--- name: "rtentry" section: "9" platform: "freebsd" package: "base" version: "16.0-CURRENT-20261027-61c70dc16a79" release: "FreeBSD 16.0-CURRENT (2026-10-27)" platforms: ["linux", "freebsd"] url: "https://new.carta.tech/rtentry" --- # rtentry(9) structure of an entry in the kernel routing table ## NAME **rtentry** – structure of an entry in the kernel routing table ## SYNOPSIS **#include \** **#include \** **#include \** ## DESCRIPTION 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)](/route/4.md) 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\_key**(*rt*) and **rt\_mask**(*rt*) macros can be used to extract this information (in the form of a *struct sockaddr \**) given a *struct rtentry \**. - *struct nhop\_object \*rt\_nhop*; The [nhop(9)](/nhop/9.md) of the route, which can either a single nexthop or a nexthop group. A nexthop can be a directly connected destination (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). - *int rt\_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. - *u\_long rt\_weight*; See description of rmx\_weight below. 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)](/route/4.md) 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)](/route/4.md) 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)](/route/8.md) 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 *struct rt\_metrics* passed with routing control messages via [route(4)](/route/4.md) API. Currently only *rmx\_mtu*, *rmx\_expire*, and *rmx\_pksent* metrics are supplied. All others are ignored. The following metrics are 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)](/time/3.md)) 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 *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\_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 RMX\_RTTUNIT per second. - *u\_long rmx\_rttvar*; The average deviation of the round-trip time to this destination, in units of RMX\_RTTUNIT per second. - *u\_long rmx\_pksent*; A count of packets successfully sent via this route. - *u\_long rmx\_weight*; The weight of nexthop in multipath route. - *u\_long rmx\_nhidx*; The kernel index of route nexthop. - *u\_long rmx\_metric*; The metric of the route, the lowest value wins. - *u\_long rmx\_filler\[1\]*; Empty space available for protocol-specific information. ## SEE ALSO [route(4)](/route/4.md), [route(8)](/route/8.md), [nhop(9)](/nhop/9.md) ## HISTORY 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. ## AUTHORS This manual page was written by Garrett Wollman.