OUTPUT packet: " --log-level 7 --log-uid
iptables -A OUTPUT -j DROP

### Set default policies to DROP
iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT DROP

### Set default policies to DROP for IPv6
ip6tables -P INPUT DROP
ip6tables -P FORWARD DROP
ip6tables -P OUTPUT DROP

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