– General Linux 2 – TCP/IP Conﬁguration and Troubleshooting
(Linux Professional Institute Certiﬁcation)

Copyright c 2003 Angus Lees. Permission is granted to make and distribute verbatim copies or modiﬁed versions of this document provided that this copyright notice and this permission notice are preserved on all copies under the terms of the GNU General Public License as published by the Free Software Foundation—either version 2 of the License or (at your option) any later version. $Id: gl2.112.3.slides.tex,v 1.1 2003/10/25 04:33:06 anguslees Exp $

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TCP/IP Conﬁguration and Troubleshooting
Objective
Candidates should be able to view, change and verify conﬁguration settings and operational status for various network interfaces. This objective includes manual and automatic conﬁguration of interfaces and routing tables. This especially means to add, start, stop, restart, delete or reconﬁgure network interfaces. It also means to change, view or conﬁgure the routing table and to correct an improperly set default route manually. Candidates should be able to conﬁgure Linux as a DHCP client and a TCP/IP host and to debug problems associated with the network conﬁguration.

Weight: 7
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TCP/IP Conﬁguration and Troubleshooting
Key ﬁles, terms, and utilities
/etc/HOSTNAME or /etc/hostname ifconfig /etc/hosts route /etc/networks netstat /etc/host.conf host /etc/resolv.conf ping /etc/nsswitch.conf tcpdump traceroute dhcpcd, dhcpclient, pump hostname (domainname, dnsdomainname) the network scripts run during system initialisation

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TCP/IP Conﬁguration and Troubleshooting
Resources of Interest
Linux Networking HOWTO by Joshua Drake : http: //www.linuxdoc.org/HOWTO/Net-HOWTO/index.html Linux Ethernet-Howto by Paul Gortmaker : http: //www.linuxdoc.org/HOWTO/Ethernet-HOWTO.html

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ifconﬁg – Low level network conﬁg
Network interface conﬁguration ifconfig eth0 192.168.7.26 netmask 255.255.255.0 broadcast 192.168.7.255 ifconfig eth0 down

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route – Low level network conﬁg
route add -net 192.168.7.0 netmask 255.255.255.0 dev eth0 route add default gw 192.168.7.1

View routing table: route -n

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netstat – See network stuff
Ports in use Routing table Interfaces Multicast groups Masqueraded connections Statistics netstat -a -u -t netstat -r netstat -i netstat -g netstat -M netstat -s

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Network debugging
ping Try to bounce an ICMP packet off a host Good for reachability, round trip delay, packet loss traceroute Show the network path to a particular host Good for testing routing problems, “which ISP screwed up” tcpdump Dump raw network trafﬁc Exceptional for diagnosing network problems involving a particular host

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tcpdump again
tcpdump is your friend, learn to use it
# tcpdump -i ppp0 not port ssh tcpdump: listening on ppp0 21:54:32.913475 10.0.128.107.1024 > 10.0.128.97.domain: 21:54:33.102745 10.0.128.97.domain > 10.0.128.107.1024: 21:54:33.103766 10.0.128.107 > 203.26.250.2: icmp: echo 21:54:33.352745 203.26.250.2 > 10.0.128.107: icmp: echo 21:54:34.102912 10.0.128.107 > 203.26.250.2: icmp: echo 21:54:34.302745 203.26.250.2 > 10.0.128.107: icmp: echo 21:56:09.908636 10.0.128.107.1068 > 203.26.250.2.www: S 21:56:10.052743 203.26.250.2.www > 10.0.128.107.1068: S 21:56:10.052869 10.0.128.107.1068 > 203.26.250.2.www: . 21:56:12.977510 10.0.128.107.1068 > 203.26.250.2.www: P

20147+ A? fatso.urnet.com.au 20147* 1/3/3 (178) (DF) request (DF) reply request (DF) reply 1245080954:1245080954(0) win 3633684004:3633684004(0) ack ack 1 win 5840 <nop,nop,times 1:2(1) ack 1 win 5840 <nop,no

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/etc/HOSTNAME or /etc/hostname
System scripts set the hostname from one of these ﬁles during boot, using the hostname command. dnsdomainname, ypdomainname, nisdomainname and domainname are variations on hostname domainname gives the NIS domainname, NOT the DNS domain

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/etc/nsswitch.conf
“Name Service Switch” conﬁguration
passwd: group: shadow: hosts: networks: protocols: services: ethers: rpc: netgroup: compat compat compat files dns files db db db db files files files files

nis

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/etc/networks
Labels for network addresses Only supports class A, B or C addresses (not CIDR) Rarely used or kept up to date localnet 192.168.1.0

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/etc/hosts
Hostname to IP address mapping, mostly superseded by DNS
127.0.0.1 localhost 192.168.1.1 cat.pasture.com.au cat # The following lines are desirable for IPv6 capable hosts ::1 fe00::0 ff00::0 ff02::1 ff02::2 ff02::3 ip6-localhost ip6-loopback ip6-localnet ip6-mcastprefix ip6-allnodes ip6-allrouters ip6-allhosts

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/etc/host.conf – Resolver conﬁguration
Various keywords to tweak non-DNS-speciﬁc resolver behaviour Rarely modiﬁed; most options no longer relevant order hosts,bind multi on

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/etc/resolv.conf – DNS conﬁguration
DNS conﬁguration for resolver Nameserver defaults to 127.0.0.1, search sufﬁx defaults to DNS domain name search pasture.com.au nameserver 10.0.128.97

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Debugging DNS
host performs various DNS queries host [options] hostname [server] Common options: -v verbose -l list all hosts in a domain (using AXFR) -t query type (“-t any” is useful)

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DHCP
“Dynamic Host Control Protocol” conﬁgures networking details, DNS, etc automatically by querying a “DHCP server” Various DHCP clients: dhcpclient Comes with ISC DHCP server, highly conﬁgurable dhcpcd pump Simple DHCP client written by RedHat udhcpc Very small DHCP client

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