A little difficult: investigate Xenix and slattach problem

I know Xenix is a little outdated, and is dangerous to use on network.
But this is a virtual machine, and I like to play retrocomputing sometime.
The question is..why ftp over slip doesn't work?

I've set this Xenix vm on libvirt

<domain type='kvm' id='5' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
  <name>Xenix</name>
  <uuid>3ee7bda7-c7f1-4581-9a95-480a1bf8a44f</uuid>
  <title>Xenix</title>
  <description>Xenix</description>
  <memory unit='KiB'>16384</memory>
  <currentMemory unit='KiB'>16384</currentMemory>
  <vcpu placement='static'>1</vcpu>
  <resource>
    <partition>/machine</partition>
  </resource>
  <os>
    <type arch='x86_64' machine='pc-i440fx-2.12'>hvm</type>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
    <kvm>
      <hidden state='on'/>
    </kvm>
  </features>
  <cpu mode='custom' match='exact' check='full'>
    <model fallback='forbid'>486</model>
    <feature policy='require' name='x2apic'/>
    <feature policy='require' name='hypervisor'/>
  </cpu>
  <clock offset='localtime'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/bin/qemu-kvm</emulator>
    <disk type='file' device='floppy'>
      <driver name='qemu'/>
      <target dev='fda' bus='fdc'/>
      <alias name='fdc0-0-0'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/home/user/.local/share/libvirt/images/xenix.raw'/>
      <backingStore/>
      <target dev='hda' bus='ide'/>
      <alias name='ide0-0-0'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/home/user/.local/share/libvirt/images/xenix2.img'/>
      <backingStore/>
      <target dev='hdd' bus='ide'/>
      <alias name='ide0-1-1'/>
      <address type='drive' controller='0' bus='1' target='0' unit='1'/>
    </disk>
    <controller type='pci' index='0' model='pci-root'>
      <alias name='pci.0'/>
    </controller>
    <controller type='fdc' index='0'>
      <alias name='fdc0'/>
    </controller>
    <controller type='ide' index='0'>
      <alias name='ide'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
    </controller>
    <controller type='virtio-serial' index='0'>
      <alias name='virtio-serial0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </controller>
    <controller type='usb' index='0' model='piix3-uhci'>
      <alias name='usb'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
    </controller>
    <serial type='pty'>
      <source path='/dev/pts/1'/>
      <target type='isa-serial' port='0'>
        <model name='isa-serial'/>
      </target>
      <alias name='serial0'/>
    </serial>
    <console type='pty' tty='/dev/pts/1'>
      <source path='/dev/pts/1'/>
      <target type='serial' port='0'/>
      <alias name='serial0'/>
    </console>
    <console type='pty'>
      <source path='/dev/pts/3'/>
      <target type='virtio' port='1'/>
      <alias name='console1'/>
    </console>
    <channel type='spicevmc'>
      <target type='virtio' name='com.redhat.spice.0' state='disconnected'/>
      <alias name='channel0'/>
      <address type='virtio-serial' controller='0' bus='0' port='1'/>
    </channel>
    <input type='mouse' bus='ps2'>
      <alias name='input0'/>
    </input>
    <input type='keyboard' bus='ps2'>
      <alias name='input1'/>
    </input>
    <graphics type='spice' port='5900' autoport='yes' listen='127.0.0.1' keymap='it'>
      <listen type='address' address='127.0.0.1'/>
      <gl enable='no' rendernode='/dev/dri/renderD128'/>
    </graphics>
    <video>
      <model type='vga' vram='16384' heads='1' primary='yes'/>
      <alias name='video0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <alias name='balloon0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </memballoon>
  </devices>
  <seclabel type='dynamic' model='dac' relabel='yes'>
    <label>+10089:+10085</label>
    <imagelabel>+10089:+10085</imagelabel>
  </seclabel>
  <qemu:commandline>
    <qemu:arg value='-L'/>
    <qemu:arg value='pc-bios'/>
  </qemu:commandline>
</domain>

Xenix works only with 3com network cards unavaliable on qemu
So I use the old slip protocol
On linux I make this

a)Ip forwading
echo 1 1>/proc/sys/net/ipv4/ip_forward

b)slattach
slattach -s 19200 -d -p slip /dev/pts/1

c)ifconfig on host
ifconfig sl0 192.168.7.1 dstaddr 192.168.7.2 netmask 255.255.255.252 mtu 576 up

d)Nat
iptables -A FORWARD -i sl0 -o bridge0 -j ACCEPT
iptables -A FORWARD -i bridge0 -o sl0 -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT

On xenix side

a)Create slip
mkdev slip


b)Reboot

shutdown 0


c)Create tcp

mkdev tcp 
/etc/tcp stop
/etc/tcp start
route add default 192.168.7.2  0

Now the problem coming!

Rlogin work?
Yes

And ping?
Yes from internal to external

And ftp?
If I transfer a text file empty work perfect.
But when I try binary...

ftp> put "La bionda-I wanna be your lover.mp3"
local: La bionda-I wanna be your lover.mp3 remote: La bionda-I wanna be your lover.mp3
227 Entering Passive Mode (192,168,7,2,4,30)
150 Opening BINARY mode data connection for La bionda-I wanna be your lover.mp3.

And..stuck.

Wireshark said

What can it be?

Xenix only supports 11.3 file names. Maybe try renaming the file to iwannabe.mp3 before transferring.

Now i'm uploading file "pup.mp3"..no way.