Network Management And
Debugging
WeeSan Lee <weesan@cs.ucr.edu>
http://www.cs.ucr.edu/~weesan/cs183/
Roadmap
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Interface Configuration
Route Configuration
Network Debugging
Network Topology
The
Internet
192.168.0.0/24
.2
Router VM
.1
Host VM
.2
10.0.0.0/24
Interface Configuration (Host VM)
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We will need:
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IP Address
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Netmask
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255.255.255.0
Broadcast
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10.0.0.2
10.0.0.255
Gateway
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10.0.0.1
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Usually
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10.0.0.0 - network
10.0.0.255 - broadcast
Interface Configuration (Router VM)
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Use ifconfig command
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$ ifconfig eth1 10.0.0.1 netmask 255.255.255.0
$ ifconfig eth1
eth1 Link encap:Ethernet HWaddr 00:C0:F0:3C:43:82
inet addr:10.0.0.1 Bcast:10.0.0.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:659988 errors:1 dropped:0 overruns:0 frame:0
TX packets:1016790 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:73459942 (70.0 Mb) TX bytes:1201693614 (1146.0 Mb)
Interrupt:10 Base address:0xd880
To bring the interface eth1 down or up via ifconfig command
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$ ifconfig eth1 down
$ ifconfig eth1 up
Interface Configuration (Router VM)
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Edit /etc/sysconfig/network-scripts/ifcfg-eth1
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To bring up the interface eth1 via ifup script
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DEVICE=eth1
ONBOOT=yes
#BOOTPROTO=dhcp
IPADDR=10.0.0.1
NETMASK=255.255.255.0
BROADCAST=10.0.0.255
$ ifup eth1
To bring down the interface eth1 via ifdown script
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$ ifdown eth1
IP Aliasing
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A way to assign multiple IP addresses on the
same interface
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$ ifconfig eth1:0 10.0.0.3 netmask 255.255.255.0
$ ifconfig eth1:1 10.0.0.4 netmask 255.255.255.0
Why?
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We could experiment new services w/out new HW
We could replace problematic HW with IP aliasing
on a healthy machine temporary
Route Configuration (Router VM)
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Default routes
Usually added by route command
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To remove a default route
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$ route add default gw 192.168.0.1
$ route del default gw 192.168.0.1
To make it persistent, edit /etc/sysconfig/network
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NETWORKING=yes
HOSTNAME=host1
DOMAINNAME=weesan.com
GATEWAY=192.168.0.1
Route Configuration (Router VM)
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$ netstat -rn
Kernel IP routing table
Destination Gateway
192.168.0.0 0.0.0.0
169.254.0.0 0.0.0.0
0.0.0.0
192.168.0.1
Genmask
255.255.255.0
255.255.0.0
0.0.0.0
Flags MSS Window irtt Iface
U
0
0
0 eth0
U
0
0
0 eth0
UG 0
0
0 eth0
Route Configuration (Router VM)
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Static routes
Usually added by ifconfig command
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$ route add -net 10.0.0.0 netmask 255.255.255.0 eth1
$ route del -net 10.0.0.0 netmask 255.255.255.0 eth1
Edit /etc/sysconfig/static-routes
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eth1 net 10.0.0.0 netmask 255.255.255.0
Route Configuration (Router VM)
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$ netstat -rn
Kernel IP routing table
Destination Gateway
10.0.0.0
0.0.0.0
192.168.0.0 0.0.0.0
169.254.0.0 0.0.0.0
0.0.0.0
192.168.0.1
Genmask
255.255.255.0
255.255.255.0
255.255.0.0
0.0.0.0
Flags MSS Window irtt Iface
U
0
0
0 eth1
U
0
0
0 eth0
U
0
0
0 eth0
UG 0
0
0 eth0
Enable IP Forwarding (Router VM)
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$ echo "1" > /proc/sys/net/ipv4/ip_forward
To make it persistent, edit /etc/sysctl.conf
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Change net.ipv4.ip_forward to 1
Network Debugging
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Can be tricky
Start from one component and work your way
through
Recommend procedures (bottom-up)
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Always check power first 
Check the LED on the devices
Check connectivity, use tools like ping, traceroute,
tcpdump, etc
Verify application protocol, use telnet
ping
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Send ICMP-REQUEST and expect ICMP-REPLY
$ ping 10.0.0.1
PING 10.0.0.1 (10.0.0.1) 56(84) bytes of data.
64 bytes from 10.0.0.1: icmp_seq=1 ttl=64 time=1.18 ms
64 bytes from 10.0.0.1: icmp_seq=2 ttl=64 time=1.57 ms
64 bytes from 10.0.0.1: icmp_seq=3 ttl=64 time=1.03 ms
--- 10.0.0.1 ping statistics --3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 1.036/1.263/1.572/0.228 ms
ping
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Start from known next hop
Not always works for remote hosts
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For example, eon.cs.ucr.edu drops ICMP packets
traceroute
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Send UDP packets to remote host with TTL 1, 2, 3, …
$ weesan@delta-1:~> traceroute www.google.com
traceroute: Warning: www.google.com has multiple addresses; using 72.14.253.99
traceroute to www.l.google.com (72.14.253.99), 30 hops max, 38 byte packets
1 138.23.211.1 (138.23.211.1) 0.286 ms 0.278 ms 0.353 ms
2 c6513telecom--te-9-4.ucr.edu (138.23.3.105) 25.070 ms 20.486 ms 1.064 ms
3 c6509telecom--te-3-3.ucr.edu (138.23.3.26) 0.384 ms 0.381 ms 0.361 ms
4 riv-dc1.riv-dc1--ucr.cenic.net (137.164.24.121) 0.311 ms 0.235 ms 0.225 ms
5 dc-lax-dc1--riv-dc1-pos.cenic.net (137.164.22.228) 1.457 ms 1.459 ms 1.446 ms
6 ***
…
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11 po-in-f99.google.com (72.14.253.99) 31.902 ms 30.762 ms 30.745 ms
tcpdump
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Originally written by Van Jacobson
$ tcpdump
$ tcpdump -i eth0
$ tcpdump host eon
$ tcpdump src net 10.0.0.0/24 and dst port 80
$ tcpdump -vvv
$ man tcpdump
Reference
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LAH
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Ch 12: TCP/IP Networking
Ch 13: Routing
Ch 19: Network Management And Debugging