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Engineering LibreTexts

10-B.2.1: Server Roles in Linux - Continued

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  • Email

    The Simple Mail Transfer Protocol (SMTP) is a communication protocol for electronic mail transmission. As an Internet standard, SMTP was first defined in 1982 by RFC 821, and updated in 2008 by RFC 5321 to Extended SMTP additions, which is the protocol variety in widespread use today. Mail servers and other message transfer agents use SMTP to send and receive mail messages. SMTP servers commonly use the Transmission Control Protocol on port number 25.

    File/Print Services

    File servers may also be categorized by the method of access: Internet file servers are frequently accessed by File Transfer Protocol or by HTTP (but are different from web servers that often provide dynamic web content in addition to static files). Servers on a LAN are usually accessed by SMB/CIFS protocol (Windows and Unix-like) or NFS protocol (Unix-like systems).

    Whereas, a print server is a type of server that connects printers to client computers over a network. It accepts print jobs from the computers and sends the jobs to the appropriate printers, queuing the jobs locally to accommodate the fact that work may arrive more quickly than the printer can actually handle. Ancillary functions include the ability to inspect the queue of jobs to be processed, the ability to reorder or delete waiting print jobs, or the ability to do various kinds of accounting (such as counting pages, which may involve reading data generated by the printer(s)). Print servers may be used to enforce administration policies, such as color printing quotas, user/department authentication, or watermarking printed documents.

    Load Balancing Services

    In computing, load balancing refers to the process of distributing a set of tasks over a set of resources (computing units), with the aim of making their overall processing more efficient. However, in our usage we are referring more to network load balancing (commonly referred to as dual-WAN routing or multihoming) as the ability to balance traffic across two or more WAN links without using complex routing protocols like BGP.

    This capability balances its network sessions like Web, email, etc. over multiple connections in order to spread out the amount of bandwidth used by each LAN user, thus increasing the total amount of bandwidth available. For example, a user has a single WAN connection to the Internet operating at 1.5Mbit/s. They wish to add a second broadband (cable, DSL, wireless, etc.) connection operating at 2.5Mbit/s. This would provide them with a total of 4Mbit/s of bandwidth when balancing sessions.

    Logging Services

    Most Linux systems already centralize logs using a syslog daemon. Linux has a special directory for storing logs called /var/log. This directory contains logs from the OS itself, services, and various applications running on the system. Syslog is actually a standard for creating and transmitting logs. The use of "syslog" can refer to either:

    • The Linux syslog service, which listens for events using a network that system applications write to. Syslog can write its messages to a local file, usually in /var/log, or forward those messages to a remote log server. There are different syslog implementations including rsyslogd and syslog-ng.
    • The syslog protocol, which is defined in RFC 5424, is a transport protocol that specifies how to transmit logs over a network. The standard also defines a data format of how messages are structured. The default network port is 514 for plaintext messages and port 6514 for encrypted messages.
    • A syslog message, which is any log formatted in the syslog message format. A syslog message consists of a standardized header and message containing the log’s contents.

    Monitoring Services

    Linux, and the Linux community, provide a significant number of tools to monitor specific aspects of the Linux system.

    Some of the tools include:

    • The Linux top command. There are many open source variants of top.
    • Network monitoring tools - ethertool, tcpdump, MRTG, traceroute to name just a few.
    • Linux server monitory tools - Linux Dash, Nagios, vmstat.
    • Log monitoring tools - Multitail, Logwatch.
    • Network Manager tools - ifconfig, GNOME Network Manager, Wicd.
    • Linux performance monitoring tools - Sysstat, Webmin, whowatch.

    Adapted from:
    "Simple Mail Transfer Protocol" by Multiple ContributorsWikipedia is licensed under CC BY-SA 3.0
    "File server" by Multiple ContributorsWikipedia is licensed under CC BY-SA 3.0
    "Print server" by Multiple ContributorsWikipedia is licensed under CC BY-SA 3.0
    "Network Load Balancing" by Multiple ContributorsWikipedia is licensed under CC BY-SA 3.0
    "Load balancing (computing)" by Multiple ContributorsWikipedia is licensed under CC BY-SA 3.0

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