SME Server:Documentation:Administration Manual:Appendix

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SME Manual Appendix

The following Appendix pages are included for your information.

Appendix A. Introduction to the Ethernet Local Area Network (LAN)

A local area network (LAN) is the system of wires and other hardware that connects the computers within your office and allows them to communicate with one another. An ethernet LAN is the most common type. Ethernet refers both to a kind of connection and to a protocol for how Internet data packets travel around your network.

The hub, a common component of an ethernet, serves as a point of interface between computers on the network. Each computer on your network is connected to the hub using an ethernet network cable. Different hubs operate at different speeds: slower hubs, operating at 10 MB/sec, are suitable for small networks; faster hubs, operating at 100 MB/sec, are suitable for larger networks. Switching 10/100 MB hubs can operate at either speed, and provide a good way to upgrade your network gradually.

An ethernet adapter, also called an ethernet card or network interface card (NIC), connects each computer to the ethernet LAN. An server with a dedicated Internet connection requires two ethernet adapters; one connects it to your LAN and the other connects it to the external network that leads to your ISP. If your server connects to your ISP using a modem or ISDN adapter, it only requires one ethernet adapter. A router ensures that Internet data packets (e.g. e-mail, web page information, etc.) reach the appropriate computers on your network. Routing is one of the functions performed by the server in server and gateway mode.

Allowing a third party, such as a systems integrator or networking company, to install your ethernet can be a good idea. It can help you select, procure and install the appropriate ethernet adapters, hub and cables. There are also various how-to guides available in bookstores if you are committed to installing it on your own.

Appendix B. DNS

DNS or the Domain Name Service is a distributed system of servers designed to translate human-readable names into computer routable IP addresses.

DNS Basics

SME, by design, does not respond to DNS queries from outside your local network, and cannot be used as a public DNS server for anyone outside your location.

If you want your SME server to be available to users outside your office using a name instead of your IP address, you MUST:

  • Register your domain name with a Registrar
  • Configure your host names on a publicly accessible DNS Server

Note: you can avoid 'Registering' your domain name if you use #Dynamic DNS Services

Imagine the following scenario: 

Root_DNS
 |     Registrar
 |    /   DNS Server
 |   /   /            Other_DNS
 |  |   /            /
 Internet---Other_ISP---Remote_User
     |
 Your_ISP---Your_ISPs_DNS
     |
    SME
     |
 Local_User


Let's assume that

  • SME has IP Address a.b.c.d
  • SME has domain name mysmeserver.com
  • Remote_User is configured to use Other_DNS for DNS lookups

If Remote_User tries to browse to http://mysmeserver.com, his computer asks Other_DNS how to find 'mysmeserver.com'. Other_DNS server then

  • asks the Root_DNS servers for the Registrar in charge of 'mysmeserver.com'
  • asks the Registrar for the DNS_Server that will answer queries about 'mysmeserver.com'
  • asks the DNS_Server for the IP address of 'mysmeserver.com'
  • saves the answer in its local cache for the amount of time specified by the administrator of the DNS record at DNS_Server.

If, on the other hand, Remote_User asks for information about your IP address (a.b.c.d), his DNS server

  • asks the Root_DNS servers where d.c.b.a.in-addr.arpa is registered.
  • asks the Registrar where to get more info about d.c.b.a.in-addr.arpa. This is probably but not necessarily Your_ISPs_DNS.
  • asks the host indicated by the Registrar (probably Your_ISPs_DNS) what name belongs to 'd.c.b.a.in-addr.arpa'. The return value is almost always a generic filler based on your IP address unless you contact your ISP and ask them to change the PTR data for your IP address.

Basically, PTR records are managed by the organization that controls the IP address (which makes sense, if you think about it).

If Local_User tries to open http://mysmeserver.com (assuming a default SME installation with DHCP and therefore DNS provided by the SME server):

  • If mysmeserver.com is configured for Local resolution, the SME server returns the data that has been configured locally.
  • If mysmeserver.com is configured to use Internet DNS Servers, the SME proceeds just as the first example from Root_DNS to Registrar to DNS_Server to local cache (actually, it checks the local cache first...)


So, for you to host a public web server at your own location you need:

  • An ISP to provide connectivity
  • A DNS Registrar where you can 'register' your domain name and publish the addresses of your DNS servers.
  • A DNS service provider who will respond to queries about your domain

Some ISP's provide registration and DNS hosting capabilities as part of the connectivity package.

Some ISP's provide DNS hosting but not Registration as part of the connectivity package.

Some Registrars provide DNS hosting as part of the registration.

Sometimes you will need 3 separate vendors for these separate services.

If you have already registered your domain name, find out if your Registrar provides DNS hosting services, and if so, how to configure them. They'll provide you with a web address where you can configure your DNS.

If they do NOT provide DNS hosting services, your ISP might. Ask them. If so, configure your DNS on their servers, then edit the Registrar page to point to the DNS servers indicated by your ISP.

If neither your Registrar nor your ISP provides DNS hosting, you'll need to find a 3rd party vendor to do this. See #DNS_Service_Providers below.

If you have not yet registered your domain name, try to find a Registrar who provides free DNS services.

PTR Records

PTR Records (or Pointer records, or Reverse DNS records) are used by internet hosts to convert an IP address into a name - sometimes for information only, sometimes for identity verification.

PTR records are constructed by reversing your IP address and appending the special suffix 'in-addr.arpa'. For example, the PTR record for a.b.c.d is d.c.b.a.in-addr.arpa.

A DNS lookup for a PTR record looks just like a DNS lookup for a domain name at this point - Root_DNS, Registrar, DNS_Server, except that the return value will be a host name instead of an IP address.

With very few exceptions all PTR records are registered to the ISP that controls the IP block in question, so frequently the ONLY way to change your PTR records is to contact your ISP and request that they be changed.

PTR records are only rarely used for their original purpose of verifying the identity of a particular computer - this is now done with SSL certificates and Trust Authorities.

The PTR record for your SME Server only becomes important if you plan to deliver email directly from your SME to recipient email servers (without using your ISP's mail server as a relay). Some email providers will not accept your email if the name returned by the 'reverse lookup' of your IP address does not in its turn result in your IP address when it, itself is looked up. It doesn't necessarily need to match your configured domain name, but it has to work both ways.

For example, if the nslookup d.c.b.a.in-addr.arpa (the reverse lookup for your IP) returns dsl-a-b-c-d.mycity.myispsname.com then before trying to send email directly from your SME to the Internet at large you want to make sure that nslookup dsl-a-b-c-d.mycity.myispsname.com returns your a.b.c.d and not an error or some other address.

Dynamic DNS Services

If your IP address is assigned dynamically, you may find it helpful to use a dynamic DNS service. A dynamic DNS service provides you with an automated way to notify them whenever your IP address changes so that they can immediately publish new DNS records for your domain. Without dynamic DNS, you would have to contact your ISP to have them change your DNS records, and your web site and other services would be unavailable for several days until the change was processed. You can easily enable the usage of a dynamic DNS service by selecting it on your server console.

A dynamic DNS service can be a great solution when used with a dedicated connection. With a typical dialup connection, your IP address changes much more frequently (possibly every time your server connects) and, because the server only connects intermittently, there is a delay in informing the dynamic DNS service of the change. This means that the risk of misdirected information is much greater with a dialup connection. For this reason, we recommend and support the use of dynamic DNS services only for dedicated connections.

Contribs.org has tested four dynamic DNS services, of which two are free services and two are commercial services: yi.org (free), dyndns.com (commercial), tzo.com (commercial) and dyndns.org (free). For simplicity, we have preprogrammed the server to work with these services (including pre-installing their client software). If you have arranged dynamic IP address assignment from your ISP and you wish to use one of these services, all you need to do is visit the appropriate web site to sign up for service, and enable that particular function in the server console. In pre-configuring the server for this particular service, we in no way interfere with nor prevent you from using another dynamic DNS service if you wish. To do so, however, would require some customized configuration on your part.

Contribs.org has tested the functionality of these services with our software. However, we accept no liability for any breach of service on their part. A failure on the part of your dynamic DNS service can result in your network becoming temporarily unreachable from the Internet. During these times, your e-mail may be undeliverable.

If your IP address is assigned dynamically and you intend to receive all your e-mail directly (rather than having it stored at an ISP and retrieving it via POP or IMAP), but you decide not to use a dynamic DNS service, you should implement multidrop e-mail as your e-mail solution as this will ensure that no e-mail is misdirected to another IP address (See Some important notes on Service list D (multidrop mail) in|Chapter 3.)


  Note:
Dynamic DNS services are not perfect. They merely point hostnames to IP addresses. If your system receives an IP address via DHCP or PPPoE, it will automatically update the dynamic DNS service each time it comes online. However, when your server disconnects from the Internet, with most dynamic DNS services your server does not indicate that it is offline in any way to the dynamic DNS service. If your system is offline for a period of time, it is possible that someone else will be assigned your IP address by your ISP. If this occurs, with most dynamic DNS services this other system will now start receiving your e-mail and web page requests until your server comes back online and updates the service with your new IP address. There is not much you can do about this, but you should be aware of this fact if there is any chance your system will be offline for a long period of time.


DNS Service Providers

Here is a brief list of vendors who provide DNS service hosting. The inclusion of a vendor here does not constitute endorsement by the SME developers.

Appendix C. Proxy Servers

The server comes with a proxy server called Squid which can proxy the web (HTTP), FTP and Gopher protocols. Proxy servers temporarily store information from the Internet on the hard drive of the server, allowing other users to access it directly from that hard drive. For example, when an employee visits a web page, the web proxy server will store that web page. Subsequent visitors to that web page will read it from your proxy server's hard drive, rather than over the Internet. This slightly reduces the network performance for the first visitor to that web page, but can enhance the performance for subsequent visitors.

Many gateway systems require the use of proxy servers, but with the server it is optional. Networked applications such as web browsers will work perfectly without proxying, due to the IP masquerading capability of the server.

In general, we recommend that proxying be disabled in your network applications. Using the proxy server can benefit the organization if you have a slow Internet connection and you've installed your server software on a fast computer. In this case, reading from the hard drive will be faster than reading from the Internet. Remember, though, that a proxy server benefits the second and subsequent visitors to a site but not the first visitor, so this benefit only applies if your users tend to visit the same sites repeatedly.

A proxy server is generally not appropriate if you have a fast Internet connection and you've installed your server software on a lower- or mid-level computer. In this case, reading from the hard drive of the computer may not be faster than over the Internet. It also offers no benefit to your organization if employees at your site do not tend to visit the same web pages.

Appendix D. Technical Support

If you are having difficulty configuring another vendor's hardware or software, we recommend you refer to the manual or contact the vendor for that product.

The SME Server is open source software, Contribs.org encourages users to freely share copies of our software.

Developers may wish to note that additional documentation, including HOWTO documents and a FAQ, can be found on our development web site - http://wiki.contribs.org/. There are also links there to other web sites relating to the server.

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