Strange default router address for interface 'ppp0'

Question

I'm using a 3G-modem to provide an Internet connection to my Linux box.

So, when I issue the ifconfig command, I see the following:

ppp0: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST>  mtu 1500
        inet 10.223.229.136  netmask 255.255.255.255  destination 10.64.64.64
        ppp  txqueuelen 3  (Point-to-Point Protocol)
        RX packets 120  bytes 33390 (32.6 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 197  bytes 23842 (23.2 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

What's wrong with destination, e.g. with the default gateway?

According to my understanding of the PPP protocol, the netmask is 255.255.255.255, so the IP address of the gateway should be 10.223.229.137 or something like that.

What do I miss? Or are we both on the same network, because the address is class A?

route -n gives me the following:

[root@server ~]# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         0.0.0.0         0.0.0.0         U     0      0        0 ppp0
10.64.64.64     0.0.0.0         255.255.255.255 UH    0      0        0 ppp0
192.168.0.0     0.0.0.0         255.255.255.0   U     0      0        0 eth0

Solution 1

pppd uses a default for "hisaddr" if it doesn't receive remote address info from the peer during IPCP. The address pppd uses is 0x0a404040 + unit where unit is the 0 in ppp0. That typically appears as 10.64.64.64. (The other default address you may see is 0xa707070 + unit = 10.112.112.112, used for dial-on-demand interfaces).

I have seen this happen with the PPP interfaces emulated by GSM cellular modems. IPCP is used to inform the host of its GPRS IP address, but there is no real peer at "the other end" so you get this fake one.

Solution 2

The destination isn't the default gateway. PPP stands for 'point-to-point protocol' and this is exactly what you're seeing: the near-end point is 10.223.229.136, and the far end is 10.64.64.64.

Check your routing table (route -n): this will display the default gateway.

The 'all 1s' netmask means that there is in effect no subnet associated with the local address. Don't bother with classes of subnet: they haven't had any real-world meaning for many years.

Comments

[maniaque]

I've updated the question, so route -n gives me the default gateway of 10.64.64.64.

[Flup]

So yes, your traffic is routed to the other side of the point-to-point connection. All looks healthy to me.