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Linux network configuration


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Linux Network Configuration

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Linux network configuration

  1. 1. Linux Network Configuration Linux TCP/IP Network Configuration Files: File Description /etc/resolve.conf List DNS servers for internet domain name resolution. Manual page for: /etc/resolv.conf /etc/hosts Lists hosts to be resolved locally (not by DNS). Manual page for: /etc/hosts /etc/nsswitch.conf List order of host name search. Typically look at local files, then NIS server, then DNS server. Manual page for: /etc/nsswitch.conf Red Hat/Fedora/CentOS: /etc/sysconfig/network Specify network configuration. eg. Static IP, DHCP, NIS, etc. Red Hat/Fedora/CentOS: /etc/sysconfig/network- Specify TCP network information. scripts/ifcfg-device Ubuntu/Debian: /etc/network/interfaces Specify network configuration and devices. eg. Static IP and info, DHCP, etc. Domain Resolution Configuration Files: File: /etc/resolv.conf - host name resolver configuration file search - Name of your domain or ISPs 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 This configures Linux so that it knows which DNS server will be resolving domain names into IP addresses. If using DHCP client, this will automatically be sent to you by the ISP and loaded into this file as part of the DHCP protocol. If using a static IP address, ask the ISP or check another machine on your network. Red Hat/Fedora GUI: /usr/sbin/system-config-network (select tab "DNS"). File: /etc/hosts - locally resolve node names to IP addresses localhost.localdomain localhost XXX.XXX.XXX.XXX node-name Note when adding hosts to this file, place the fully qualified name first. (It helps sendmail identify your server correctly) i.e.: XXX.XXX.XXX.XXX superserver This informs Linux of local systems on the network which are not handled by the DNS server. (or for all systems in your LAN if you are not using DNS or NIS) The file format for the hosts file is specified by RFC 952. Red Hat/Fedora configuration GUI: /usr/sbin/system-config-network (select tab "Hosts"). File: /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. Fedora / Red Hat Network Configuration Files: /etc/sysconfig/network Red Hat network configuration file used by the system during the boot process. File: /etc/sysconfig/network-scripts/ifcfg-eth0 Configuration settings for your first ethernet port (0). Your second port is eth1. File: /etc/modprobe.conf (kernel 2.6) /etc/modules.conf (kernel 2.4) (or for older systems: /etc/conf.modules) Example statement for Intel ethernet card: alias eth0 eepro100 Modules for other devices on the system will also be listed. This tells the kernel which device driver to use if configured as a loadable module. (default for Red Hat) Fedora / Red Hat Network GUI Configuration Tools: The following GUI tools edit the system configuration files. There is no difference in the configuration developed with the GUI tools and that developed by editing system configuration files directly. TCP/IP ethernet configuration:1 of 19 21/02/2012 08:18 a.m.
  2. 2. Linux Network Configuration TCP/IP ethernet configuration: Network configuration: /usr/sbin/system-config-network (FC-2/3) GUI shown here ---> /usr/bin/redhat-config-network (/usr/bin/neat) (RH 7.2+ FC-1) Text console configuration tool: /usr/sbin/system-config-network-tui (Text User Interface (TUI) for Fedora Core 2/3) /usr/bin/redhat-config-network-tui (RH 9.0 - FC-1) Text console network configuration tool. First interface only - eth0: /usr/sbin/netconfig /usr/bin/netcfg (GUI) (last available with RH 7.1) Gnome Desktop: Gnome Desktop Network Configuration /usr/bin/gnome-network-preferences (RH 9.0 - FC-3) Proxy configuration. Choose one of three options: 1. Direct internet connection 2. Manual proxy configuration (specify proxy and port) 3. Automatic proxy configuration (give URL) Assigning an IP address: Computers may be assiged a static IP address or assigned one dynamically. Typically a server will require a static IP while a workstation will use DHCP (dynamic IP assignment). The Linux server requires a static IP so that those who wish to use its resources can find the system. It is more easily found if the IP address does not change and is static. This is not important for the Linux client workstation and thus it is easier to use an automated Dynamic Host Configuration Protocol (DHCP) for IP address assignment. Static IP address assignment: Choose one of the following methods: Command Line: /sbin/ifconfig eth0 netmask broadcast Network address by convention would be the lowest: Broadcast address by convention would be the highest: The gateway can be anything, but following convention: Note: the highest and lowest addresses are based on the netmask. The previous example is based on a netmask of Red Hat / Fedora GUI tools: /usr/bin/neat Gnome GUI network administration tool. Handles all interfaces. Configure for Static IP or DHCP client. (First available with Red Hat 7.2.) /usr/bin/netcfg (Handles all interfaces) (last available in Red Hat 7.1) Red Hat / Fedora Console tools: /usr/sbin/system-config-network-tui (Text User Interface) /usr/sbin/netconfig (Only seems to work for the first network interface eth0 but not eth1,...) Directly edit configuration files/scripts. See format below. The ifconfig command does NOT store this information permanently. Upon reboot this information is lost. Manually add the network configuration to /etc/sysconfig/network-scripts/ifcfg-eth0 (Red Hat/Fedora/CentOS) for the first NIC, ifcfg-eth1 for the second, etc, or /etc/network/interfaces (Ubuntu) as shown below. Any other commands you may want to add to the system boot sequence can be added to the end of the file /etc/rc.d/rc.local. The commands netcfg and netconfig make permanent changes to system network configuration files located in /etc/sysconfig/network-scripts/, so that this information is retained and used upon system boot. The IANA has allocated IP addresses in the range of to for private networks. Helpful tools: Network Calculators: Subnet mask calculator, node calculator, mask inverter, ... IP subnet calculator Command line IP Configuration: ifconfig ifconfig interface [aftype] options | address ... where: interface: eth0, eth1, eth2 represent the computer ethernet interfaces aftype: inet (TCP/IP, default), inet6 (IPv6), ax25 (AMPR Packet Radio), ddp (Appletalk Phase 2), ipx (Novell IPX) or netrom (AMPR Packet radio) Options: Option Description2 of 19 21/02/2012 08:18 a.m.
  3. 3. Linux Network Configuration Option Description up Activate the interface. Implied if IP addresses are specified. down Shut down interface Enable ARP protocol on this interface. Allow ARP to detect the addresses of computer hosts arp attached to the network. -arp Disable ARP protocol on this interface Enable promiscuous mode. Receive all packets on the network not just those destined for this promisc interface. -promisc Disable promiscuous mode. Specify the Maximum Transfer Unit (MTU) of the interface. The MTU is the maximum number of mtu ## octets the interface is able to handle in a single transaction. Defaults: Ethernet: 1500 SLIP: 296 broadcast Set the network broadcast address for this interface. XXX.XXX.XXX.XXX netmask Set the IP network mask for this interface. XXX.XXX.XXX.XXX Man page: ifconfig Ubuntu / Debian IP Configuration Files: File: /etc/network/interfaces Static IP example: auto lo iface lo inet loopback auto eth0 iface eth0 inet static address netmask broadcast network gateway Dynamic IP (DHCP) example: auto lo iface lo inet loopback auto eth0 iface eth0 inet dhcp auto eth1 iface eth1 inet dhcp auto eth2 iface eth2 inet dhcp auto ath0 iface ath0 inet dhcp auto wlan0 iface wlan0 inet dhcp Interfaces: lo: Loopback interface (network within your system without slowing down for the real ethernet based network) eth0: First ethernet interface card wlan0: First wireless network interface Also see " man interfaces" Ubuntu GUI Network Tools: /usr/bin/gnome-nettool (apt-get install gnome-nettool)3 of 19 21/02/2012 08:18 a.m.
  4. 4. Linux Network Configuration /usr/bin/network-admin (apt-get install gnome-network-admin) Red Hat / Fedora Core IP Configuration Files: The Red Hat configuration tools store the configuration information in the file /etc/sysconfig/network. They will also allow one to configure routing information. File: /etc/sysconfig/network Static IP address Configuration: (Configure gateway address) NETWORKING=yes HOSTNAME=my-hostname - Hostname is defined here and by command hostname FORWARD_IPV4=true - True for NAT firewall gateways and linux routers. False for everyone else - desktops and servers. GATEWAY="XXX.XXX.XXX.YYY" - Used if your network is connected to another network or the internet. Static IP configuration. Gateway not defined here for DHCP client. OR for DHCP client configuration: NETWORKING=yes HOSTNAME=my-hostname - Hostname is defined here and by command hostname (Gateway is assigned by DHCP server.) OR for NIS client configuration: NETWORKING=yes HOSTNAME=my-hostname - Hostname is defined here and by command hostname NISDOMAIN=NISProject1 - NIS domain to attach File (Red Hat/Fedora): /etc/sysconfig/network-scripts/ifcfg-eth0 (S.u.s.e.: /etc/sysconfig/network/ifcfg-eth-id-XX:XX:XX:XX:XX) This file used by the command scripts ifup and ifdown Static IP address configuration: DEVICE=eth0 BOOTPROTO=static BROADCAST=XXX.XXX.XXX.255 IPADDR=XXX.XXX.XXX.XXX NETMASK= NETWORK=XXX.XXX.XXX.0 ONBOOT=yes - Will activate upon system boot RHEL4/FC3 additions: TYPE=Ethernet HWADDR=XX:XX:XX:XX:XX:XX GATEWAY=XXX.XXX.XXX.XXX OR for DHCP client configuration: DEVICE=eth0 ONBOOT=yes BOOTPROTO=dhcp RHEL4/FC3 additions: IPV6INIT=no USERCTL=no PEERDNS=yes TYPE=Ethernet4 of 19 21/02/2012 08:18 a.m.
  5. 5. Linux Network Configuration TYPE=Ethernet HWADDR=XX:XX:XX:XX:XX:XX (Used by script /etc/sysconfig/network-scripts/ifup to bring the various network interfaces on-line) To disable DHCP change BOOTPROTO=dhcp to BOOTPROTO=none In order for updated information in any of these files to take effect, one must issue the command: service network restart (or: /etc/init.d/network restart) Network IP aliasing: Assign more than one IP address to one ethernet card: ifconfig eth0 XXX.XXX.XXX.XXX netmask broadcast XXX.XXX.XXX.255 ifconfig eth0:0 netmask broadcast ifconfig eth0:1 netmask broadcast route add -host XXX.XXX.XXX.XXX dev eth0 route add -host dev eth0 route add -host dev eth0 In this example 0 and 1 are aliases in addition to the regular eth0. The result of the ifconfig command: eth0 Link encap:Ethernet HWaddr 00:10:4C:25:7A:3F inet addr:XXX.XXX.XXX.XXX Bcast:XXX.XXX.XXX.255 Mask: UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:14218 errors:0 dropped:0 overruns:0 frame:0 TX packets:1362 errors:0 dropped:0 overruns:0 carrier:0 collisions:1 txqueuelen:100 Interrupt:5 Base address:0xe400 eth0:0 Link encap:Ethernet HWaddr 00:10:4C:25:7A:3F inet addr: Bcast: Mask: UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 Interrupt:5 Base address:0xe400 eth0:1 Link encap:Ethernet HWaddr 00:10:4C:25:7A:3F inet addr: Bcast: Mask: UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 Interrupt:5 Base address:0xe400 Config file: /etc/sysconfig/network-scripts/ifcfg-eth0:0 DEVICE=eth0:0 ONBOOT=yes BOOTPROTO=static BROADCAST= IPADDR= NETMASK= NETWORK= ONBOOT=yes Aliases can also be shut down independently. i.e.: ifdown eth0:0 The option during kernel compile is: CONFIG_IP_ALIAS=y (Enabled by default in Redhat) Note: The Apache web server can be configured so that different IP addresses can be assigned to specific domains being hosted. See Apache configuration and "configuring an IP based virtual host" in the YoLinux Web site configuration tutorial. DHCP Linux Client: get connection info: /sbin/pump -i eth0 --status (Red Hat Linux 7.1 and older) Device eth0 IP: 4.XXX.XXX.XXX Netmask: Broadcast: 4.XXX.XXX.255 Network: 4.XXX.XXX.0 Boot server 131.XXX.XXX.4 Next server Gateway: 4.XXX.XXX.1 Domain: Nameservers: 4.XXX.XXX.1 4.XXX.XXX.2 4.XXX.XXX.3 Renewal time: Sat Aug 11 08:28:55 2001 Expiration time: Sat Aug 11 11:28:55 2001 Changing the host name: This is a three step process: 1. Issue the command: hostname new-host-name 2. Change network configuration file: /etc/sysconfig/network Edit entry: HOSTNAME=new-host-name 3. Restart systems which relied on the hostname (or reboot): Restart network services: service network restart (or: /etc/init.d/network restart) Restart desktop: Bring down system to console mode: init 3 Bring up X-Windows: init 5 One may also want to check the file /etc/hosts for an entry using the system name which allows the system to be self aware. The hostname may be changed at runtime using the command: sysctl -w kernel.hostname="superserver" Note that hostnames may only contain alphanumeric characters, minus signs ("-"), and periods ("."). They must begin with an alphabetic character and end with an alphanumeric character.5 of 19 21/02/2012 08:18 a.m.
  6. 6. Linux Network Configuration alphabetic character and end with an alphanumeric character. Change the host name using GUI tool: /usr/sbin/system-config-network (Red Hat / Fedora / CentOS) Hostname entries are made in two places: Select the "DNS" tab. Select the "Devices" tab + "Edit" + the "General" tab. Activating and De-Activating your NIC: Commands for starting and stopping TCP/IP network services on a Network Interface Card (NIC): Activate: /sbin/ifup eth0 (Also: ifconfig eth0 up - Note: Even if no IP address is assigned you can listen.) De-Activate: /sbin/ifdown eth0 (Also: ifconfig eth0 down) These scripts use the scripts and NIC config files in /etc/sysconfig/network-scripts/ GUI Interface control/configuration: Start/Stop network interfaces /usr/bin/system-control-network (Fedora Core 2/3) /usr/bin/redhat-control-network (RH 9.0 - FC-1) Configure Ethernet, ISDN, modem, token Ring, Wireless or DSL network connection: /usr/sbin/system-config-network-druid (FC2/3) /usr/sbin/redhat-config-network-druid (RH 9 - FC-1) Subnets: M # OF CLASS C CLASS CLASS A SUB Slash CLASS A CLASS B CLASS B CLASS C SUB CLASS C SUB A C S NETS Fmt HOSTS HOSTS MASK MASK HOSTS MASK MASK HOSTS K 1 Invalid 255 or /32 16,777,214 65,534 254 1 address 256 Invalid 254 128 /31 33,554,430 131,070 510 2 addresses 2 hosts 252 64 /30 67,108,862 262,142 1,022 4 addresses 6 hosts 248 32 /29 134,217,726 524,286 2,046 8 addresses 14 hosts 240 16 /28 268,435,454 1,048,574 4,094 16 addresses6 of 19 21/02/2012 08:18 a.m.
  7. 7. Linux Network Configuration addresses 30 hosts 224 8 /27 536,870,910 2,097,150 8,190 32 addresses 62 hosts 192 4 /26 1,073,741,822 4,194,302 16,382 64 addresses 126 hosts 128 2 /25 2,147,483,646 8,388,606 32,766 128 addresses Binary position 8 7 6 5 4321 Value 128 64 32 16 8 4 2 1 Example: 192 1 1 0 0 0000 Example 192=128+64 Some addresses are reserved and outside this scope. Loopback (, reserved class C 192.168.XXX.XXX, reserved class B 172.31.XXX.XXX and reserved class A 10.XXX.XXX.XXX. Subnet Example: Your ISP assigns you a subnet mask of for your office. Network Base address Computer 1 Computer 2 Computer 3 Computer 4 Computer 5 DSL router/Gateway Broadcast address Of the eight addresses, there are six assigned to hardware systems and ultimately only five usable addresses. Links: Subnet calculator Table of subnets IP Subnetting, Variable Subnetting, and CIDR (Supernetting) Subnet Masking and Addressing Network Classes: The concept of network classes is a little obsolete as subnets are now used to define smaller networks using CIDR (Classless Inter-Domain Routing) as detailed above. These subnets may be part of a class A, B, C, etc network. For historical reference the network classes are defined as follows: Class A: Defined by the first 8 bits with a range of 0 - 127. First number (8 bits) is defined by Internic i.e. 77.XXX.XXX.XXX One class A network can define 16,777,214 hosts. Range: - Class B: Defined by the first 8 bits with a range from 128 - 191 First two numbers (16 bits) are defined by Internic i.e. 182.56.XXX.XXX One class B network can define 65,534 hosts. Range: - Class C: Defined by the first 8 bits with a range from 192 - 223 First three numbers (24 bits) are defined by Internic i.e. 220.56.222.XXX One class B network can define 254 hosts. Range: - Class D: Defined by the first 8 bits with a range from 224 - 239 This is reserved for multicast networks (RFC988) Range: - Class E: Defined by the first 8 bits with a range from 240 - 255 This is reserved for experimental use. Range: - Enable Forwarding: Forwarding allows the network packets on one network interface (i.e. eth0) to be forwarded to another network interface (i.e. eth1). This will allow the Linux computer to conect ("ethernet bridge") or route network traffic. The bridge configuration will merge two (or several) networks into one single network topology. IpTables firewall rules can be used to filter traffic. A router configuration can support multicast and basic IP routing using the "route" command. IP masquerading (NAT) can be used to connect private local area networks (LAN) to the internet or load balance servers. Turn on IP forwarding to allow Linux computer to act as a gateway or router. echo 1 > /proc/sys/net/ipv4/ip_forward Default is 0. One can add firewall rules by using ipchains. Another method is to alter the Linux kernel config file: /etc/sysctl.conf Set the following value: net.ipv4.ip_forward = 17 of 19 21/02/2012 08:18 a.m.
  8. 8. Linux Network Configuration See file /etc/sysconfig/network for storing this configuration. FORWARD_IPV4=true Change the default "false" to "true". All methods will result in a proc file value of "1". Test: cat /proc/sys/net/ipv4/ip_forward The TCP Man page - Linux Programmers Manual and /usr/src/linux/Documentation/proc.txt (Kernel 2.2 RH 7.0-) cover /proc/sys/net/ipv4/* file descriptions. Also see: (YoLinux tutorials) Configure Linux as an internet gateway router: Using Linux and iptables/ipchains to set up an internet gateway for home or office (iptables) Load balancing servers using LVS (Linux Virtual Server) (ipvsadm) Adding a network interface card (NIC): Manual method: This does not alter the permanent configuration and will only configure support until the next reboot. cd /lib/modules/2.2.5-15/net/ - Use kernel version for your system. This example uses 2.2.5-15 (Fedora Core 3: /lib/modules/2.6.12-1.1381_FC3/kernel/net/) Here you will find the modules supported by your system. It can be permanently added to: /etc/modprobe.conf (kernel 2.6) /etc/modules.conf (kernel 2.4) (or for older systems: /etc/conf.modules) Example: alias eth0 3c59x /sbin/insmod 3c59x (For a 3Com ethernet card) This inserts the specified module into the kernel. /sbin/modprobe 3c59x This also loads a module into the system kernel. Modprobe command line options: -r : to unload the module. /sbin/modprobe -l * : list all modules. /sbin/modprobe -lt net * : List only network modules /sbin/modprobe -t net * : Try loading all network modules and see what sticks. (act of desperation) ifconfig ... The easy way: Red Hat versions 6.2 and later, ship with Kudzu, a device detection program which runs during system initialization. (/etc/rc.d/init.d/kudzu) This can detect a newly installed NIC and load the appropriate driver. Then use /usr/sbin /netconfig to configure the IP address and network settings. The configuration will be stored so that it will be utilized upon system boot. Systems with two NIC cards: Typically two cards are used when connecting to two networks. In this case the device must be defined using one of three methods: 1. Use the Red Hat GUI tool /usr/bin/netcfg OR 2. Define network parameters in configuration files: Define new device in file (Red Hat/Fedora) /etc/sysconfig/network-scripts/ifcfg-eth1 (S.u.s.e 9.2: /etc/sysconfig/network/ifcfg-eth-id-XX:XX:XX:XX:XX) DEVICE=eth1 BOOTPROTO=static IPADDR= NETMASK= GATEWAY=XXX.XXX.XXX.XXX Special routing information may be specified, if necessary, in the file (Red Hat/Fedora): /etc/sysconfig/static-routes (S.u.s.e. 9.2: /etc/sysconfig/network/routes) Example: eth1 net XXX.XXX.XXX.0 netmask gw XXX.XXX.XXX.XXX OR 3. Define network parameters using Unix command line interface: Define IP address: ifconfig eth0 XXX.XXX.XXX.XXX netmask broadcast XXX.XXX.XXX.2558 of 19 21/02/2012 08:18 a.m.
  9. 9. Linux Network Configuration ifconfig eth0 XXX.XXX.XXX.XXX netmask broadcast XXX.XXX.XXX.255 ifconfig eth1 netmask broadcast If necessary, define route with with the route command: Examples: route add default gw XXX.XXX.XXX.XXX dev eth0 route add -net XXX.XXX.XXX.0 netmask gw XXX.XXX.XXX.XXX dev eth0 Where XXX.XXX.XXX.XXX is the gateway to the internet as defined by your ISP or network operator. If a mistake is made just repeat the route command substituting "del" in place of "add". Configuring your NIC: Speed and Duplex settings: This is usually not necessary because most ethernet adapters can auto-negotiate link speed and duplex setting. List NIC speed and configuration: mii-tool eth0: negotiated 100baseTx-FD flow-control, link ok Verbose mode: mii-tool -v eth0: negotiated 100baseTx-FD flow-control, link ok product info: Intel 82555 rev 4 basic mode: autonegotiation enabled basic status: autonegotiation complete, link ok capabilities: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD advertising: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD flow-control link partner: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD flow-control Set NIC configuration: mii-tool -F option Option Parameters -F 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD -A 100baseT4 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD Query NIC with ethtool: Command Description ethtool -g eth0 Queries ethernet device for rx/tx ring parameter information. ethtool -a eth0 Queries ethernet device for pause parameter information. ethtool -c eth0 Queries ethernet device for coalescing information. ethtool -i eth0 Queries ethernet device for associated driver information. ethtool -d eth0 Prints a register dump for the specified ethernet device. ethtool -k eth0 Queries ethernet device for offload information. ethtool -S eth0 Queries ethernet device for NIC and driver statistics. Man Pages: mii-tool - view, manipulate media-independent interface status ethtool - Display or change ethernet card settings Route: Static routes: IP (Internet Protocol) uses a routing table to determine where packets should be sent. First the packet is examined to see if its destination is for the local or remote network. If it is to be sent to a remote network, the routing table is consulted to determine the path. If there is no information in the routing table then the packet is sent to the default gateway. Static routes are set with the route command and with the configuration file (Red Hat/Fedora): /etc/sysconfig/network-scripts/route-eth0 or (Red Hat 7: /etc/sysconfig/static-routes) (S.u.s.e. 9.2: /etc/sysconfig/network/routes): via See command: /etc/sysconfig/network-scripts/ifup-routes eth0 Dynamic routes: RIP (Routing Information Protocol) is used to define dynamic routes. If multiple routes are possible, RIP will choose the shortest route. (Fewest hops between routers not physical distance.) Routers use RIP to broadcast the routing table over UDP port 520. The routers would then add new or improved routes to their routing tables. Man pages: route - show / manipulate the IP routing table (Static route) Show routes: Option Description -n display IP addresses. Do not resolve host names for faster results.9 of 19 21/02/2012 08:18 a.m.
  10. 10. Linux Network Configuration -e Print more extensive information about routes. -v Verbose. --help Route command information. Manipulate routes: Option Description add or del or neither Add or delete route information. If not specified then print route table information. -host XXX.XXX.XXX.XXX Add a single computer host identified by the IP address. -net XXX.XXX.XXX.XXX Add a network identified by the network address, to the route. gw XXX.XXX.XXX.XXX Specify the network gateway. netmask XXX.XXX.XXX.XXX Specify the network netmask. default Of all the routes specified, identify one as the default network route. (typically the gateway is specified as the default route) Examples: Show routing table: route -e Access individual computer host specified via network interface card eth1: route add -host eth1 Access ISP network identified by the network address and netmask using network interface card eth0: route add -net netmask gw eth0 Conversly: route del -net netmask gw eth0 Specify default gateway to use to access remote network via network interface card eth0: route add default gw eth0 (Gateway can also be defined in /etc/sysconfig/network) Specify two gateways for two network destinations: (i.e. one external, one internal private network. Two routers/gateways will be specified.) Add internet gateway as before: route add default gw eth0 Add second private network: route add -net netmask gw eth0 routed - network routing daemon. Uses RIP protocol to update routing table. ipx_route - show / manipulate the IPX routing table - IPX is the Novell networking protocol (Not typically used unless your office has Novell servers) ifuser - Identify destinations routed to a particular network interface. VPN, Tunneling: Commercial VPN Linux software solutions - YoLinux - IPSec VPN for Linux - IPSec VPN for Linux FreeSWAN tutorial - howto OpenVPN - SSL VPN solution for site to site, WiFi security, and enterprise-scale remote access with load balancing, failover, and fine-grained access-controls. SSL-Explorer - Java SLL based VPN Quagga dynamic routing suite VLAN n2n pier to pier within a private fabric CIPE: Crypto IP Encapsulation (Easiest way to configure two Linux gateways connecting two private networks over the internet with encryption.) CIPE Home page - CIPE is a simple encapsulation system that securely connects two subnets. GRE Tunneling - Generic Routing Encapsulation - Hugo Samayoa VPN HowTo - Matthew D. Wilson Installing and Running PPTP on Linux L2TP Extensions (l2tpext) Internet Drafts. 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 List externally connected processes: netstat -punta -a: Show both listening and non-listening sockets. -p: Show PID of process owning socket -u: Show UDP -t: Show TCP -n: Show IP addresses only. Dont resolve host names -g: Show multi-cast group membership info -c: Continuous mode - update info every second -v: Verbose -e: Extended information -o: show network timer information List all connected processes: netstat -nap Show network statistics: netstat -s Display routing table info: netstat -rn $ netstat -nr Kernel IP routing table Destination Gateway Genmask Flags MSS Window irtt Iface U 0 0 0 eth0 U 0 0 0 eth0 UG 0 0 0 eth0 Flags: G: route uses gateway U: Interface is "up" H: Only a single host is accessible (eg. loopback) D: Entry generated by ICMP redirect message M: Modified by ICMP redirect message10 of 19 21/02/2012 08:18 a.m.
  11. 11. Linux Network Configuration M: Modified by ICMP redirect message Display interface statistics: netstat -i $ netstat -i Kernel Interface table Iface MTU Met RX-OK RX-ERR RX-DRP RX-OVR TX-OK TX-ERR TX-DRP TX-OVR Flg eth0 1500 0 2224 0 0 0 1969 0 0 0 BMRU lo 16436 0 1428 0 0 0 1428 0 0 0 LRU Where: RX-OK/TX-OK: number of packets transmitted/received error free RX-ERR/TX-ERR: number of dammaged/error packets transmitted/received RX-DRP/TX-DRP: number of dropped packets RX-OVR/TX-OVR: number of packets dropped because of a buffer overrun Flags: B: A broadcast address has been set L: This interface is a loopback device M: All packets are received N: Trailers are avoided O: ARP is turned off for this interface P: Point-to-point connection R: Interface is running U: Interface is up ping - send ICMP ECHO_REQUEST packets to network hosts. Use Cntl-C to stop ping. traceroute - print the route packets take to network host. (Ubuntu Note: Typically Ubuntu installs tracepath for IPv4 and traceroute6 for IPv6. One can install traceroute: apt-get install traceroute) traceroute IP-address-of-server traceroute domain-name-of-server mtr - a network diagnostic tool introduced in Fedora - Like traceroute except it gives more network quality and network diagnostic info. Leave running to get real time stats. Reports best and worst round trip times in milliseconds. mtr IP-address-of-server mtr domain-name-of-server whois - Lookup a domain name in the internic whois database. finger - Display information on a system user. i.e. finger user@host Uses $HOME/.plan and $HOME/.project user files. iptables - IP firewall administration (Linux kernel 2.6/2.4) See YoLinux firewall/gateway configuration. ipchains - IP firewall administration (Linux kernel 2.2) See YoLinux firewall/gateway configuration. socklist - Display list of open sockets, type, port, process id and the name of the process. Kill with fuser or kill. host - Give a host name and the command will return IP address. Unlike nslookup, the host command will use both /etc/hosts as well as DNS. Example: host domain-name-of-server nslookup - Give a host name and the command will return IP address. Also see Testing your DNS (YoLinux Tutorial) Note that nslookup does not use the /etc/hosts file. inetd/xinetd: Network Socket Listener Daemons: The network listening daemons listen and respond to all network socket connections made on the TCP/IP ports assigned to it. The ports are defined by the file /etc/services. When a connection is made, the listener will attempt to invoke the assigned program and pipe the data to it. This simplified matters by allowing the assigned program to read from stdin instead of making its own sockets connection. The listener hadles the network socket connection. Two network listening and management daemons have been used in Red Hat Linux distributions: inetd: Red Hat 6.x and older xinetd: Red Hat 7.0-9.0, Fedora Core inetd: Configuration file: /etc/inetd.conf Entries in this file consist of a single line made up of the following fields: service socket-type protocol wait user server cmdline service: The name assigned to the service. Matches the name given in the file /etc/services socket-type: stream: connection protocols (TCP) dgram: datagram protocols (UDP) raw rdm seqpacket protocol: Transport protocol name which matches a name in the file /etc/protocols. i.e. udp, icmp, tcp, rpc/udp, rpc/tcp, ip, ipv6 wait: Applies only to datagram protocols (UDP). wait[.max]: One server for the specified port at any time (RPC) nowait[.max]: Continue to listen and launch new services if a new connection is made. (multi-threaded) Max refers to the maximum number of server instances spawned in 60 seconds. (default=40) user[.group]: login id of the user the process is executed under. Often nobody, root or a special restricted id for that service. server: Full path name of the server program to be executed. cmdline: Command line to be passed to the server. This includes argument 0 (argv[0]), that is the command name. This field is empty for internal services. Example of internal TCP services: echo, discard, chargen (character generator), daytime (human readable time), and time (machine readable time). (see RFC) Sample File: /etc/inetd.conf #echo stream tcp nowait root internal #echo dgram udp wait root internal11 of 19 21/02/2012 08:18 a.m.
  12. 12. Linux Network Configuration #echo dgram udp wait root internal ftp stream tcp nowait root /usr/sbin/tcpd in.ftpd -l -a #pop-3 stream tcp nowait root /usr/sbin/tcpd ipop3d #swat stream tcp nowait.400 root /usr/sbin/swat swat A line may be commented out by using a # as the first character in the line. This will turn the service off. The maximum length of a line is 1022 characters. The inet daemon must be restarted to pick up the changes made to the file: /etc/rc.d/init.d/inetd restart For more information see the man pages "inetd" and "inetd.conf". xinetd: Extended Internet Services Daemon: Xinetd has access control machanisms, logging capabilities, the ability to make services available based on time, and can place limits on the number of servers that can be started, redirect services to different ports and network interfaces (NIC) or even to a different server, chroot a service etc... and thus a worthy upgrade from inetd. Use the command chkconfig --list to view all system services and their state. It will also list all network services controlled by xinetd and their respective state under the title "xinetd based services". (Works for xinetd (RH7.0+) but not inetd) The xinetd network daemon uses PAM also called network wrappers which invoke the /etc/hosts.allow and /etc/hosts.deny files. Configuration file: /etc/xinetd.conf which in turn uses configuration files found in the directory /etc/xinetd.d/. To turn a network service on or off: Edit the file /etc/xinetd.d/service-name Set the disable value: disable = yes or disable = no Restart the xinetd process using the signal: SIGUSR1 (kill -SIGUSR1 process-id) - Soft reconfiguration does not terminate existing connections. (Important if you are connected remotely) SIGUSR2 - Hard reconfiguration stops and restarts the xinetd process. (Note: Using the HUP signal will terminate the process.) OR Use the chkconfig command: chkconfig service-name on (or off) This command will also restart the xinetd process to pick up the new configuration. The file contains entries of the form: service service-name { attribute assignment-operator value value ... ... { Where: attribute: disable: yes no type: RPC INTERNAL: UNLISTED: Not found in /etc/rpc or /etc/services id: By default the service id is the same as the service name. socket_type: stream: TCP dgram: UDP raw: Direct IP access seqpacket: service that requires reliable sequential datagram transmission flags: Combination of: REUSE, INTERCEPT, NORETRY, IDONLY, NAMEINARGS, NODELAY, DISABLE, KEEPALIVE, NOLIBWRAP. See the xinetd man page for details. protocol: Transport protocol name which matches a name in the file /etc/protocols. wait: no: multi-threaded yes: single-threaded - One server for the specified port at any time (RPC) user: See file : /etc/passwd group: See file : /etc/group server: Program to execute and recieve data stream from socket. (Fully qualified name - full pathe name of program) server_args: Unlike inetd, arg[0] or the name of the service is not passed. only_from: IP address, factorized address, netmask range, hostname or network name from file /etc/networks. no_access: Deny from ... (inverse of only_from) access_times port: See file /etc/services Also: log_type, log_on_success, log_on_failure (Log options: += PID,HOST,USERID,EXIT,DURATION,ATTEMPT and RECORD), rpc_version, rpc_number, env, passenv, redirect, bind, interface, banner, banner_success, banner_fail, per_source, cps, max_load, groups, enabled, include, includedir, rlimit_as, rlimit_cpu, rlimit_data,12 of 19 21/02/2012 08:18 a.m.
  13. 13. Linux Network Configuration banner_fail, per_source, cps, max_load, groups, enabled, include, includedir, rlimit_as, rlimit_cpu, rlimit_data, rlimit_rss, rlimit_stack. The best source of information is the man page and its many examples. assignment-operator: = +=: add a value to the set of values -=: delete a value from the set of values Then restart the daemon: /etc/rc.d/init.d/xinetd restart Example from man page: Limit telnet sessions to 8 Mbytes of memory and a total 20 CPU seconds for child processes. service telnet { socket_type = stream wait = no nice = 10 user = root server = /usr/etc/in.telnetd rlimit_as = 8M rlimit_cpu = 20 } [Pitfall] Red Hat 7.1 with updates as of 07/06/2001 required that I restart the xinetd services before FTP would work properly even though xinetd had started without failure during the boot sequence. I have no explanation as to why this occurs or how to fix it other than to restart xinetd: /etc/rc.d/init.d/xinetd restart. Man Pages: xinetd xinetd.conf xinetd.log tcpd For more info see: xinetd - Frederic Raynal Controlling Access to Services See RFCs: 862, 863, 864, 867, 868, 1413. man page xinetd, xinetd.conf, xinetd.log Remote commands: rcp, rsh, rlogin, rwho, ... Most of the original Unix remote commands have been superceeded by secure shell equivalents. Instead of telnet, rsh or rlogin, one should use the encrypted connection ssh. telnet - user interface to the TELNET protocol rlogin - remote login rsh - remote shell to execute a command and return results uux - Remote command execution over UUCP rcp - remote file copy uucp - Unix to Unix copy uuxqt - UUCP execution daemon uucico - UUCP file transfer daemon cu - Call up another system (cu is an old legacy command which is reported to not work very well) See the secure shell tutorial for use of ssh, rssh, scp and sftp RWHO: Remote Who daemon - rwhod The "rwho" command is used to display users logged into computers on your LAN. By default, Red Hat Linux has the network interface to the rwhod disabled. Thus if one issues the command " rwho", you will only see who is logged into the system you are logged into and not remote systems on the network. This is a safe approach for internet servers as it reduces the exposure of a service which could be exploited by hackers. If you wish to use rwhod on a local private and firewall protected network, here is how: Allow broacast capabilities. Edit /etc/init.d/rwhod change from: daemon rwhod to: daemon rwhod -b Start service: Set service to start with system boot: chkconfig --level 345 rwhod on Start rwhod service: service rwhod start (or: service rwhod restart) Man pages: rwho: who is logged in on local network machines rwhod: system status server who: show who is logged on to the same system RPC: Remote Procedure Calls (Portmapper)13 of 19 21/02/2012 08:18 a.m.
  14. 14. Linux Network Configuration Portmapper is a network service required to support RPCs. Many services such as NFS (file sharing services) and NIS (Network Information Services) require portmapper. An RPC server makes available a collection of procedures (programs) that a client system may call and then receive the returned results. The list of services available is listed in /etc/rpc on the server. The message communication is in a machine independent form called XDR (External Data Representation format). List RPC services supported: [root]# rpcinfo -p localhost Starting portmap server: /etc/rc.d/init.d/portmap start service portmap start (Red Hat/Fedora Core) Man Pages: portmap - DARPA port to RPC program number mapper rpcinfo - report RPC information pmap_dump - print a list of all registered RPC programs pmap_set - set the list of registered RPC programs /etc/rpc - rpc program number data base PAM: Network Wrappers: Pluggable Authentication Modules for Linux (TCP Wrappers) This system allows or denies network access. One can reject or allow specific IP addresses or subnets to access your system. File: /etc/hosts.allow in.ftpd: This specifically allows the given IP address to ftp to your system. One can also specify an entire domain. i.e. .name- Note the beginning ".". File: /etc/hosts.deny ALL:ALL This generally denies any access. See the pam man page. File: /etc/inetd.conf ftp stream tcp nowait root /usr/sbin/tcpd in.ftpd -l -a The inet daemon accepts the incoming network stream and assigns it to the PAM TCP wrapper, /usr/sbin/tcpd, which accepts or denies the network connection as defined by /etc/hosts.allow and /etc/hosts.deny and then passes it along to ftp. This is logged to /var/log/secure Advanced PAM: More specific access can be assigned and controlled by controlling the level of authentication required for access. Files reflect the inet service name. Rules and modules are stacked to achieve the level of security desired. See the files in /etc/pam.d/... (some systems use /etc/pam.conf) The format: service type control module-path module-arguments auth - (type) Password is required for the user nullok - Null or non-existatant password is acceptable shadow - encrypted passwords kept in /etc/shadow account - (type) Verifies password. Can track and force password changes. password - (type) Controls password update retry=3 - Sets the number of login attempts minlen=8 - Set minimum length of password session - (type) Controls monitoring Modules: /lib/security/ - password database module /lib/security/ - /lib/security/ - checks is password is crackable /lib/security/ After re-configuration, restart the inet daemon: killall -HUP inetd For more info see: Wietses Papers Pluggable Authentication Modules for Linux (PAM) Home Page ICMP:14 of 19 21/02/2012 08:18 a.m.
  15. 15. Linux Network Configuration ICMP is the network protocol used by the ping and traceroute commands. ICMP redirect packets are sent from the router to the host to inform the host of a better route. To enable ICMP redirect, add the following line to /etc/sysctl.conf : net.ipv4.conf.all.accept_redirects = 1 Add the following to the file: /etc/rc.d/rc.local for f in /proc/sys/net/ipv4/conf/*/accept_redirects do echo 1 > $f done Command to view Kernel IP routing cache: /sbin/route -Cn NOTE: This may leave you vulnerable to hackers as attackers may alter your routes. Blocking ICMP and look invisible to ping: 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 OR drop all incomming pings: echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_all This is sometimes necessary to look invisible to DOS (Denial Of Service) attackers who use ping to watch your machine and launch an attack when its pressence is detected Network Monitoring Tools: tcpdump - dump traffic on a network. See discussion below. Command line option Description -c Exit after receiving count packets. -C Specify size of output dump files. -i Specify interface if multiple exist. Lowest used by default. i.e. eth0 -w file-name Write the raw packets to file rather than parsing and printing them out. They can later be printed with the -r option. -n Improve speed by not performing DNS lookups. Report IP addresses. -t Dont print a timestamp on each dump line. Filter expressions: primitive Description host host-name If host has multiple IPs, all will be checked. net network-number Network number. net network-number mask mask Network number and netmask specified. port port-number Port number specified. tcp Sniff TCP packets. udp Sniff UDP packets. icmp Sniff icmp packets. Examples: tcpdump tcp port 80 and host server-1 tcpdump ip host server-1 and not server-2 iptraf - Interactive Colorful IP LAN Monitor nmap - Network exploration tool and security scanner List pingable nodes on network: nmap -sP Scans network for IP addresses to using ping. Ethereal - Network protocol analyzer. Examine data from a live network. RPMs required: ethereal-0.8.15-2.i386.rpm - Red Hat 7.1 Powertools CD RPM ucd-snmp-4.2-12.i386.rpm - Red Hat 7.1 binary CD 1 ucd-snmp-utils-4.2-12.i386.rpm - Red Hat 7.1 binary CD 1 Also: gtk+, glib, glibc, XFree86-libs-4.0.3-5 (base install) There is an error in the ethereal package because it does not show the snmp libraries as a dependancies, but you can deduce this from the errors that you get if the ucd-snmp libraries are not installed. EtherApe - Graphical network monitor for Unix modeled after etherman. This is a great network discovery program with cool graphics. (Red Hat Powertools CD 7.1) Gkrellm - Network and system monitor. Good for monitoring your workstation. (Red Hat Powertools CD) IPTraf - ncurses-based IP LAN monitor. (Red Hat Powertools CD)15 of 19 21/02/2012 08:18 a.m.
  16. 16. Linux Network Configuration IPTraf - ncurses-based IP LAN monitor. (Red Hat Powertools CD) Cheops - Network discovery, location, diagnosis and management. Cheops can identify all of the computers that are on your network, their IP address, their DNS name, the operating system they are running. Cheops can run a port scan on any system on your network. (Red Hat Powertools CD) ntop - Shows network usage in a way similar to what top does for processes. Monitors how much data is being sent and received on your network. (Red Hat Powertools CD) MRTG - Multi Router Traffic Grapher - Monitor network traffic load using SNMP and generate an HTML/GIF report. (See sample output) dnsad - IP traffic capture. Export to Cisco Netflow for network analysis reporting. Big Brother - Monitoring ans services availablility. - Network Management using SNMP. Nagios - host, service and network monitoring Angel network monitor Using tcpdump to monitor the network: [root]# ifconfig eth0 promisc - Put nic into promiscuous mode to sniff traffic. [root]# tcpdump -n host not XXX.XXX.XXX.XXX | more - Sniff net but ignore IP which is your remote session. [root]# ifconfig eth0 -promisc - Pull nic out of promiscuous mode. Network Intrusion and Hacker Detection Systems: SNORT: Monitor the network, performing real-time traffic analysis and packet logging on IP networks for the detection of an attack or probe. InterSect Alliance - Intrusiuon analysis. Identifies malicious or unauthorized access attempts. ARP: Address Resolution Protocol Ethernet hosts use the Address Resolution Protocol (ARP) to convert a 32-bit internet IP addresses into a 48-bit Ethernet MAC address used by network hardware. (See: RFC 826) ARP broadcasts are sent to all hosts on the subnet by the data transmitting host to see who replies. The broadcast is ignored by all except the intended receiver which recognizes the IP address as its own. The MAC addresses are remembered (APR cache) for future network communications. Computers on the subnet typically keep a cache of ARP responses. ARP broadcasts are passed on by hubs and switches but are blocked by routers. Reverse ARP (See: RFC 903) is a bootstrap protocol which allows a client to broadcast requesting a server to reply with its IP address. View ARP tables: Shows other systems on your network (including IP address conflicts): /sbin/arp -a Show ARP table Linux style: /sbin/arp -e List ARP table: cat /proc/net/arp Note that the use of a switch instead of a hub will limit your view of other hosts. Typically all you will see in the arp table is your router or gateway. Set/Configure ARP tables: Add a hosts IP address: /sbin/arp -s hostname XX:XX:XX:XX:XX:XX pub Delete a host from the table: /sbin/arp -d hostname This can be used to remove a duplicate IP or force a new interface to provide info. Man pages: arp (8) man page - manipulate the system ARP cache arpwatch (8) man page - keep track of ethernet/ip address pairings arpsnmp (8) man page - keep track of ethernet/ip address pairings. Reads information generated by snmpwalk arping (8) man page - send ARP REQUEST to a neighbor host Print ARP reply (similar to arp -a): arping ip (8) man page - show / manipulate routing, devices, policy routing and tunnels View ARP table: ip neighbor ARP is something that simply works. No Linux system configuration is necessary. Its all part of the ethernet and IP protocol. The aforementioned information is just part of the Linux culture of full visibility into what is going on. Configuring Linux For Network Multicast: Regular network exchanges of data are peer to peer unicast transactions. An HTTP request to a web server (TCP/IP), email SNMP (TCP/IP), DNS (UDP), FTP (TCP/IP), ... are all peer to peer unicast transactions. If one wants to transmit a video, audio or data stream to multiple nodes with one transmission stream instead of multiple individual peer to peer connections, one for each node, one may use multicasting to reduce network load. Note that multicast and a network broadcast are different. Multicast messages are only "heard" by the nodes on the network that have "joined the multicast group" which are those that are interested in the information. The Linux kernel is Level-2 Multicast-Compliant. It meets all requirements to send, receive and act as a router for multicast datagrams. For a process to receive multicast datagrams it has to request the kernel to join the multicast group and bind the port receiving the datagrams. When a process is no longer interested in the multicast group, a request is made to the kernel to leave the group. It is the kernel/host which joins the multicast group and not the process. Kernel configuration requires "CONFIG_IP_MULTICAST=y". In order for the Linux kernel to support multicast routing, set the following in the kernel config: CONFIG_IP_MULTICAST=y CONFIG_IP_ROUTER=y CONFIG_IP_MROUTE=y CONFIG_NET_IPIP=y16 of 19 21/02/2012 08:18 a.m.