.\"
.\" Copyright (c) 2026 Pouria Mousavizadeh Tehrani <pouria@FreeBSD.org>
.\"
.\" SPDX-License-Identifier: BSD-2-Clause
.\"
.Dd July 30, 2026
.Dt NHOP 9
.Os
.Sh NAME
.Nm nhop ,
.Nm nhop_object
.Nd structure of a nexthop entry in the kernel routing table
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In net/route.h
.In net/route/nhop.h
.Sh DESCRIPTION
The kernel provides a common mechanism by which all routes can store
and retrieve nexthop entries from a central table of routes.
The
.In net/route/nhop.h
header file defines the structures and manifest constants used in this
facility.
.Pp
The data plane structure of a nexthop is defined by
.Vt "struct nhop_object" ,
which includes the following fields:
.Bl -tag -offset indent -width 6n
.It Vt "uint16_t flags" ;
See below.
.It Vt "uint16_t nh_mtu" ;
MTU for this nexthop.
.It Vt "char gw_buf[28]";
The gateway of the nexthop.
An storage suitable to hold AF_INET, AF_INET6, or AF_NET gateway.
.It Vt "struct ifnet *nh_ifp" ;
.It Vt "struct ifaddr *nh_ifa" ;
These two fields represent the
.Dq 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 nexthop represents.
.It Vt "struct ifnet *nh_aifp" ;
Interface of the source address.
It is same as
.Va nh_ifp
except for IPv6 loopback routes, or when the
.Dv NHF_PREFSRC
is set, which means the source address is borrowed from another interface.
.It Vt "counter_u64_t nh_pksent";
A count of packets successfully sent via this nexthop.
.It Vt "uint8_t nh_prepend_len";
link-level prepend length which is currently unused.
.It Vt "uint8_t spare[3]";
.It Vt "uint32_t spare1";
These two fields are unused.
.It Vt "char nh_prepend[48]";
link-level prepend data which is currently unused.
.It Vt "struct nhop_priv *nh_priv";
Pointer to control plane data of nexthop.
.El
.Pp
The following flag bits are defined:
.Bl -tag -offset indent -width "NHF_BLACKHOLE" -compact
.It Dv NHF_MULTIPATH
The nexthop is a nexthop group.
.It Dv NHF_REJECT
Same as
.Dv RTF_REJECT .
The destination is presently unreachable.
.It Dv NHF_BLACKHOLE
Same as
.Dv RTF_BLACKHOLE
Requests that output sent via this nexthop be discarded.
.It Dv NHF_REDIRECT
Same as
.Dv RTF_DYNAMIC
and
.Dv RTF_MODIFIED
flags together.
.It Dv NHF_DEFAULT
The nexthop is used for default route.
.It Dv NHF_BROADCAST
Same as
.Dv RTF_BROADCAST
Indicates that the destination is a broadcast address.
.It Dv NHF_GATEWAY
Same as
.Dv RTF_GATEWAY
The route points to an intermediate destination and not the ultimate
recipient; the
.Va gw4_sa
and
.Va gw6_sa
fields name that destination.
.It Dv NHF_HOST
Same as
.Dv RTF_HOST
This is a nexthop to a host route.
This should result in an
.Er EHOSTUNREACH
error from output routines.
.It Dv NHF_INVALID
This nexthop is currently down.
.It Dv NHF_PREFSRC
Used only in the
.Xr rtnetlink 4
indicating that the source address is manually overwritten.
.El
.Sh SEE ALSO
.Xr route 4 ,
.Xr route 8 ,
.Xr rtentry 9
.Sh HISTORY
The
.Nm
stack is a rework of routing subsystem by
.An Alexander V. Chernikov Aq melifaro@FreeBSD.org
.Sh AUTHORS
.An -nosplit
This manual page was written by
.An Seyed Pouria Mousavizadeh Tehrani Aq pouria@FreeBSD.org