1. COP 4343 Unix System Administration
Unit 12: basic network services
concepts
dhcpd, named
superserver

2. Basic Network Services
service listens on port for requests
examples:
http uses port 80
ssh uses port 22
firewall must allow IP packets through
/etc/sysconfig/iptables
service iptables restart

3. /etc/sysconfig/iptables
# ftp
-A RH-Firewall-1-INPUT -p tcp --dport 20 -j ACCEPT
-A RH-Firewall-1-INPUT -p tcp --dport 21 -j ACCEPT
# ssh
-A RH-Firewall-1-INPUT -p tcp --dport 22 -j ACCEPT
# http
-A RH-Firewall-1-INPUT -p tcp --dport 80 -j ACCEPT
# mail
-A RH-Firewall-1-INPUT -p tcp --dport 25 -j ACCEPT
-A RH-Firewall-1-INPUT -p tcp --dport 143 -j ACCEPT
-A RH-Firewall-1-INPUT -p tcp --dport 587 -j ACCEPT

4. Basic Network Services
examples:
dhcpd: UPD ports 67 and 68
named: UPD port 53
superserver:
can be configured to listen on any port

5. dhcpd server install dhcpd package configuration file: /etc/dhcpd.conf
enable in run-level

6. dhcpd configuration example
subnet netmask {
range ;
Range of available IP addresses
default-lease-time ;
Default lease time in seconds
max-lease-time ;
Maximum time a lease will be given
option subnet-mask ;
subnetmask given to clients
option domain-name-servers , ;
List of name server IP addresses
option domain-name "your.domain.name";
option routers ;
Gateway router }

7. named server implements domain name server (DNS)
Configuration file: /var/named/chroot/etc/named.conf
example:
zone "ege.com" {
type master;
file "ege.hosts"; };

8. Example: ege.hosts @ IN SOA ege.com. raimund.ege.com. ( 75
8H ; refresh
2H ; retry
1W ; expire
1H) ; min time to live
NS ns3.ege.com.
NS ns4.ege.com.
MX mail.ege.com.
TXT "Ege Consulting Inc" A
mail A
ns A
ns A
instructor A A

9. Superserver configurable server to handle multiple network services
xinetd
xinetd.conf
xinetd.d
listens on multiple network ports and starts the appropriate service when a client connects to that port
traditional Unix: inetd

10. example configuration file
service echo {
type = INTERNAL
id = echo-stream
socket_type = stream
protocol = tcp
user = root
wait = no
disable = yes }

11. example configuration file
service ntalk {
disable = yes
socket_type = dgram
wait = yes
user = root
group = tty
server = /usr/bin/ktalkd }

12. xinetd security enable port in firewall host specific: iptables
/etc/hosts.allow
/etc/hosts.deny

13. xinetd to enable: disable = no reload xinetd service
service xinetd reload
test via
telnet host port

14. example services port 7: echo port 19: chargen port 9: discard
repeats back whatever you type
port 19: chargen
returns a character stream until control-D
port 9: discard
like /dev/null
port 37: time
returns a number corresponding to the current time
port 13: daytime
returns the current date/time in human-readable form

15. Services Beyond the Superserver
services do not rely on superservers:
Apache Web
SMTP
FTP
...
run as service in run level