Howto Setup Linux as router for ppp dial up (PSTN/ISDN) connections

Posted on in Categories Linux, Networking, RedHat/Fedora Linux, Ubuntu Linux last updated November 23, 2004

This step by step howto will help you to setup Linux as router only in two (2) minutes. I’m assuming that your setup is as follows:

(A) You are using RedHat Linux, FC linux/Debian Linux (this should work with ALL Other distros as well 😀 )

(B) ppp0 is internet interface (connected to Modem for example) and eth0 connected to your internal lan (connected to your HUB/Switch for example)

My Linux  --> eth0
Box      --> ppp0 (comes up when you dial up into ISP)

Step # 1 Turn on ip forwarding in kernel

1) Open linux kernel configuration file:
# vi /etc/sysctl.conf

2) Add/modify following line:
net.ipv4.ip_forward = 1

Step # 2 Restart network
# /etc/init.d/network restart
OR
# service network restart

(Note first dial to ISP using wvdial or similar program )

Step # 3 Setup IP forwarding and Masquerading (to act as router), you need to use NAT option of iptables as follows:
# iptables --table nat --append POSTROUTING --out-interface ppp0 -j MASQUERADE
# iptables --append FORWARD --in-interface eth0 -j ACCEPT

Step # 4 You are done! Test it with ping or dig:
# ping your-isp.com
# dig www.nixcraft.com
# dig yahoo.com

Step # 5 Point all client to your eth0 IP address as Router/Gateway.

Step # 6 Put code described in step # 3 to script and call it from /etc/rc.local

How to setup Linux as a router for DSL, T1 line etc

Posted on in Categories Howto, Iptables, Linux, Networking, RedHat/Fedora Linux, Tips, Ubuntu Linux last updated November 23, 2004

There are a few ways to set up a Linux machine as route. Here is a relatively straight forward and common method. This method requires that the system use iptables for Network Address Translation (NAT).

This step by step small howto will help you to setup Linux router only in 2 minutes.

Configuration steps

=> First enable packet forwarding
=> Next setup Network Address Translation using IPTABLES MASQUERADE targets
=> Save the changes
=> Verify everything is working

I’m assuming that your setup is as follows:
A) You are using any Linux distro

B) eth0 is internet interface (connected to router for example) and eth1 connected to your internal lan (connected to your HUB/Switch for example).

My Linux   eth0  --> Internet
box       eth1  --> Lan

Step # 1 Turn on ip forwarding in kernel

1) Open linux kernel configuration file (you must be a root user or use su – command to become a root user):
# vi /etc/sysctl.conf

2) Add/modify following line:
net.ipv4.ip_forward = 1

Step # 2 Restart network
# /etc/init.d/network restartOR# service network restart

Step # 3 Setup IP forwarding and Masquerading (to act as router), you need to use NAT option of iptables as follows (add following rules to your iptables shell script) :
# iptables --table nat --append POSTROUTING --out-interface eth0 -j MASQUERADE
# iptables --append FORWARD --in-interface eth1 -j ACCEPT

Step # 4 You are done! Test it with ping or dig:
# ping your-isp.com
# dig yahoo.com

Step # 5 Point all desktop client to your eth1 IP address as Router/Gateway. Or use DHCP to distribute this information (recommended)

Step # 6 Put code described in step # 3 to script and call it from /etc/rc.local file.

How to: Troubleshoot UNIX / Linux BIND DNS server problems

Posted on in Categories BIND Dns, CentOS, Debian Linux, FreeBSD, Gentoo Linux, GNU/Open source, Howto, Linux, OpenBSD, RedHat/Fedora Linux, Suse Linux, Sys admin, Troubleshooting, Tuning, UNIX last updated November 19, 2004

BIND is the Berkeley Internet Name Domain, DNS server. It is wildly used on UNIX and Linux like oses. You can use following tools to troubleshoot bind related problems under UNIX or Linux oses.

Task: Port 53 open and listing requests

By default BIND listen DNS queries on port 53. So make sure port 53 is open and listing user requests. by running any one of the following tests. See if you can telnet to port 53 from remote computer:
$ telnet remote-server-ip 53
OR
telnet ns1.nixcraft.org domain
Output:

Trying 192.168.0.5...
Connected to ns1.nixcraft.org.
Escape character is '^]'.

If you cannot connect make sure firewall is not blocking your requests. Next use netstat command to list open and listing port 53 on server itself:
$ netstat -tulpn | grep :53
OR
# netstat -atve
Output:

Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       User       Inode
tcp        0      0 ns1.nixcraft.org:domain *:*                     LISTEN      named      10386
tcp        0      0 rhx.test.com:domain     *:*                     LISTEN      named      10384
tcp        0      0 *:ssh                   *:*                     LISTEN      root       1785
tcp        0      0 rhx.test.com:rndc       *:*                     LISTEN      named      10388
tcp        0      0 rhx.test.com:smtp       *:*                     LISTEN      root       1873
tcp        0      0 ns1.nixcraft.org:ssh    w2k.nixcraft.org:1057   ESTABLISHED root       10501
tcp        0      0 rhx.test.com:32773      rhx.test.com:domain     TIME_WAIT   root       0
tcp        0      0 ns1.nixcraft.org:32775  ns1.nixcraft.org:domain TIME_WAIT   root       0
tcp        0      0 rhx.test.com:32774      rhx.test.com:domain     TIME_WAIT   root       0

Make sure iptables firewall is not blocking request on server:
# iptables -L -n
OR
# iptables -L -n | less
Make sure named is running:
# /etc/init.d/named status
If not start named:
# chkconfig named on
# service named start

Task: Use log files

You can use log files after starting/restarting bind to see error messages:
# tail –f /var/log/message
Output:

Nov 17 16:50:25 rhx named[3539]: listening on IPv4 interface lo, 127.0.0.1#53
Nov 17 16:50:25 rhx named[3539]: listening on IPv4 interface eth0, 192.168.0.5#53
Nov 17 16:50:25 rhx named[3539]: command channel listening on 127.0.0.1#953
Nov 17 16:50:25 rhx named[3539]: zone 0.0.127.in-addr.arpa/IN: loaded serial 1997022700
Nov 17 16:50:25 rhx named[3539]: nixcraft.org.rev:1: no TTL specified; using SOA MINTTL instead
Nov 17 16:50:25 rhx named[3539]: zone 0.168.192.in-addr.arpa/IN: loaded serial 12
Nov 17 16:50:25 rhx named[3539]: zone localhost/IN: loaded serial 42
Nov 17 16:50:25 rhx named[3539]: zone nixcraft.org/IN: loaded serial 12
Nov 17 16:50:25 rhx named[3539]: running

Task: Check zone file for errors

You can check zone file syntax and /etc/named.conf file using following utilities. named-checkconf command is named (BIND) configuration file syntax checking tool.
# named-checkconf /etc/named.conf
Output:

/etc/named.conf:32: missing ';' before 'zone'

Plesse note that if named-checkconf did not find any errors it will not display in output on screen.

Check zone file syntax for errors. named-checkzone is zone file validity checking tool. named-checkzone checks the syntax and integrity of a zone file. It performs the same checks as named does when loading a zone. This makes named checkzone useful for checking zone files before configuring them into a name server.
# named-checkzone localhost /var/named/localhost.zone
OR
#named-checkzone nixcraft.org /var/named/nixcraft.org.zone
Output:

zone nixcraft.org/IN: loaded serial 12
OK

Task: Testing BIND/DNS with utilities

You can use host and dig utilties to test your bind configuration.

  • host: host is a simple utility for performing DNS lookups. It is normally used to convert names to IP addresses and vice versa.
  • dig: dig (domain information groper) is a flexible tool for interrogating DNS name servers. It performs DNS lookups and displays the answers that are returned from the name server(s) that were queried. Most DNS administrators use dig to troubleshoot DNS problems because of its flexibility, ease of use and clarity of output. Other lookup tools tend to have less functionality than dig.

List IP address associated with host names:
# host nixcraft.org
OR
# host www
Output:

www.nixcraft.org has address 192.168.0.6

Perform a zone transfer for zone name using -l option:
# host -l nixcraft.org

nixcraft.org SOA ns1.nixcraft.org. admin.nixcraft.org. 12 10800 900 604800 86400
nixcraft.org name server ns1.nixcraft.org.
nixcraft.org mail is handled by 10 mail.nixcraft.org.
nixcraft.org has address 192.168.0.5
gw.nixcraft.org has address 192.168.0.254
mail.nixcraft.org has address 192.168.0.7
ns1.nixcraft.org has address 192.168.0.5
w2k.nixcraft.org has address 192.168.0.1
www.nixcraft.org has address 192.168.0.6
nixcraft.org SOA ns1.nixcraft.org. admin.nixcraft.org. 12 10800 900 604800 86400

Other examples
# dig mail.nixcraft.org
# dig 192.168.0.5

List of LINUX troubleshooting commands/tools part # 1

Posted on in Categories News last updated November 18, 2004

Generic troubleshooting tips

1) Use tail –f to watch log file in real time, advantage is simple you can spot error or warning message in real time.
tail –f /path/to/log/file
Example(s):
# tail –f /var/log/maillog

2) Use telnet command to see if you get response or not. Sometime you will also see some informative message:
telnet ip port

Example(s):
# telnet localhost 53
# telnet localhost 25

3) Make sure you can see PID of your service.
pidof service-name
cat /var/run/service.pid

Example(s):
# pidof sshd
# cat /var/run/sshd.pid

4) You need to make sure that your DNS server or third party DNS server (ISP) is accessible. This is an important step, as many network services depend upon DNS; especially sendmail/postfix or Squid etc for example. Run dig or nslookup. No timeout should occurred.

# dig your-domain.com
# nslookup gw.isp.com
# more /etc/resolv.conf

5) For networking troubleshooting, make sure your ip address configuration is right, gateway, routine, hostname etc all configured. Here is list of tools on RedHat Linux to verify or modify information:

Hostname verification or setup tools

  • hostname : To get hostname of server.
  • hostname –s : To get FQDN hostname of server
  • more /etc/sysconfig/network : To setup hostname and networking can enabled or disabled.
  • dnsdomainname : List or setup domainname.
  • more /etc/hosts :Make sure at least localhost entry do exist.

Ethernet configuration tools

  • ifconfig : To see running network card information.
  • ifconfig eth0 up|down : To enable|disable network interface
  • service network reload|restart|stop|start : To reload (after changed made in ip config file)|restart|stop|start network interface with all properties.
  • route|netstat –rn : To print routing table
  • ping ip-address : To see if host is alive or dead
  • more /etc/modules.conf : To see your network card configuration alias for eth0 exists or not.
  • lsmod : To list loaded modules (read as drivers), here you need to see that eth0 module is loaded or not, if not loaded then use insmod to insert (load) driver.
  • dhclient : Dynamic Host Configuration Protocol Client, run this if your Ethernet card is not getting ip from DHCP box on startup; this command does by default shows useful information.

To see if service blocked because of access control

  • iptables –n –L : To list all iptable rules; useful to see if firewall blocks service or not.
  • service iptables stop|start : To start|stop iptables
  • more /etc/xinetd.conf

OR

  • more /etc/xinetd.conf/SERVICENAME = To list configuration of xinetd server. Again useful to see if firewall xinetd based security blocks service or not (xinetd includes host-based and time-based access control)
  • more /etc/hosts.allow : To see list of hosts allowed to access service.
  • more /etc/hosts.deny : To see list of hosts NOT allowed to access service. NOTE first TCP wrappers (hosts.allow|hosts.deny) checked and then xinetd-based access control checked.
  • more /etc/path/to/application.conf : See your application configuration file for access control. For example smb.conf and many other applications/services got own access control list in application. You need to check that as well.

Read man pages of all above command for more details on syntax and usage.