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 structrtentry, which includes the following fields:
structradix_nodert_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_key(rt)
and rt_mask(rt) macros can be used to extract this information (in the form of a structsockaddr*) given a structrtentry*.
structsockaddr*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).
intrt_flags;
See below. If the RTF_UP flag is not present, the rtfree() function will delete the route
from the radix tree when the last reference drops.
intrt_refcnt;
Route entries are reference-counted; this field indicates the number of external (to the
radix tree) references.
structifnet*rt_ifp;
structifaddr*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.
u_longrt_mtu;
See description of rmx_mtu below.
u_longrt_weight;
See description of rmx_weight below.
u_longrt_expire;
See description of rmx_expire below.
counter64_trt_pksent;
See description of rmx_pksent below.
structrtentry*rt_gwroute;
This member is a reference to a route whose destination is rt_gateway. It is only used for
RTF_GATEWAY routes.
structmtxrt_mtx;
Mutex to lock this routing entry.
The following flag bits are defined:
RTF_UP The route is not deleted.
RTF_GATEWAY The route points to an intermediate destination and not the ultimate recipient; the
rt_gateway and rt_gwroute fields name that destination.
RTF_HOST This is a host route.
RTF_REJECT The destination is presently unreachable. This should result in an EHOSTUNREACH
error from output routines.
RTF_DYNAMIC This route was created dynamically by rtredirect().
RTF_MODIFIED This route was modified by rtredirect().
RTF_DONE Used only in the route(4) protocol, indicating that the request was executed.
RTF_XRESOLVE 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.
RTF_STATIC Indicates that this route was manually added by means of the route(8) command.
RTF_BLACKHOLE Requests that output sent via this route be discarded.
RTF_PROTO1
RTF_PROTO2
RTF_PROTO3 Protocol-specific.
RTF_PINNED Indicates that this route is immutable to a routing protocol.
RTF_LOCAL Indicates that the destination of this route is an address configured as belonging
to this system.
RTF_BROADCAST Indicates that the destination is a broadcast address.
RTF_MULTICAST Indicates that the destination is a multicast address.
Several metrics are supplied in structrt_metrics passed with routing control messages via route(4) API.
Currently only rmx_mtu, rmx_expire, and rmx_pksent metrics are supplied. All others are ignored.
The following metrics are defined by structrt_metrics:
u_longrmx_locks;
Flag bits indicating which metrics the kernel is not permitted to dynamically modify.
u_longrmx_mtu;
MTU for this path.
u_longrmx_hopcount;
Number of intermediate systems on the path to this destination.
u_longrmx_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_longrmx_recvpipe;
Nominally, the bandwidth-delay product for the path from the destination to 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_longrmx_sendpipe;
As before, but in the opposite direction.
u_longrmx_ssthresh;
The slow-start threshold used in TCP congestion-avoidance.
u_longrmx_rtt;
The round-trip time to this destination, in units of RMX_RTTUNIT per second.
u_longrmx_rttvar;
The average deviation of the round-trip time to this destination, in units of RMX_RTTUNIT
per second.
u_longrmx_pksent;
A count of packets successfully sent via this route.
u_longrmx_filler[4];
Empty space available for protocol-specific information.
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