Introduction to Linux Networking in Linux

Internet In 1970's, DARPA (Defence Advanced Research Projects Agency) wanted something to link their computers together. They chose BSD to implement the first internet, arpa net because if Unix's portability. Thus, all networking protocols nowadays are all based on the original BSD implementation of networking.

Basic networking In a networked environment, each computer has its own IP address and MAC address. There is also a subnet mask associated with the IP address. Basic routing is that when a computer receives an IP address that is not known, it passes it upstream to its router.

Simple Networking Setup

Setting up the network on Linux To assign an IP address to a computer, you use the ifconfig command. There are many options to ifconfig, typical parameters are: ifconfig IP netmask After assigning an IP address, the computer needs a default gateway so it can pass packets upstream. To add a default route, you use the route command: route add default IP

Hosts IP addresses are hard to remember. Who remebers the IP address to yahoo.com? So there is a hosts file all unix. Windows actually have this file too, but windows don't normally uses it because it uses NetBUI. An entry in /etc/hosts contains the following: IP Address (IPv4 or IPv6)alias domain

DNS and /etc/resolv.conf In Local Network, hosts files works fine, but on the internet, hosts files would not work. So DNS is created. In DNS, there is a DNS name server that resolves the IP address of a domain name. The file /etc/resolv.conf is responsible for DNS resolution in a workstation (non-server). Normally, /etc/hosts is querying a DNS server, however, that option may be changed. An entry in /etc/resolv.conf: nameserverIP address searchdomain

DHCP Assigning an IP address to each computer is not a feasible solution if clients are not always connected. DHCP or Dynamic Host Configuration Protocol is developed. Most Linux distributions is configured to automatically asks for an IP address from a DHCP server when it boots up. To asks for an IP address manually, you use the dhclient command: dhclient interface

More on routing To do routing with linux, the kernel usually needs to be recompiled with ip forwarding. But a loadable module may be loaded instead of recompile if the kernel supports loading additional modules. Benefits of using monolithic kernel is added security. Once ip forwarding is enabled, the machine can act as a basic router. Routed is the routing daemon.

Routing protocols Base installation of Unix supports the RIP protocol. (Routing Information Protocol) There are a few more protocols: IGRP (Interior Gateway Routing Protocol) EGP (Exterior Gateway Protocol) BGP (Border Gateway Protocol) To use the above routing protocols, third party programs will be needed: Zebra (GNU) Bgpd Gated Quagga (Told to be the best)

Netstat The command netstat show network statistics. (Note: different unixes shows different outputs) The options -r shows the routing table. -n shows numeric IPs instead of hosts. -i shows configured network interfaces. -t shows active TCP connections, -u for udp, -w for raw and -x for unix sockets. -a shows listening sockets.

Routing Table Routing tables Internet: Destination Gateway Flags Refs Use Netif Expire default UGS rl UH 1 6 lo /22 link#1 UC 0 0 rl :00:02:56:93:f0 UHLW 1 0 rl UGHS 0 0 lo0 Flags G – uses a gateway U – interface up H – only single host

Network statistics on connections Active Internet connections Proto Recv-Q Send-Q Local Address Foreign Address (state) tcp fukakyon.ssh ESTABLISHED udp4 0 0 localhost localhost udp4 0 0 localhost localhost udp4 0 0 localhost.domain *.* udp4 0 0 fukakyon.domain *.*

Active Internet connections (including servers) Proto Recv-Q Send-Q Local Address Foreign Address (state) tcp fukakyon.ssh ESTABLISHED tcp4 0 0 *.3128 *.* LISTEN tcp4 0 0 *.http *.* LISTEN tcp4 0 0 *.https *.* LISTEN tcp4 0 0 *.3127 *.* LISTEN tcp4 0 0 localhost.8005 *.* LISTEN tcp4 0 0 *.8009 *.* LISTEN tcp4 0 0 *.8008 *.* LISTEN tcp4 0 0 *.ftp *.* LISTEN tcp4 0 0 localhost *.* LISTEN tcp4 0 0 *.smtp *.* LISTEN tcp4 0 0 *.postgresql *.* LISTEN tcp4 0 0 *.3306 *.* LISTEN tcp4 0 0 *.3129 *.* LISTEN tcp4 0 0 *.imaps *.* LISTEN tcp4 0 0 *.imap *.* LISTEN tcp4 0 0 localhost *.* LISTEN tcp4 0 0 *.ssh *.* LISTEN tcp4 0 0 localhost.rndc *.* LISTEN tcp4 0 0 localhost.domain *.* LISTEN tcp4 0 0 fukakyon.domain *.* LISTEN

More on DNS DNS is Domain Name System. There are different types of records in DNS (Common types): A recordDirect address translation PTR recordIP->host translation MX recordMail Exchange record CNAME recordAlias of an A record

Types of quering for a DNS record When asking for a reply to a DNS query. (i.e. Asking for the IP address to yahoo.com), there are 2 types of queries: Recursive lookup(default) Non-recursive lookup In recursive lookup, when the reply to a query is not in a nameserver cache, that nameserver will ask another nameserver for the answer. That nameserver will then do the same thing if the reply is not in the cache. In recursive lookup, you will always get the IP address for a valid domain. In non-recursive lookup, if the reply is not in the cache, the nameserver will just return with a possible link to a nameserver that might have the reply.

Example replies Say we search for the IP address of gpro.com non-recursive: ; > DiG > +norecurse gpro.com ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ;; flags: qr ra; QUERY: 1, ANSWER: 0, AUTHORITY: 13, ADDITIONAL: 14 ;; QUESTION SECTION: ;gpro.com. IN A ;; AUTHORITY SECTION: com IN NS M.GTLD-SERVERS.NET. com IN NS A.GTLD-SERVERS.NET. ;; ADDITIONAL SECTION: A.GTLD-SERVERS.NET IN A ;; Query time: 6 msec ;; SERVER: #53( ) ;; WHEN: Wed Mar 16 18:42: ;; MSG SIZE rcvd: 498

We are asked to query another server, which gives us the following: ; > DiG norecurse gpro.com ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ;; flags: qr; QUERY: 1, ANSWER: 0, AUTHORITY: 2, ADDITIONAL: 1 ;; QUESTION SECTION: ;gpro.com. IN A ;; AUTHORITY SECTION: gpro.com IN NS ns-tk022.ocn.ad.jp. gpro.com IN NS ns.gpro.com. ;; ADDITIONAL SECTION: ns.gpro.com IN A ;; Query time: 152 msec ;; SERVER: #53( ) ;; WHEN: Wed Mar 16 18:54: ;; MSG SIZE rcvd: 91

One more query and we get the answer: ; > DiG norecurse gpro.com ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ;; flags: qr aa ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 2 ;; QUESTION SECTION: ;gpro.com. IN A ;; ANSWER SECTION: gpro.com IN A ;; AUTHORITY SECTION: gpro.com IN NS ns-tk022.ocn.ad.jp. gpro.com IN NS ns.gpro.com. ;; ADDITIONAL SECTION: ns-tk022.ocn.ad.jp IN A ns.gpro.com IN A ;; Query time: 306 msec ;; SERVER: #53( ) ;; WHEN: Wed Mar 16 18:55: ;; MSG SIZE rcvd: 123