HPUX routing[7]
routing(7) routing(7)
NAME
routing - system support for local network packet routing
DESCRIPTION
The network facilities provide general packet routing, leaving most
routing table maintenance to applications processes.
A simple set of data structures comprise a routing table used in
selecting the appropriate remote host or gateway when transmitting
packets. The table contains a single entry for each route to a
specific network or host.
The table contains the entry lo0 for the local loopback after system
boot-up, and an entry ( lan0, lan1, lan2,...) for each interface card
after the ifconfig command is executed (see ifconfig(1m)).
The super-user can change the table by using the route(1m) command
(see route(1M), or by information received in Internet Control Message
Protocol (ICMP) redirect messages.
To display the routing table, use the netstat -r command (see
netstat(1)) which displays the destination internet address, which
gateway to use to get to that destination, and flags. A routing table
contains three types of entries: entries for a specific host, entries
for all hosts on a specific network, and entries for any destination
not matched by entries of the first two types (a wildcard route). The
various types of routes are determined by the flags field of the
display from netstat. The flags field is either U, UG, UH, or UGH.
The U flag is always present. The G flag indicates a route using a
gateway and is accompanied by a hop count. If a route is via a remote
gateway, the hop count must be greater than zero. If no G flag is
present, it indicates a route that does not use a remote gateway. The
H flag indicates a route to a host. If the H flag is absent, it
indicates a route to a network.
The keyword, default, in the destination field indicates a wildcard
gateway. This is used as a last resort if no information exists in
the table about how to get to a particular remote network. Routes
that are not valid are not displayed (see Flags below for details).
First, an attempt is made to find a route to the specific host. If
that search fails, the algorithm looks for a route to the host's
network. If both searches fail, the wildcard gateway is returned if
one is in the table. If there are multiple routes of the same type
(in other words, two routes to a host via different gateways, two
routes to a network via different gateways, or two default routes),
the packet is routed over the first route of the same type found in
the route table (i.e, the first route of the same type displayed by
netstat -r).
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If all of the searches fail, an error is returned.
Flags
The following truth table shows the relationship between the count
parameter used with the route command and the destination type, flags,
and route type.
tab(*); cB cB cB cBw(2i) c c l l. Count*Destination
Type*Flags*Route Type _ =0*network*1=U*T{ route to a network via
a gateway which is the local host itself T}
>0*network*3=UG*T{ route to a network via a gateway which is a
remote host T}
=0*host*5=UH*T{ route to a host via a gateway which is the local
host itself T}
>0*host*7=UGH*T{ route to a host via a gateway which is a remote
host T}
=0*"default"*1=U*T{ wildcard route via the local host T}
>0*"default"*3=UG*T{ wildcard route via a remote gateway T}
The routing algorithm used includes the ability to recognize subnets.
Subnet addresses are similar to the network address portion of
Internet addresses; network addresses identify physically distinct
networks; subnet addresses identify physically distinct subnetworks of
the same network. Subnets allow a network manager to partition the
host number space associated with a given network number into discrete
subnetworks. This facility is desirable if it is necessary for
several physical networks to share a single network number. An
example is a facility with a single class B network number and several
Ethernet-like physical networks. The host space of a class B address
is 16 bits, while a single physical network can have a limitation of
200-300 hosts. If subnets are used it is possible for all of the
networks to have the same network number while each host recognizes
that another host with the same network number is not necessarily on
the same physical network. The routing algorithm attempts to find a
gateway for a host if it is not on the same subnet, although it may
have the same network number.
The subnet for a given host is specified in the ifconfig command (see
ifconfig(1M)). It is specified as a 32-bit subnet mask. The next
paragraph outlines an example use of subnets.
An example class C network number is 192.34.17.0, with the last field
specifying the host number. Normally all hosts with the prefix
192.34.17 are recognized as being on the same logical and physical
network. If subnets are not in use, the default mask used is
255.255.255.0. When routing, bit-wise logical ANDs are performed
between the mask and the Internet address of the remote host, and
between the mask and the local Internet address. If the result is
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non-zero, it is assumed that the remote host is on the same subnet as
the local host. If subnets are to be used and the 8-bit host field is
to be partitioned into 2 bits of subnet and 6 bits of host, the subnet
mask would be 255.255.255.192. Note that a subnet mask of all zeroes
causes the routing mechanism to assume that all hosts, whether local
or remote, are on the local physical network.
If a subnet mask is not specified in the ifconfig command, the default
mask is used to indicate that subnets are not in use. The default
masks for the various classes of Internet addresses are as follows:
Class A: 255.0.0.0
Class B: 255.255.0.0
Class C: 255.255.255.0
WARNINGS
Reciprocal route commands must be executed on the local host and the
destination host, as well as all intermediate hosts, if routing is to
succeed in the cases of virtual circuit connections or bidirectional
datagram transfers.
AUTHOR
routing was developed by the University of California, Berkeley.
FILES
/etc/hosts
/etc/networks
SEE ALSO
netstat(1), ifconfig(1m), route(1m).
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