2

I always thought that the default gateways has to be on the same network as the server/client trying to connect to the internet. However today I rented a server from Hetzner and the server is able to connect to the internet even though, as it seems to me, the default gateway is definitely not connected to the same network. Also the netmask seems to be set to /32.

root@test:~# ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 138.201.187.120  netmask 255.255.255.255  broadcast 138.201.187.120
        inet6 fe80::9400:ff:fe3b:eece  prefixlen 64  scopeid 0x20<link>
        inet6 2a01:4f8:c17:69b0::1  prefixlen 64  scopeid 0x0<global>
        ether 96:00:00:3b:ee:ce  txqueuelen 1000  (Ethernet)
        RX packets 1166  bytes 638244 (638.2 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 653  bytes 92539 (92.5 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 112  bytes 8824 (8.8 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 112  bytes 8824 (8.8 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

root@test:~# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         172.31.1.1      0.0.0.0         UG    0      0        0 eth0
172.31.1.1      0.0.0.0         255.255.255.255 UH    0      0        0 eth0
root@test:~# ip route
default via 172.31.1.1 dev eth0
172.31.1.1 dev eth0 scope link
root@test:~# ping ipv4.google.com
PING ipv4.l.google.com (172.217.21.238) 56(84) bytes of data.
64 bytes from fra16s13-in-f14.1e100.net (172.217.21.238): icmp_seq=1 ttl=54 time=4.83 ms
64 bytes from fra16s13-in-f14.1e100.net (172.217.21.238): icmp_seq=2 ttl=54 time=4.96 ms
64 bytes from fra16s13-in-f14.1e100.net (172.217.21.238): icmp_seq=3 ttl=54 time=5.01 ms
^C
--- ipv4.l.google.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2003ms
rtt min/avg/max/mdev = 4.837/4.938/5.010/0.073 ms
root@test:~#
root@test:~# traceroute ipv4.google.com
traceroute to ipv4.google.com (216.58.207.78), 30 hops max, 60 byte packets
 1  _gateway (172.31.1.1)  0.139 ms  0.091 ms  0.062 ms
 2  12643.your-cloud.host (136.243.182.17)  0.320 ms  0.103 ms  0.075 ms
 3  * * *
 4  213-239-251-233.clients.your-server.de (213.239.251.233)  1.479 ms 213-239-251-237.clients.your-server.de (213.239.251.237)  0.722 ms  1.320 ms
 5  core21.fsn1.hetzner.com (213.239.239.125)  0.417 ms  0.310 ms  0.278 ms
 6  core4.fra.hetzner.com (213.239.245.18)  5.536 ms core4.fra.hetzner.com (213.239.245.14)  4.880 ms core0.fra.hetzner.com (213.239.252.33)  5.539 ms
 7  72.14.218.94 (72.14.218.94)  5.094 ms  5.085 ms  5.113 ms
 8  108.170.251.193 (108.170.251.193)  5.018 ms * 108.170.252.65 (108.170.252.65)  6.311 ms
 9  108.170.235.246 (108.170.235.246)  7.428 ms 72.14.232.50 (72.14.232.50)  4.903 ms 72.14.234.227 (72.14.234.227)  4.951 ms
10  108.170.252.18 (108.170.252.18)  5.287 ms fra16s25-in-f14.1e100.net (216.58.207.78)  4.804 ms 108.170.251.145 (108.170.251.145)  5.646 ms
root@test:~#

Thanks

  • 2
    Sadly, host configuration is off topic here. But briefly. you're right 99% of the time. In this case the host does a recursive lookup and uses the interface, rather than the gateway IP. – Ron Trunk Jan 8 at 16:13
  • a point-to-point link does not require an overlapping network range. – Jasen Jan 9 at 8:10

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