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I'm facing a strange problem. I have a Debian squeeze machine connected to Internet through a 3G USB modem. The machine connects as expected, and I can resolve domain names and establish HTTP and SSH connections.

The problem is when I try to communicate to that machine from another PC connected to Internet. The machine doesn't respond to PING and I can't connect to the SSHD installed in it. If I connect the machine to the local network, it works right.

Any idea?


Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         *              U     0      0        0 ppp0

iptables -L:

Chain INPUT (policy ACCEPT)
target     prot opt source               destination         

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination   

netstat -n -l:

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State      
tcp        0      0 *               LISTEN     
tcp        0      0   *               LISTEN     
tcp        0      0    *               LISTEN     
tcp6       0      0 :::22                   :::*                    LISTEN     
udp        0      0   *                          
Active UNIX domain sockets (only servers)
Proto RefCnt Flags       Type       State         I-Node   Path
unix  2      [ ACC ]     STREAM     LISTENING     1531     /var/run/dbus/system_bus_socket


ppp0      Link encap:Point-to-Point Protocol  
          inet addr:  P-t-P:  Mask:
          RX packets:531 errors:1 dropped:0 overruns:0 frame:0
          TX packets:565 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3 
          RX bytes:87991 (85.9 KiB)  TX bytes:40721 (39.7 KiB)
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migrated from Jul 6 '11 at 10:32

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Do you have a routable IP? What does ifconfig say? – Nikolai N Fetissov Jul 5 '11 at 13:17
I updated the question with ifconfig's output – fjyaniez Jul 6 '11 at 7:16
up vote 1 down vote accepted

If you're on 3G then you are almost certainly behind at least one layer of NAT (often more than one) and all incoming connections will effectively be blocked.

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Fairly probable – fjyaniez Jul 6 '11 at 7:17

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