TCP/IP over null-modem

This tiny document describes how I managed to establish a TCP/IP connection between two Debian GNU/Linux boxes using a null-modem cable.

Software

You need the net-tools and ifupdown packages installed (you’ll probably have those installed anyway). On non-debian systems, take care to have /sbin/slattach installed.

Kernel

You need “Network device support -> SLIP (serial line) support” (CONFIG_SLIP) enabled.

/etc/network/interfaces

On machine 1, add the following lines:

iface sl0 inet static
    address 192.168.0.1
    netmask 255.255.255.255
    pointopoint 192.168.0.2

On the other box, the lines look like:

iface sl0 inet static
    address 192.168.0.2
    netmask 255.255.255.255
    pointopoint 192.168.0.1

Notice the absence of a second (third) t in pointopoint!

slattach

Now it’s time to run slattach -s <speed> -p slip <device>, e.g. slattach -s 115200 -p slip /dev/ttyS0 on both machines.

Bring up the interface

You can now ifup sl0 on both machines et voilà – here’s your tcp/ip connection.

Sources

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