Updated: May 06, 2022

System packet forwarding database


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

int socket( PF_ROUTE,
            int family );


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.

This error occurs: If:
EEXIST Requested to duplicate an existing entry.
ESRCH Requested to delete a nonexistent entry.
ENOBUFS Insufficient resources were available to install a new route.

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 or flags */
#define RTM_GET         0x4     /* Report Metrics */
#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_LOCK        0x8     /* fix specified metrics */
#define RTM_OLDADD      0x9     /* caused by SIOCADDRT */
#define RTM_OLDDEL      0xa     /* caused by SIOCDELRT */
#define RTM_RESOLVE     0xb     /* req 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_OIFINFO     0xe     /* Old (pre-1.5) RTM_IFINFO message */
#define RTM_IFINFO      0xf     /* iface/link going up/down etc. */
#define RTM_IFANNOUNCE  0x10    /* iface arrival/departure */
#define RTM_IEEE80211   0x11    /* IEEE80211 wireless event */

A message header consists of one of the following:

struct rt_msghdr {
    u_short rtm_msglen;     /* to skip over non-understood messages */
    u_char  rtm_version;    /* future binary compatibility */
    u_char  rtm_type;       /* message type */
    u_short rtm_index;      /* index for associated ifp */
    u_short zero;           /* align structure */
    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 are initializing */
    struct  rt_metrics rtm_rmx; /* metrics themselves */

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

struct ifa_msghdr {
    unsigned short  ifam_msglen;    /* to skip over non-understood messages */
    unsigned char   ifam_version;   /* future binary compatibility */
    unsigned char   ifam_type;      /* message type */
    int             ifam_addrs;     /* like rtm_addrs */
    int             ifam_flags;     /* value of ifa_flags */
    unsigned 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          0x0001          /* route usable */
#define RTF_GATEWAY     0x0002          /* destination is a gateway */
#define RTF_HOST        0x0004          /* host entry (net otherwise) */
#define RTF_REJECT      0x0008          /* host or net unreachable */
#define RTF_DYNAMIC     0x0010          /* created dynamically (by redirect) */
#define RTF_MODIFIED    0x0020          /* modified dynamically (by redirect) */
#define RTF_DONE        0x0040          /* message confirmed */
#define RTF_MASK        0x0080          /* subnet mask present */
#define RTF_CLONING     0x0100          /* generate new routes on use */
#define RTF_XRESOLVE    0x0200          /* external daemon resolves name */
#define RTF_LLINFO      0x0400          /* generated by ARP or ESIS */
#define RTF_STATIC      0x0800          /* manually added */
#define RTF_BLACKHOLE   0x1000          /* just discard pkts (during updates) */
#define RTF_CLONED      0x2000          /* this is a cloned route */
#define RTF_PROTO2      0x4000          /* protocol specific routing flag */
#define RTF_PROTO1      0x8000          /* protocol specific routing flag */
#define RTF_SRC         0x10000         /* route has fixed source address */
#define RTF_BROADCAST   0x80000         /* route represents a bcast address */

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 */


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)
                  "Usage: %s: <ip_addr_of_default_gateway>\n",
          return 1;

      if((s = socket(AF_ROUTE, SOCK_RAW, 0)) == -1)
              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);

      if(write(s, rtm, rtm->rtm_msglen) < 0)
                        return 1;
      return 0;