# A Multi-Path Bidirectional Layer 3 Proxy

## Setup Notes
### Linux
#### Policy Based Routing

    ip route flush table 10
    ip route add table 10 to 1.1.1.0/24 dev eth1
    ip rule add from 1.1.1.4 table 10 priority 10

    ip route flush table 11
    ip route add table 11 to 1.1.1.0/24 dev eth2
    ip rule add from 1.1.1.5 table 11 priority 11

#### ARP Flux

    sysctl -w net.ipv4.conf.all.arp_announce=1
    sysctl -w net.ipv4.conf.all.arp_ignore=1
    
See http://kb.linuxvirtualserver.org/wiki/Using_arp_announce/arp_ignore_to_disable_ARP

### Systemd unit

    [Unit]
    Description=NetCombiner for interface %i
    After=network-online.target

    [Service]
    Type=forking

    ExecStartPre=/etc/netcombiner/%i.pre
    ExecStart=/usr/local/sbin/netcombiner %i
    ExecStartPost=/etc/netcombiner/%i.post

    User=root
    Group=root

    Restart=always

    [Install]
    WantedBy=multi-user.target
    
### Setup Scripts
These are functional setup scripts that make the application run as intended on Linux.

### Remote Portal
#### Pre-Start

    #!/bin/bash
    set -e

    ## Set up variables
    REMOTE_PORTAL_ADDRESS=A.B.C.D

    ## IPv4 Forwarding
    sysctl -w net.ipv4.ip_forward=1
    sysctl -w net.ipv4.conf.eth0.proxy_arp=1
    
    ## Transfer the local routing table to a much lower priority
    (ip rule show | grep '20:') > /dev/null || ip rule add from all table local priority 20
    ip rule del priority 0 2> /dev/null || true
    
    ## Ports to route locally

    ### MPBL3P
    ip rule del priority 1 2> /dev/null || true
    ip rule add to "$REMOTE_PORTAL_ADDRESS" dport 1234 table local priority 1

    ### SSH
    ip rule del priority 2 2> /dev/null || true
    ip rule add to "$REMOTE_PORTAL_ADDRESS" dport 22   table local priority 2

#### Post-Start

    #!/bin/bash
    set -e

    ## Set up variables
    REMOTE_PORTAL_ADDRESS=A.B.C.D

    ## Tunnel addr/up
    ip addr add 172.19.152.2/31 dev nc0
    ip link set up nc0
    
    # Route packets to the interface but not for nc via the tunnel
    ip route flush table 19
    ip route add table 19 to "$REMOTE_PORTAL_ADDRESS" via 172.19.152.3 dev nc0
    ip rule del priority 19 2> /dev/null || true
    ip rule add to "$REMOTE_PORTAL_ADDRESS" table 19 priority 19
    
### Local Portal
#### Pre-Start

    #!/bin/bash
    set -e

    ## Set up variables
    GATEWAY_INTERFACE=eth0
    GATEWAY_ADDRESS=10.36.12.1
    
    ## Fix ARP
    sysctl -w net.ipv4.conf.all.arp_announce=1
    sysctl -w net.ipv4.conf.all.arp_ignore=1
    
    ## IPv4 Forwarding
    sysctl -w net.ipv4.ip_forward=1
    
    ## Gateway Interface Setup
    ip addr flush dev "$GATEWAY_INTERFACE"
    ip addr add "$GATEWAY_ADDRESS"/32 dev "$GATEWAY_INTERFACE"
    ip link set up "$GATEWAY_INTERFACE"
    
    ## Per-Interface Routing Tables
    
    ### 10.10.0.0/24
    ip route flush table 10
    ip route add table 10 default via 10.10.0.1
    ip rule del priority 10 2> /dev/null || true
    ip rule add from 10.10.0.0/24 table 10 priority 10
    
    ### 192.168.0.0/24
    ip route flush table 11
    ip route add table 11 default via 192.168.0.1
    ip rule del priority 11 2> /dev/null || true
    ip rule add from 192.168.0.0/24 table 11 priority 11

#### Post-Start

    #!/bin/bash
    set -e
    
    ## Set up variables
    REMOTE_PORTAL_ADDRESS=A.B.C.D
    GATEWAY_INTERFACE=eth0
    
    ## Tunnel Address and Enable
    ip addr add 172.19.152.3/31 dev nc0
    ip link set up nc0
    
    ## Route Outbound Packets Correctly
    ip route flush table 20
    ip route add table 20 default via 172.19.152.2 dev nc0
    ip rule del priority 20 2> /dev/null || true
    ip rule add from "$REMOTE_PORTAL_ADDRESS" iif "$GATEWAY_INTERFACE" table 20 priority 20
    
    ## Route Inbound Packets Correctly
    ip route flush table 21
    ip route add table 21 to "$REMOTE_PORTAL_ADDRESS" dev "$GATEWAY_INTERFACE"
    ip rule del priority 21 2> /dev/null || true
    ip rule add to "$REMOTE_PORTAL_ADDRESS" table 21 priority 21
    
#### Client

Connect to `GATEWAY_INTERFACE` and set the IP to `REMOTE_PORTAL_ADDRESS`/32 with a gateway of `GATEWAY_ADDRESS`.