Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

SocketAddrV6 is not roundtrip serializable

sunshowers.io

Post reply on HN
Print view View original

1–10 of 16 posts

Re: SocketAddrV6 is not roundtrip serializable

#3

Post by majke »

Falsehoods programmers think about addresses:

- parsing addresses is well defined (try parsing ::1%3)

- since 127.0.0.2 is on loopback, ::2 surely also would be

- interface number on Linux is unique

- unix domain socket names are zero-terminated (abstract are not)

- sin6_flowinfo matters (it doens;t unless you opt-in with setsockopt)

- sin6_scope_id matters (it doesn't unless on site-local range)

(I wonder if scope_id would work on ipv4-mapped-IPv6, but if I remember right I checked and it didn't)

- In ipv4, scope_id doesnt exist (true but it can be achieved by binding to interface)

and so on...

Years ago I tried to document all the quirks I knew about https://idea.popcount.org/2019-12-06-addressing/

Re: SocketAddrV6 is not roundtrip serializable

#4

Post by sunshowers »

majke wrote: post #3

Falsehoods programmers think about addresses: - parsing addresses is well defined (try parsing ::1%3) - since 127.0.0.2 is on loopback, ::2 surely also would be - interface number on Linux is unique - unix domain socket names are zero-terminated (abstract are not) - sin6_flowinfo matters (it doens;t unless you opt-in with setsockopt) - sin6_scope_id matters (it doesn't unless on site-local range) (I wonder if scope_i…

Thanks. At Oxide we do use the scope ID quite a bit, as my colleague Cliff Biffle says here: https://hachyderm.io/@cliffle/115492946627058792

Re: SocketAddrV6 is not roundtrip serializable

#6

Post by koito17 »

Unrelated to the article, but the scope ID in IPv6 addresses is super useful for the networking software I write.

The socket API for IPv4 requires a strange ceremony of calling if_nametoindex to map an interface name to an index then setting IP_BOUND_IF in order to bind a socket to a particular network interface. (Sorry, this only works on BSD and Mac OS; Linux instead uses SO_BINDTODEVICE for whatever reason).

When you don't supply a network interface, the OS "helpfully" guesses an interface, and that interface is usually the wrong one for multicast and broadcast packets.

I made a PXE boot server in Rust recently, and I lost at least 30 minutes of my time figuring out why DHCP responses wouldn't be receieved by an EFI client. The problem was that 255.255.255.255:68 is inherently ambiguous when multiple network interfaces exist.

In the case of the IPv6 API, you have to specify the interface up front (e.g. ff05::1:3%en0) or else you won't even be able to send a packet. I used to find this design tedious, but I prefer writing scope IDs explicitly in an address rather than having libc (or my own code) iterate over a linked list provided by the kernel to get an integer then supply that integer to a setsockopt (and remember that Linux has a quirk here).

Re: SocketAddrV6 is not roundtrip serializable

#7

Post by o11c »

majke wrote: post #3

Falsehoods programmers think about addresses: - parsing addresses is well defined (try parsing ::1%3) - since 127.0.0.2 is on loopback, ::2 surely also would be - interface number on Linux is unique - unix domain socket names are zero-terminated (abstract are not) - sin6_flowinfo matters (it doens;t unless you opt-in with setsockopt) - sin6_scope_id matters (it doesn't unless on site-local range) (I wonder if scope_i…

You can use ::ffff:127.0.0.2 for most purposes, but you can't ping it.

Re: SocketAddrV6 is not roundtrip serializable

#8

Post by namibj »

majke wrote: post #3

Falsehoods programmers think about addresses: - parsing addresses is well defined (try parsing ::1%3) - since 127.0.0.2 is on loopback, ::2 surely also would be - interface number on Linux is unique - unix domain socket names are zero-terminated (abstract are not) - sin6_flowinfo matters (it doens;t unless you opt-in with setsockopt) - sin6_scope_id matters (it doesn't unless on site-local range) (I wonder if scope_i…

It's sad that the only other loopback v6's appear to be v4's /8 in the form mapped into a slice of v7 address space

Re: SocketAddrV6 is not roundtrip serializable

#9

Post by theamk »

I think the answer is very obvious here: Debug should not omit fields. serde should not skip fields. Which means that there needs to be a way to put flowinfo into textual representation. Since there are already incompatibilities in how ipv6 addresses are handled ("%enp4s0f0" is supported by ping but not by Rust), might as well make something up, like "1234::5678%9-Q0x42". Tools like "ping" accept "-Q 0x42" option already, so most network engineers would be able to guess what does "-Q0x42" tacked to the end of address means.

Since this is backward-compatible when QoS is not set, I'd just change Display to emit it. But I am guessing some people may complain, so special implementation for Debug would work too...

Re: SocketAddrV6 is not roundtrip serializable

#10

Post by mananaysiempre »

o11c wrote: post #7
majke wrote: post #3

Falsehoods programmers think about addresses: - parsing addresses is well defined (try parsing ::1%3) - since 127.0.0.2 is on loopback, ::2 surely also would be - interface number on Linux is unique - unix domain socket names are zero-terminated (abstract are not) - sin6_flowinfo matters (it doens;t unless you opt-in with setsockopt) - sin6_scope_id matters (it doesn't unless on site-local range) (I wonder if scope_i…

You can use ::ffff:127.0.0.2 for most purposes, but you can't ping it.

> you can't ping it

WTF?..

(My Linux machine can, but I’ve no clue if I should trust that now.)

Post reply on HN