VPP配置SNAT，内网接口GigabitEthernet2/0/0，外网接口GigabitEthernet3/0/0，NAT之后的地址为GigabitEthernet3/0/0接口的地址。
vpp# set interface state GigabitEthernet2/0/0 up vpp# set interface state GigabitEthernet3/0/0 up vpp# vpp# set interface ip address GigabitEthernet2/0/0 30.1.1.1/24 vpp# set interface ip address GigabitEthernet3/0/0 192.168.1.203/24 vpp# vpp# nat44 plugin enable vpp# set interface nat44 in GigabitEthernet2/0/0 out GigabitEthernet3/0/0 vpp# nat44 add interface address GigabitEthernet3/0/0
NAT44-ED插件使用的feature和node节点如下。
ip4-output: ip4-sv-reassembly-output-feature nat44-ed-in2out-output nat44-in2out-output-worker-handoff nat-pre-in2out-output ip4-unicast: ip4-sv-reassembly-feature nat44-ed-out2in nat44-ed-in2out nat44-out2in-worker-handoff nat44-in2out-worker-handoff nat44-handoff-classify nat44-ed-classify nat-pre-out2in nat-pre-in2out
nat44-ed处理节点流程如下。VPP配置了多个worker线程的话，需要nat44-in2out-worker-handoff和nat44-out2in-worker-handoff节点；否则，使用nat-pre-in2out和nat-pre-out2in节点。
|-----------------------|
| ip4-input-no-checksum |
|-----------------------|
|
|---------------------------| GigabitEthernet3/0/0 |-----------------------------|
| ip4-sv-reassembly-feature |-------------------------------| nat44-out2in-worker-handoff |
| | | / nat-pre-out2in |
|---------------------------| |-----------------------------|
GigabitEthernet2/0/0 | |
|-----------------------------| |-----------------|
| nat44-in2out-worker-handoff | | nat44-ed-out2in |
| / nat-pre-in2out | | |
|-----------------------------| |-----------------|
| |
|-------------------| nonexist |----------------------------| |
| nat44-ed-in2out |-----------| nat44-ed-in2out-slowpath | |
|-------------------| |----------------------------| |
existing session | | |
| | |
|----------------| | |
| ip4-lookup |--------------------------|------------------------|
|----------------|
一. 报文重组功能
IP报文分片之后，仅在第一个分片中可见4层的端口号信息，对于后续分片由于没有端口号，将不能进行NAT操作，所以需要IP报文重组功能。在ARC集合ip4-unicast中，nat-pre-in2out和nat-pre-out2in都是位于ip4-sv-reassembly-feature之后，报文先经过重组处理。
VNET_FEATURE_INIT (nat_pre_in2out, static) = {
.arc_name = "ip4-unicast",
.node_name = "nat-pre-in2out",
.runs_after = VNET_FEATURES ("acl-plugin-in-ip4-fa",
"ip4-sv-reassembly-feature"),
};
VNET_FEATURE_INIT (nat_pre_out2in, static) = {
.arc_name = "ip4-unicast",
.node_name = "nat-pre-out2in",
.runs_after = VNET_FEATURES ("acl-plugin-in-ip4-fa",
"ip4-dhcp-client-detect",
"ip4-sv-reassembly-feature"),
};
节点nat_pre_nat的下一个节点索引默认为NAT_NEXT_IN2OUT_ED_FAST_PATH，对应于节点nat44-ed-in2out。
VLIB_NODE_FN (nat_pre_in2out_node)
(vlib_main_t * vm, vlib_node_runtime_t * node, vlib_frame_t * frame)
{
return nat_pre_node_fn_inline (vm, node, frame,
NAT_NEXT_IN2OUT_ED_FAST_PATH);
}
在函数nat_pre_node_fn_inline中，主要的工作就是记录ARC集合的下一个节点索引，记录在报文接口的nat.arc_next中，以便在NAT44模块处理完成之后，继续ARC集中的处理流程。通常情况下一个feature为ip4-lookup。
static inline uword
nat_pre_node_fn_inline (vlib_main_t *vm, vlib_node_runtime_t *node,
vlib_frame_t *frame, u32 def_next)
{
while (n_left_from > 0) {
vlib_buffer_t *b0;
b0 = *b;
b++;
next0 = def_next;
vnet_feature_next (&arc_next0, b0);
vnet_buffer2 (b0)->nat.arc_next = arc_next0;
n_left_from--;
next[0] = next0;
next++;
}
二. NAT44模块使能
对应命令nat44 plugin enable。如果VPP配置的worker线程数量大于1，需要创建frame_queue队列，在worker线程之间传递报文，保证同一个会话流在同一个worker上处理，避免使用锁。内网接口和外网接口分别注册fq_in2out_index和fq_out2in_index为索引的队列，对应node节点nat44-ed-in2out和nat44-ed-out2in。
int nat44_plugin_enable (nat44_config_t c)
{
snat_main_t *sm = &snat_main;
if (sm->num_workers > 1) {
vlib_main_t *vm = vlib_get_main ();
vlib_node_t *node;
if (sm->fq_in2out_index == ~0) {
node = vlib_get_node_by_name (vm, (u8 *) "nat44-ed-in2out");
sm->fq_in2out_index = vlib_frame_queue_main_init (node->index, sm->frame_queue_nelts);
}
if (sm->fq_out2in_index == ~0) {
node = vlib_get_node_by_name (vm, (u8 *) "nat44-ed-out2in");
sm->fq_out2in_index = vlib_frame_queue_main_init (node->index, sm->frame_queue_nelts);
}
三. NAT44接口
设置接口的inside/outside属性。首先，判断如果接口已经在nat44的接口中，并且接口的inside/outside属性没有改变，结束处理。
int nat44_ed_add_interface (u32 sw_if_index, u8 is_inside)
{
snat_main_t *sm = &snat_main;
nat_fib_t *outside_fib;
snat_interface_t *i;
i = nat44_ed_get_interface (sm->interfaces, sw_if_index);
if (i){
if ((nat44_ed_is_interface_inside (i) && is_inside) ||
(nat44_ed_is_interface_outside (i) && !is_inside))
return 0;
否则，这里是要修改接口的inside/outside属性，分两种情况。第一，worker线程数量大于1，接口从inside修改为outside，需要删除nat44-in2out-worker-handoff；否则，删除nat44-out2in-worker-handoff。
第二，worker线程小于等于1，接口从inside修改为outside，需要删除nat-pre-in2out，否则，删除nat-pre-out2in。
最后，开启ip4-sv-reassembly-feature，如果已经开启，增加引用计数。并且，删除记录下来的接口旧的feature，使能新的feature。以上提到的这些feature都位于ARC集合ip4-unicast。
if (sm->num_workers > 1) {
del_feature_name = !is_inside ? "nat44-in2out-worker-handoff" :
"nat44-out2in-worker-handoff";
feature_name = "nat44-handoff-classify";
} else {
del_feature_name = !is_inside ? "nat-pre-in2out" : "nat-pre-out2in";
feature_name = "nat44-ed-classify";
}
rv = ip4_sv_reass_enable_disable_with_refcnt (sw_if_index, 1);
if (rv)
return rv;
vnet_feature_enable_disable ("ip4-unicast", del_feature_name, sw_if_index, 0, 0, 0);
vnet_feature_enable_disable ("ip4-unicast", feature_name, sw_if_index, 1, 0, 0);
}
以下处理nat44模块中从未添加过此接口的情况。在worker线程数量大于1时，对于inside接口，使能nat44-in2out-worker-handoff；对于outside接口，使能nat44-out2in-worker-handoff。
使能报文重组feature：ip4-sv-reassembly-feature。将接口添加到nat44模块的接口池中（sm->interfaces）。
else {
if (sm->num_workers > 1)
feature_name = is_inside ? "nat44-in2out-worker-handoff" : "nat44-out2in-worker-handoff";
else
feature_name = is_inside ? "nat-pre-in2out" : "nat-pre-out2in";
nat_validate_interface_counters (sm, sw_if_index);
rv = ip4_sv_reass_enable_disable_with_refcnt (sw_if_index, 1);
if (rv) return rv;
vnet_feature_enable_disable ("ip4-unicast", feature_name, sw_if_index, 1, 0, 0);
pool_get (sm->interfaces, i);
i->sw_if_index = sw_if_index;
i->flags = 0;
}
如下命令显示nat44模块的接口池内容：
vpp# show nat44 interfaces NAT44 interfaces: GigabitEthernet2/0/0 in GigabitEthernet3/0/0 out
四. nat44添加地址
分配snat_address_resolve_t地址结构，记录接口索引等。
int
nat44_ed_add_interface_address (u32 sw_if_index, u8 twice_nat)
{
snat_main_t *sm = &snat_main;
ip4_main_t *ip4_main = sm->ip4_main;
ip4_address_t *first_int_addr;
snat_address_resolve_t *ap;
vec_add2 (sm->addr_to_resolve, ap, 1);
ap->sw_if_index = sw_if_index;
ap->is_twice_nat = twice_nat;
ap->is_resolved = 0;
获取接口的首个IP地址，添加到nat44的地址池中。添加成功之后，设置is_resolved为真。
first_int_addr = ip4_interface_first_address (ip4_main, sw_if_index, 0);
if (first_int_addr) {
rv = nat44_ed_add_address (first_int_addr, ~0, twice_nat);
if (0 != rv) {
nat44_ed_del_addr_resolve_record (sw_if_index, twice_nat);
return rv;
}
ap->is_resolved = 1;
}
这里不涉及两次NAT的情况，twice_nat为false。添加的地址保存在nat44模块的addresses地址向量中，先检测其中是否已有此地址，避免重复添加。
int
nat44_ed_add_address (ip4_address_t *addr, u32 vrf_id, u8 twice_nat)
{
snat_main_t *sm = &snat_main;
snat_address_t *ap, *addresses;
addresses = twice_nat ? sm->twice_nat_addresses : sm->addresses;
// check if address already exists
vec_foreach (ap, addresses) {
if (ap->addr.as_u32 == addr->as_u32) {
nat_log_err ("address exist");
return VNET_API_ERROR_VALUE_EXIST;
}
}
在addresses向量中分配新成员，填入新地址。
if (twice_nat)
vec_add2 (sm->twice_nat_addresses, ap, 1);
else
vec_add2 (sm->addresses, ap, 1);
ap->addr_len = ~0;
ap->fib_index = ~0;
ap->addr = *addr;
五. 节点流程
测试VPP配置了2个worker线程。如下，thread2在inside接口GigabitEthernet2/0/0接收到ICMP请求报文，不是IP分片，节点nat44-in2out-worker-handoff通过处理，将报文handoff到thread1。
------------------- Start of thread 2 vpp_wk_1 -------------------
Packet 2
00:06:05:665438: dpdk-input
GigabitEthernet2/0/0 rx queue 1
ICMP: 30.1.1.2 -> 192.168.12.254
ICMP echo_request checksum 0x4bde id 1
00:06:05:665440: ethernet-input
IP4: 50:7b:9d:c7:03:73 -> 00:60:e0:65:b5:c7
00:06:05:665440: ip4-input-no-checksum
ICMP: 30.1.1.2 -> 192.168.12.254
ICMP echo_request checksum 0x4bde id 1
00:06:05:665441: ip4-sv-reassembly-feature
[not-fragmented]
00:06:05:665441: nat44-in2out-worker-handoff
fragment id 0x3b72
NAT44_IN2OUT_WORKER_HANDOFF : next-worker 1 trace index 1
以上通过frame-queue：fq_in2out_index，将报文handoff到了thread1的nat44-ed-in2out节点。对于第一个报文，session并不存在，需要进入慢速节点处理：nat44-ed-in2out-slowpath。
------------------- Start of thread 1 vpp_wk_0 ------------------- Packet 2 00:06:05:665444: handoff_trace HANDED-OFF: from thread 2 trace index 1 00:06:05:665444: nat44-ed-in2out NAT44_IN2OUT_ED_FAST_PATH: sw_if_index 5, next index 3 search key local 30.1.1.2:1 remote 192.168.12.254:1 proto ICMP fib 0 thread-index 0 session-index 0 00:06:05:665447: nat44-ed-in2out-slowpath NAT44_IN2OUT_ED_SLOW_PATH: sw_if_index 5, next index 10, session 4, translation result 'success' via i2of i2of match: saddr 30.1.1.2 sport 1 daddr 192.168.12.254 dport 1 proto ICMP fib_idx 0 rewrite: saddr 192.168.12.12 daddr 192.168.12.254 icmp-id 28931 txfib 0 o2if match: saddr 192.168.12.254 sport 28931 daddr 192.168.12.12 dport 28931 p roto ICMP fib_idx 0 rewrite: daddr 30.1.1.2 icmp-id 1 txfib 0 00:06:05:665453: ip4-lookup fib 0 dpo-idx 3 flow hash: 0x00000000 ICMP: 192.168.12.12 -> 192.168.12.254
另外，对于由外到内的报文，即由GigabitEthernet3/0/0接口接收的报文。节点nat44-out2in-worker-handoff匹配到session，并且session位于thread1，即当前的thread，不需要handoff处理；也不需要进入慢速路径节点nat44-ed-out2in-slowpath。
------------------- Start of thread 1 vpp_wk_0 ------------------- Packet 14 00:06:10:263962: ip4-input-no-checksum ICMP: 192.168.12.254 -> 192.168.12.12 ICMP echo_reply checksum 0xe2da id 28931 00:06:10:263962: ip4-sv-reassembly-feature [not-fragmented] 00:06:10:263963: nat44-out2in-worker-handoff NAT44_OUT2IN_WORKER_HANDOFF : next-worker 1 trace index 13 00:06:10:263965: nat44-ed-out2in NAT44_OUT2IN_ED_FAST_PATH: sw_if_index 6, next index 10, session 4, translation result 'success' via o2if i2of match: saddr 30.1.1.2 sport 1 daddr 192.168.12.254 dport 1 proto ICMP fib_idx 0 rewrite: saddr 192.168.12.12 daddr 192.168.12.254 icmp-id 28931 txfib 0 o2if match: saddr 192.168.12.254 sport 28931 daddr 192.168.12.12 dport 28931 p roto ICMP fib_idx 0 rewrite: daddr 30.1.1.2 icmp-id 1 txfib 0 search key local 192.168.12.254:28931 remote 192.168.12.12:28931 proto ICMP fib 0 thread-index 0 session-index 0 no reason for slow path 00:06:10:263966: ip4-lookup fib 0 dpo-idx 4 flow hash: 0x00000000 ICMP: 192.168.12.254 -> 30.1.1.2 ICMP echo_reply checksum 0x53dd id 1 00:06:10:263967: ip4-rewrite tx_sw_if_index 5 dpo-idx 4 : ipv4 via 30.1.1.2 GigabitEthernet2/0/0: mtu:9000 next:3 flags:[] 507b9dc703730060e065b5c70800 flow hash: 0x00000000 00000000: 507b9dc703730060e065b5c708004500003cb15400003f01ddc3c0a80cfe1e01 00000020: 0102000053dd0001017e6162636465666768696a6b6c6d6e6f707172 00:06:10:263968: GigabitEthernet2/0/0-output
对于由GigabitEthernet2/0/0接口接收的第二个icmp请求报文，由于session已经存在，也不再需要进入慢速路径节点。由于报文被网卡送到了thread2线程，这里显示为handoff之后的处理。
------------------- Start of thread 1 vpp_wk_0 ------------------- Packet 13 00:06:10:263324: handoff_trace HANDED-OFF: from thread 2 trace index 5 00:06:10:263324: nat44-ed-in2out NAT44_IN2OUT_ED_FAST_PATH: sw_if_index 5, next index 10, session 4, translation result 'success' via i2of i2of match: saddr 30.1.1.2 sport 1 daddr 192.168.12.254 dport 1 proto ICMP fib _idx 0 rewrite: saddr 192.168.12.12 daddr 192.168.12.254 icmp-id 28931 txfib 0 o2if match: saddr 192.168.12.254 sport 28931 daddr 192.168.12.12 dport 28931 proto ICMP fib_idx 0 rewrite: daddr 30.1.1.2 icmp-id 1 txfib 0 lookup skipped - cached session index used 00:06:10:263327: ip4-lookup fib 0 dpo-idx 6 flow hash: 0x00000000 ICMP: 192.168.12.12 -> 192.168.12.254 ICMP echo_request checksum 0xdada id 28931