Linux Network tips

TCP/IP Network Configuration Files

/etc/resolv.conf - host name resolver configuration file

search name-of-domain.com 

  - Name of your domain or ISP's domain if using their name server 

nameserver XXX.XXX.XXX.XXX

   - IP address of primary name server 

nameserver XXX.XXX.XXX.XXX 

   - IP address of secondary name server 

Ex:

   nameserver 135.252.44.227

Note: for DHCP, this file will be loaded automatically.

/etc/hosts - locally resolve node names to IP addresses

  127.0.0.1         your-node-name.your-domain.com  localhost.localdomain  localhost 
  XXX.XXX.XXX.XXX   node-name 

Ex:

  127.0.0.1       localhost.localdomain   localhost
  135.252.142.150     qdcvs.gdc.lucent.com

/etc/sysconfig/network

  Red Hat network configuration file used by the system during the boot process. 

/etc/nsswitch.conf - System Databases and Name Service Switch configuration file

  hosts:   files dns nisplus nis 

  This example tells Linux to first resolve a host name by looking at the local hosts file
 (/etc/hosts), then if the name is not found look to your DNS server as defined by 
  /etc/resolv.conf and if not found there look to your NIS server. 
  In the past this file has had the following names: /etc/nsswitch.conf, 
  /etc/svc.conf, /etc/netsvc.conf, ... depending on the distribution. 
  

/etc/sysconfig/network-scripts/ifcfg-eth0

Configuration settings for your first ethernet port (0).

Assigning an IP address

Static IP address assignment

command Line assignment

   /sbin/ifconfig eth0 192.168.10.12 netmask 255.255.255.0 broadcast 192.168.10.255 

  note: ifconfig command does NOT store this information permanently.

Edit configuration files/scripts

  1. /etc/sysconfig/network

    For static IP configuration

    NETWORKING=yes 
    HOSTNAME=my-hostname
      - Hostname is defined here and by command hostname 
    FORWARD_IPV4=true
      - True for NAT firewall gateways and linux routers. 
    GATEWAY="XXX.XXX.XXX.YYY" 
      - Used if your network is connected to another network or the internet. 
  

    for DHCP client configuration: 

    NETWORKING=yes 
    HOSTNAME=my-hostname
      - Hostname is defined here and by command hostname 

  2. /etc/sysconfig/network-scripts/ifcfg-eth0

   Static IP address configuration: 

   DEVICE=eth0 
   BOOTPROTO=static 
   BROADCAST=XXX.XXX.XXX.255 
   IPADDR=XXX.XXX.XXX.XXX 
   NETMASK=255.255.255.0 
   NETWORK=XXX.XXX.XXX.0 
   ONBOOT=yes 

   DHCP client configuration: 

   DEVICE=eth0 
   ONBOOT=yes 
   BOOTPROTO=dhcp 

   note: use /sbin/ifup to bring interface(such as eth0) up.
         /sbin/ifdown bring interface down.

route

Ex: 

route -e
  Show routing table 

route add -host 123.213.221.231 eth1 
  Access individual computer host specified via 
  network interface card eth1: 

route add -net 10.13.21.0 netmask 255.255.255.0 gw 192.168.10.254 eth0 

  Access ISP network identified by the network address 
  and netmask using network interface card eth0.

route del -net 10.13.21.0 netmask 255.255.255.0 gw 192.168.10.254 eth0 
  Specify default gateway to use to access remote network via 
  network interface card eth0.

route add default gw 201.51.31.1 eth0 

Usefull Linux networking commands

  /etc/rc.d/init.d/network   start 
    - command to start, restart or stop the network 
 
  netstat 
   - Display connections, routing tables, stats etc 

  netstat -punta 
   - List externally connected processes: 

  netstat -nap 
   - List all connected processes: 
 
  netstat -s 
   - Show network statistics

  netstat -a -i eth0 
   - Kernel interface table info

  ping 
   - send ICMP ECHO_REQUEST packets to network hosts. 

  traceroute 
   - print the route packets take to network host 
   traceroute IP-address-of-server 
   traceroute domain-name-of-server 

  whois 
   - Lookup a domain name in the internic whois database. 

  finger 
   - Display information on a system user. 

  iptables 
   - IP firewall administration 
  
  host 
   - Give a host name and the command will return IP address.

  nslookup 
   - Give a host name and the command will return IP address. 

  The following firewall rules will drop ICMP requests. 

    Iptables: 
    iptables -A OUTPUT -p icmp -d 0/0 -j DROP 

    Ipchains: 
    ipchains -A output -p icmp -d 0/0 -j DENY 

 Drop all incomming pings: 

   echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_all 

 nmap - Network exploration tool and security scanner 
   List pingable nodes on network: nmap -sP 192.168.0.0/24 
   Scans network for IP addresses 192.168.0.0 to 192.168.0.255 using ping. 

 tcpdump - dump traffic on a network.
   tcpdump tcp port 80 and host server-1 
   tcpdump ip host server-1 and not server-2 

 note: use tcpdump must put interface to promisc mode.
       ifconfig eth0 promisc

       cancel promisc mode
       ifconfig eth0 -promisc

 chkconfig 
  -list to view all system services and their st