Linux IP Masquerade HOWTO
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# 5.6. Testing External MASQ server Internet connectivity

- Step Six: Testing external MASQ server to Internet connectivity

  From the MASQ server, ping the external IP address of the MASQ server's EXTERNAL network interface that is connected to the Internet. This address might be a Ethernet interface, a PPP interface, etc. connection to your ISP. If you don't know what this external IP address is, run the Linux command "/sbin/ifconfig" on the MASQ server itself to get the Internet address. The output should look something like the following (we are looking for the IP address of eth0):

      ------------------------------------
      eth0      Link encap:Ethernet  HWaddr 00:08:C7:A4:CC:5B
                inet addr:12.13.14.15  Bcast:12.13.14.255  Mask:255.255.255.0
                UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
                RX packets:6108459 errors:0 dropped:0 overruns:0 frame:0
                TX packets:5422798 errors:8 dropped:0 overruns:0 carrier:8
                collisions:4675 txqueuelen:100
                Interrupt:11 Base address:0xfcf0
      ------------------------------------

  As you can see from the above, the external IP address is "12.13.14.15" for this example. So, now that you have your IP address after running the "ipconfig" command, ping your external IP address. This will confirm that the MASQ server has full network connectivity. The output should look something like the following (hit Control-C to abort the ping):

      -------------------------------------
      masq-server# ping 12.13.14.15
      PING 12.13.14.15 (12.13.14.15): 56 data bytes
      64 bytes from 12.13.14.15: icmp_seq=0 ttl=255 time=0.8 ms
      64 bytes from 12.13.14.15: icmp_seq=1 ttl=255 time=0.4 ms
      64 bytes from 12.13.14.15: icmp_seq=2 ttl=255 time=0.4 ms
      64 bytes from 12.13.14.15: icmp_seq=3 ttl=255 time=0.5 ms
      ^C

      --- 12.13.14.15 ping statistics ---
      4 packets transmitted, 4 packets received, 0% packet loss
      round-trip min/avg/max = 0.4/0.5/0.8 ms
      -------------------------------------

  If either of these tests doesn't work, you need to go back and double check your network cabling and verify that the two network interfaces on the MASQ server are seen in "dmesg". An example of this output would be the following towards the END of the "dmesg" command:

      -------------------------------------
      .
      .
      PPP: version 2.3.7 (demand dialling)
      TCP compression code copyright 1989 Regents of the University of California
      PPP line discipline registered.
      3c59x.c:v0.99H 11/17/98 Donald Becker
      http://cesdis.gsfc.nasa.gov/linux/drivers/
      vortex.html
      eth0: 3Com 3c905 Boomerang 100baseTx at 0xfe80,  00:60:08:a7:4e:0e, IRQ 9
        8K word-wide RAM 3:5 Rx:Tx split, autoselect/MII interface.
        MII transceiver found at address 24, status 786f.
        Enabling bus-master transmits and whole-frame receives.
      eth1: 3Com 3c905 Boomerang 100baseTx at 0xfd80,  00:60:97:92:69:f8, IRQ 9
        8K word-wide RAM 3:5 Rx:Tx split, autoselect/MII interface.
        MII transceiver found at address 24, status 7849.
        Enabling bus-master transmits and whole-frame receives.
      Partition check:
       sda: sda1 sda2 < sda5 sda6 sda7 sda8 >
       sdb:
      .
      .
      -------------------------------------

  Also be sure that the cabling is correct (Ethernet: the NICs connecting the external MASQ server to your ISP has the "link" light lit up). Finally, make sure that TCP/IP is correctly configured on the MASQed Server as described by the various Network HOWTOs (URLs can be found in the requirements section for your 2.4.x kernel in Section 2.6, 2.2.x kernel in Section 2.7, or 2.0.x kernel in Section 2.8).

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