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Linux Basics

Linux Basics. Reading: Chap 1-2 [WFR05] Linux Command Manual. About Linux. Linux is the name of the kernel Linux is Open Source Software (OSS) Linux is licensed through the General Public License (version 2, aka GPL2)

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Linux Basics

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  1. Linux Basics Reading: Chap 1-2 [WFR05] Linux Command Manual

  2. About Linux • Linux is the name of the kernel • Linux is Open Source Software (OSS) • Linux is licensed through the General Public License (version 2, aka GPL2) • The right to redistribute is granted only if the distribution is licensed under the terms of the GPL and either includes, or unconditionally offers to include at the moment of distribution, the source code • The Linux kernel by itself can serve as a firewall, router, access point, and even a static web page server • Typically, Linux is packaged with a great number of applications and utilities, also OSS

  3. Components of a Linux System • Kernel (can be monolithic or modular) • Modules (if modular kernel) • Filesystem(s) • Boot Loader • Libraries and Dynamic Linker • Init and rc system • Utilities • Applications

  4. Components of a Linux System • Kernel (can be monolithic or modular) • Modules (if modular kernel) • Filesystem(s) • Boot Loader • Libraries and Dynamic Linker • Init and rc system • Utilities • Applications

  5. Linux Kernel • A kernel is the central component of most computer operating systems (OS). Its responsibilities include managing the system's resources • Monolithic architecture includes much of OS functionality in kernel • Memory and process management • Device drivers • File systems • Network • In contrast, microkernels (e.g., Mach and NT) includes minimal functionality • Inter-process communication and memory management • Pros and cons

  6. Linux Kernel • Since V1.2, a combination of • Base kernel • Loadable kernel modules

  7. Linux Kernel Configuration • Monolithic architecture includes much of OS functionality in kernel • Memory and process management • Device drivers • File systems • Network

  8. Linux Kernel Configuration • Configuration in a tree structure to decide which files to be compiled into the kernel

  9. Linux Kernel Configuration • Configuration in a tree structure to decide which files to be compiled into the kernel • Options to compile directly in or as a module

  10. Linux Kernel Configuration • Configuration in a tree structure to decide which files to be compiled into the kernel • Options to compile directly in or as a module • Online help to explain choices

  11. Components of a Linux System • Kernel (can be monolithic or modular) • Modules (if modular kernel) • Filesystem(s) • Boot Loader • Libraries and Dynamic Linker • Init and rc system • Utilities • Applications

  12. Linux Loadable Kernel Modules (LKM) • Linux supports kernel modules as an option • Modules are loaded at run time • Reduce memory requirements • Add functionality to Linux kernel • Run in privileged kernel mode • As fast as base kernel • Doesn't require a reboot to add or remove functionality or develop your own module • LKMs are used for • Device drivers • Filesystem drivers • Network drivers • …

  13. LKM utilities • ismod – insert LKM • rmmod – remove LKM • lsmod – list LKM • modinfo • modprob – can read /etc/modules; insert/remove a set of LKMs intelligently

  14. Components of a Linux System • Kernel (can be monolithic or modular) • Modules (if modular kernel) • File system(s) • Boot Loader • Libraries and Dynamic Linker • Init and rc system • Utilities • Applications

  15. Linux File System Support • Linux uses the virtual file system (VFS) interface to modularize file system support • File systems may be compiled in as modules (but watch out for catch-22) • “you need to mount the root filesystem to add the module that lets you mount the root filesystem” • In addition to file systems that manage disk partitions, there are also pseudo file systems

  16. Pseudo File Systems • A 'pseudo' file system provides a file style interface to the inner workings of the kernel. • Most important is the /proc file system which provides many important interfaces to the kernel and running processes • /proc can be used to set parameters in the running kernel as well as to read states • e.g. echo “1” > /proc/sys/net/ipv4/ip_forward

  17. Components of a Linux System • Kernel (can be monolithic or modular) • Modules (if modular kernel) • Filesystem(s) • Boot Loader • Libraries and Dynamic Linker • Init and rc system • Utilities • Applications

  18. Boot Loader • Takes over from BIOS after POST • Usually on master boot record (MBR) of hard drive • the 512-byte boot sector that is the first sector of a partitioned disk • Can offer choice of different OSes (dual boot) • Linux typically uses GRUB (LILO in the past)

  19. GRUB • GRand Unified Boot loader • Two stages • The first being small with the sole purpose of loading the second one. • Understands several file system types • Provides for changing of boot options at boot time (useful for testing new kernel features) For more information: http://www.gnu.org/software/grub/ Which partition contains the kernel 1st partition on first hard disk root (hd0,0)kernel /vmlinuz-i686-up-4GB root=/dev/hda9boot Partition containing /sbin/init, which becomes the root partition File name of the kernel

  20. Boot process on Linux • BIOS -> bootloader -> kernel • The first process to start is a script /etc/rc.d/rc.sysinit • 6 run-time levels • /etc/rc.d/rc?.d/ • Runtime 5 is used for boot the system into GUI mode using XDM and X-Windows. • Runtime 3 is used for single-user mode • Scripts with S for startup and K for shutdown

  21. Init and RC System • Takes over once kernel loads • Brings system up to ready state • Starts different services • Can be used after boot to start and stop services e.g. /etc/init.d/httpd start • boot the system into GUI mode using XDM and X-Windows.

  22. Components of a Linux System • Kernel (can be monolithic or modular) • Modules (if modular kernel) • Filesystem(s) • Boot Loader • Libraries and Dynamic Linker • Init and rc system • Utilities • Applications

  23. Unix and the toolkit approach • /bin and /sbin (/usr/bin and /usr/sbin too) • STDIN, STDOUT, STDERR • Redirection and Pipes • e.g. dmesg | head -l

  24. Practices (cont’d) Hints: • If the commands are not in the default paths, try /sbin or /usr/sbin • A number of ways for finding out linux distributions • dmesg | head –l • Cat /proc/versions • “man” is your friend!

  25. Top Network Utilities • ifconfig • route • ping • traceroute (tcptraceroute) • nmap • netstat • ssh (scp, sftp) • telnet • nc • tcpdump

  26. Components of a Linux System • Kernel (can be monolithic or modular) • Modules (if modular kernel) • Filesystem(s) • Boot Loader • Libraries and Dynamic Linker • Init and rc system • Utilities • Applications

  27. Applications • Anything more complex than a utility? • System services (daemons) • X Windowing system • Interactive programs

  28. Practices ssh to linux01~04.cs.uh.edu • Find out the followings: • what Linux distribution is used? • Processor type, memory, CPU speed, # of CPUs • Which boot loader is used? • Try the following command • ifconfig • route • ping www.uh.edu • traceroute www.google.com • Netstat • Explain the results from ping, netstat

  29. Linux Networking Tools

  30. ping traceroute host, (nslookup) dig nmap telnet Top Network Utilities • ifconfig • iwconfig • route • iptables • iwconfig • netstat • ssh (scp, sftp) • tcpdump

  31. ifconfig • Configure a network interface • Without options, ifconfig shows current settings • can bring interface up or down • example: • ifconfig eth1 up • pump -i eth1 --- dhcp client program • ifconfig eth1

  32. ifconfig (CS Firewall) eth0 Link encap:Ethernet HWaddr 00:E0:81:2A:9D:C3 inet addr:129.7.240.254 Bcast:129.7.240.255 Mask:255.255.255.192 inet6 addr: fe80::2e0:81ff:fe2a:9dc3/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:222210314 errors:0 dropped:0 overruns:0 frame:0 TX packets:194237844 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:2468437723 (2354.0 Mb) TX bytes:1403836636 (1338.8 Mb) Base address:0xdc00 Memory:fe9e0000-fea00000 eth1 Link encap:Ethernet HWaddr 00:04:23:A8:58:82 inet addr:129.7.254.188 Bcast:129.7.254.191 Mask:255.255.255.192 inet6 addr: fe80::204:23ff:fea8:5882/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:397766811 errors:0 dropped:0 overruns:0 frame:0 TX packets:521981776 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:2719493949 (2593.5 Mb) TX bytes:217572585 (207.4 Mb) Base address:0xc880 Memory:fe8c0000-fe8e0000 eth2 Link encap:Ethernet HWaddr 00:04:23:A8:58:83 inet addr:192.168.10.254 Bcast:192.168.10.255 Mask:255.255.255.0 inet6 addr: fe80::204:23ff:fea8:5883/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:334616505 errors:0 dropped:0 overruns:0 frame:0 TX packets:238180941 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:2097863118 (2000.6 Mb) TX bytes:2193856536 (2092.2 Mb) Base address:0xcc00 Memory:fe8e0000-fe900000

  33. IP-Aliasing • “IP-aliases are additional IP-addresses/masks hooked up to a base interface by adding a colon and a string when running ifconfig.” • example: • ifconfig eth0:0 192.168.100.1 • ifconfig eth0:1 192.168.101.1 • Remove an aliasing • Ifconfig eth0:0 down • linux/Documentation/networking/alias.txt

  34. route • Show and/or manipulate the IP routing table • Commonly used in determining or setting default routers for a machine on network • example: • route add default gw 129.7.243.254 • route add -net 192.168.1.0 gw 10.0.0.10 netmask 255.255.0.0 • route del -net 192.168.1.0 gw 10.0.0.10 netmask 255.255.0.0 • To remove all routes: ifconfig eth0 down

  35. Static Routes • Routes can be static or dynamic • Most host-based routes are static • Static routes are layer 3 clues as to where to find hosts on a complicated network. • They include a destination network and a next-hop IP address. • The default route's destination network is a wildcard

  36. route (CS Firewall) Computer Science department firewall configuration $ /sbin/route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 129.7.240.0 192.168.10.253 255.255.255.192 UG 0 0 0 eth2 129.7.240.64 192.168.10.253 255.255.255.192 UG 0 0 0 eth2 129.7.240.128 192.168.10.253 255.255.255.192 UG 0 0 0 eth2 129.7.240.192 0.0.0.0 255.255.255.192 U 0 0 0 eth0 129.7.241.0 192.168.10.253 255.255.255.192 UG 0 0 0 eth2 129.7.254.128 0.0.0.0 255.255.255.192 U 0 0 0 eth1 129.7.242.0 192.168.10.253 255.255.255.0 UG 0 0 0 eth2 129.7.243.0 192.168.10.253 255.255.255.0 UG 0 0 0 eth2 192.168.10.0 0.0.0.0 255.255.255.0 U 0 0 0 eth2 loopback 127.0.0.1 255.0.0.0 UG 0 0 0 lo 0.0.0.0 129.7.254.190 0.0.0.0 UG 0 0 0 eth1 Flag U. This flag indicates that the route entry is up and running or ACTIVE. Flag G. This flag indicates that the route entry specifies an indirect route. Flag H. This flag indicates that the destination field in this route entry specifies a host route.

  37. tcpdump • Prints out headers of packets on a network interface • Provides for filtering output, and can also do some protocol analysis • example • tcpdump -i eth0 • tcpdump -i eth0 host [hostname]

  38. init scripts • Scripts for starting services are in /etc/init.d/ • Arguments are required for these scripts • (start, stop, restart, status) • To run a service at boot time • update-rc.d xxx defaults • To remove a service at boot time • update-rc.d -f xxx remove

  39. netstat • Prints information about various parts of the networking subsystem • Current network connections • Routing tables • Interface statistics • Masqueraded connections • Multicast memberships Alternatively, cat /proc/net/xxx

  40. netstat examples • netstat -r (provides same result as route command) • netstat -a (shows all connections) • netstat -tulp (shows all services) • gives programs listening for TCP and UDP connections • t for TCP, u for udp, l for listening sockets, -p for program (show the PIC and name of the program)

  41. Try this Run as root: # netstat -tulp # /etc/init.d/apache start # netstat -tulp Compare the results

  42. HTTP (WWW) • HyperText Transport Protocol • Uses TCP connections on port 80* • Commands are plaintext; human readable (if you don't mind html) • example: telnet www.uh.edu 80 • Try the following: telnet localhost 80 Trying 127.0.0.1... Connected to Cougar. Escape character is '^]'. GET /apache2-default/ HTTP/1.1 * Typically. Other ports such as 8080, 443 for SSL, etc. can also be used.

  43. Configuring Apache • Typically, Apache configuration files can be found under /etc/apache/conf • Knoppix and Debian create a symbolic link so everything is under /etc/apache • Most of the configuration is in httpd.conf • Additional configurations can be included from other files with the “Include” directive • Most distributions break this up into multiple files to provide for ease of management

  44. Common Apache Directives • Apache.conf contains two basic types of options • Directives are one-liner Attribute Value pairs • DocumentRoot /var/www • ServerName www.example.com • Blocks (also considered directives in apache documentation) define sections where directives have a limited scope • <Directory /var/www/> ... </Directory> • <IfModule SSL> ... </IfModule>

  45. Name Services • Provides a map from human readable address space (hostnames) to machine readable address space (IP) • Hierarchical system checks local resources before querying remote ones • /etc/hosts • optional local network naming systems • DNS • DNS works off a hierarchy as well.

  46. DNS and BIND • The internet's most common DNS server is BIND. • BIND consists of a set of configuration under /etc/bind and a daemon called named • For further information, O'Reilly has a great book, DNS and BIND (4th ed.) • The default install creates a caching nameserver

  47. Querying DNS • Several utilities provide the ability to perform name resolution using DNS • The most simple is the host command. • example • host www.uh.edu • host 129.7.1.1 • For more power and flexibility in interrogating DNS servers, use the dig command.

  48. dig $ dig @129.7.240.1 www.cs.uh.edu ; <<>> DiG 9.2.5 <<>> @129.7.240.1 www.cs.uh.edu ; (1 server found) ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 35927 ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 2 ;; QUESTION SECTION: ;www.cs.uh.edu. IN A ;; ANSWER SECTION: www.cs.uh.edu. 3600 IN A 129.7.228.92 ;; AUTHORITY SECTION: cs.uh.edu. 3600 IN NS dns.cs.uh.edu. cs.uh.edu. 3600 IN NS ns2.uh.edu. ;; ADDITIONAL SECTION: dns.cs.uh.edu. 3600 IN A 129.7.240.1 ns2.uh.edu. 34494 IN A 129.7.1.6 ;; Query time: 0 msec ;; SERVER: 129.7.240.1#53(129.7.240.1) ;; WHEN: Wed Feb 8 12:25:20 2006 ;; MSG SIZE rcvd: 115

  49. DHCP server • Set up the configuration file • Edit /etc/dhcp3/dhcpd.conf • /etc/init.d/dhcp3-server start • Set route to broadcast address • route add 255.255.255.255 dev eth0

  50. Formation of an Ad Hoc Network • Plug in the wireless card. • Bring your wireless card online using ifconfig eth1 up, but do not set it up with an IP address. (Don't use pump) • Set the card in ad-hoc mode using • iwconfig eth1 mode "ad-hoc" • iwconfig eth1 essid COSC6397sp07 channel 6 • ifconfig eth1 192.168.0.x • route add default gw 192.168.0.1

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