TCP/IP 详解 卷1 ch9 IP Routing

1. For the host which is not configured as router, datagrams received through the network interfaces that are not ours are dropped.

2. the steps that IP performs when it searches its routing table.
1) Search for a matching host address.
2) Search for a matching network address.
3) Search for a default entry. (The default entry is normally specified in the routing table as a network entry, with a network ID of 0.)
A matching host address is always used before a matching network address.

先匹配主机地址，然后匹配网络地址

3. netstat -rn

/home/a/j/nomad2:netstat -r Routing Table: IPv4 Destination Gateway Flags Ref Use Interface -------------------- -------------------- ----- ----- ------ --------- 192.168.1.0 x4100.unix-center.net U 1 5016 e1000g0 224.0.0.0 x4100.unix-center.net U 1 0 e1000g0 default 192.168.1.1 UG 1 31181 localhost localhost UH 2 1581 lo0 Routing Table: IPv6 Destination/Mask Gateway Flags Ref Use If --------------------------- --------------------------- ----- --- ------ ----- fe80::/10 fe80::214:4fff:fe45:f196 U 1 0 e1000g0 ff00::/8 fe80::214:4fff:fe45:f196 U 1 0 e1000g0 default fe80::214:4fff:fe45:f196 U 1 0 e1000g0 localhost localhost UH 1 843 lo0 /home/a/j/nomad2:netstat -n TCP: IPv4 Local Address Remote Address Swind Send-Q Rwind Recv-Q State -------------------- -------------------- ----- ------ ----- ------ ----------- 192.168.1.10.22 123.185.162.221.36531 16448 0 50400 0 ESTABLISHED 192.168.1.10.22 120.82.220.22.53947 261528 0 49980 0 ESTABLISHED 192.168.1.10.823 192.168.2.5.2049 64128 0 49640 0 ESTABLISHED 192.168.1.10.38284 192.168.2.5.389 32640 0 49640 0 ESTABLISHED 192.168.1.10.22 192.168.1.18.61797 65535 0 49640 0 ESTABLISHED 192.168.1.10.22 123.185.162.221.38773 16448 0 50400 0 LAST_ACK 192.168.1.10.57425 192.168.2.5.389 393728 0 49640 0 ESTABLISHED 192.168.1.10.22 123.185.162.221.35909 16448 0 50400 0 ESTABLISHED 192.168.1.10.22 117.88.79.188.51905 64980 0 50400 0 ESTABLISHED 192.168.1.10.22 221.215.188.13.3721 15816 35 49284 0 ESTABLISHED 192.168.1.10.48229 192.168.2.5.389 6912 0 49640 0 TIME_WAIT 192.168.1.10.48230 192.168.2.5.389 6912 0 49640 0 TIME_WAIT 192.168.1.10.48231 192.168.2.5.389 6912 0 49640 0 TIME_WAIT 192.168.1.10.48232 192.168.2.5.389 6912 0 49640 0 TIME_WAIT 192.168.1.10.48233 192.168.2.5.389 6912 0 49640 0 TIME_WAIT 192.168.1.10.48234 192.168.2.5.389 6912 0 49640 0 TIME_WAIT 192.168.1.10.48235 192.168.2.5.389 6912 0 49640 0 TIME_WAIT 192.168.1.10.48236 192.168.2.5.389 6912 0 49640 0 TIME_WAIT 192.168.1.10.48237 192.168.2.5.389 6912 0 49640 0 TIME_WAIT 192.168.1.10.48238 192.168.2.5.389 6912 0 49640 0 TIME_WAIT 192.168.1.10.22 120.82.220.80.32140 261352 0 49980 0 ESTABLISHED Active UNIX domain sockets Address Type Vnode Conn Local Addr Remote Addr fffffe913273a760 stream-ord 0000000 0000000 ffffffffad9b8750 stream-ord 0000000 0000000 ffffffffacadf748 stream-ord ffffffff9c8f5d40 0000000 /tmp/ssh-OgS27674/agent.27674 ffffffff95203900 stream-ord 0000000 0000000 ffffffff8777e398 stream-ord 0000000 0000000 ffffffff8777ec80 stream-ord ffffffff8777fbc0 0000000 /var/run/.inetd.uds /home/a/j/nomad2:netstat -rn Routing Table: IPv4 Destination Gateway Flags Ref Use Interface -------------------- -------------------- ----- ----- ------ --------- 192.168.1.0 192.168.1.10 U 1 5016 e1000g0 224.0.0.0 192.168.1.10 U 1 0 e1000g0 default 192.168.1.1 UG 1 31181 127.0.0.1 127.0.0.1 UH 2 1581 lo0 Routing Table: IPv6 Destination/Mask Gateway Flags Ref Use If --------------------------- --------------------------- ----- --- ------ ----- fe80::/10 fe80::214:4fff:fe45:f196 U 1 0 e1000g0 ff00::/8 fe80::214:4fff:fe45:f196 U 1 0 e1000g0 default fe80::214:4fff:fe45:f196 U 1 0 e1000g0 ::1 ::1 UH 1 843 lo0

for a given route, the flag is like following,

U The route is up. G The route is to a gateway (router). If this flag is not set, the destination is directly connected. H The route is to a host, that is, the destination is a complete host address. If this flag is not set, the route is to a network, and the destination is a network address: a net ID, or a combination of a net ID and a subnet ID. D The route was created by a redirect (Section 9.5). M The route was modified by a redirect <br>

4. indirect route VS. direct route

A packet going out a direct route has both the IP address and the link-layer address specifying the destination (without G flag);

When a packet is sent out an indirect route, the IP address specifies the final destination but the link-layer address specifies the gateway (that is, the next-hop router).

5. The H flag, however, specifies that the destination address (the first column of netstat output) is a complete host address. The absence of the H flag means the destination address is a network address (the host ID portion will be 0).

如果没有H，说明是一个网络号，例如3中的最后一条

6. The ICMP "host unreachable" error message is sent by a router when it receives an IP datagram that it cannot deliver or forward.

for example,

/home/a/j/nomad2:ping 192.82.14.1 ICMP Communication Administratively Prohibited from gateway 75.101.32.66 for icmp from x4100.unix-center.net (192.168.1.10) to 192.82.14.1 no answer from 192.82.14.1

7. ICMP Redirect Errors

ICMP redirects allow TCP/IP hosts to be dumb when it comes to routing, with all the intelligence in the routers