1 上面的文章读了net/ipv4/route.c 中的ip_route_input函数，是协议站对收到报文的路由查找函数。继续阅读一下协议栈发包的时候路由查找的调用函数ip_roue_output_key。
2
3 int ip_route_output_key(struct net *net, struct rtable **rp, struct flowi *flp)
4 {
5 return ip_route_output_flow(net, rp, flp, NULL, 0);
6 }
7 只是一个函数封装，真正的处理函数是ip_route_output_flow.
8 int ip_route_output_flow(struct net *net, struct rtable **rp, struct flowi *flp,
9 struct sock *sk, int flags)
10 {
11 int err;
12
13 /*路由查找*/
14 if ((err = __ip_route_output_key(net, rp, flp)) != 0)
15 return err;
16
17 /*IPSec 的处理代码*/
18 if (flp->proto) {
19 if (!flp->fl4_src)
20 flp->fl4_src = (*rp)->rt_src;
21 if (!flp->fl4_dst)
22 flp->fl4_dst = (*rp)->rt_dst;
23 err = __xfrm_lookup(net, (struct dst_entry **)rp, flp, sk,
24 flags ? XFRM_LOOKUP_WAIT : 0);
25 if (err == -EREMOTE)
26 err = ipv4_dst_blackhole(net, rp, flp);
27
28 return err;
29 }
30
31 return 0;
32 }
33
34 很眼熟的__ip_route_output_key函数。
35 int __ip_route_output_key(struct net *net, struct rtable **rp,
36 const struct flowi *flp)
37 {
38 unsigned hash;
39 struct rtable *rth;
40
41 if (!rt_caching(net))
42 goto slow_output;
43
44 /*类似于ip_route_input,先在cache中查找路由， 找到就返回*/
45 hash = rt_hash(flp->fl4_dst, flp->fl4_src, flp->oif, rt_genid(net));
46
47 rcu_read_lock_bh();
48 for (rth = rcu_dereference(rt_hash_table[hash].chain); rth;
49 rth = rcu_dereference(rth->u.dst.rt_next)) {
50 if (rth->fl.fl4_dst == flp->fl4_dst &&
51 rth->fl.fl4_src == flp->fl4_src &&
52 rth->fl.iif == 0 &&
53 rth->fl.oif == flp->oif &&
54 rth->fl.mark == flp->mark &&
55 !((rth->fl.fl4_tos ^ flp->fl4_tos) &
56 (IPTOS_RT_MASK | RTO_ONLINK)) &&
57 net_eq(dev_net(rth->u.dst.dev), net) &&
58 !rt_is_expired(rth)) {
59 dst_use(&rth->u.dst, jiffies);
60 RT_CACHE_STAT_INC(out_hit);
61 rcu_read_unlock_bh();
62 *rp = rth;
63 return 0;
64 }
65 RT_CACHE_STAT_INC(out_hlist_search);
66 }
67 rcu_read_unlock_bh();
68
69 /*不支持cache 或在cache中没找到相应的路由信息，在路由表中查找*/
70 slow_output:
71 return ip_route_output_slow(net, rp, flp);
72 }
73
74 继续探险
75 /*
76 * Major route resolver routine.
77 */
78
79 static int ip_route_output_slow(struct net *net, struct rtable **rp,
80 const struct flowi *oldflp)
81 {
82 u32 tos = RT_FL_TOS(oldflp); /*获取tos和当前的RTO_ONLINK（？）标志*/
83 struct flowi fl = { .nl_u = { .ip4_u =
84 { .daddr = oldflp->fl4_dst,
85 .saddr = oldflp->fl4_src,
86 .tos = tos & IPTOS_RT_MASK,
87 .scope = ((tos & RTO_ONLINK) ? /*根据这个标志，得出路由的scope*/
88 RT_SCOPE_LINK :
89 RT_SCOPE_UNIVERSE),
90 } },
91 .mark = oldflp->mark,
92 .iif = net->loopback_dev->ifindex, /*设备号为lo的设备号?*/
93 .oif = oldflp->oif };
94 struct fib_result res;
95 unsigned flags = 0;
96 struct net_device *dev_out = NULL;
97 int free_res = 0;
98 int err;
99
100
101 res.fi = NULL;
102 #ifdef CONFIG_IP_MULTIPLE_TABLES
103
104
105 res.r = NULL;
106 #endif
107
108 /*先是对源地址， 发包接口号和目的地址进行判断分类处理。下面的每一个红色跳转就是一种情况*/
109 if (oldflp->fl4_src) { /*源*/
110 err = -EINVAL;
111 if (ipv4_is_multicast(oldflp->fl4_src) ||
112 ipv4_is_lbcast(oldflp->fl4_src) ||
113 ipv4_is_zeronet(oldflp->fl4_src))
114 goto out;
115
116 /*上面是对报文源地址的合理性检查，源地址是多播，广播或0地址时，返回错误*/
117
118 /* I removed check for oif == dev_out->oif here.
119 It was wrong for two reasons:
120 我在这里删去检查oif == dev_out->oif是否成立，因为有两个原因说明这个检查时错误的：
121 1. ip_dev_find(net, saddr) can return wrong iface, if saddr
122 is assigned to multiple interfaces
123 如果源地址是一个多播接口的地址，函数ip_dev_find(net, saddr)可能返回错误的设备接口。
124 2. Moreover, we are allowed to send packets with saddr
125 of another iface. --ANK
126 而且可以用另外设备接口的源地址发送报文
127 */
128
129 if (oldflp->oif == 0
130 && (ipv4_is_multicast(oldflp->fl4_dst) ||
131 oldflp->fl4_dst == htonl(0xFFFFFFFF))) { /*发包接口为lo，目的地址是广播或多播时查找发包设备，ip_dev_find返回与所给定的源地址相等的第一个设备*/
132 /* It is equivalent to inet_addr_type(saddr) == RTN_LOCAL */
133 dev_out = ip_dev_find(net, oldflp->fl4_src);
134 if (dev_out == NULL)
135 goto out;
136
137 /* Special hack: user can direct multicasts
138 and limited broadcast via necessary interface
139 without fiddling with IP_MULTICAST_IF or IP_PKTINFO.
140 This hack is not just for fun, it allows
141 vic,vat and friends to work.
142 They bind socket to loopback, set ttl to zero
143 and expect that it will work.
144 From the viewpoint of routing cache they are broken,
145 because we are not allowed to build multicast path
146 with loopback source addr (look, routing cache
147 cannot know, that ttl is zero, so that packet
148 will not leave this host and route is valid).
149 Luckily, this hack is good workaround.
150 */
151
152 /*当报文初始化的出接口为lo接口源地址不为空目的地址是多播或广播地址时，找到源地址所对应的接口重新为出接口赋值， 然后创建cache路由项*/
153 fl.oif = dev_out->ifindex;
154 goto make_route;
155 }
156
157 /*?????*/
158 if (!(oldflp->flags & FLOWI_FLAG_ANYSRC)) {
159 /* It is equivalent to inet_addr_type(saddr) == RTN_LOCAL */
160 dev_out = ip_dev_find(net, oldflp->fl4_src);
161 if (dev_out == NULL)
162 goto out;
163 dev_put(dev_out);
164 dev_out = NULL;
165 }
166 }
167
168
169 if (oldflp->oif) {/*发包设备不为空*/
170 /*检测出接口是否存在*/
171 dev_out = dev_get_by_index(net, oldflp->oif);
172 err = -ENODEV;
173 if (dev_out == NULL)
174 goto out;
175
176 /* RACE: Check return value of inet_select_addr instead. */
177 /*看设备是否是多地址*/
178 if (__in_dev_get_rtnl(dev_out) == NULL) {
179 dev_put(dev_out);
180 goto out; /* Wrong error code */
181 }
182
183 /*当目的地址是本地多播地址或广播地址，并且报文源地址为空时，找出出接口设备上IP地址scope小于RT_SCOPE_LINK的地址，并赋值，然后往cache中添加路由表项*/
184 if (ipv4_is_local_multicast(oldflp->fl4_dst) ||
185 oldflp->fl4_dst == htonl(0xFFFFFFFF)) {
186 if (!fl.fl4_src)
187 fl.fl4_src = inet_select_addr(dev_out, 0,
188 RT_SCOPE_LINK);
189 goto make_route;
190 }
191 /*目的地址是单播地址或空，源地址为空，那就选一个小于特定scope的IP地址*/
192 if (!fl.fl4_src) {
193 if (ipv4_is_multicast(oldflp->fl4_dst))
194 fl.fl4_src = inet_select_addr(dev_out, 0,
195 fl.fl4_scope);
196 else if (!oldflp->fl4_dst)
197 fl.fl4_src = inet_select_addr(dev_out, 0,
198 RT_SCOPE_HOST);
199 }
200 }
201
202 if (!fl.fl4_dst) {/*目的地址为空*/
203 fl.fl4_dst = fl.fl4_src;
204 if (!fl.fl4_dst)
205 fl.fl4_dst = fl.fl4_src = htonl(INADDR_LOOPBACK);/*目的和源地址都是空，则赋值为lo接口地址*/
206 if (dev_out)
207 dev_put(dev_out);
208 dev_out = net->loopback_dev;
209 dev_hold(dev_out);
210 fl.oif = net->loopback_dev->ifindex;
211 res.type = RTN_LOCAL;
212 flags |= RTCF_LOCAL;
213 /*为发给本机的报文添加cache路由*/
214 goto make_route;
215 }
216
217 /*一种情况是源地址目的地址不为空，目的地址为空，出接口为lo*/
218 /*还有其他几种情况，就是目的地址和出接口必须对应*/
219 if (fib_lookup(net, &fl, &res)) {
220 res.fi = NULL;
221 if (oldflp->oif) {
222 /* Apparently, routing tables are wrong. Assume,
223 that the destination is on link.
224
225 WHY? DW.
226 Because we are allowed to send to iface
227 even if it has NO routes and NO assigned
228 addresses. When oif is specified, routing
229 tables are looked up with only one purpose:
230 to catch if destination is gatewayed, rather than
231 direct. Moreover, if MSG_DONTROUTE is set,
232 we send packet, ignoring both routing tables
233 and ifaddr state. --ANK
234
235
236 We could make it even if oif is unknown,
237 likely IPv6, but we do not.
238 */
239
240 if (fl.fl4_src == 0)
241 fl.fl4_src = inet_select_addr(dev_out, 0,
242 RT_SCOPE_LINK);
243 res.type = RTN_UNICAST;
244 /*没有查到路由，并且出接口不为lo*/
245 goto make_route;
246 }
247 if (dev_out)
248 dev_put(dev_out);
249 err = -ENETUNREACH;
250 goto out;
251 }
252 /*找到路由*/
253 free_res = 1;
254
255 /*路由指向本地*/
256 if (res.type == RTN_LOCAL) {
257 if (!fl.fl4_src)
258 fl.fl4_src = fl.fl4_dst;
259 if (dev_out)
260 dev_put(dev_out);
261 dev_out = net->loopback_dev;
262 dev_hold(dev_out);
263 fl.oif = dev_out->ifindex;
264 if (res.fi)
265 fib_info_put(res.fi);
266 res.fi = NULL;
267 flags |= RTCF_LOCAL;
268 goto make_route;
269 }
270
271 /*是否支持多路径路由*/
272 #ifdef CONFIG_IP_ROUTE_MULTIPATH
273 if (res.fi->fib_nhs > 1 && fl.oif == 0)
274 fib_select_multipath(&fl, &res);
275 else
276 #endif
277 if (!res.prefixlen && res.type == RTN_UNICAST && !fl.oif)
278 fib_select_default(net, &fl, &res);
279
280 if (!fl.fl4_src)
281 fl.fl4_src = FIB_RES_PREFSRC(res);
282
283 if (dev_out)
284 dev_put(dev_out);
285 dev_out = FIB_RES_DEV(res);
286 dev_hold(dev_out);
287 fl.oif = dev_out->ifindex;
288
289 /*往cache中添加相应的路由项*/
290 make_route:
291 err = ip_mkroute_output(rp, &res, &fl, oldflp, dev_out, flags);
292
293
294 if (free_res)
295 fib_res_put(&res);
296 if (dev_out)
297 dev_put(dev_out);
298 out: return err;
299 }
