UDP package failed to send back from the ssserver

[jackli0127]

shadowsocks-rust verison: v.1.17.1
OS version: Centos 9 Stream or Ubuntu 20.4

We upgraded our Shadowsocks from shadowsocks-libev 3.3.4 on CentOS 7 to shadowsocks-rust 1.17.1 on CentOS Stream 9 and Ubuntu 20.4, and then I found a large number of errors in the ssserver log as attached, which resulted in the loss of video during our users' Zoom meeting.

If I deploy shadowsocks-libev 3.3.5 on CentOS Stream 9 or Ubuntu 20.04, the same error messages are found. However, if I deploy shadowsocks-libev 3.3.4, both the log messages and our meeting video return to normal.

Any ideas on how to resolve the problem? Thanks!

[database64128]

It seems like an MTU misconfiguration somewhere in your network. You didn't discover the issue with shadowsocks-libev because it didn't bother with disabling IP fragmentation.

[jackli0127]

However, even when I configure a smaller MTU value on the network interface or in the ss-server parameters (shadowsocks-libev 3.3.5), I still encounter this error message.

The error only occurs when using the UDP protocol, and even UDP packets of 500 bytes fail to send

[zonyitoo]

EMSGSIZE should only occurs when packet size is greater than MTU. But interestingly that those ~500 bytes packets still fails.

The error was reported here:

shadowsocks-rust/crates/shadowsocks-service/src/server/udprelay.rs

Lines 791 to 797 in 1789dfa

	warn!(
	"udp failed to send back {} bytes to client {}, from target {}, error: {}",
	data.len(),
	self.peer_addr,
	addr,
	err
	);

And you may also try to comment these lines and see if EMSGSIZE goes away:

shadowsocks-rust/crates/shadowsocks/src/net/sys/unix/mod.rs

Lines 55 to 57 in 1789dfa

	if let Err(err) = set_disable_ip_fragmentation(addr_family, &socket) {
	warn!("failed to disable IP fragmentation, error: {}", err);
	}

[nasaboy]

I've also encountered this error, but it doesn't seem to have much impact

2024-03-26T17:14:38.953329076+08:00  WARN tokio-runtime-worker ThreadId(02) shadowsocks_service::server::udprelay: udp failed to send back 1455 bytes to client [::ffff:223.*.*.*]:6753, from target 122.10.255.159:8130, error: Message too long (os error 90)
2024-03-26T17:18:19.833890636+08:00  WARN tokio-runtime-worker ThreadId(03) shadowsocks_service::server::udprelay: udp failed to send back 1455 bytes to client [::ffff:223.*.*.*]:14735, from target 122.10.255.159:8130, error: Message too long (os error 90)
2024-03-26T17:19:53.901777502+08:00  WARN tokio-runtime-worker ThreadId(03) shadowsocks_service::server::udprelay: udp failed to send back 1455 bytes to client [::ffff:223.*.*.*]:14735, from target 122.10.255.159:8130, error: Message too long (os error 90)