Just after reboot, can use internal network. However, once dialup modem
is used internal network fails. And internal network continues to fail
and _even_after_disconnecting_ the dialup modem. I use "kppp" to connect.

In particular, during and after use of dialup modem, ping gives:

ping: sendto: Operation not permitted

---------Details follow---------

Am using crossover cable to connect two Knoppix PCs at 192.168.0.1 and
192.168.0.2 and submask 255.255.255.0. Suppose 192.168.0.1 is PC1 and
192.168.0.1 is PC2. PC1 is a Knoppix3.8 harddrive install (but many
packages have been added since). PC2 is Knoppix4.0.2 live-DVD. Did
network configuration on both PCs from KDE menu with:

KNOPPIX > Network/Internet > Network card configuration

Just after rebooting PC1 and starting up PC2, can do networking.
As well as more advance operations, I can do this from PC1:
$ ping -c 4 192.168.0.2
PING 192.168.0.2 (192.168.0.2): 56 data bytes
64 bytes from 192.168.0.2: icmp_seq=0 ttl=64 time=1.7 ms
64 bytes from 192.168.0.2: icmp_seq=1 ttl=64 time=0.4 ms
64 bytes from 192.168.0.2: icmp_seq=2 ttl=64 time=0.4 ms
64 bytes from 192.168.0.2: icmp_seq=3 ttl=64 time=0.4 ms

--- 192.168.0.2 ping statistics ---
4 packets transmitted, 4 packets received, 0% packet loss
round-trip min/avg/max = 0.4/0.7/1.7 ms

However, I then use "kppp" with the dialup modem with PC1 (my
harddrive install). After this, the internal network is broken. For
instance, I can't ping PC2 from PC1 anymore (and vice versa):

# ping -c 3 192.168.0.2
PING 192.168.0.2 (192.168.0.2): 56 data bytes
ping: sendto: Operation not permitted
ping: wrote 192.168.0.2 64 chars, ret=-1
ping: sendto: Operation not permitted
ping: wrote 192.168.0.2 64 chars, ret=-1
ping: sendto: Operation not permitted
ping: wrote 192.168.0.2 64 chars, ret=-1

--- 192.168.0.2 ping statistics ---
3 packets transmitted, 0 packets received, 100% packet loss

Also, with eth0 active, I can't access the dialup internet
but access returns after I shut down eth0:

$ ping -c 1 sf.net
(...I wait 10 seconds but nothing happens...
so I Ctrl-C but get no report...)

$ su -c "ifdown eth0"
Password:
$ ping -c 1 sf.net
PING sf.net (66.35.250.203): 56 data bytes
64 bytes from 66.35.250.203: icmp_seq=0 ttl=52 time=311.6 ms

--- sf.net ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 311.6/311.6/311.6 ms

Here is extra info from PC1 (notice no ppp0 entry in "interfaces):

$ cat /etc/network/interfaces
# /etc/network/interfaces -- configuration file for ifup(8), ifdown(8)

# The loopback interface
# automatically added when upgrading
auto lo eth0
iface lo inet loopback

iface eth0 inet static
address 192.168.0.1
netmask 255.255.255.0
network 192.168.0.0
broadcast 192.168.0.255
gateway 192.168.0.10
$ su -c "ifup eth0"
Password:
$ ifconfig -a
eth0 Link encap:Ethernet HWaddr 00:01:29:FA:C6:15
inet addr:192.168.0.1 Bcast:192.168.0.255 Mask:255.255.255.0
inet6 addr: fe80::201:29ff:fefa:c615/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:767 errors:0 dropped:0 overruns:0 frame:0
TX packets:1314 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:77371 (75.5 KiB) TX bytes:133720 (130.5 KiB)
Interrupt:23 Base address:0xa000

lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:383898 errors:0 dropped:0 overruns:0 frame:0
TX packets:383898 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:78166138 (74.5 MiB) TX bytes:78166138 (74.5 MiB)

ppp0 Link encap:Point-to-Point Protocol
inet addr:66.203.170.142 P-t-P:10.1.1.152 Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
RX packets:6510 errors:21 dropped:0 overruns:0 frame:0
TX packets:5793 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:8100282 (7.7 MiB) TX bytes:342900 (334.8 KiB)

sit0 Link encap:IPv6-in-IPv4
NOARP MTU:1480 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)