MFC r363888:
Handle delayed checksums if needed in NAT64.

Upper level protocols defer checksums calculation in hope we have
checksums offloading in a network card. CSUM_DELAY_DATA flag is used
to determine that checksum calculation was deferred. And IP output
routine checks for this flag before pass mbuf to lower layer. Forwarded
packets have not this flag.

NAT64 uses checksums adjustment when it translates IP headers.
In most cases NAT64 is used for forwarded packets, but in case when it
handles locally originated packets we need to finish checksum calculation
that was deferred to correctly adjust it.

Add check for presence of CSUM_DELAY_DATA flag and finish checksum
calculation before adjustment.

MFC r345263:
Add SPDX-License-Identifier and update year in copyright.

MFC r345262:
Modify struct nat64_config.

Add second IPv6 prefix to generic config structure and rename another
fields to conform to RFC6877. Now it contains two prefixes and length:
PLAT is provider-side translator that translates N:1 global IPv6 addresses
to global IPv4 addresses. CLAT is customer-side translator (XLAT) that
algorithmically translates 1:1 IPv4 addresses to global IPv6 addresses.
Use PLAT prefix in stateless (nat64stl) and stateful (nat64lsn)
translators.

Modify nat64_extract_ip4() and nat64_embed_ip4() functions to accept
prefix length and use plat_plen to specify prefix length.

Retire net.inet.ip.fw.nat64_allow_private sysctl variable.
Add NAT64_ALLOW_PRIVATE flag and use "allow_private" config option to
configure this ability separately for each NAT64 instance.

Obtained from: Yandex LLC
Sponsored by: Yandex LLC

MFC r339542:
Retire IPFIREWALL_NAT64_DIRECT_OUTPUT kernel option. And add ability
to switch the output method in run-time. Also document some sysctl
variables that can by changed for NAT64 module.

NAT64 had compile time option IPFIREWALL_NAT64_DIRECT_OUTPUT to use
if_output directly from nat64 module. By default is used netisr based
output method. Now both methods can be used, but they require different
handling by rules.

Obtained from: Yandex LLC
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D16647

MFC r345003:
Add NULL pointer check to nat64_output().

It is possible that a processed packet was originated by local host,
in this case m->m_pkthdr.rcvif is NULL. Check and set it to V_loif to
avoid NULL pointer dereference in IP input code, since it is expected
that packet has valid receiving interface when netisr processes it.

Obtained from: Yandex LLC
Sponsored by: Yandex LLC

Revert r340541. It requires VNET_DEFINE_STATIC() macro that is not yet
merged into stable/11.

MFC r339542:
Retire IPFIREWALL_NAT64_DIRECT_OUTPUT kernel option. And add ability
to switch the output method in run-time. Also document some sysctl
variables that can by changed for NAT64 module.

NAT64 had compile time option IPFIREWALL_NAT64_DIRECT_OUTPUT to use
if_output directly from nat64 module. By default is used netisr based
output method. Now both methods can be used, but they require different
handling by rules.

Obtained from: Yandex LLC
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D16647

MFC r333403:
Bring in some last changes in NAT64 implementation:

o Modify ipfw(8) to be able set any prefix6 not just Well-Known,
and also show configured prefix6;
o relocate some definitions and macros into proper place;
o convert nat64_debug and nat64_allow_private variables to be
VNET-compatible;
o add struct nat64_config that keeps generic configuration needed
to NAT64 code;
o add nat64_check_prefix6() function to check validness of specified
by user IPv6 prefix according to RFC6052;
o use nat64_check_private_ip4() and nat64_embed_ip4() functions
instead of nat64_get_ip4() and nat64_set_ip4() macros. This allows
to use any configured IPv6 prefixes that are allowed by RFC6052;
o introduce NAT64_WKPFX flag, that is set when IPv6 prefix is
Well-Known IPv6 prefix. It is used to reduce overhead to check this;
o modify nat64lsn_cfg and nat64stl_cfg structures to use nat64_config
structure. And respectivelly modify the rest of code;
o remove now unused ro argument from nat64_output() function;
o remove __FreeBSD_version ifdef, NAT64 was not merged to older versions;
o add commented -DIPFIREWALL_NAT64_DIRECT_OUTPUT flag to module's Makefile
as example.

MFC r333406:
Update NAT64 documentation, now we support any IPv6 prefixes.

MFC r332456:
Migrate NAT64 to FIB KPI.

MFC r320479:
Fix IPv6 extension header parsing. The length field doesn't include the
first 8 octets.

Obtained from: Yandex LLC

MFC r304041:
Move logging via BPF support into separate file.

* make interface cloner VNET-aware;
* simplify cloner code and use if_clone_simple();
* migrate LOGIF_LOCK() to rmlock;
* add ipfw_bpf_mtap2() function to pass mbuf to BPF;
* introduce new additional ipfwlog0 pseudo interface. It differs from
ipfw0 by DLT type used in bpfattach. This interface is intended to
used by ipfw modules to dump packets with additional info attached.
Currently pflog format is used. ipfw_bpf_mtap2() function uses second
argument to determine which interface use for dumping. If dlen is equal
to ETHER_HDR_LEN it uses old ipfw0 interface, if dlen is equal to
PFLOG_HDRLEN - ipfwlog0 will be used.

Obtained from: Yandex LLC
Sponsored by: Yandex LLC

MFC r304043:
Add three helper function to manage tables from external modules.

ipfw_objhash_lookup_table_kidx does lookup kernel index of table;
ipfw_ref_table/ipfw_unref_table takes and releases reference to table.

Obtained from: Yandex LLC
Sponsored by: Yandex LLC

MFC r304046, 304108:
Add ipfw_nat64 module that implements stateless and stateful NAT64.

The module works together with ipfw(4) and implemented as its external
action module.

Stateless NAT64 registers external action with name nat64stl. This
keyword should be used to create NAT64 instance and to address this
instance in rules. Stateless NAT64 uses two lookup tables with mapped
IPv4->IPv6 and IPv6->IPv4 addresses to perform translation.

A configuration of instance should looks like this:
1. Create lookup tables:
# ipfw table T46 create type addr valtype ipv6
# ipfw table T64 create type addr valtype ipv4
2. Fill T46 and T64 tables.
3. Add rule to allow neighbor solicitation and advertisement:
# ipfw add allow icmp6 from any to any icmp6types 135,136
4. Create NAT64 instance:
# ipfw nat64stl NAT create table4 T46 table6 T64
5. Add rules that matches the traffic:
# ipfw add nat64stl NAT ip from any to table(T46)
# ipfw add nat64stl NAT ip from table(T64) to 64:ff9b::/96
6. Configure DNS64 for IPv6 clients and add route to 64:ff9b::/96
via NAT64 host.

Stateful NAT64 registers external action with name nat64lsn. The only
one option required to create nat64lsn instance - prefix4. It defines
the pool of IPv4 addresses used for translation.

A configuration of instance should looks like this:
1. Add rule to allow neighbor solicitation and advertisement:
# ipfw add allow icmp6 from any to any icmp6types 135,136
2. Create NAT64 instance:
# ipfw nat64lsn NAT create prefix4 A.B.C.D/28
3. Add rules that matches the traffic:
# ipfw add nat64lsn NAT ip from any to A.B.C.D/28
# ipfw add nat64lsn NAT ip6 from any to 64:ff9b::/96
4. Configure DNS64 for IPv6 clients and add route to 64:ff9b::/96
via NAT64 host.

Obtained from: Yandex LLC
Relnotes: yes
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D6434

MFC r304048:
Replace __noinline with special debug macro NAT64NOINLINE.

MFC r304061:
Use %ju to print unsigned 64-bit value.

MFC r304076:
Make statistics nat64lsn, nat64stl an nptv6 output netstat-like:
"@value @description" and fix build due to -Wformat errors.

MFC r304378 (by bz):
Try to fix gcc compilation errors (which are right).
nat64_getlasthdr() returns an int, which can be -1 in case of error,
storing the result in an uint8_t and then comparing to < 0 is not
helpful. Do what is done in the rest of the code and make proto an
int here as well.

MFC r309187:
Fix ICMPv6 Time Exceeded error message translation.

MFC r314718:
Use new ipfw_lookup_table() in the nat64 too.

MFC r315204,315233:
Use memset with structure size.