ROUTE

System packet forwarding database

Synopsis:

#include <sys/socket.h>
#include <net/if.h>
#include <net/route.h>

int socket( PF_ROUTE,
            SOCK_RAW,
            int family );

Description:

QNX TCP/IP provides some packet routing facilities.

The socket manager maintains a routing information database that's used in selecting the appropriate network interface when transmitting packets.

A user process (or possibly multiple cooperating processes) maintains this database by sending messages over a special kind of socket. This supplants fixed-size ioctl()s used in earlier releases. Routing table changes may be carried out only by the superuser.

This interface may spontaneously emit routing messages in response to external events, such as receipt of a redirect or of a failure to locate a suitable route for a request. The message types are described in greater detail below.

Routing database entries

Routing database entries come in two flavors: for a specific host or for all hosts on a generic subnetwork (as specified by a bit mask and value under the mask). The effect of wildcard or default routing may be achieved by using a mask of all zeros. There may be hierarchical routes.

When the system is booted and addresses are assigned to the network interfaces, each protocol family installs a routing table entry for each interface when it's ready for traffic. Normally the protocol specifies the route through each interface as a “direct” connection to the destination host or network. If the route is direct, the transport layer of a protocol family usually requests the packet be sent to the same host specified in the packet. Otherwise, the interface is requested to address the packet to the gateway listed in the routing entry (i.e. the packet is forwarded).

Routing packets

When routing a packet, the kernel attempts to find the most specific route matching the destination. (If there are two different mask and value-under-the-mask pairs that match, the more specific is the one with more bits in the mask. A route to a host is regarded as being supplied with a mask of as many ones as there are bits in the destination). If no entry is found, the destination is declared to be unreachable, and a routing-miss message is generated if there are any listeners on the routing control socket described below.

A wildcard routing entry is specified with a zero destination address value and a mask of all zeroes. Wildcard routes are used when the system fails to find other routes matching the destination. The combination of wildcard routes and routing redirects can provide an economical mechanism for routing traffic.

Routing control messages

To open the channel for passing routing control messages, use the socket call shown in the synopsis above.

The family parameter may be AF_UNSPEC, which provides routing information for all address families, or it can be restricted to a specific address family. There can be more than one routing socket open per system.

Messages are formed by a header followed by a small number of sockaddrs (with variable length) interpreted by position and delimited by the new length entry in the sockaddr. An example of a message with four addresses might be a redirect: Destination, Netmask, Gateway, and Author of the redirect. The interpretation of which addresses are present is given by a bit mask within the header; the sequence is least-significant to most-significant bit within the vector.

Any messages sent to the socket manager are returned, and copies are sent to all interested listeners. The interface provides the process ID for the sender. To distinguish between outstanding messages, the sender may use an additional sequence field. However, message replies may be lost when socket manager buffers are exhausted.

The interface may reject certain messages, as indicated in the rtm_errno field.

In the current implementation, all routing processes run locally, and the values for rtm_errno are available through the normal errno mechanism, even if the routing reply message is lost.

A process may avoid the expense of reading replies to its own messages by calling setsockopt(), to turn off the SO_USELOOPBACK option at the SOL_SOCKET level. A process may ignore all messages from the routing socket by doing a shutdown() system call for further input.

If a route is in use when it's deleted, the routing entry is marked down and removed from the routing table, but the resources associated with it won't be reclaimed until all references to it are released. User processes can obtain information about the routing entry to a specific destination by using a RTM_GET message or by calling sysctl().

The messages are:

#define RTM_ADD       0x1  /* Add Route */
#define RTM_DELETE    0x2  /* Delete Route */
#define RTM_CHANGE    0x3  /* Change Metrics, Flags, or Gateway */
#define RTM_GET       0x4  /* Report Information */
#define RTM_LOSING    0x5  /* Kernel Suspects Partitioning */
#define RTM_REDIRECT  0x6  /* Told to use different route */
#define RTM_MISS      0x7  /* Lookup failed on this address */
#define RTM_RESOLVE   0xb  /* request to resolve dst to LL addr */
#define RTM_NEWADDR   0xc  /* address being added to iface */
#define RTM_DELADDR   0xd  /* address being removed from iface */
#define RTM_IFINFO    0xe  /* iface going up/down etc. */

A message header consists of one of the following:

struct rt_msghdr {
    u_short rtm_msglen;    /* skip over non-understood msgs */
    u_char  rtm_version;   /* future binary compatibility */
    u_char  rtm_type;      /* message type */
    u_short rtm_index;     /* index for associated ifp */
    int     rtm_flags;     /* flags, incl kern & message, e.g. DONE */
    int     rtm_addrs;     /* bitmask identifying sockaddrs in msg */
    pid_t   rtm_pid;       /* identify sender */
    int     rtm_seq;       /* for sender to identify action */
    int     rtm_errno;     /* why failed */
    int     rtm_use;       /* from rtentry */
    u_long  rtm_inits;     /* which metrics we're initializing */
    struct  rt_metrics rtm_rmx; /* metrics themselves */
};

struct if_msghdr {
    u_short ifm_msglen;    /* to skip over non-understood msgs */
    u_char  ifm_version;   /* future binary compatibility */
    u_char  ifm_type;      /* message type */
    int     ifm_addrs;     /* like rtm_addrs */
    int     ifm_flags;     /* value of if_flags */
    u_short ifm_index;     /* index for associated ifp */
    struct  if_data ifm_dat /* statistics and other data about if */
};

struct ifa_msghdr {
    u_short ifam_msglen;   /* to skip over non-understood msgs */
    u_char  ifam_version;  /* future binary compatibility */
    u_char  ifam_type;     /* message type */
    int     ifam_addrs;    /* like rtm_addrs */
    int     ifam_flags;    /* value of ifa_flags */
    u_short ifam_index;    /* index for associated ifp */
    int     ifam_metric;   /* value of ifa_metric */
};

The RTM_IFINFO message uses an if_msghdr header. The RTM_NEWADDR and RTM_DELADDR messages use an ifa_msghdr header. All other messages use the rt_msghdr header.

The metrics structure is:

struct rt_metrics {
    u_long rmx_locks;      /* Kernel must leave these values alone */
    u_long rmx_mtu;        /* MTU for this path */
    u_long rmx_hopcount;   /* max hops expected */
    u_long rmx_expire;     /* lifetime for route, e.g. redirect */
    u_long rmx_recvpipe;   /* inbound delay-bandwidth product */
    u_long rmx_sendpipe;   /* outbound delay-bandwidth product */
    u_long rmx_ssthresh;   /* outbound gateway buffer limit */
    u_long rmx_rtt;        /* estimated round trip time */
    u_long rmx_rttvar;     /* estimated rtt variance */
    u_long rmx_pksent;     /* packets sent using this route */
};

Flags include the values:

#define RTF_UP        0x1    /* route usable */
#define RTF_GATEWAY   0x2    /* destination is a gateway */
#define RTF_HOST      0x4    /* host entry (net otherwise) */
#define RTF_REJECT    0x8    /* host or net unreachable */
#define RTF_DYNAMIC   0x10   /* created dynamically (by redirect) */
#define RTF_MODIFIED  0x20   /* modified dynamically (by redirect) */
#define RTF_DONE      0x40   /* message confirmed */
#define RTF_MASK      0x80   /* subnet mask present */
#define RTF_CLONING   0x100  /* generate new routes on use */
#define RTF_XRESOLVE  0x200  /* external daemon resolves name */
#define RTF_LLINFO    0x400  /* generated by ARP or ESIS */
#define RTF_STATIC    0x800  /* manually added */
#define RTF_BLACKHOLE 0x1000 /* just discard pkts (during updates) */
#define RTF_PROTO2    0x4000 /* protocol specific routing flag */
#define RTF_PROTO1    0x8000 /* protocol specific routing flag */

Specifiers for metric values in rmx_locks and rtm_inits are:

#define RTV_MTU       0x1   /* init or lock _mtu */
#define RTV_HOPCOUNT  0x2   /* init or lock _hopcount */
#define RTV_EXPIRE    0x4   /* init or lock _expire */
#define RTV_RPIPE     0x8   /* init or lock _recvpipe */
#define RTV_SPIPE     0x10  /* init or lock _sendpipe */
#define RTV_SSTHRESH  0x20  /* init or lock _ssthresh */
#define RTV_RTT       0x40  /* init or lock _rtt */
#define RTV_RTTVAR    0x80  /* init or lock _rttvar */

Specifiers for which addresses are present in the messages are:

#define RTA_DST      0x1  /* destination sockaddr present */
#define RTA_GATEWAY  0x2  /* gateway sockaddr present */
#define RTA_NETMASK  0x4  /* netmask sockaddr present */
#define RTA_GENMASK  0x8  /* cloning mask sockaddr present */
#define RTA_IFP      0x10 /* interface name sockaddr present */
#define RTA_IFA      0x20 /* interface addr sockaddr present */
#define RTA_AUTHOR   0x40 /* sockaddr for author of redirect */
#define RTA_BRD      0x80 /* for NEWADDR, */
                          /*      broadcast or p-p dest addr */

Examples:

Use the following code to set the default route:

 
#include <sys/socket.h>
#include <sys/uio.h>
#include <unistd.h>
#include <net/route.h>
#include <netinet/in.h>
#include <stdio.h>
#include <libgen.h>
#include <arpa/inet.h>
#include <process.h>
#include <errno.h>

struct my_rt
{
      struct rt_msghdr rt;
      struct sockaddr_in dst;
      struct sockaddr_in gate;
      struct sockaddr_in mask;
};

int main(int argc, char **argv)
{
      int s;
      struct rt_msghdr *rtm;
      struct sockaddr_in *dst, *gate, *mask;
      struct my_rt my_rt;

      if(argc < 2)
      {
          fprintf(stderr,
                  "Usage: %s: <ip_addr_of_default_gateway>\n",
                  basename(argv[0]));
          return 1;
      }

      if((s = socket(AF_ROUTE, SOCK_RAW, 0)) == -1)
      {
              perror("socket");
              return 1;
      }

      memset(&my_rt, 0x00, sizeof(my_rt));

      rtm  = &my_rt.rt;

      dst  = &my_rt.dst;
      gate = &my_rt.gate;
      mask = &my_rt.mask;

      rtm->rtm_type = RTM_ADD;
      rtm->rtm_flags = RTF_UP | RTF_GATEWAY | RTF_STATIC;
      rtm->rtm_msglen = sizeof(my_rt);
      rtm->rtm_version = RTM_VERSION;
      rtm->rtm_seq = 1234;
      rtm->rtm_addrs = RTA_DST | RTA_GATEWAY | RTA_NETMASK;
      rtm->rtm_pid = getpid();      
      
      dst->sin_len    = sizeof(*dst);
      dst->sin_family = AF_INET;

      mask->sin_len    = sizeof(*mask);
      mask->sin_family = AF_INET;

      gate->sin_len    = sizeof(*gate);
      gate->sin_family = AF_INET;
      inet_aton(argv[1], &gate->sin_addr);

AGAIN:
      if(write(s, rtm, rtm->rtm_msglen) < 0)
      {
               if(errno == EEXIST && rtm->rtm_type == RTM_ADD)
               {
                        rtm->rtm_type = RTM_CHANGE;
                        goto AGAIN;
               }
                        perror("write");
                        return 1;
      }
      return 0;
}

See also:

setsockopt(), socket(), sysctl()

io-pkt* in the Utilities Reference