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      1 /*
      2  * Copyright 2012 Daniel Drown
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  * http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  *
     16  * getroute.c - get an ip route
     17  */
     18 #include <string.h>
     19 #include <errno.h>
     20 
     21 #include <sys/socket.h>
     22 #include <linux/netlink.h>
     23 #include <linux/rtnetlink.h>
     24 #include <arpa/inet.h>
     25 
     26 #include <netlink/handlers.h>
     27 #include <netlink/msg.h>
     28 
     29 #include "getroute.h"
     30 #include "netlink_callbacks.h"
     31 #include "netlink_msg.h"
     32 
     33 /* function: get_default_route_cb
     34  * finds the default route with the request family and out interface and saves the gateway
     35  * msg  - netlink message
     36  * data - (struct default_route_data) requested filters and response storage
     37  */
     38 static int get_default_route_cb(struct nl_msg *msg, void *data) {
     39   struct rtmsg *rt_p;
     40   struct rtattr *rta_p;
     41   int rta_len;
     42   struct default_route_data *default_route = data;
     43   union anyip *this_gateway = NULL;
     44   ssize_t this_gateway_size;
     45   int this_interface_id = -1;
     46 
     47   if(default_route->reply_found_route) { // we already found our route
     48     return NL_OK;
     49   }
     50 
     51   rt_p = (struct rtmsg *)nlmsg_data(nlmsg_hdr(msg));
     52   if(rt_p->rtm_dst_len != 0) { // not a default route
     53     return NL_OK;
     54   }
     55   if((rt_p->rtm_family != default_route->request_family) || (rt_p->rtm_table != RT_TABLE_MAIN)) { // not a route we care about
     56     return NL_OK;
     57   }
     58 
     59   rta_p = (struct rtattr *)RTM_RTA(rt_p);
     60   rta_len = RTM_PAYLOAD(nlmsg_hdr(msg));
     61   for(; RTA_OK(rta_p, rta_len); rta_p = RTA_NEXT(rta_p, rta_len)) {
     62     switch(rta_p->rta_type) {
     63       case RTA_GATEWAY:
     64         this_gateway = RTA_DATA(rta_p);
     65         this_gateway_size = RTA_PAYLOAD(rta_p);
     66         break;
     67       case RTA_OIF:
     68         this_interface_id = *(int *)RTA_DATA(rta_p);
     69         break;
     70       default:
     71         break;
     72     }
     73   }
     74 
     75   if(this_interface_id == default_route->request_interface_id) {
     76     default_route->reply_found_route = 1;
     77     if(this_gateway != NULL) {
     78       memcpy(&default_route->reply_gateway, this_gateway, this_gateway_size);
     79       default_route->reply_has_gateway = 1;
     80     } else {
     81       default_route->reply_has_gateway = 0;
     82     }
     83   }
     84   return NL_OK;
     85 }
     86 
     87 /* function: error_handler
     88  * error callback for get_default_route
     89  * nla  - where the message came from
     90  * err  - netlink message
     91  * arg  - (int *) storage for the error number
     92  */
     93 static int error_handler(struct sockaddr_nl *nla, struct nlmsgerr *err, void *arg) {
     94   int *retval = arg;
     95   if(err->error < 0) { // error_handler called even on no error (NLMSG_ERROR reply type used)
     96     *retval = err->error;
     97   }
     98   return NL_OK;
     99 }
    100 
    101 /* function: get_default_route
    102  * finds the first default route with the given family and interface, returns the gateway (if it exists) in the struct
    103  * default_route - requested family and interface, and response storage
    104  */
    105 int get_default_route(struct default_route_data *default_route) {
    106   struct rtmsg msg;
    107   struct nl_cb *callbacks = NULL;
    108   struct nl_msg *nlmsg = NULL;
    109   int retval = 0;
    110 
    111   default_route->reply_has_gateway = 0;
    112   default_route->reply_found_route = 0;
    113 
    114   memset(&msg,'\0',sizeof(msg));
    115   msg.rtm_family = default_route->request_family;
    116   msg.rtm_table = RT_TABLE_MAIN;
    117   msg.rtm_protocol = RTPROT_KERNEL;
    118   msg.rtm_scope = RT_SCOPE_UNIVERSE;
    119 
    120   callbacks = nl_cb_alloc(NL_CB_DEFAULT);
    121   if(!callbacks) {
    122     retval = -ENOMEM;
    123     goto cleanup;
    124   }
    125   // get_default_route_cb sets the response fields in default_route
    126   nl_cb_set(callbacks, NL_CB_VALID, NL_CB_CUSTOM, get_default_route_cb, default_route);
    127   nl_cb_err(callbacks, NL_CB_CUSTOM, error_handler, &retval);
    128 
    129   nlmsg = nlmsg_alloc_rtmsg(RTM_GETROUTE, NLM_F_REQUEST | NLM_F_ROOT, &msg);
    130   if(!nlmsg) {
    131     retval = -ENOMEM;
    132     goto cleanup;
    133   }
    134   send_netlink_msg(nlmsg, callbacks);
    135 
    136 cleanup:
    137   if(callbacks)
    138     nl_cb_put(callbacks);
    139   if(nlmsg)
    140     nlmsg_free(nlmsg);
    141 
    142   return retval;
    143 }
    144