HOWTO Ubuntu Networking

[ajmorris]

Here is a networking tutorial with what I think are some handy CLI commands to know.
Feel free to comment and/or make suggestions. Make sure you know what a command does before
issuing it, especially in the security orientated ones.

You may also like to see my tutorial on getting to know your ubuntu system better: http://ubuntuforums.org/showthread.php?t=842307


Debugging

# Show the ethernet status

ethtool eth0

# Force 100Mbit Full duplex

ethtool -s eth0 speed 100 duplex full

# Disable auto negotiation

ethtool -s eth0 autoneg off

# Blink the ethernet led

ethtool -p eth0

# Display all interfaces (similar to ifconfig)

ip link show

# Bring device up (or down). Same as "ifconfig eth0 up"

ip link set eth0 up

# Display all IP addresses (similar to ifconfig)

ip addr show

# Similar to arp -a

ip neigh show

# Ping on ethernet layer

arping 192.168.16.254

# uses tcp instead of icmp to trace throught firewalls (install via sudo apt-get install tcptraceroute)

tcptraceroute -f 5 cb.vu

Routing

Print routing table

# use "ip route"

route -n

netstat -rn

Add and delete a route

route add -net 192.168.20.0 netmask 255.255.255.0 gw 192.168.16.254

# same as above with ip route

ip route add 192.168.20.0/24 via 192.168.16.254

route add -net 192.168.20.0 netmask 255.255.255.0 dev eth0

route add default gw 192.168.51.254

# same as above with ip route

ip route add default via 192.168.51.254 dev eth0

route delete -net 192.168.20.0 netmask 255.255.255.0

Configure additional IP addresses

# First IP

ifconfig eth0 192.168.50.254 netmask 255.255.255.0

# Second IP

ifconfig eth0:0 192.168.51.254 netmask 255.255.255.0

# Equivalent Commands:

ip addr add 192.168.50.254/24 dev eth0

ip addr add 192.168.51.254/24 dev eth0 label eth0:1

Change MAC address

#Normally you have to bring the interface down before the change.

ifconfig eth0 down

ifconfig eth0 hw ether 00:01:02:03:04:05

Ports in use

#Listening on open ports:

netstat -an | grep LISTEN

# lists all Internet connections

lsof -i

# displays list of open sockets (use apt-get install procinfo)

socklist

netstat -anp --udp --tcp | grep LISTEN

# List active connections to/from system

netstat -tup

# List listening ports from system

netstat -tupl

Firewall (iptables)

# For status

iptables -L -n -v

# Open everything

iptables -P INPUT       ACCEPT

iptables -P FORWARD     ACCEPT

iptables -P OUTPUT      ACCEPT

# Zero the packet and byte counters in all chains

iptables -Z

# Flush all chains

iptables -F

# Delete all chains

iptables -X

IP Forward for routing

# Check and then enable IP forward with:

# Check IP forward 0=off, 1=on

nano -w /proc/sys/net/ipv4/ip_forward

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

# or edit /etc/sysctl.conf with:

net.ipv4.ip_forward = 1

NAT Network Address Translation

# to activate NAT

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

# Port forward 20022 to internal IP port ssh

iptables -t nat -A PREROUTING -p tcp -d 78.31.70.238 --dport 20022 -j DNAT --to 192.168.16.44:22

# Port forward of range 993-995

iptables -t nat -A PREROUTING -p tcp -d 78.31.70.238 --dport 993:995 -j DNAT --to 192.168.16.254:993-995

ip route flush cache

# Check NAT status

iptables -L -t nat

NOTE: You can delete a port forward with -D instead of -A.


DNS

On *nix the DNS entries are valid for all interfaces and are stored in /etc/resolv.conf.
The domain to which the host belongs is also stored in this file. A minimal configuration is:

nameserver 78.31.70.238
search mydomain.net intern.lab
domain mydomain.net

Check the system domain name with:

# Same as dnsdomainname

hostname -d

Forward queries

Dig is used to test the DNS settings.
See from which server the client receives the answer (simplified answer).
in this example, we use google.com

dig google.com
google.com.267INA64.233.187.99
;; SERVER: 192.168.1.254#53(192.168.1.254)


The router 192.168.1.254 answered and the response is the A entry.
Any entry can be queried and the DNS server can be selected with @:

# To test the local server

dig @127.0.0.1 NS sun.com

# Query an external server

dig @204.97.212.10 NS MX heise.de

# Get the full zone (zone transfer)

dig AXFR @ns1.xname.org cb.vu

The program host is also quite powerful.

# Get the mail MX entry

host -t MX google.com

# Get the NS record over a TCP connection

host -t NS -T google.com

# Get everything

host -a google.com

Reverse queries

Find the name belonging to an IP address (in-addr.arpa.). This can be done with dig, host and nslookup:

dig -x 78.31.70.238

host 78.31.70.238

nslookup 78.31.70.238

Single hosts can be configured in the file /etc/hosts instead of running named locally
to resolve the hostname queries. The format is simple, for example:

64.233.187.99 google.com google


DHCP

The default ubuntu dhcp client is dhclient, however, i like dhcpcd a lot better,
and that is what i will use in my examples

apt-get install dhcpcd to install it.

# Trigger a renew (does not always work)

dhcpcd -n eth0

# release and shutdown

dhcpcd -k eth0

The lease with the full information is stored in:

/var/lib/dhcpcd/dhcpcd-eth0.info


For dhclient:

dhclient eth0

The lease with the full information is stored in:

/var/db/dhclient.leases.eth0


Use

/etc/dhclient.conf

to prepend options or force different options:

nano -w /etc/dhclient.conf

interface "eth0" {
prepend domain-name-servers 127.0.0.1;
default domain-name "google.com";
supersede domain-name "google.com";
}


Traffic analysis

Bmon http://people.suug.ch/~tgr/bmon/ is a small console bandwidth monitor and can display the
flow on different interfaces. You can install it on ubuntu with apt-get install bmon

Sniff with tcpdump (tcpdump comes with ubuntu)

tcpdump -nl -i eth0 not port ssh and src \(192.168.16.121 or 192.168.16.54\)

# select to/from a single IP

tcpdump -n -i eth0 net 192.168.16.121

# select traffic to/from a network

tcpdump -n -i eth0 net 192.168.16.0/24

# Buffered output

tcpdump -l > dump && tail -f dump

# Write traffic headers in binary file

tcpdump -i eth0 -w traffic.eth0

# Write traffic + payload in binary file

tcpdump -i eth0 -s 0 -w traffic.eth0

# Read from file (also for ethereal

tcpdump -r traffic.eth0

# The two classic commands

tcpdump port 80

# Check if pop or imap is secure

tcpdump host google.com

tcpdump -i eth0 -X port \(110 or 143\)

# Only catch pings

tcpdump -n -i eth0 icmp

# -s 0 for full packet -A for ASCII

tcpdump -i eth0 -s 0 -A port 80 | grep GET

Additional important options:


* -A Print each packets in clear text (without header)

* -X Print packets in hex and ASCII

* -l Make stdout line buffered

* -D Print all interfaces available

Scan with nmap

Nmap http://insecure.org/nmap/ is a port scanner with OS detection,
it is usually installed on most distributions. Install it in ubuntu
with apt-get install nmap

# scans all reserved TCP ports on the host

nmap google.com

# Find out which IP are used and by which host on 0/24

nmap -sP 192.168.16.0/24

# Do a stealth SYN scan with version and OS detection

nmap -sS -sV -O google.com

Other non standard but useful tools are hping (www.hping.org) an IP packet assembler/analyzer
and fping (fping.sourceforge.net). fping can check multiple hosts in a round-robin fashion.

Traffic control (QoS)

Traffic control manages the queuing, policing, scheduling, and other traffic parameters for a network.
The following examples are simple practical uses of the Linux capabilities to better use the available
bandwidth.

Limit upload

DSL or cable modems have a long queue to improve the upload throughput.
However filling the queue with a fast device (e.g. ethernet) will dramatically
decrease the interactivity. It is therefore useful to limit the device
upload rate to match the physical capacity of the modem, this should
greatly improve the interactivity. Set to about 90% of the modem maximal (cable) speed.

For a 512 Kbit upload modem:

tc qdisc add dev eth0 root tbf rate 480kbit latency 50ms burst 1540

# Status

tc -s qdisc ls dev eth0

# Delete the queue

tc qdisc del dev eth0 root

[code[tc qdisc change dev eth0 root tbf rate 220kbit latency 50ms burst 1540[/code]


Quality of service

Priority queuing with tc to optimize VoIP. See the full example on voip-info.org or
www.howtoforge.com. Suppose VoIP uses udp on ports 10000:11024 and device eth0
(could also be ppp0 or so). The following commands define the QoS to three queues
and force the VoIP traffic to queue 1 with QoS 0x1e (all bits set).
The default traffic flows into queue 3 and QoS Minimize-Delay flows into queue 2.

tc qdisc add dev eth0 root handle 1: prio priomap 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 0

tc qdisc add dev eth0 parent 1:1 handle 10: sfq

tc qdisc add dev eth0 parent 1:2 handle 20: sfq

tc qdisc add dev eth0 parent 1:3 handle 30: sfq

tc filter add dev eth0 protocol ip parent 1: prio 1 u32

# use server port range (added after u32 above)

match ip dport 10000 0x3C00 flowid 1:1

# or/and use server IP (added after u32 above)

match ip dst 123.23.0.1 flowid 1:1

Status and remove with

# queue status

tc -s qdisc ls dev eth0

# delete all QoS

tc qdisc del dev eth0 root

Calculate port range and mask

The tc filter defines the port range with port and mask which you have to calculate.
Find the 2^# ending of the port range, deduce the range and convert to HEX.
This is your mask. Example for 10000 -> 11024, the range is 1024.

# ending is 2^14 = 16384

2^13 (8192) < 10000 < 2^14 (16384)

# mask is 0x3C00

echo "obase=16;(2^14)-1024" | bc

NIS Debugging

Some commands which should work on a well configured NIS client:

# get the connected NIS server name (apt-get install nis to use)

ypwhich

# The NIS domain name as configured

domainname

# should display the group from the NIS server

ypcat group

# Rebuild the yp database

cd /var/yp && make

Is ypbind running?

ps auxww | grep ypbind

/usr/sbin/ypbind

yppoll passwd.byname

Map passwd.byname has order number 1190635041. Mon Sep 24 13:57:21 2007
The master server is servername.domain.net.

nano -w /etc/yp.conf

ypserver servername

domain domain.net broadcast

Netcat

Netcat http://netcat.sourceforge.net (nc) is better known as the "network Swiss Army Knife",
it can manipulate, create or read/write TCP/IP connections. Here some useful examples,
there are many more on the net, for example g-loaded.eu[...]
http://www.g-loaded.eu/2006/11/06/ne...seful-examples and here
http://www.terminally-incoherent.com...-netcat-tricks.


File transfer

Copy a large folder over a raw tcp connection. The transfer is very quick (no protocol overhead) and you don't need to mess up with NFS or SMB or FTP or so, simply make the file available on the server, and get it from the client. Here 192.168.1.1 is the server IP address.

# Serve tar folder on port 4444

server: tar -cf - -C VIDEO_TS . | nc -l -p 4444

# Pull the file on port 4444

client: nc 192.168.1.1 4444 | tar xpf - -C VIDEO_TS

# Server a single file

server: cat largefile | nc -l 5678

# Pull the single file

client: nc 192.168.1.1 5678 > largefile

# Server partition image

server: dd if=/dev/da0 | nc -l 4444

# Pull partition to clone

client: nc 192.168.1.1 4444 | dd of=/dev/da0

# Pull partition to file

client: nc 192.168.1.1 4444 | dd of=da0.img

Other hacks


Remote shell

# Provide a remote shell on port 4444 (aserver backdoor)
[code]nc -lp 4444 -e /bin/bash[code]

Emergency web server

Serve a single file on port 80 in a loop.

while true; do nc -l -p 80 < unixtoolbox.xhtml; done

Chat

Joe and Carter can chat over a simple TCP socket. The text is transferred with the enter key.

Joe: nc -lp 4444

Carter: nc 192.168.1.1 4444

AJ

[Rocket2DMn]

Very nice detailed guide for terminal networking.
You have a few code tages that need to be closed at the end (and one or two throughout), just FYI. You might also consider a little table of contents at the top and bigger, bolder subtitles on sections.
Good work.

[ajmorris]

Very nice detailed guide for terminal networking.
You have a few code tages that need to be closed at the end (and one or two throughout), just FYI. You might also consider a little table of contents at the top and bigger, bolder subtitles on sections.
Good work.

Thanks, i was in a hurry when i posted it... i'll start making it easier to read/follow

[GeneralMao]

Wow great guide. I have saved it in my bookmark and printed a copy.

Great job, very handy.

[zo934]

HEy HOw do I put the firewall back to default,
I was messing with it lastnight, trying to configure firewall in Ubuntu,
I think I set it back to default but want to be positive. Now when I try and login, I can login, but all I get is a blank screen, and i have to power down my machine, could I have changed something in the firewall to cause this? Help please

[eival]

question about the ToS code.(im not on ubuntu to test right now)

[tc qdisc add dev eth0 root tbf rate 480kbit latency 50ms burst 1540]

for "latency" and "burst"

whats the lowest/fastest time you can set the latency? will it accept 0ms(zero)?

and what is "burst" and what does the number 1540 represent?(packets/ping?)

an if the burst can be changed what would the result be/which would increase speed(setting it higher than 1540 or lower)?

thanks.

[ajmorris]

HEy HOw do I put the firewall back to default,
I was messing with it lastnight, trying to configure firewall in Ubuntu,
I think I set it back to default but want to be positive. Now when I try and login, I can login, but all I get is a blank screen, and i have to power down my machine, could I have changed something in the firewall to cause this? Help please

I doubt the editing of your firewall will have prevented you from being able to log in, could you please start a new thread, rather than discussing it here, about your issue Feel free to pm me with the link of your thread.

AJ

[ajmorris]

question about the ToS code.(im not on ubuntu to test right now)

[tc qdisc add dev eth0 root tbf rate 480kbit latency 50ms burst 1540]

for "latency" and "burst"

whats the lowest/fastest time you can set the latency? will it accept 0ms(zero)?

and what is "burst" and what does the number 1540 represent?(packets/ping?)

an if the burst can be changed what would the result be/which would increase speed(setting it higher than 1540 or lower)?

thanks.

You must have a latency or limit set. Thus the lowest setting for latency is 1ms.

As for burst, burst is a paramater, similar to the cburst paramater. Essentially, it controls the max amount of data that can be accumulated if the real speed rate is lower than the 'ceil'. By default, it is about 1 packet, in the example, i have set it to 1540 packets. You can also use it in collaboration with ceil to limit speeds when it gets to a certain number of packets, etc.

I hope that helps,

AJ

[A_M_S]

Great compilation.


Thank you!

[raja.genupula]

good thread . its have nice information .