Example: tinc from behind a masquerading firewall

When running tinc from behind a masquerading firewall (not on the firewall itself), one must be careful to configure the firewall so that it allows the tinc traffic to pass through without altering the source and destination ports. Example firewall rules are included in this example. They are written for iptables (Linux 2.4 firewall code), but commented so that you may apply the same kind of rules to other firewalls.

  1. Example: tinc from behind a masquerading firewall
    1. Overview
    2. Configuration of the host running tinc
    3. Configuration of tinc
    4. Configuration of the firewall

Overview

The network setup is as follows:

Configuration of the host running tinc

host# ifconfig
eth0      Link encap:Ethernet  HWaddr 00:20:30:40:50:60
          inet addr:10.20.30.42  Bcast:10.20.30.255  Mask:255.255.255.0
          UP BROADCAST RUNNING  MTU:1500  Metric:1
          ...

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          UP LOOPBACK RUNNING  MTU:3856  Metric:1
          ...

vpn       Link encap:Point-to-Point Protocol
          inet addr:192.168.10.20  P-t-P:192.168.10.20  Mask:255.255.0.0
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          ...

host# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.20.30.0      *               255.255.255.0   U     0      0        0 eth0
192.168.0.0     *               255.255.0.0     U     0      0        0 vpn
default         10.20.30.1      0.0.0.0         UG    0      0        0 eth0

host# iptables -L -v
Chain INPUT (policy ACCEPT 1234 packets, 123K bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy ACCEPT 2161K packets, 364M bytes)
 pkts bytes target     prot opt in     out     source               destination

host# iptables -L -v -t nat
Chain PREROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Configuration of tinc

host# cat /etc/tinc/vpn/tinc.conf
Name = atwork
ConnectTo = home

host# cat /etc/tinc/vpn/tinc-up
#!/bin/sh

ifconfig $INTERFACE 192.168.10.20 netmask 255.255.0.0

host# ls /etc/tinc/vpn/hosts
atwork  home

host# cat /etc/tinc/vpn/hosts/atwork
Address = 123.234.123.1
Subnet = 192.168.10.20/32
-----BEGIN RSA PUBLIC KEY-----
...
-----END RSA PUBLIC KEY-----

host# cat /etc/tinc/vpn/hosts/home
Address = 200.201.202.203
Subnet = 192.168.1.0/24
-----BEGIN RSA PUBLIC KEY-----
...
-----END RSA PUBLIC KEY-----

Configuration of the firewall

firewall# ifconfig
ppp0      Link encap:Point-to-Point Protocol
          inet addr:123.234.123.1  P-t-P:123.234.120.1  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP  MTU:1500  Metric:1
          ...

eth0      Link encap:Ethernet  HWaddr 00:20:13:14:15:16
          inet addr:10.20.30.1  Bcast:10.20.30.255  Mask:255.255.255.0
          UP BROADCAST RUNNING  MTU:1500  Metric:1
          ...

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          UP LOOPBACK RUNNING  MTU:3856  Metric:1
          ...

firewall# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.20.30.0      *               255.255.255.0   U     0      0        0 eth0
default         123.234.120.1   0.0.0.0         UG    0      0        0 ppp0

firewall# iptables -L -v
Chain INPUT (policy ACCEPT 1234 packets, 123K bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain FORWARD (policy DROP 1234 packets, 123K bytes)
 pkts bytes target     prot opt in     out     source               destination
 1234  123K ACCEPT     any  --  ppp0   eth0    anywhere             10.20.30.0/24
 1234  123K ACCEPT     any  --  eth0   ppp0    10.20.30.0/24        anywhere

Chain OUTPUT (policy ACCEPT 2161K packets, 364M bytes)
 pkts bytes target     prot opt in     out     source               destination

firewall# iptables -L -v -t nat
Chain PREROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
 1234  123K DNAT       tcp  --  ppp0   any     anywhere             anywhere           tcp dpt:655 to:10.20.30.42:655
 1234  123K DNAT       udp  --  ppp0   any     anywhere             anywhere           udp dpt:655 to:10.20.30.42:655

Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
 1234  123K MASQUERADE all  --  eth0   ppp0    anywhere             anywhere

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

firewall# cat /etc/init.d/firewall
#!/bin/sh

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

iptables -P FORWARD DROP
iptables -F FORWARD
iptables -A FORWARD -j ACCEPT -i ppp0 -o eth0 -d 10.20.30.0/24
iptables -A FORWARD -j ACCEPT -i eth0 -o ppp0 -s 10.20.30.0/24

iptables -t nat -F POSTROUTING
# Next rule prevents masquerading from altering source port of outbound tinc packets
iptables -t nat -A POSTROUTING -p udp -m udp --sport 655 -j MASQUERADE -o ppp0 --to-ports 655
iptables -t nat -A POSTROUTING -j MASQUERADE -o ppp0

iptables -t nat -F PREROUTING
# Next two rules forward incoming tinc packets to the host behind the firewall running tinc
iptables -t nat -A PREROUTING -j DNAT -i ppp0 -p tcp --dport 655 --to 10.20.30.42:655
iptables -t nat -A PREROUTING -j DNAT -i ppp0 -p udp --dport 655 --to 10.20.30.42:655