Option to disable DNS cache.

[fernvenue]

Hi, I'm using shadowsocks-rust with my own local DNS server, and it does have DNS cache already, so I think maybe we can add an option to disable DNS cache for shadowsocks-rust? If shadowsocks-rust already supports this feature or I've got something wrong, please correct me, thx :)

[zonyitoo]

https://docs.rs/trust-dns-resolver/0.22.0/trust_dns_resolver/config/struct.ResolverOpts.html#structfield.cache_size

trust-dns has cache by default.

[fernvenue]

Hi @zonyitoo, thanks for your reply, and sorry for the late response. I'm a little bit confused here. Do you mean we can't disable DNS cache? Actually, I don't have any DNS pollution issues, and trust-dns is causing me some problems. Is there any way to make shadowsocks-rust just use the system DNS directly without any changes?

[zonyitoo]

https://github.com/shadowsocks/shadowsocks-rust/blob/master/crates/shadowsocks/src/dns_resolver/trust_dns_resolver.rs#L102

You could set cache_size = 0 right here and see if it solves your problem.

[fernvenue]

Thanks, I'll give it a shot :)

[fernvenue]

Hi @zonyitoo, I just trid zedifen@9aba921 and it worked perfectly fine for me, maybe we can add a flag like --disable-dns-cache for this feature?

[zedifen]

Looks like shadowsocks-rust allows user to use their system's default DNS (instead of trust-dns), as described in README - Configuration Environment Variables; to enable this, use --dns "system" ¹ together with export SS_SYSTEM_DNS_RESOLVER_FORCE_BUILTIN=1 (to force use the DnsResolver::System as fallback — which is tokio's builtin resolver — instead of trust-dns backend) ².

Footnotes

1. According to code at crates/shadowsocks-service/src/config.rs#L2070, this yields a DnsConfig::System enum. ↩

2. When called by crates/shadowsocks-service/src/local/mod.rs#L168, build_dns_resolver returns a None is both are set, which makes things fall back to the default DnsResolver::System as in crates/shadowsocks/src/context.rs#L39 ↩

[zedifen]

Anyway, I also proposed a simple patch, allowing the change of cache_size in trust-dns through command-line (as well as config file?). Build artifacts may be downloaded from the workflow 😊.

Update: I fixed a typo in clap configuration, which causes a runtime type mismatch failure.

[fernvenue]

Hi @zedifen, I just tested it, and --dns-cache-size 0 worked perfectly fine for me :)

[fernvenue]

Here's how I tested it, start a pure socks server first:

~# ssservice local --dns-cache-size 0 --dns udp://[::1]:53 -b [::1]:1080
xxxxxxxxxxxxx INFO  shadowsocks socks TCP listening on [::1]:1080

Send a request to check if it works:

~# curl -sI https://example.com -x socks5h://[::1]:1080 | grep HTTP
HTTP/2 200

After that, I changed example.com's resolution to :: on my local DNS server, then:

~# curl -sI https://example.com -x socks5h://[::1]:1080
curl: (97) Can't complete SOCKS5 connection to example.com. (5)

So, it seems worked perfectly fine. @zonyitoo can we merge this change to the master branch?

[fernvenue]

Seems we can close this issue for now :)