The symmetric NAT limits one external port to be used with only one peer ip:port.
Note:
1. To avoid using too much #ifdef, we're always passing peer_ip and peer_port as arguments, but won't use them under full cone NAT, let the compiler optimize them.
2. We need to find port binding without peer ip:port, so don't add peer ip:port into hash key.
3. Symmetric NAT needs to *select another external port if a port is used by any other NAT entry*, this behavior is exactly same as Full Cone NAT, so we don't need to change anything related to `ipv4_nat_port_inuse`.
Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>
Support ICMP ECHO REQUEST & REPLY. Id of ICMP is processed like port of TCP in NAT. However, our ICMP stack doesn't have a method to manage id allocation like tcp_selectport(), the id is set by apps (like icmp_ping.c) without conflict avoidance, so not adding such conflict avoidance logic to ICMP stack when implementing NAT.
Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>
Add basic functions for NAT (NAPT), remaining some logic unimplemented (UDP, ICMP, port assignment, etc). NAT for TCP can work now (unless port conflicts).
Outbound: LAN -> Forward -> NAT(only if targeting at WAN) -> WAN
Inbound: WAN -> NAT(only from WAN, change dest) -> Forward -> LAN
Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>
