• The Netstat command
• netstat has many uses
• Implementation and usage vary by OS
• Some options:
• Viewing Network Activity with netstat
• OpenBSD Config
• Configure network interfaces with /etc/hostname.<int>
• Configure a default route with /etc/mygate
• DHCP Client hints
• The ifconfig Command
• View network interface parameters
• Configure network interface parameters
• Resolving Hostnames
• Hosts must be told how and from where
• Sys-V hosts tend to look in /etc/nsswitch.conf
• Client DNS Configuration
• Client DNS is configured in /etc/resolv.conf
• Static Routing
• Use netstat to view system's route table
• IP routing decisions
• Static routing is good for
• Static routes are manipulated with the route command

The Netstat command

netstat has many uses

View the system's routing table
View active or listening TCP and UDP services
Other network statistics

Implementation and usage vary by OS

Some options:

    -I <int>  show stats for specific NIC
    -a        show 'server' socket state too
    -f inet   only show IPv4 info
    -n        don't do IP -> name (DNS) lookups
    -r        show system routing table

Viewing Network Activity with netstat

netstat -a -n -f inet
Use netstat to see established and open TCP/UDP sockets

netstat -a -f unix
Similar information for Unix sockets

netstat -q -i
Usage statistics for interfaces by protocol
The -q limits output to interfaces in use

OpenBSD Config

Configure network interfaces with /etc/hostname.<int>

Static IP config looks like this:

    feanor % cat /etc/hostname.fxp0
    inet 128.138.220.44 255.255.255.0 NONE

DHCP looks like this:

    feanor % cat /etc/hostname.wi0
    dhcp NONE NONE NONE

Configure a default route with /etc/mygate

    feanor % cat /etc/mygate
    128.138.220.1

DHCP Client hints

Client lease info found in /var/db/dhclient.leases
Configuration info in /etc/dhclient.conf
You don't have to touch this usually
Add your own DNS domains or requested hostname

   send host-name "feanor";
   append domain-name " cs.colorado.edu colorado.edu";

The ifconfig Command

View network interface parameters

Use -a to see info about all interfaces
Use netstat -i if -a doesn't work (e.g. on HP/UX)

Specify an interface to view it only

Configure network interface parameters

Typically:

    % ifconfig <int> down

    % ifconfig <int> up <ip> netmask <nm>

Resolving Hostnames

Hosts must be told how and from where

DNS - the Domain Name System
/etc/hosts - the traditional local file
NIS / NIS+ - Sun's Problem..

Sys-V hosts tend to look in /etc/nsswitch.conf

A 'hosts' entry designates one or more sources

    feanor % ssh bfs grep hosts /etc/nsswitch.conf
    hosts:      files dns

bfs will look in its own /etc/hosts file first

 and then (via /etc/resolv.conf) try DNS

Client DNS Configuration

Client DNS is configured in /etc/resolv.conf

'search' line gives domains to look in
'nameserver' lines give IP addrs of DNS nameservers to use
Specify up to three, they are tried in order

OpenBSD has an optional 'lookup' line where you specify to look in /etc/hosts first

    coatl % cat /etc/resolv.conf
    search cs.colorado.edu colorado.edu
    nameserver 128.138.242.197
    nameserver 128.138.192.220
    nameserver 128.138.192.205
    lookup file bind

Static Routing

Use netstat to view system's route table

     % netstat -rn -f inet

Each line describes a DESTINATION known to the system
Flags may be:

     U  Usable route
     G  Destination will forward
     R  Destination un-Reachable
     S  Static route
     L  Link address known
     H  Destination is a Host

IP routing decisions

Next Hop routing
Outgoing packet's destination IP addr is matched in route table
Longest match determines route chosen

Static routing is good for

Small networks with few hosts
Simple network topologies

Static routes are manipulated with the route command

General syntax of route is:

    % route  <cmd>  [<args>]

Commands are:

    add        Add a route.
    delete     Delete a specific route.
    change     Change aspects of a route
    flush      Remove all routes.

There are commands for viewing routes too
See the manpage..