On Mon, Jan 23, 2023 at 07:47 PM +02, Leon Romanovsky wrote:
> On Mon, Jan 23, 2023 at 03:44:39PM +0100, Jakub Sitnicki wrote:
>> Users who want to share a single public IP address for outgoing connections
>> between several hosts traditionally reach for SNAT. However, SNAT requires
>> state keeping on the node(s) performing the NAT.
>>
>> A stateless alternative exists, where a single IP address used for egress
>> can be shared between several hosts by partitioning the available ephemeral
>> port range. In such a setup:
>>
>> 1. Each host gets assigned a disjoint range of ephemeral ports.
>> 2. Applications open connections from the host-assigned port range.
>> 3. Return traffic gets routed to the host based on both, the destination IP
>> and the destination port.
>>
>> An application which wants to open an outgoing connection (connect) from a
>> given port range today can choose between two solutions:
>>
>> 1. Manually pick the source port by bind()'ing to it before connect()'ing
>> the socket.
>>
>> This approach has a couple of downsides:
>>
>> a) Search for a free port has to be implemented in the user-space. If
>> the chosen 4-tuple happens to be busy, the application needs to retry
>> from a different local port number.
>>
>> Detecting if 4-tuple is busy can be either easy (TCP) or hard
>> (UDP). In TCP case, the application simply has to check if connect()
>> returned an error (EADDRNOTAVAIL). That is assuming that the local
>> port sharing was enabled (REUSEADDR) by all the sockets.
>>
>> # Assume desired local port range is 60_000-60_511
>> s = socket(AF_INET, SOCK_STREAM)
>> s.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)
>> s.bind(("192.0.2.1", 60_000))
>> s.connect(("1.1.1.1", 53))
>> # Fails only if 192.0.2.1:60000 -> 1.1.1.1:53 is busy
>> # Application must retry with another local port
>>
>> In case of UDP, the network stack allows binding more than one socket
>> to the same 4-tuple, when local port sharing is enabled
>> (REUSEADDR). Hence detecting the conflict is much harder and involves
>> querying sock_diag and toggling the REUSEADDR flag [1].
>>
>> b) For TCP, bind()-ing to a port within the ephemeral port range means
>> that no connecting sockets, that is those which leave it to the
>> network stack to find a free local port at connect() time, can use
>> the this port.
>>
>> IOW, the bind hash bucket tb->fastreuse will be 0 or 1, and the port
>> will be skipped during the free port search at connect() time.
>>
>> 2. Isolate the app in a dedicated netns and use the use the per-netns
>> ip_local_port_range sysctl to adjust the ephemeral port range bounds.
>>
>> The per-netns setting affects all sockets, so this approach can be used
>> only if:
>>
>> - there is just one egress IP address, or
>> - the desired egress port range is the same for all egress IP addresses
>> used by the application.
>>
>> For TCP, this approach avoids the downsides of (1). Free port search and
>> 4-tuple conflict detection is done by the network stack:
>>
>> system("sysctl -w net.ipv4.ip_local_port_range='60000 60511'")
>>
>> s = socket(AF_INET, SOCK_STREAM)
>> s.setsockopt(SOL_IP, IP_BIND_ADDRESS_NO_PORT, 1)
>> s.bind(("192.0.2.1", 0))
>> s.connect(("1.1.1.1", 53))
>> # Fails if all 4-tuples 192.0.2.1:60000-60511 -> 1.1.1.1:53 are busy
>>
>> For UDP this approach has limited applicability. Setting the
>> IP_BIND_ADDRESS_NO_PORT socket option does not result in local source
>> port being shared with other connected UDP sockets.
>>
>> Hence relying on the network stack to find a free source port, limits the
>> number of outgoing UDP flows from a single IP address down to the number
>> of available ephemeral ports.
>>
>> To put it another way, partitioning the ephemeral port range between hosts
>> using the existing Linux networking API is cumbersome.
>>
>> To address this use case, add a new socket option at the SOL_IP level,
>> named IP_LOCAL_PORT_RANGE. The new option can be used to clamp down the
>> ephemeral port range for each socket individually.
>>
>> The option can be used only to narrow down the per-netns local port
>> range. If the per-socket range lies outside of the per-netns range, the
>> latter takes precedence.
>>
>> UAPI-wise, the low and high range bounds are passed to the kernel as a pair
>> of u16 values in host byte order packed into a u32. This avoids pointer
>> passing.
>>
>> PORT_LO = 40_000
>> PORT_HI = 40_511
>>
>> s = socket(AF_INET, SOCK_STREAM)
>> v = struct.pack("I", PORT_HI << 16 | PORT_LO)
>> s.setsockopt(SOL_IP, IP_LOCAL_PORT_RANGE, v)
>> s.bind(("127.0.0.1", 0))
>> s.getsockname()
>> # Local address between ("127.0.0.1", 40_000) and ("127.0.0.1", 40_511),
>> # if there is a free port. EADDRINUSE otherwise.
>>
>> [1] https://github.com/cloudflare/cloudflare-blog/blob/232b432c1d57/2022-02-connectx/connectx.py#L116
>>
>> v3 -> v4:
>> * Clarify that u16 values are in host byte order (Neal)
>>
>> v2 -> v3:
>> * Make SCTP bind()/bind_add() respect IP_LOCAL_PORT_RANGE option (Eric)
>>
>> v1 -> v2:
>> * Fix the corner case when the per-socket range doesn't overlap with the
>> per-netns range. Fallback correctly to the per-netns range. (Kuniyuki)
>
> Please put changelog after "---" trailer, so it will be stripped while
> applying patch.
I've put the changelog above the "---" on purpose. AFAIK, it is (was?)
preferred by netdev maintainers to keep the changelog in the
description.
Do you know if this convention is now a thing of the past? I might have
missed something.
