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RTENTRY(9) Kernel Developer's Manual RTENTRY(9)

rtentry — structure of an entry in the kernel routing table

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

The kernel provides a common mechanism by which all protocols can store and retrieve entries from a central table of routes. Parts of this mechanism are also used to interact with user-level processes by means of a socket in the route(4) pseudo-protocol family. The <net/route.h> header file defines the structures and manifest constants used in this facility.

The basic structure of a route is defined by “struct rtentry”, which includes the following fields:

struct radix_node rt_nodes[2];
Glue used by the radix-tree routines. These members also include in their substructure the key (i.e., destination address) and mask used when the route was created. The (rt) and (rt) macros can be used to extract this information (in the form of a “struct sockaddr *”) given a struct rtentry *.
struct sockaddr *rt_gateway;
The “target” of the route, which can either represent a destination in its own right (some protocols will put a link-layer address here), or some intermediate stop on the way to that destination (if the RTF_GATEWAY flag is set).
long rt_refcnt;
Route entries are reference-counted; this field indicates the number of external (to the radix tree) references. If the RTF_UP flag is not present, the () function will delete the route from the radix tree when the last reference drops.
u_long rt_flags;
See below.
struct ifnet *rt_ifp;
 
struct ifaddr *rt_ifa;
These two fields represent the “answer”, as it were, to the question posed by a route lookup; that is, they name the interface and interface address to be used in sending a packet to the destination or set of destinations which this route represents.
struct sockaddr *rt_genmask;
When the () family of functions performs a cloning operation as requested by the RTF_CLONING or RTF_PRCLONING flag, this field is used as the mask for the new route which is inserted into the table. If this field is a null pointer, then a host route is generated.
caddr_t rt_llinfo;
When the RTF_LLINFO flag is set, this field contains information specific to the link layer represented by the named interface address. (It is normally managed by the () routine.) Protocols such as arp(4) use this field to reference per-destination state internal to that protocol.
struct rt_metrics rt_rmx;
See below.
struct rtentry *rt_gwroute;
This member is a reference to a route whose destination is rt_gateway. It is only used for RTF_GATEWAY routes.
struct rtentry *rt_parent;
A reference to the route from which this route was cloned, or a null pointer if this route was not generated by cloning. See also the RTF_WASCLONED flag.

The following flag bits are defined:

The route is not deleted.
The route points to an intermediate destination and not the ultimate recipient; the rt_gateway and rt_gwroute fields name that destination.
This is a host route.
The destination is presently unreachable. This should result in an EHOSTUNREACH error from output routines.
This route was created dynamically by ().
This route was modified by rtredirect().
Used only in the route(4) protocol, indicating that the request was executed.
When this route is returned as a result of a lookup, automatically create a new route using this one as a template and rt_genmask (if present) as a mask.
When this route is returned as a result of a lookup, send a report on the route(4) interface requesting that an external process perform resolution for this route. (Used in conjunction with RTF_CLONING.)
Indicates that this route represents information being managed by a link layer's adaptation layer (e.g., ARP).
Indicates that this route was manually added by means of the route(8) command.
Requests that output sent via this route be discarded.
 
 
Protocol-specific.
Like RTF_CLONING, only managed by an entire protocol. (E.g., IP uses this flag to manage a per-host cache integrated with the routing table, for those destinations which do not have a link layer performing this function.)
Indicates that this route was generated as a result of cloning requested by the RTF_CLONING or RTF_PRCLONING flag. When set, the rt_parent field indicates the route from which this one was generated.
(Reserved for future use to indicate routes which are not to be modified by a routing protocol.)
Indicates that the destination of this route is an address configured as belonging to this system.
Indicates that the destination is a broadcast address.
Indicates that the destination is a multicast address.

Every route has associated with it a set of metrics, defined by struct rt_metrics:

u_long rmx_locks;
Flag bits indicating which metrics the kernel is not permitted to dynamically modify.
u_long rmx_mtu;
MTU for this path.
u_long rmx_hopcount;
Number of intermediate systems on the path to this destination.
u_long rmx_expire;
The time (a la time(3)) at which this route should expire, or zero if it should never expire. It is the responsibility of individual protocol suites to ensure that routes are actually deleted once they expire.
u_long rmx_recvpipe;
Nominally, the bandwidth-delay product for the path the destination this system. In practice, this value is used to set the size of the receive buffer (and thus the window in sliding-window protocols like TCP).
u_long rmx_sendpipe;
As before, but in the opposite direction.
u_long rmx_ssthresh;
The slow-start threshold used in TCP congestion-avoidance.
u_long rmx_rtt;
The round-trip time to this destination, in units of RTM_RTTUNIT per second.
u_long rmx_rttvar;
The average deviation of the round-type time to this destination, in units of RTM_RTTUNIT per second.
u_long rmx_pksent;
A count of packets successfully sent via this route.
u_long rmx_filler[4];
Empty space available for protocol-specific information.

route(4), route(8), rtalloc(9)

The rtentry structure first appeared in 4.2BSD. The radix-tree representation of the routing table and the rt_metrics structure first appeared in 4.3BSD-Reno. The RTF_PRCLONING mechanism first appeared in FreeBSD 2.0.

This manual page was written by Garrett Wollman.

There are a number of historical relics remaining in this interface. The rt_gateway and rmx_filler fields could be named better.

There is some disagreement over whether it is legitimate for RTF_LLINFO to be set by any process other than rt_ifa->ifa_rtrequest().

October 8, 1996 DragonFly-5.6.1