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The settings for a chat window appear in a popover attached to the window. You can open and close the settings popover using the main menu, the toolbar, or the lock icon in the bottom corner of the chat window.

Show Settings

Settings toolbar item

lock icon

Save settings

If you select "Save settings" in the settings popover, then those settings will be remembered and available every time you open Underpass.

Ssave settings

You can see all the saved chats by selecting the "Saved Chats" item in the main menu.

Saved Chats


If you want Underpass to stop remembering a saved chat, just deselect "Save settings" in the settings popover. You can also select a chat in the Chats window and use the "Delete" command.


If a saved chat window is open when you quit Underpass, then the window will be automatically reopened when you run Underpass again. On the other hand, if you open a new chat and don't select "Save settings", then the new window will be forgotten when you quit Underpass.

A chat's password is not saved along with the other settings. All passwords are stored securely in your keychain, so if you want to save the password, you need to select "Save password" too.

Save password

As long as "Save" is selected, any changes that you make to saved settings or passwords will be saved automatically.


A chat password is highly recommended. Both devices must use the same password for the chat. You can leave the password blank, but in that case the password must be left blank on the other device too. See Passwords and encryption for more information on passwords. Your password can contain any UTF-8 characters.


Port (Internet chats)

For internet chats, a port number is required. The port can be any integer between 1024 and 65535. Underpass provides a default value that you may use if you wish. See Basic concepts for more information on internet ports.


Partner's address (Internet chats)

For internet chats, entering your partner's internet address is optional, but it is required in order for you to initiate a chat with your partner. If you leave the partner's address blank, then you must select "Automatically listen for partner" instead.

Partner's address

You can both enter an address and select "Automatically listen for partner" to enable both partners to initiate a chat with the other.

Underpass accepts IPv4 addresses such as, IPv6 addresses such as 2001:db8:ffff:1:201:02ff:fe03:0405, and domain names such as

Partner's computer (LAN chats)

For LAN chats, selecting your partner's computer name is optional, but it is required in order for you to initiate a chat with your partner. If you don't select a partner in the popup, then you must select "Automatically listen for partner" instead.

Partner's computer

You can both select your partner's computer and select "Automatically listen for partner" to enable both partners to initiate a chat with the other.

Underpass automatically detects other devices on your LAN that are running Underpass and listening for chats. The popup menu list changes dynamically as devices join or leave the network. If you have already selected a partner, but your partner is not currently available on the LAN, then your partner's computer name will appear disabled in the popup.

Partner's computer unavailable

Automatically listen for partner

Select "Automatically listen for partner" to allow your partner to start a chat with you whenever the chat window is open. If you close the window or quit Underpass, then Underpass stops listening, but it will start listening again when you reopen the window.

In a LAN chat, when you automatically listen for a partner, your computer name will be displayed below the checkbox. Your computer name then needs to be selected in your partner's Underpass settings.

Automatically listen for partner

In an internet chat, when you automatically listen for a partner, you will see links to web sites that allow you to test your listening port and obtain your IP address. The listening port test links to the Gibson Research Corporation Shields Up! internet probe. "What is my IP address?" links to the What Is My IP Address web site.

Underpass attempts to automatically detect your IPv4 address. If your IPv4 address is detected, it is also displayed in the settings popover. This works for many but not all models of router. If Underpass is unable to detect your IPv4 address, and you don't already know the address, then you can use the "What is my IP address?" link.

Automatically listen for partner

Your partner can use your Mac's IP address to start a chat if the IP address is entered in the other device's Underpass settings.

Chat Nicknames

You can customize the appearance of your chat by setting the window title and the screen names of the two chat partners that appear in the messages list. These nicknames are all local to your Mac and are never transmitted over the network.

Chat Nicknames

Each of these nickname settings is optional. If you don't set the window title, then Underpass will give the chat window a default title. If you leave a name setting blank, then the name will not appear before messages, just a time stamp.

Your Name Optional

Your Name Optional

Next: Frequently Asked Questions

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