ROUTE(4) OpenBSD Programmer's Manual ROUTE(4)NAMEroute - kernel packet forwarding database
SYNOPSIS
#include <sys/socket.h>
#include <net/if.h>
#include <net/route.h>
int
socket(PF_ROUTE, SOCK_RAW, family);
DESCRIPTION
OpenBSD provides some packet routing facilities. The kernel maintains a
routing information database, which is used in selecting the appropriate
network interface when transmitting packets.
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 ioctl(2)'s used in earlier releases. Routing table
changes may only be carried out by the super user.
The operating system may spontaneously emit routing messages in response
to external events, such as receipt of a redirect, or failure to locate a
suitable route for a request. The message types are described in greater
detail below.
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.
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 ``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).
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.
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. Routes created by redirects
from wildcard routes and other routes will be marked cloned, until their
``parent'' from which they were created has disappeared.
Route labels can be attached to routes and may contain arbitrary
information about the route. Labels are sent over the routing socket
(see below) as sockaddr_rtlabel structures.
The Routing Socket
One opens the channel for passing routing control messages by using the
socket(2) call shown in the SYNOPSIS above.
The family parameter may be 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.
Messages are formed by a header followed by a small number of sockaddr
structures (which are variable length), interpreted by position, and
delimited by the length entry in the sockaddr. An example of a message
with four addresses might be an IPv4 route addition: the destination,
netmask, gateway, and label, since both netmasks and labels are sent over
the routing socket as sockaddr structures. The interpretation of which
addresses are present is given by a bit mask within the header, and the
sequence is least significant to most significant bit within the vector.
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.
The kernel may reject certain messages, and will indicate this by filling
in the rtm_errno field. The routing code returns EEXIST if requested to
duplicate an existing entry, ESRCH if requested to delete a non-existent
entry, or ENOBUFS if insufficient resources were available to install a
new route. In the current implementation, all routing processes run
locally, and the values for rtm_errno are available through the normal
errno mechanism, even if the routing reply message is lost.
A process may avoid the expense of reading replies to its own messages by
issuing a setsockopt(2) call indicating that the SO_USELOOPBACK option at
the SOL_SOCKET level is to be turned off. A process may ignore all
messages from the routing socket by doing a shutdown(2) system call for
further input.
A process can specify an alternate routing table by using the SO_RTABLE
setsockopt(2).
A process can specify which route message types it's interested in by
using ROUTE_FILTER(int type) and issuing a setsockopt call with the
ROUTE_MSGFILTER option at the AF_ROUTE level. For example, to only get
interface specific messages:
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)");
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 RTM_GET message or via the PF_ROUTE
sysctl(3).
Messages include:
#define RTM_ADD 0x1 /* Add Route */
#define RTM_DELETE 0x2 /* Delete Route */
#define RTM_CHANGE 0x3 /* Change Metrics or flags */
#define RTM_GET 0x4 /* Report Metrics */
#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 /* req 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_IFANNOUNCE 0xf /* iface arrival/departure */
#define RTM_DESYNC 0x10 /* route socket buffer overflow */
A message header consists of one of the following:
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_hdrlen; /* sizeof(rt_msghdr) to skip over the header */
u_short rtm_index; /* index for associated ifp */
u_short rtm_tableid; /* routing table id */
u_char rtm_priority; /* routing priority */
u_char rtm_mpls; /* MPLS additional infos */
int rtm_addrs; /* bitmask identifying sockaddrs in msg */
int rtm_flags; /* flags, incl. kern & message, e.g. DONE */
int rtm_fmask; /* bitmask used in RTM_CHANGE message */
pid_t rtm_pid; /* identify sender */
int rtm_seq; /* for sender to identify action */
int rtm_errno; /* why failed */
u_int 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_hdrlen; /* sizeof(if_msghdr) to skip over the header */
u_short ifm_index; /* index for associated ifp */
u_short ifm_tableid; /* routing table id */
u_char ifm_pad1;
u_char ifm_pad2;
int ifm_addrs; /* like rtm_addrs */
int ifm_flags; /* value of if_flags */
int ifm_xflags;
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_hdrlen; /* sizeof(ifa_msghdr) to skip over the header */
u_short ifam_index; /* index for associated ifp */
u_short ifam_tableid; /* routing table id */
u_char ifam_pad1;
u_char ifam_pad2;
int ifam_addrs; /* like rtm_addrs */
int ifam_flags; /* value of ifa_flags */
int ifam_metric; /* value of ifa_metric */
};
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_hdrlen; /* sizeof(ifa_msghdr) to skip over the header */
u_short ifan_index; /* index for associated ifp */
u_short ifan_what; /* what type of announcement */
char ifan_name[IFNAMSIZ]; /* if name, e.g. "en0" */
};
The RTM_IFINFO message uses an if_msghdr header, the RTM_NEWADDR and
RTM_DELADDR messages use an ifa_msghdr header, the RTM_IFANNOUNCE message
uses an if_announcemsghdr header, and all other messages use the
rt_msghdr header.
The metrics structure is:
struct rt_metrics {
u_int64_t rmx_pksent; /* packets sent using this route */
u_int rmx_locks; /* Kernel must leave these values */
u_int rmx_mtu; /* MTU for this path */
u_int rmx_expire; /* lifetime for route, e.g. redirect */
u_int rmx_refcnt; /* # references hold */
u_int rmx_hopcount; /* max hops expected */
u_int rmx_recvpipe; /* inbound delay-bandwidth product */
u_int rmx_sendpipe; /* outbound delay-bandwidth product */
u_int rmx_ssthresh; /* outbound gateway buffer limit */
u_int rmx_rtt; /* estimated round trip time */
u_int rmx_rttvar; /* estimated rtt variance */
};
Only rmx_mtu, rmx_expire, rmx_pksent, and rmx_locks are used by the
kernel routing table. All other values will be ignored when inserting
them into the kernel and are set to zero in routing messages sent by the
kernel. They are left for compatibility reasons with other systems.
Flags include the values:
#define RTF_UP 0x1 /* route usable */
#define RTF_GATEWAY 0x2 /* destination is a gateway */
#define RTF_HOST 0x4 /* host entry (net otherwise) */
#define RTF_REJECT 0x8 /* host or net unreachable */
#define RTF_DYNAMIC 0x10 /* created dynamically (by redirect) */
#define RTF_MODIFIED 0x20 /* modified dynamically (by redirect) */
#define RTF_DONE 0x40 /* message confirmed */
#define RTF_MASK 0x80 /* subnet mask present */
#define RTF_CLONING 0x100 /* generate new routes on use */
#define RTF_XRESOLVE 0x200 /* external daemon resolves name */
#define RTF_LLINFO 0x400 /* generated by ARP or ESIS */
#define RTF_STATIC 0x800 /* manually added */
#define RTF_BLACKHOLE 0x1000 /* just discard pkts (during updates) */
#define RTF_PROTO2 0x4000 /* protocol specific routing flag */
#define RTF_PROTO1 0x8000 /* protocol specific routing flag */
#define RTF_CLONED 0x10000 /* this is a cloned route */
#define RTF_MPATH 0x40000 /* multipath route or operation */
#define RTF_JUMBO 0x80000 /* try to use jumbo frames */
#define RTF_MPLS 0x100000 /* MPLS additional infos */
Specifiers for metric values in rmx_locks and rtm_inits are:
#define RTV_MTU 0x1 /* init or lock _mtu */
#define RTV_HOPCOUNT 0x2 /* init or lock _hopcount */
#define RTV_EXPIRE 0x4 /* init or lock _hopcount */
#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 */
Only RTV_MTU and RTV_EXPIRE should be used; all other flags are ignored.
Specifiers for which addresses are present in the messages are:
#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, bcast or p-p dest addr */
#define RTA_LABEL 0x400 /* route label present */
SEE ALSOnetstat(1), socket(2), sysctl(3), mygate(5), route(8)HISTORY
A PF_ROUTE protocol family first appeared in 4.3BSD-Reno.
OpenBSD 4.9 February 12, 2011 OpenBSD 4.9
