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 100 Mb/sec, are suitable for small networks; faster hubs, operating at 1 Gb/sec, are suitable for larger networks. Switching 100Mb/1 Gb 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:

 |     Registrar
 |    /   DNS Server
 |   /   /            Other_DNS
 |  |   /            /

Let's assume that

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

If Remote_User tries to browse to, his computer asks Other_DNS how to find ''. Other_DNS server then

  • asks the Root_DNS servers for the Registrar in charge of ''
  • asks the Registrar for the DNS_Server that will answer queries about ''
  • asks the DNS_Server for the IP address of ''
  • 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 is registered.
  • asks the Registrar where to get more info about This is probably but not necessarily Your_ISPs_DNS.
  • asks the host indicated by the Registrar (probably Your_ISPs_DNS) what name belongs to ''. 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 (assuming a default SME installation with DHCP and therefore DNS provided by the SME server):

  • If is configured for Local resolution, the SME server returns the data that has been configured locally.
  • If 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 ''. For example, the PTR record for a.b.c.d is

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 (the reverse lookup for your IP) returns then before trying to send email directly from your SME to the Internet at large you want to make sure that nslookup returns your a.b.c.d and not an error or some other address.

SPF Records

SPF (Sender Policy Framework) records are added to the DNS zone record for domain names. Many receiving mail servers now require sending mail servers to have properly configured SPF records for the domain(s) being sent from. Failure to have SPF records can result in mail being rejected by mail servers eg Hotmail servers will reject mail that comes from mail servers without SPF records.

The SPF entries are added to your external DNS records which are hosted by your connectivity ISP. They are not configured on the sme server.

See here.

Here is a test site where you can check if SPF records are configured for your domain.

Different providers have different mail acceptance policies.

Look for tech support pages for the provider.


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.

Warning.png Warning:
Pre-configured Dynamic DNS Service is no longer integrated into SME Server core functionality due to its rapidly changing providers. If you need this service, please refer to the "smeserver-dyndns" contrib.

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.)

Important.png 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.

Important.png Note:
If you are using an IP address whether dynamically allocated or fixed, but located within a DSL block assigned to a provider of home connectivity services it may very well be blocked by mainline ISPs, so that you are not able to send email from it. The solution is to use a smarthost to relay your email, this being provided by your connectivity provider, check their documentation. The smarthost settings can be configured in the Server Manager email settings. This issue as noted applies to both fixed and dynamic IP addresses.

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, 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 - There are also links there to other web sites relating to the server.