Notes:
1. This version of NAT66 is a stateful one like NAT44, corresponding to Linux's MASQUERADE target of ip6tables. We can support stateless NAT66 & NPTv6 later by slightly modify the address & port selection logic (maybe just match the rules and skip the entry find).
2. We're using same flag `IFF_NAT` for both NAT44 & NAT66 to make control easier. Which means, if we enable NAT, both NAT44 & NAT66 will be enabled. If we don't want one of them, we can just disable that one in Kconfig.
3. Maybe we can accelerate the checksum adjustment by pre-calculate a difference of checksum, and apply it to each packet, instead of calling `net_chksum_adjust` each time. Just a thought, maybe do it later.
4. IP fragment segments on NAT66 connections are not supported yet.
Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>
To prepare for future IPv6 NAT functions.
- Rename common ipv4_nat_xxx to nat_xxx
- Move some common definitions into header
Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>
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>
