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Low-Cost Email Connection between UNIX and Macs

Huiqi Liu

Despite the proliferation of the Internet and ever growing popularity of email, in many workplaces, email is largely restricted to people in technical departments (who are generally familiar with networked UNIX workstations and text editors such as vi), while people in sales, marketing, and administrative departments (who may be Mac or PC users) do not have easy access.

The Problem

In this case, the network consists of an Ethernet-based TCP/IP environment comprising just over a dozen UNIX workstations of various flavors, mostly Sun SPARCs. A small Macintosh LocalTalk network of a similar number of machines is also established serving mainly word processing, spreadsheets, and FileMaker Pro database management. A dial-up PPP connection to the Internet is already in place on a Sun SPARC server. All users can physically, or through dumb terminals, log onto the UNIX server and access email. The objective was to get email to the Mac users.

This process proved to be more complex and time consuming than anticipated. The first step was to get the two networks (LocalTalk and TCP/IP) to talk to each other. The size of the configuration prohibited the use of hardware gateways, although many systems integrators recommended this route. We chose to install a PowerMac running off-the-shelf software packages as a "gateway," which so far has proved to be stable and effective. Once the network connection is established, either a graphical-based package or terminal emulation can be used as the front-end for email.

The Solution

Linking Macs and UNIX is not a new problem. Many common solutions, however, tend to use hardware gateways, such as Cayman Systems' GatorRouter and Shiva's FastPath (see for example, Stone 1995), which are not cost-effective for our purpose. In many ways, the various functionalities offered by hardware gateways are more than we need now or will need in the foreseeable future. Another possible way to achieve our goal would be to put all the Macs on Ethernet. Since most of our Macs are 68000-based machines without built-in Ethernet cards, this method would require an Ethernet card for each Mac. This is not very practical, and more importantly, it does not protect our investment toward future upgrade as most new PowerPC-based machines have built-in Ethernet connections. Besides, many of our existing peripherals are LocalTalk based. Admittedly, the benefit of the second method is the much higher bandwidth that Ethernet provides (10 Mbits/sec against 230.4 Kbits/sec for LocalTalk).

Neither of the above solutions was suitable for us. We wanted to fully utilize our existing LocalTalk network, thus preserving our investment in current equipment, and if possible use software rather than hardware for routing between LocalTalk and TCP/IP. As far as the speed of the network is concerned, it is preferable to gradually evolve into Ethernet, rather than to adopt a "big bang" approach. A graphical-based email package will be used instead of running telnet and then using UNIX text-based applications. Our solution of a gateway Mac is very economical, and it also gives us an easily manageable network set-up.

Figure 1 shows a schematic of final configuration of our network. The gateway PowerMac physically sits on both the TCP/IP network and the LocalTalk network. It has both a built-in Ethernet card (for TCP/IP) and a LocalTalk port. For our solution, this is the only Mac that needs an Ethernet connection. Additionally, the gateway has a number of software packages installed that allow it both to retain its link to the current LocalTalk network and to act as a translator between LocalTalk and TCP/IP.

Individual Macs can only support one type of network by default, that is, either EtherTalk to run on an Ethernet connection or LocalTalk to run on a LocalTalk connection. To allow the gateway computer to "talk" both, we installed Apple LocalTalk Bridge software. This software facilitates the use of EtherTalk as the network driver without losing the link to LocalTalk. With MacTCP properly configured, the gateway Mac could now talk to TCP/IP as well as LocalTalk hosts. However, other nodes on the LocalTalk network still could not talk to TCP/IP, so we installed and configured the Apple IP Gateway software on the gateway PowerMac, and configured MacTCP on all nodes. This allowed transparent exchange of IP packets and LocalTalk packets between UNIX workstations and all Macs on the network. Figure 2 summarizes the gateway computer configuration.

We still needed an email package to run on the Macs in order to complete our task. Most common packages are POP3 (Post Office Protocol, Version 3) based and simply require the UNIX mail server to be configured as POP server. After evaluating a number of them, we chose Z-Mail from NCD. It runs across all major platforms; thus it offers a uniform user interface no matter which platform (Mac or UNIX) a user logs on to. Z-Mail is also available on the PC for possible future expansion.

Set Up and Administration

The Apple LocalTalk Bridge is a Control Panel extension that requires a minimum amount of configuration. You simply run the installation program, restart the computer, and it is done. Once installed, the bridge can be dynamically switched on and off when necessary.

The Apple IP Gateway is also very easy to set up. To configure the gateway, first configure MacTCP to assign an IP address for the machine (see below). Next, gateway configuration is done through Gateway Manager, a front-end utility supplied with the package. The gateway has three options for IP addressing: automatic addressing, manual addressing, or both. Automatic addressing means clients are assigned addresses by the gateway each time they connect through it. Manual addressing means clients use preassigned addresses. The "both" option either automatic or manual addresses.

With the manual addressing scheme, you specify only those nodes with predefined IPaddresses that are allowed to connect to the gateway. To keep the network more secure, you can limit access to specific LocalTalk networks; this is very useful, especially when automatic addressing is used.

The Gateway Manager provides a graphical display to show current connections that can be used to monitor its usage. It is also helpful to know if there are any active connections before doing a reboot!

MacTCP is also a Control Panel extension. MacTCP allows you to assign an IP address to the computer either manually or through a server. In the manual mode, an IP address needs to be pre-assigned. In the server mode, the machine gets its IP address from the server. In our case, the gateway Mac must have a manually assigned address; other Macs can use either mode. Only the gateway Mac needs to be included in the host table of the UNIX server; all others are hidden from it a further measure to ensure network security and simplicity.

Configuring POP3 Server--Z-POP

To enable the use of Z-Mail, our Sun SPARC server, (which has the dial-up PPP connection to the Internet) is configured as a POP3 server. POP3 was designed to permit client computers to dynamically access mailboxes stored on a central server. A POP3 service daemon has to be installed for this purpose. An enhanced version called Z-POP comes with Z-Mail. It has some features not common on standard servers, such as mailbox synchronization and centralized storage of configuration files. If you already have a POP3 server configured, it may be used for Z-Mail also (but you will not be able to use the extra functions of Z-POP).

Configuring Z-POP is straightforward. Edit the /etc/services to define the port number: 15232 for Z-POP by default (standard POP3 uses port number 110):

zpop    15232/tcp

Then, edit the /etc/inetd.conf file to include the Z-POP server:

zpop stream tcp nowait root /usr/local/etc/ zpoplib/zpop zpop

The inetd has to be restarted (i.e., kill -HUP process id) or a system reboot will ensure that the POP server gets initialized. To check that the configuration is correct, use telnet to open a session with the server:

telnet <name of server host> <port number of Z-POP>

A successful connection will respond with a greeting such as:

+OK Welcome to ZPOP server version 1.0.

After Z-POP has been properly set up, loading Z-Mail itself is easy. The first time Z-Mail starts, some menu-driven configuration will need to be performed. You need to specify the IP address of the mail server and the port number to connect to (e.g., 15232 for Z-POP), the directory to store mail files on the local machine, and a couple of other simple parameters. Z-Mail is then ready to use.


We encountered some recurrent problems when users started to run Z-Mail on their Macs. These were narrowed down to a few causes, and the following is a summary of the problems and fixes.


Those of you who have used MacTCP may be familiar with the number of times it requires reconfiguration and subsequent rebooting of the machine. We found that quite often it would die for no apparent reason. The machine involved would refuse to connect to the gateway, not to mention the server, with a message of: "can't open MacTCP driver." When this happened, changing the IP address allocation from server mode to manual mode (and giving the machine an IP address) and rebooting would cure the fault. When switched back to server mode, it would still function without any problems.

MacTCP sometimes failed to release the driver when it was not being used and thus caused problems next time around. Fortunately, a small system extension called ZapTCP written by Steve Falkenburg of MacDTS is available in the public domain. This works by installing a system-wide patch that takes control of MacTCP when an application quits or crashes. Putting ZapTCP in the extensions folder (in addition to MacTCP itself) drastically reduced the number of reboots. From time to time, however, MacTCP still hangs.

Since we only have a dozen or so machines, it does not take much time or effort for us to use manual addressing. In the end, we opted to give each Mac its own IP address (and correspondingly set the IP Gateway to manual addressing mode), which together with ZapTCP almost eliminated the problem with MacTCP.

POP3 server

Z-Mail, like other POP3-compliant mail packages, must successfully open a TCP session with the POP server before doing anything else. Therefore, if the server failed to respond in a certain given time limit, Z-Mail would not be able to establish a TCP session and could not start the application.

The easiest way to check this is by opening a telnet session onto the POP server port and measuring the response time. (Do this first on the mail server locally and then through the network from a Mac.) Normally, the response should be instant (1 or 2 seconds). If it takes longer than 10 seconds, then the mail application will be very slow to start (if at all). In Z-Mail binary, there is a hard-coded default "NetWork Timeout" value of 60 seconds, which may be increased with a resource editing program such as ResEdit.

If the response is not quick enough, the load on the server may be too high or the network has too much traffic. In our setup, this "slow response" seems to be linked to the PPP connection (as our POP server is also the PPP server). When PPP is up (which happens every 30 minutes) and actively transmitting data, the machine is always slower to service a POP request.

Gateway PowerMac

Occasionally, the gateway PowerMac hangs. This happened initially with a very high frequency that was traced to the After Dark screensaver utility. Disabling the screensaver reduced the system crashes considerably. (We have version 2.0 of After Dark, which reportedly has problems on the PowerMac. We have yet to source a later version.) Sometimes, when the machine stays on for several days gradually a "dirty" system will result. With system 7.5, it is possible to schedule automatic reboot during the night, which again cuts down system problems.


This article documents a low-cost solution to getting email on AppleTalk network through a UNIX mail server. We used affordable Apple software packages to integrate LocalTalk and TCP/IP and found it stable, effective and easy to manage. Note that this setup is not restricted just to email; once the network connection is up, other services such as ftp, sharing disks, and printers, can also be achieved.


Stone, D. 1995. UNIX, TCP/IP, and Macs. Sys Admin 4(3):63 - 72.

About the Author

Huiqi Liu holds a Ph.D. in Geology from Royal Holloway, University of London. Whilst still completing his thesis, he found computers were much more interesting than rocks and decided to take part in a Master's course in computer science. He now works as systems support specialist for CogniSeis Development Inc., a software house to the oil industry, UK Office. He can be reached at