man/man4/route.4

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.\"     From: @(#)route.4	8.6 (Berkeley) 4/19/94
.\" $FreeBSD: src/share/man/man4/route.4,v 1.9.2.6 2002/03/17 09:12:44 schweikh Exp $
.\" $DragonFly: src/share/man/man4/route.4,v 1.3 2004/03/11 12:28:55 hmp Exp $
.\"
.Dd September 12, 2019
.Dt ROUTE 4
.Os
.Sh NAME
.Nm route
.Nd kernel packet forwarding database
.Sh SYNOPSIS
.In sys/types.h
.In sys/time.h
.In sys/socket.h
.In net/if.h
.In net/route.h
.Ft int
.Fn socket PF_ROUTE SOCK_RAW "int family"
.Sh DESCRIPTION
.Dx
provides some packet routing facilities.
The kernel maintains a routing information database, which
is used in selecting the appropriate network interface when
transmitting packets.
.Pp
A user process (or possibly multiple co-operating processes)
maintains this database by sending messages over a special kind
of socket.
This supplants fixed size
.Xr ioctl 2 Ns 's
used in earlier releases.
Routing table changes may only be carried out by the super user.
.Pp
The operating system may spontaneously emit routing messages in response
to external events, such as receipt of a re-direct, or failure to
locate a suitable route for a request.
The message types are described in greater detail below.
.Pp
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 route may be achieved by using
a mask of all zeros, and there may be hierarchical routes.
.Pp
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 is ready for traffic.
Normally the protocol specifies the route
through each interface as a
.Dq 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).
.Pp
When routing a packet,
the kernel will attempt 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.
.Pp
A wildcard routing entry is specified with a zero
destination address value, and a mask of all zeroes.
Wildcard routes will be 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.
.Pp
One opens the channel for passing routing control messages
by using the socket call shown in the synopsis above:
.Pp
The
.Fa family
parameter may be
.Dv AF_UNSPEC
which will provide
routing information for all address families, or can be restricted
to a specific address family by specifying which one is desired.
There can be more than one routing socket open per system.
.Pp
Messages are formed by a header followed by a small
number of sockaddrs (now variable length particularly
in the
.Tn ISO
case), interpreted by position, and delimited
by the new length entry in the sockaddr.
An example of a message with four addresses might be an
.Tn ISO
redirect:
Destination, Netmask, Gateway, and Author of the redirect.
The interpretation of which address are present is given by a
bit mask within the header, and the sequence is least significant
to most significant bit within the vector.
.Pp
Any messages sent to the kernel are returned, and copies are sent
to all interested listeners.  The kernel will provide the process
ID for the sender, and the sender may use an additional sequence
field to distinguish between outstanding messages.  However,
message replies may be lost when kernel buffers are exhausted.
.Pp
The kernel may reject certain messages, and will indicate this
by filling in the
.Ar rtm_errno
field.
The routing code returns
.Er EEXIST
if
requested to duplicate an existing entry,
.Er ESRCH
if
requested to delete a non-existent entry,
or
.Er ENOBUFS
if insufficient resources were available
to install a new route.
In the current implementation, all routing processes run locally,
and the values for
.Ar rtm_errno
are available through the normal
.Em errno
mechanism, even if the routing reply message is lost.
.Pp
A process may avoid the expense of reading replies to
its own messages by issuing a
.Xr setsockopt 2
call indicating that the
.Dv SO_USELOOPBACK
option
at the
.Dv SOL_SOCKET
level is to be turned off.
A process may ignore all messages from the routing socket
by doing a
.Xr shutdown 2
system call for further input.
.Pp
A process can specify which route message types it's interested in by passing
an array of route message types to the
.Xr setsockopt 2
call with the
.Dv ROUTE_MSGFILTER
option at the
.Dv PF_ROUTE
level.
For example, to only get specific messages:
.Bd -literal -offset indent
unsigned int rtfilter;

rtfilter = ROUTE_FILTER(RTM_IFINFO) | ROUTE_FILTER(RTM_IFANNOUNCE);

if (setsockopt(routefd, PF_ROUTE, ROUTE_MSGFILTER,
    &rtfilter, sizeof(rtfilter)) == -1)
	err(1, "setsockopt(ROUTE_MSGFILTER)");
.Ed
.Pp
If a route is in use when it is deleted,
the routing entry will be marked down and removed from the routing table,
but the resources associated with it will not
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
.Dv RTM_GET
message, or by calling
.Xr sysctl 3 .
.Pp
Messages include:
.Bd -literal
#define	RTM_ADD		0x1    /* Add Route */
#define	RTM_DELETE	0x2    /* Delete Route */
#define	RTM_CHANGE	0x3    /* Change Metrics, Flags, or Gateway */
#define	RTM_GET		0x4    /* Report Information */
#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_RESOLVE	0xb    /* request 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_IFINFO	0xe    /* iface going up/down etc. */
#define	RTM_NEWMADDR	0xf    /* mcast group membership being added to if */
#define	RTM_DELMADDR	0x10   /* mcast group membership being deleted */
#define	RTM_IFANNOUNCE	0x11   /* iface arrival/departure */
.Ed
.Pp
A message header consists of one of the following:
.Bd -literal
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 */
    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 {
    u_short ifm_msglen;         /* to skip over non-understood messages */
    u_char  ifm_version;        /* future binary compatibility */
    u_char  ifm_type;           /* message type */
    u_short ifm_index;          /* index for associated ifp */
    int     ifm_flags;          /* value of if_flags */
    int     ifm_addrs;          /* like rtm_addrs */
    struct  if_data ifm_data;   /* statistics and other data about if */
};

struct ifa_msghdr {
    u_short ifam_msglen;        /* to skip over non-understood messages */
    u_char  ifam_version;       /* future binary compatibility */
    u_char  ifam_type;          /* message type */
    u_short ifam_index;         /* index for associated ifp */
    int     ifam_flags;         /* value of ifa_flags */
    int     ifam_addrs;         /* like rtm_addrs */
    int     ifam_addrflags;	/* family specific address flags */
    int     ifam_metric;        /* value of ifa_metric */
};

struct ifma_msghdr {
    u_short ifmam_msglen;       /* to skip over non-understood messages */
    u_char  ifmam_version;      /* future binary compatibility */
    u_char  ifmam_type;         /* message type */
    u_short ifmam_index;        /* index for associated ifp */
    int     ifmam_flags;        /* value of ifa_flags */
    int     ifmam_addrs;        /* like rtm_addrs */
};

struct if_announcemsghdr {
	u_short	ifan_msglen;	/* to skip over non-understood messages */
	u_char	ifan_version;	/* future binary compatibility */
	u_char	ifan_type;	/* message type */
	u_short	ifan_index;	/* index for associated ifp */
	char	ifan_name[IFNAMSIZ]; /* if name, e.g. "en0" */
	u_short	ifan_what;	/* what type of announcement */
};
.Ed
.Pp
The
.Dv RTM_IFINFO
message uses a
.Ar if_msghdr
header, the
.Dv RTM_NEWADDR
and
.Dv RTM_DELADDR
messages use a
.Ar ifa_msghdr
header, the
.Dv RTM_NEWMADDR
and
.Dv RTM_DELMADDR
messages use a
.Vt ifma_msghdr
header, the
.Dv RTM_IFANNOUNCE
message uses a
.Vt if_announcemsghdr
header,
and all other messages use the
.Ar rt_msghdr
header.
.Pp
The
.Dq Li "struct rt_metrics"
and the flag bits are as defined in
.Xr rtentry 9 .
.Pp
Specifiers for metric values in rmx_locks and rtm_inits are:
.Bd -literal
#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 */
.Ed
.Pp
Specifiers for which addresses are present in the messages are:
.Bd -literal
#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 */
.Ed
.Sh SEE ALSO
.Xr sysctl 3 ,
.Xr route 8 ,
.Xr rtentry 9
.Sh HISTORY
A
.Dv PF_ROUTE
protocol family first appeared in
.Bx 4.3 reno .