Table of Contents


If you're having trouble establishing a chat with your partner, the first thing to check is whether you're using the same password. Note that if one partner's password setting is blank, then the other partner's password setting must be blank too.

Besides mismatched passwords, most of the issues with chats involve the "Listen for partner" setting. See Settings for more information about that setting. In technical terms, each chat connection has a client and a server: the server listens for connections, and the client connects to the listening server. When you select "Start Chat", Underpass becomes a client. When you turn on the "Listen for partner" setting, Underpass becomes a server. The client side is usually not a problem, because your device acts as a client all the time, for example when you load a web site in your browser or check and send email. Configuring a server properly is more complex, so the following sections apply mostly to the "Listen for partner" setting.

Cellular Connection

Many cell phone providers block incoming network connections from the internet. Thus, if your iOS device is currently using a cellular connection for internet, and you're unable to receive a chat, your cellular provider may be preventing it. You shouldn't have any trouble starting a chat, but the "Listen for partner" setting may not work. If your provider won't allow it, then you'll need to switch to a wifi network with an internet connection in order to listen for chats over the internet.

Internet Ports

In a home or an office, your iOS device is typically not connected directly to the internet. Instead, your device is behind a router. A router allows multiple computers to share a single internet connection. A router also protects your computers by blocking unexpected traffic from the internet from reaching your computers; in this way, the router is a kind of firewall. Although this protection is usually desirable, it could prevent you from receiving Underpass chats behind a router. Therefore, if your device is behind a router, then your router must be configured to allow Underpass chats from the internet.

When you select "Listen for partner", Underpass attempts to perform port mapping on your router. Port mapping allows traffic coming from the internet destined for a certain port to reach that port on your device. Underpass supports port mapping for a number of models of router, including Apple's AirPort Extreme and AirPort Express base stations. If the port mapping fails, Underpass shows an error message in the chat window.

Router port mapping unavailable

Not all router models support automatic port mapping, so you may need to check your router documentation or contact your router vendor to learn how to set up port mapping manually. Also, it's possible to disable port mapping on some routers, so make sure port mapping is enabled. For Apple's AirPort base stations, you can check the port mapping status with the AirPort Utility app. Make sure that NAT-PMP (Network Address Translation - Port Mapping Protocol) is enabled in the Advanced section when you edit the router settings:

Enable NAT Port Mapping Protocol

Multiple chat windows cannot simultaneously listen for a partner on the same port. If you try, you'll see an error like this:

Port is already in use.

When you see that error, please choose a different port for your chat. Each chat that is listening must use a different port. You may also see that error if some other application on your device is already listening for a connection on that port. The use of a port is usually exclusive, which is why your computer has many ports available.

Internet Addresses

When you select "Listen for partner", Underpass attempts to automatically detect your IPv4 address. If your IPv4 address is detected, then it is displayed in the settings. This works with many but not all models of router. If Underpass cannot detect your IPv4 address, and you don't already know it, then you can use the "What is my IP address?" link in the settings to discover it. This link opens a third party web site in your web browser. The web site is a service that will show your IP address.


Your IP address may or may not change over time. Thankfully, many Internet Service Providers (ISP) give you the same IP address all of the time. Some ISPs allow you to specially request a static IP address, perhaps with an additional service charge. If for some reason your ISP changes your IP address, your partners will have to change the "Partner's address" in their settings in order to chat with you again. If your ISP changes your IP address, and you can't obtain a static IP address from your ISP, one solution is to sign up for a third party Dynamic Domain Name Service (DDNS). Some of these services are free, others are paid. Dynamic DNS services give you a static domain name through which your device can always be reached, i.e., Your chat partners would enter your domain name in the "Partner's address" instead of entering your IP address. The Dynamic DNS service updates the DNS entry for your domain name automatically to point to your new IP address whenever your IP address changes.

As mentioned above, your IP address might not be reachable by partners if you're on a cellular connection. Also, your IP address is more likely to change on cellular than on a broadband internet connection.


LAN chats in Underpass use the Bonjour network service discovery protocol to allow chat partners to find each other on the network. Bonjour is built into the iOS operating system and runs by default. In most cases, Bonjour should just work, but there are a few circumstances in which it may not work:

  1. Your router could block Bonjour services.

    Check your router to make sure that Bonjour traffic is allowed. Bonjour uses port 5353, so this port must not be blocked.

  2. The LAN itself could block Bonjour services.

    This may happen on a network that you do not control, such as in an office, coffee shop, or school. In that situation, if automatic discovery of chat partners does not appear to be working, ask the local network administrator about Bonjour support.

Note that your own computer name will not appear in your "Partner's computer" menu. Underpass automatically filters it out of the list, to avoid confusion and selecting the wrong name.

Bonjour Settings

Next: Contact support

Table of Contents