Last week I posted a question about an interaction I saw when trying to
use LinuxNode to establish an AX.25 connection with another LinuxNode
over an axip link. I didn't receive any replies that explained what
was happening, so I did my best to figure it out. The paragraphs below
are a first draft of the explanation I plan to put into the gateway
section of the book. I'd really, really, appreciate any feedback on
whether this explanation is correct, or coherent.
Thanks,
John N8UR
[EMAIL PROTECTED]
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Extract from _Networking_Without_Wires_, copyright 1999 John Ackermann
N8UR:
"Once you figure it out, configuring ax25ipd is quite straightforward,
largely because (unlike xNOS) all the axip routes share a single
interface. However, there is a downside to this simplicity. You need
to be careful about using the LinuxNode software to gateway AX.25
connections across an axip link. This came to light recently when we
were configuring a pair of Linux axip gateway systems to link two
PacketCluster nodes, and I think it's worth taking a couple of
paragraphs to show what happened; if nothing else, you'll gain an
appreciation of the interactions that can cause problems in computer
networking!
For various reasons, the link between the two Clusters was going to be
done using straight AX.25 connections via the Linux node program. In
other words, the station initiating the link would first connect via RF
to the LinuxNode at his end of the gateway, and from there issue a
command like "c axip W8APR" to connect to the LinuxNode at the other
end of the tunnel. From W8APR, he would issue another connect command
to reach the remote cluster. Although we had everything configured
correctly, the script didn't seem to work reliably. Some test
configurations would work, while others resulted in no connection being
made between the two LinuxNodes via the axip link.
Further digging showed that the only time the axip link worked was when
the callsign of the user was the same as that of the node. In other
words, W8APR-5 could establish a connection to K8MK via the W8APR
gateway, but a user with a different callsign could not. Very strange,
but there is a reason (if not a solution).
Here's the first part of the problem. Since ax25ipd can support many
axip links on a single interface, it relies on callsigns to determine
to which IP address the packet should go. If the destination callsign
doesn't match a route defined in ax25ipd.conf, the packet goes into
never-never-land; although it is sent to the axip interface, it never
comes out. This is different than an xNOS gateway, which can ignore
callsigns because it uses a different port name for each axip link.
The second part of the problem is that the LinuxNode program quite
properly does callsign substitution when you use its "(C)onnect <port>
<callsign>" command. In other words, if I am connected to the W8APR
node and issue the command "c axip K8MK", to reach Jim's Linux axip
gateway, the packet will be sent with N8UR as the source address,* not
W8APR.
My connect request will make it to K8MK, but what happens then? K8MK
attempts to respond, but to N8UR and not W8APR -- it has no way of
knowing that I connected using the W8APR node. All it knows is that
the packet came in on the axip interface. So, it sends the
acknowledgement back to N8UR via axip. Since W8APR, not N8UR, is
defined in K8MK's ax25ipd.conf routing table, the daemon doesn't know
where to send the packet and it goes off into limbo. The result is
that I never get the "Connected to…" message.
This means that ax25ipd and node don't make a good combination for user
connections. A workaround is to run NET/ROM at each end of the AXIP
link and instruct users to connect to the remote LinuxNode using
NET/ROM syntax (no port name in the connect command). Since the layer
3 NET/ROM packets will use the node callsigns (the user's callsign is
encapsulated as part of the NET/ROM frame), the connection will work
properly.
* Actually, the callsign will have an "inverted SSID" attached to it,
so N8UR will be sent as "N8UR-15".
