Verizon DSL traffic blocking explained

As is so often the case, this Internet access problem was firewall related

Michael Horowitz
Michael Horowitz

Michael Horowitz wrote his first computer program in 1973 and has been a computer nerd ever since. He spent more than 20 years working in an IBM mainframe (MVS) environment. He has worked in the research and development group of a large Wall Street financial company, and has been a technical writer for a mainframe software company.

He teaches a large range of self-developed classes, the underlying theme being Defensive Computing. Michael is an independent computer consultant, working with small businesses and the self-employed. He can be heard weekly on The Personal Computer Show on WBAI.


4 min read

As is so often the case with networking problems, the firewall was source of the Verizon DSL problem I wrote about recently.

I had experienced problems making outbound connections at two Verizon DSL business customers and was told by another Verizon DSL customer that they too had a similar problem.

The problem first came up when trying to use NetMeeting from a Verizon DSL customer to remotely control a computer. Despite there being no firewall on the receiving computer NetMeeting still couldn't make a connection. Even a simple ping of the target computer failed.

I suspected Verizon was the source of the problem when, a few days later, from another Verizon DSL customer, Real VNC failed to connect to a computer (another remote control attempt). Again, a ping of the target computer failed, but so too, did pings of websites such as yahoo.com, cnet.com and cbs.com that normally respond to pings (not all websites do).

When Verizon tech support and press relations made it clear that they don't block outgoing traffic, the problem had to be with the configuration of their modem/router.

In a standard consumer grade router, the firewall has a simple task: block all unsolicited incoming traffic. It doesn't try to govern outgoing traffic at all. Thus, any connection to the Internet that starts from a computer on the LAN is allowed. This is similar to the way the Windows XP firewall works, except that the XP firewall is likely to have some pre-defined holes in it.

The firewall in the Verizon Westell 7500 router/modem is a bit more ambitious, it tries to also exert control over outgoing connections that originate from the LAN. In some circumstances this is a good thing, but it caused me problems.

The actions of firewalls are easily quantified. They control a TCP/IP networking concept; a port. Ports are assigned numbers ranging from zero up to roughly 65,000. Some port numbers are reserved for specific types of traffic, others can be used by any networking software for any purpose. For example, you requested this web page using port 80. When you request a secure web page you are using port 443.

To see this for yourself, try to go to www.cnet.com:80 (the colon 80 may not show in your web browser status line when hovering over this link, but it is in the link). Everything works fine, the colon 80 is explicitly stating that port 80 should be used. Normally, the port number is implied when using the HTTP protocol. If you use any port number other than 80, you'll get an error message from your browser rather than the CNET home page.

Each port is either:
-- inbound or outbound
-- used by TCP or UDP or both (low level protocols)
-- open, closed or stealthed (stealth is the best)

That's it. Everything a firewall is doing can be quantified with rules about ports that are allowed and ports that blocked.

The Verizon DSL problems that I experienced stemmed from their using vague words to describe the functioning of the firewall. Nothing about the actions of the firewall in the Westell 7500 is explained in terms of ports. Thus, no one is sure exactly what the firewall is doing (I spoke to tech support twice).

When you configure the firewall in the Westell 7500, you get the choices shown below (full size image).

Take, for example, the "Minimum Security (Low)" setting which "allows all traffic except for known attacks". Is it allowing everything coming in or everything going out or both? And, what is a known attack? Firewalls control ports, not attacks. A given piece of malicious software may use one port number to phone home this week and a new variant can use a different port number next week.

Then it says "your modem is visible by other computers on the Internet". First off, the Westell 7500 is not just a modem, if it were, this posting wouldn't exist. Then, it's not clear if this means that no incoming ports are blocked or if it just means that the 7500 will respond to pings.

The bottom line is that these words have no meaning. Think of it as a gas station with pumps labeled "best", "medium" and "worst" without the octane rating.

In my case, the term "Typical Security (Medium)" tripped me up. That's what one modem was set to when I couldn't do ping or traceroute or Real VNC remote control. Lowering the setting to "Minimum Security (Low)" fixed the immediate problem.

What's the difference, in terms of ports being blocked, between Medium and Low? Even Verizon doesn't know.

In a scenario very reminiscent of WiFi routers shipping with encryption disabled, Verizon normally uses the "low" and "none" firewall settings. "Typical Security (Medium)" is not, according to tech support, typical. They rely on security software on the computers of their customers.

Shields Up!

A great service for testing ports is Shields Up! from Steve Gibson at grc.com. It too, pointed out how vague the firewall security description is.

With the Westell 7500 set to "MaxiumSecurity (High)" Shields Up! reported that the FTP port (21) was closed rather than stealthed. This is not maximum security. The boring, ordinary, years-old, dusty Belkin router that sits between me and the Internet as I write this, is, according to Shields Up!, fully stealthing all the common ports.

During a recent installation of a new Verizon business DSL line, the customer was not given a choice as to the equipment Verizon would provide. Later, tech support said they do offer dumb modems, presumably without firewalls. That may be the better way to go in terms of Defensive Computing as it lets you chose a router with better documentation.

See a summary of all my Defensive Computing postings.