Multiplayer

DOS multiplayer games used two main networking approaches: serial connections (modem dial-up or direct null modem cable) for head-to-head play, and IPX for LAN parties. dosbox-automation emulates both over modern internet connections.

Serial multiplayer

Games like Doom, Rise of the Triad, and Terminal Velocity supported modem or null modem play. dosbox-automation's serial port emulation handles both.

Modem mode

Modem mode emulates a Hayes-compatible dial-up modem. One player listens for incoming calls; the other dials their IP address (or hostname) using the game's built-in dial-up interface - the same ATDT commands real modems used, just routed over the internet instead of a phone line.

The host listens on a port:

[serial]
serial1 = modem listenport:5000 sock:1

The client dials the host's address from within the game, e.g., ATDT 203.0.113.5:5000.

Some games have restrictive phone number input fields that won't accept IP addresses. You can work around this with a phonebook file - map fake phone numbers to real addresses in phonebook.txt (or whatever filename you've set via phonebookfile):

5551234 203.0.113.5:5000

Now dialling 5551234 in the game connects to your friend's machine.

Nullmodem mode

Nullmodem mode creates a direct serial link between two DOSBox instances, skipping the modem handshake entirely. This is the simplest option when both players can coordinate outside the game.

The host listens on a port:

[serial]
serial1 = nullmodem port:1250 sock:1

The client connects to the host:

[serial]
serial1 = nullmodem server:203.0.113.5 port:1250 sock:1

The connection is established as soon as both sides start DOSBox - no dialling required. The game sees a standard serial connection on COM1.

ENet reliable UDP transport

By default, both modem and nullmodem modes use TCP. Adding sock:1 switches to ENet, a reliable UDP-based transport. For internet play, ENet is strongly preferred - it has lower latency than TCP and is more resilient on unreliable connections. Both sides must use the same transport.

IPX multiplayer

LAN-party games like Duke Nukem 3D, WarCraft II, and Command & Conquer used the IPX protocol. dosbox-automation tunnels IPX over modern UDP/IP. One player starts a server, others connect to it:

IPXNET STARTSERVER
IPXNET CONNECT 203.0.113.5

See the IPX page for configuration details.

Port forwarding and firewalls

In both serial and IPX multiplayer, the host needs the chosen port to be reachable from the internet. If the host is behind a router or NAT, this typically means setting up port forwarding for that single port (UDP when using sock:1 or IPX, TCP otherwise).

If neither player can forward a port, a VPN such as ZeroTier or Tailscale is the easiest alternative - both players join the same virtual network and connect using their VPN-assigned addresses, with no port forwarding needed.