Why sendmail Is So Complex?

In its simplest role, that of transporting mail from a user on one machine to another user on the same machine, sendmail is almost trivial. All vendors supply a sendmail (and a configuration file) that will accomplish this. But as your needs increase, the job of sendmail becomes more complicated, and its configuration file becomes more complex. On hosts that are connected to the Internet, for example, sendmail should use the Domain Name System (DNS) to translate hostnames into network addresses. Machines with UUCP connections, on the other hand, need to have sendmail run the uux program.

The sendmail program needs to transport mail between a wide variety of machines. Consequently, its configuration file is designed to be very flexible. This concept allows a single binary to be distributed to many machines, where the configuration file can be customized to suit particular needs. This configurability contributes to making sendmail complex.

When mail needs to be delivered to a particular user, for example, the sendmail program decides on the appropriate delivery method based on its configuration file. The decision process might include the following steps:

·         If the recipient receives mail on the same machine as the sender, sendmail delivers the mail using the /usr/sbin/mail.local program.

·         If the recipient’s machine is connected to the sending machine using UUCP, it uses uux to send the mail message.

·         If the recipient’s machine is on the Internet, the sending machine transports the mail using SMTP.

·         Otherwise, the mail message might need to be transported over another network (such as Bitnet) or possibly rejected.

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Defination of MUA, MTA & MSA (sendmail)

MUA

A mail user agent (MUA) is any of the many programs that users run to read, reply to, compose, and dispose of email.

Examples of MUAs also exist for PCs. Eudora and Claris-Works are two standalone MUAs. Netscape and Explorer are web browsers that canalso act as MUAs. Thunderbird is an open source MUA from the folks at Mozilla. Many MUAs can exist on a single machine. MUAs sometimes perform limited mail transport, but this is usually a very complex task for which they are not suited.

MTA

A mail transfer agent (MTA) is a highly specialized program that delivers mail and transports it between machines, like the post office does. Usually, there is only one MTA on a machine. The sendmail program is an MTA.

MSA

sendmail also recognizes the role of a mail submission agent (MSA), as defined in RFC2476. MTAs are not supposed to alter an email’s text, except to add Received:, Return-Path:, and other required headers. Email submitted by an MUA might require more modification than is legal for an MTA to perform, so the new role of an MSA was created. An MSA accepts messages from an MUA, and has the legal right to heavily add to, subtract from, and screen or alter all such email. An MSA, for example, can ensure that all hostnames are fully qualified, and that headers, such as Date:, are always included.

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Mail Server Basic

Imagine yourself withpen and paper, writing a letter to a friend far away. You finish the letter and sign it, reflect on what you’ve written, then tuck the letter into an envelope. You put your friend’s address on the front, your return address in the lefthand corner, and a stamp in the righthand corner, and the letter is ready for mailing. Electronic mail (email for short) is prepared in much the same way, but a computer is used instead of pen and paper.

The post office transports real letters in real envelopes, whereas sendmail transports electronic letters in electronic envelopes. If your friend (the recipient) is in the same neighborhood (on the same machine), only a single post office (sendmail running locally) is involved. If your friend is in a distant location, the mail message will be forwarded from the local post office (sendmail running locally) to a distant one (sendmail running remotely) for delivery. Although sendmail is similar to a post office in many ways, it is superior in others:

·         Delivery typically takes seconds rather than days.

·         Address changes (forwarding) take effect immediately, and mail can be forwarded anywhere in the world.

·         Host addresses are looked up dynamically. Therefore, machines can be moved or renamed and email delivery will still succeed.

·         Mail can be delivered through programs that access other networks (such as Unix to Unix Communication Protocol [UUCP] and Bitnet). This would be like the post office using United Parcel Service to deliver an overnight letter.

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Configuring a DHCP Client

To configure a DHCP client manually, modify the /etc/sysconfig/network file to enable networking and the configuration file for each network device in the  /etc/sysconfig/network-scripts directory. In this directory, each device should have a configuration file named ifcfg-eth0, where eth0 is the network device name.

The /etc/sysconfig/network-scripts/ifcfg-eth0 file should contain the following lines:

DEVICE=eth0

BOOTPROTO=dhcp

ONBOOT=yes

A configuration file is needed for each device to be configured to use DHCP.

Other options for the network script includes:

• DHCP_HOSTNAME — Only use this option if the DHCP server requires the client to specify a hostname before receiving an IP address. (The DHCP server daemon in Red Hat Enterprise Linux  does not support this feature.)
• PEERDNS=<answer> , where <answer> is one of the following:
• yes — Modify /etc/resolv.conf with information from the server. If using DHCP, then yes is the default.
• no — Do not modify /etc/resolv.conf.

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Starting and Stopping the DHCP Server

When the DHCP server is started for the first time, it fails unless the dhcpd.leases file exists. Use the command touch /var/lib/dhcpd/dhcpd.leases to create the file if it does not exist.

If the same server is also running BIND as a DNS server, this step is not necessary, as starting the named service automatically checks for a dhcpd.leases file.

To start the DHCP service, use the command /sbin/service dhcpd start. To stop the DHCP server, use the command /sbin/service dhcpd stop.

By default, the DHCP service does not start at boot time.  To configure the daemon to start automatically at boot time.

If more than one network interface is attached to the system, but the DHCP server should only be started on one of the interfaces, configure the DHCP server to start only on that device. In /etc/sysconfig/dhcpd, add the name of the interface to the list of DHCPDARGS:

# Command line options here

DHCPDARGS=eth0

This is useful for a firewall machine with two network cards. One network card can be configured as a DHCP client to retrieve an IP address to the Internet. The other network card can be used as a DHCP server for the internal network behind the firewall. Specifying only the network card connected to the internal network makes the system more secure because users can not connect to the daemon via the Internet.

Other command line options that can be specified in /etc/sysconfig/dhcpd include:

• -p <portnum> — Specifies the UDP port number on which dhcpd should listen. The default is port 67. The DHCP server transmits responses to the DHCP clients at a port number one greater than the UDP port specified. For example, if the default port 67 is used, the server listens on port 67 for requests and responses to the client on port 68. If a port is specified here and the DHCP relay agent is used, the same port on which the DHCP relay agent should listen must be specified.
• -f — Runs the daemon as a foreground process. This is mostly used for debugging.
• -d — Logs the DHCP server daemon to the standard error descriptor. This is mostly used for debugging. If this is not specified, the log is written to /var/log/messages.
• -cf <filename> — Specifies the location of the configuration file. The default location is /etc/dhcp/dhcpd.conf.
• -lf <filename> — Specifies the location of the lease database file. If a lease database file already exists, it is very important that the same file be used every time the DHCP server is started. It is strongly recommended that this option only be used for debugging purposes on non-production machines. The default location is /var/lib/dhcpd/dhcpd.leases.
• -q — Do not print the entire copyright message when starting the daemon.

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DHCP Lease Database

On the DHCP server, the file /var/lib/dhcpd/dhcpd.leases stores the DHCP client lease database. Do not change this file. DHCP lease information for each recently assigned IP address is automatically stored in the lease database. The information includes the length of the lease, to whom the IP address has been assigned, the start and end dates for the lease, and the MAC address of the network interface card that was used to retrieve the lease.

All times in the lease database are in Coordinated Universal Time (UTC), not local time.

The lease database is recreated from time to time so that it is not too large. First, all known leases are saved in a temporary lease database. The dhcpd.leases file is renamed dhcpd.leases~ and the temporary lease database is written to dhcpd.leases.

The DHCP daemon could be killed or the system could crash after the lease database has been renamed to the backup file but before the new file has been written. If this happens, the dhcpd.leases file does not exist, but it is required to start the service. Do not create a new lease file. If you do, all old leases are lost which causes many problems. The correct solution is to rename the dhcpd.leases~ backup file to dhcpd.leases and then start the daemon.

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Dhcp configuration file Example

Example Subnet Declaration

subnet 192.168.1.0 netmask 255.255.255.0 {

        option routers                  192.168.1.254;

        option subnet-mask              255.255.255.0;

        option domain-search              "example.com";

        option domain-name-servers       192.168.1.1;

        option time-offset              -18000;     # Eastern Standard Time

        range 192.168.1.10 192.168.1.100;

}

To configure a DHCP server that leases a dynamic IP address to a system within a subnet, modify “Range Parameter” with your values. It declares a default lease time, maximum lease time, and network configuration values for the clients. This example assigns IP addresses in the range192.168.1.10 and 192.168.1.100 to client systems.

Range Parameter

default-lease-time 600;

max-lease-time 7200;

option subnet-mask 255.255.255.0;

option broadcast-address 192.168.1.255;

option routers 192.168.1.254;

option domain-name-servers 192.168.1.1, 192.168.1.2;

option domain-search "example.com";

subnet 192.168.1.0 netmask 255.255.255.0 {

   range 192.168.1.10 192.168.1.100;

}

 To assign an IP address to a client based on the MAC address of the Network interface card, use the hardware ethernet parameter within a hostdeclaration. As demonstrated in "Static IP Address using DHCP", the host apex declaration specifies that the network interface card with the MAC address 00:A0:78:8E:9E:AA always receives the IP address 192.168.1.4.

Note that the optional parameter host-name can also be used to assign a host name to the client.

Static IP Address using DHCP

host apex {

   option host-name "apex.example.com";

   hardware ethernet 00:A0:78:8E:9E:AA;

   fixed-address 192.168.1.4;

}

All subnets that share the same physical network should be declared within a shared-network declaration as shown in "Shared-network Declaration". Parameters within the shared-network, but outside the enclosed subnet declarations, are considered to be global parameters. The name of the shared-network must be a descriptive title for the network, such as using the title 'test-lab' to describe all the subnets in a test lab environment.

Shared-network Declaration

shared-network name {

    option domain-search              "test.redhat.com";

    option domain-name-servers      ns1.redhat.com, ns2.redhat.com;

    option routers                  192.168.0.254;

    more parameters for EXAMPLE shared-network

    subnet 192.168.1.0 netmask 255.255.252.0 {

        parameters for subnet

        range 192.168.1.1 192.168.1.254;

    }

    subnet 192.168.2.0 netmask 255.255.252.0 {

        parameters for subnet

        range 192.168.2.1 192.168.2.254;

    }

}

As demonstrated in "Group Declaration", the group declaration is used to apply global parameters to a group of declarations. For example, shared networks, subnets, and hosts can be grouped.

Group Declaration

group {

   option routers                  192.168.1.254;

   option subnet-mask              255.255.255.0;

   option domain-search              "example.com";

   option domain-name-servers       192.168.1.1;

   option time-offset              -18000;     # Eastern Standard Time

   host apex {

      option host-name "apex.example.com";

      hardware ethernet 00:A0:78:8E:9E:AA;

      fixed-address 192.168.1.4;

   }

   host raleigh {

      option host-name "raleigh.example.com";

      hardware ethernet 00:A1:DD:74:C3:F2;

      fixed-address 192.168.1.6;

   }

}

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Dhcp Configuration file Logic

The first step in configuring a DHCP server is to create the configuration file that stores the network information for the clients. Use this file to declare options and global options for client systems.

The configuration file can contain extra tabs or blank lines for easier formatting. Keywords are case-insensitive and lines beginning with a hash sign (#) are considered comments.

There are two types of statements in the configuration file:

• Parameters — State how to perform a task, whether to perform a task, or what network configuration options to send to the client.
• Declarations — Describe the topology of the network, describe the clients, provide addresses for the clients, or apply a group of parameters to a group of declarations.

The parameters that start with the keyword option are reffered to as options. These options control DHCP options; whereas, parameters configure values that are not optional or control how the DHCP server behaves.

Parameters (including options) declared before a section enclosed in curly brackets ({ }) are considered global parameters. Global parameters apply to all the sections below it.


"If the configuration file is changed, the changes do not take effect until the DHCP daemon is restarted with the command service dhcpd restart."

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How to install DHCP?

The dhcp package contains an ISC DHCP server. First, install the package as the superuser:

# yum install dhcp

Installing the dhcp package creates a file, /etc/dhcp/dhcpd.conf, which is merely an empty configuration file:

# cat /etc/dhcp/dhcpd.conf

#

# DHCP Server Configuration file.

#   see /usr/share/doc/dhcp*/dhcpd.conf.sample

The sample configuration file can be found at 




/usr/share/doc/dhcp <version>/dhcpd.conf.sample.

You should use this file to help you configure/etc/dhcp/dhcpd.conf, which is explained in detail below.

DHCP also uses the file /var/lib/dhcpd/dhcpd.leases to store the client lease database.

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What is dhcp ?

Dynamic Host Configuration Protocol (DHCP) is a network protocol that automatically assigns TCP/IP information to client machines. Each DHCP client connects to the centrally located DHCP server, which returns that client's network configuration (including the IP address, gateway, and DNS servers).

Why use DHCP?

DHCP is useful for automatic configuration of client network interfaces. When configuring the client system, the administrator chooses DHCP instead of specifying an IP address, netmask, gateway, or DNS servers. The client retrieves this information from the DHCP server. DHCP is also useful if an administrator wants to change the IP addresses of a large number of systems. Instead of reconfiguring all the systems, he can just edit one DHCP configuration on the server for the new set of IP addresses. If the DNS servers for an organization changes, the changes are made on the DHCP server, not on the DHCP clients. When the administrator restarts the network or reboots the clients, the changes will go into effect. If an organization has a functional DHCP server properly connected to a network, laptops and other mobile computer users can move these devices from office to office.

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ls command

About ls

Lists the contents of a directory.

Syntax

ls [Option] [pathnames]

Option

-a

Shows you all files, even files that are hidden (these files begin with a dot.)

-A

List all files including the hidden files. However, does not display the working directory (.) or the parent directory (..).

-b

Force printing of non-printable characters to be in octal \ddd notation.

-c

Use time of last modification of the i-node (file created, mode changed, and so forth) for sorting (-t) or printing (-l or -n).

-C

Multi-column output with entries sorted down the columns. Generally this is the default option.

-d     

If an argument is a directory it only lists its name not its contents.

-f      

Force each argument to be interpreted as a directory and list the name found in each slot. This option turns off -l, -t, -s, and -r, and turns on -a; the order is the order in which entries appear in the directory.

-F

Mark directories with a trailing slash (/), doors with a trailing greater-than sign (>), executable files with a trailing asterisk (*), FIFOs with a trailing vertical bar (|), symbolic links with a trailing at-sign (@), and AF_Unix address family sockets with a trailing equals sign (=).

-g

Same as -l except the owner is not printed.

-I

For each file, print the i-node number in the first column of the report.

-l

Shows you huge amounts of information (permissions, owners, size, and when last modified.)

-L

If an argument is a symbolic link, list the file or directory the link references rather than the link itself.

-m

Stream output format; files are listed across the page, separated by commas.

-n

The same as -l, except that the owner's UID and group's GID numbers are printed, rather than the associated character strings.

-o

The same as -l, except that the group is not printed.

-p

Displays a slash ( / ) in front of all directories.

-q

Force printing of non-printable characters in file names as the character question mark (?).

-r

Reverses the order of how the files are displayed.

-R

Includes the contents of subdirectories.

-s

Give size in blocks, including indirect blocks, for each entry.

-t

Shows you the files in modification time.

-u

Use time of last access instead of last modification for sorting (with the -t option) or printing (with the -l option).

-x

Displays files in columns.

-1

Print one entry per line of output.

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