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The whole /8 is "on-net" for the loopback interface, but that doesn't mean it's usable to applications. On macOS, I can't ping or bind to 127.0.0.2, for exampl

The whole /8 is "on-net" for the loopback interface, but that doesn't mean it's usable to applications. On macOS, I can't ping or bind to 127.0.0.2, for example. Linux's behavior of allowing the whole /8 to be used, even when only a single address is assigned, is a bit of an oddity here.

akira2501 2y ago
> I can't ping or bind to 127.0.0.2, for example. You can as soon as you define it as an alias. > Linux's behavior of allowing the whole /8 to be used, even when only a single address is assigned, is a bit of an oddity here. Per the RFC: "127.0.0.0/8 - This block is assigned for use as the Internet host loopback address. A datagram sent by a higher level protocol to an address anywhere within this block should loop back inside the host."
- greyface- 2y ago
  Yes, exactly. If I connect a physical loopback adapter to a network interface, I can't just ping any on-net address and expect a response. I need to add it as an explicit interface address/alias first. Just because the packet loops back on the interface doesn't mean that it will then be treated as if it were addressed to the host. Linux special-cases 127/8 on the virtual loopback interface and will treat all looped-back packets as if they were addressed to the host, even when the destination isn't explicitly configured as an alias. This behavior is not required by the quote from RFC3330.
RKearney 2y ago
> On macOS, I can't ping or bind to 127.0.0.2 uname -a Darwin LT-JT6692XPXJ 23.5.0 Darwin Kernel Version 23.5.0: Wed May 1 20:12:58 PDT 2024; root:xnu-10063.121.3~5/RELEASE_ARM64_T6000 arm64 ~ ifconfig lo0 lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384 options=1203<RXCSUM,TXCSUM,TXSTATUS,SW_TIMESTAMP> inet 127.0.0.1 netmask 0xff000000 inet6 ::1 prefixlen 128 inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1 nd6 options=201<PERFORMNUD,DAD> ~ ping -c 1 127.0.0.2 PING 127.0.0.2 (127.0.0.2): 56 data bytes --- 127.0.0.2 ping statistics --- 1 packets transmitted, 0 packets received, 100.0% packet loss ~ 11s sudo ifconfig lo0 add 127.0.0.2 Password: ~ ping -c 1 127.0.0.2 PING 127.0.0.2 (127.0.0.2): 56 data bytes 64 bytes from 127.0.0.2: icmp_seq=0 ttl=64 time=0.824 ms --- 127.0.0.2 ping statistics --- 1 packets transmitted, 1 packets received, 0.0% packet loss round-trip min/avg/max/stddev = 0.824/0.824/0.824/nan ms ~ ifconfig lo0 lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384 options=1203<RXCSUM,TXCSUM,TXSTATUS,SW_TIMESTAMP> inet 127.0.0.1 netmask 0xff000000 inet6 ::1 prefixlen 128 inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1 inet 127.0.0.2 netmask 0xff000000 nd6 options=201<PERFORMNUD,DAD> ~

akira2501 2y ago

> I can't ping or bind to 127.0.0.2, for example. You can as soon as you define it as an alias. > Linux's behavior of allowing the whole /8 to be used, even when only a single address is assigned, is a bit of an oddity here. Per the RFC: "127.0.0.0/8 - This block is assigned for use as the Internet host loopback address. A datagram sent by a higher level protocol to an address anywhere within this block should loop back inside the host."

greyface- 2y ago
Yes, exactly. If I connect a physical loopback adapter to a network interface, I can't just ping any on-net address and expect a response. I need to add it as an explicit interface address/alias first. Just because the packet loops back on the interface doesn't mean that it will then be treated as if it were addressed to the host. Linux special-cases 127/8 on the virtual loopback interface and will treat all looped-back packets as if they were addressed to the host, even when the destination isn't explicitly configured as an alias. This behavior is not required by the quote from RFC3330.

RKearney 2y ago

> On macOS, I can't ping or bind to 127.0.0.2 uname -a Darwin LT-JT6692XPXJ 23.5.0 Darwin Kernel Version 23.5.0: Wed May 1 20:12:58 PDT 2024; root:xnu-10063.121.3~5/RELEASE_ARM64_T6000 arm64 ~ ifconfig lo0 lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384 options=1203<RXCSUM,TXCSUM,TXSTATUS,SW_TIMESTAMP> inet 127.0.0.1 netmask 0xff000000 inet6 ::1 prefixlen 128 inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1 nd6 options=201<PERFORMNUD,DAD> ~ ping -c 1 127.0.0.2 PING 127.0.0.2 (127.0.0.2): 56 data bytes --- 127.0.0.2 ping statistics --- 1 packets transmitted, 0 packets received, 100.0% packet loss ~ 11s sudo ifconfig lo0 add 127.0.0.2 Password: ~ ping -c 1 127.0.0.2 PING 127.0.0.2 (127.0.0.2): 56 data bytes 64 bytes from 127.0.0.2: icmp_seq=0 ttl=64 time=0.824 ms --- 127.0.0.2 ping statistics --- 1 packets transmitted, 1 packets received, 0.0% packet loss round-trip min/avg/max/stddev = 0.824/0.824/0.824/nan ms ~ ifconfig lo0 lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384 options=1203<RXCSUM,TXCSUM,TXSTATUS,SW_TIMESTAMP> inet 127.0.0.1 netmask 0xff000000 inet6 ::1 prefixlen 128 inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1 inet 127.0.0.2 netmask 0xff000000 nd6 options=201<PERFORMNUD,DAD> ~