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<== Date ==> <== Thread ==>
Subject: Re: Can't properly restrict IOC CA server to an interface
From: Jure Varlec via Tech-talk <tech-talk at aps.anl.gov>
To: EPICS tech-talk <tech-talk at aps.anl.gov>
Date: Wed, 13 Apr 2022 17:50:18 +0000
Hi Michael,

> Binding a socket to a specific interface doesn't always work so well
> when that interface disappears and reappears.  You mention "wifi" and
> "after a reboot or several", this makes me wonder if your network
> interface(s) are being deleted and recreated.

I agree that could be a problem if the IOC would be started during boot.
But it is not. This is my development system and I'm starting the IOC
manually, after all the interfaces are already set up. So I don't think this
is it, or at least, I don't see how it could be.

I brought out the debugger and dug into the code today. The socket is
bound, so I don't see why a "random" source address would be chosen
by the OS. I went a step further and extended cas_send_dg_msg(): I
replaced sendto() with sendmsg(), adding ancillary message data
via IP_PKTINFO socket option in order to override the source address.
It "works", in the sense that if I use a non-existent address, the sending
fails. However, if I use a valid source address, the IP header will contain
the address of the WiFi interface instead.

This is totally bizarre. I'm now convinced that the OS is rewriting the
address after the packet leaves the IOC, but I don't know how or why.
I quote the relevant part of systemd-networkd config below if anyone
has experience with a similar setup. I don't see anything special. Is it
possible that a bridge interface with an IP behaves differently than
a physical interface? AFAIK, assigning an IP address to a bridge is
standard practice and recommended, but I could be mistaken.

Best,
Jure

==> 20-machines.netdev <==
[Match]
Host=*

[NetDev]
Kind=bridge
Name=machines


==> 20-machines.network <==
[Match]
Name=machines

[Network]
Address=10.1.1.1/24
DHCPServer=true
IPForward=true
IPMasquerade=true

[DHCPServer]
DNS=10.1.1.1
EmitDNS=true
PoolOffset=128
PoolSize=100

Replies:
Re: Can't properly restrict IOC CA server to an interface Michael Davidsaver via Tech-talk
References:
Can't properly restrict IOC CA server to an interface Jure Varlec via Tech-talk
Re: Can't properly restrict IOC CA server to an interface Michael Davidsaver via Tech-talk
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