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Unformatted text preview: s participate in a routing protocol with other routers, and the routing
protocol should obviate the need for redirects. (This means that in Figure 9.1 the routing file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/ip_rout.htm (13 of 17) [12/09/2001 14.46.51] Chapter 9. IP Routing table should be updated by either a routing daemon or redirects, but not by both.)
4.4BSD, when acting as a router, performs the following checks, all of which must be true
before an ICMP redirect is generated.
1. The outgoing interface must equal the incoming interface.
2. The route being used for the outgoing datagram must not have been created or
modified by an ICMP redirect, and must not be the router's default route.
3. The datagram must not be source routed.
4. The kernel must be configured to send redirects.
The kernel variable is named ip_sendredirects, or something similar. (See Appendix E.)
Most current systems (4.4BSD, SunOS 4.1.x, Solaris 2.x, and AIX 3.2.2, for example) enable this
variable by default. Other systems such as SVR4 disable it by default. Additionally, a 4.4BSD host that receives an ICMP redirect performs some checks before
modifying its routing table. These are to prevent a misbehaving router or host, or a
malicious user, from incorrectly modifying a system's routing table.
4. The new router must be on a directly connected network.
The redirect must be from the current router for that destination.
The redirect cannot tell the host to use itself as the router.
The route that's being modified must be an indirect route. Our final point about redirects is that routers should send only host redirects (codes 1 or 3
from Figure 9.5) and not network redirects. Subnetting makes it hard to specify exactly
when a network redirect can be sent instead of a host redirect. Some hosts treat a received
network redirect as a host redirect, in case a router sends the wrong type. 9.6 ICMP Router Discovery Messages
We mentioned earlier in...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12