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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?

route:

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         *               0.0.0.0         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      0 127.0.0.1:707           0.0.0.0:*               LISTEN     
tcp        0      0 0.0.0.0:111             0.0.0.0:*               LISTEN     
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN     
tcp6       0      0 :::22                   :::*                    LISTEN     
udp        0      0 0.0.0.0:111             0.0.0.0:*                          
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

ifconfig

ppp0      Link encap:Point-to-Point Protocol  
          inet addr:46.6.170.222  P-t-P:10.64.64.64  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          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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1  
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

1 Answer 1

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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