Network Address Translation (NAT)

NuttX supports full cone NAT logic, which currently supports

TCP
UDP
ICMP
- ECHO (REQUEST & REPLY)
- Error Messages (DEST_UNREACHABLE & TIME_EXCEEDED & PARAMETER_PROBLEM)

Workflow

Local Network (LAN)                          External Network (WAN)
                  |----------------|
     <local IP,   |                | <external IP,             <peer IP,
       -----------|                |-----------------------------
      local port> |                |  external port>            peer port>
                  |----------------|

Outbound
- LAN -> Forward -> NAT (only if targeting at WAN) -> WAN
- All packets from LAN and targeting at WAN will be masqueraded with local ip:port changed to external ip:port.
Inbound
- WAN -> NAT (only from WAN, change destination) -> Forward -> LAN
- Packets from WAN will try to be changed back from external ip:port to local ip:port and send to LAN.

Configuration Options

CONFIG_NET_NAT: Enable or disable Network Address Translation (NAT) function. Depends on CONFIG_NET_IPFORWARD.
CONFIG_NET_NAT_HASH_BITS: The bits of the hashtable of NAT entries, hashtable has (1 << bits) buckets.
CONFIG_NET_NAT_TCP_EXPIRE_SEC: The expiration time for idle TCP entry in NAT. The default value 86400 is suggested by RFC2663, Section 2.6, Page 5. But we may set it to shorter time like 240s for better performance.
CONFIG_NET_NAT_UDP_EXPIRE_SEC: The expiration time for idle UDP entry in NAT.
CONFIG_NET_NAT_ICMP_EXPIRE_SEC: The expiration time for idle ICMP entry in NAT.
CONFIG_NET_NAT_ENTRY_RECLAIM_SEC: The time to auto reclaim all expired NAT entries. A value of zero will disable auto reclaiming. Expired entries will be automatically reclaimed when matching inbound/outbound entries, so this config does not have significant impact when NAT is normally used, but very useful when the hashtable is big and there are only a few connections using NAT (which will only trigger reclaiming on a few chains in hashtable).

Usage

ipv4_nat_enable()

ipv4_nat_disable()

int ipv4_nat_enable(FAR struct net_driver_s *dev);

Enable NAT function on a network device, on which the outbound packets will be masqueraded.

Returns:: Zero is returned if NAT function is successfully enabled on the device; A negated errno value is returned if failed.

int ipv4_nat_disable(FAR struct net_driver_s *dev);

Disable NAT function on a network device.

Returns:: Zero is returned if NAT function is successfully disabled on the device; A negated errno value is returned if failed.

Validation

Validated on Ubuntu 22.04 x86_64 with NuttX SIM by following steps:

Configure NuttX with >=2 TAP devices (host route mode) and NAT enabled:

CONFIG_NET_IPFORWARD=y
CONFIG_NET_NAT=y
# CONFIG_SIM_NET_BRIDGE is not set
CONFIG_SIM_NETDEV_NUMBER=2

Call ipv4_nat_enable on one dev on startup

/* arch/sim/src/sim/up_netdriver.c */
int netdriver_init(void)
{
  ...
  ipv4_nat_enable(&g_sim_dev[0]);
  ...
}

Set IP Address for NuttX on startup

ifconfig eth0 10.0.1.2
ifup eth0
ifconfig eth1 10.0.10.2
ifup eth1

Configure IP & namespace & route on host side (maybe need to be root, then try sudo -i)

IF_HOST="enp1s0"
IF_0="tap0"
IP_HOST_0="10.0.1.1"
IF_1="tap1"
IP_HOST_1="10.0.10.1"
IP_NUTTX_1="10.0.10.2"

# add net namespace LAN for $IF_1
ip netns add LAN
ip netns exec LAN sysctl -w net.ipv4.ip_forward=1
ip link set $IF_1 netns LAN
ip netns exec LAN ip link set $IF_1 up
ip netns exec LAN ip link set lo up

# add address and set default route
ip addr add $IP_HOST_0/24 dev $IF_0
ip netns exec LAN ip addr add $IP_HOST_1/24 dev $IF_1
ip netns exec LAN ip route add default dev $IF_1 via $IP_NUTTX_1

# nat to allow NuttX to access the internet
iptables -t nat -A POSTROUTING -o $IF_HOST -j MASQUERADE
iptables -A FORWARD -i $IF_HOST -o $IF_0 -j ACCEPT
iptables -A FORWARD -i $IF_0 -o $IF_HOST -j ACCEPT
sysctl -w net.ipv4.ip_forward=1

Do anything in the LAN namespace will go through NAT

# Host side
iperf -B 10.0.1.1 -s -i 1
# LAN side
sudo ip netns exec LAN iperf -B 10.0.10.1 -c 10.0.1.1 -i 1

# Host side
python3 -m http.server
# LAN side
for i in {1..20000}; do sudo ip netns exec LAN curl 'http://10.0.1.1:8000/' > /dev/null 2>1; done

# LAN side
sudo ip netns exec LAN ping 8.8.8.8

# LAN side
sudo ip netns exec LAN traceroute -n 8.8.8.8     # ICMP error msg of UDP
sudo ip netns exec LAN traceroute -n -T 8.8.8.8  # ICMP error msg of TCP

# Host side
tcpdump -nn -i tap0
# LAN side
sudo ip netns exec LAN tcpdump -nn -i tap1