Can't connect to `localhost`, but `127.0.0.1` works

[tyilo]

Versions/Environment

1. What version of Rust are you using?
   1.94.1

2. What operating system are you using?
   Linux

3. What versions of the driver and its dependencies are you using? (Run cargo pkgid mongodb & cargo pkgid bson)

registry+https://github.com/rust-lang/crates.io-index#mongodb@3.5.2
registry+https://github.com/rust-lang/crates.io-index#bson@3.1.0

1. What version of MongoDB are you using? (Check with the MongoDB shell using db.version())
   8.0.20

2. What is your MongoDB topology (standalone, replica set, sharded cluster, serverless)?
   standalone

Describe the bug

I can connect to mongodb://127.0.0.1:27017/test but connecting to mongodb://localhost:27017/test doesn't work. Maybe this is a IPv6 vs IPv4 issue?

To Reproduce

Cargo.toml:

[package]
name = "test-connection"
version = "0.1.0"
edition = "2024"

[dependencies]
mongodb = { version = "3.5.2", default-features = false, features = ["dns-resolver", "rustls-tls", "compat-3-3-0", "bson-3"] }
tokio = { version = "1.50.0", features = ["full"] }

src/main.rs:

use mongodb::bson::{Document, doc};

async fn connect_to_database() -> mongodb::error::Result<mongodb::Database> {
    let client_options =
        mongodb::options::ClientOptions::parse(std::env::var("MONGODB_URL").unwrap()).await?;
    let default_database = client_options.default_database.as_ref().unwrap().clone();

    let client = mongodb::Client::with_options(client_options)?;
    Ok(client.database(&default_database))
}

#[tokio::main]
async fn main() {
    let db = connect_to_database().await.unwrap();
    let collection = db.collection::<Document>("test");
    let docs = collection.count_documents(doc! {}).await.unwrap();
    println!("{docs} documents in test collection");
}

---

Run a mongo server using podman:

$ podman run --rm -it -p 27017:27017 mongo:8.0.20

See that it is running:

$ podman ps
CONTAINER ID  IMAGE                        COMMAND     CREATED         STATUS         PORTS                     NAMES
9898fac8d819  docker.io/library/mongo:8.0  mongod      22 seconds ago  Up 22 seconds  0.0.0.0:27017->27017/tcp  agitated_lewin

See that connecting to 127.0.0.1 works:

$ MONGODB_URL=mongodb://127.0.0.1:27017/test cargo run
...
0 documents in test collection

See that connecting to localhost doesn't work (fails after 30 seconds):

$ MONGODB_URL=mongodb://localhost:27017/test cargo run
...
thread 'main' (141261) panicked at src/main.rs:16:58:
called `Result::unwrap()` on an `Err` value: Error { kind: ServerSelection { message: "Server selection timeout: No available servers. Topology: { Type: Unknown, Servers: [ { Address: localhost:27017, Type: Unknown, Error: Kind: I/O error: Connection reset by peer (os error 104), labels: {}, source: None, server response: None } ] }" }, labels: {}, wire_version: None, source: None, server_response: None }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[tyilo]

Both mongosh mongodb://127.0.0.1:27017/test and mongosh mongodb://localhost:27017/test works.

[abr-egn]

$ MONGODB_URL=mongodb://127.0.0.1:27017/test cargo run
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.30s
     Running `target/debug/test-connection`
0 documents in test collection

$ MONGODB_URL=mongodb://localhost:27017/test cargo run
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.28s
     Running `target/debug/test-connection`
0 documents in test collection

Something odd happening with your local resolver or /etc/hosts maybe?

[tyilo]

Maybe localhost resolves to both ::1 and 127.0.0.1 and the rust driver keeps trying to connect to ::1, but never tries 127.0.0.1?

[tyilo]

I can confirm that adding the following to /etc/hosts:

127.0.0.1        foo
::1              foo

also causes it to happen for mongodb://foo:27017/test.

If I remove the ::1 line after the process has started, but before it finishes, it will actually connect.

[tyilo]

Debugging some more it seems like tcp_connect succeeds with the [::1]:27017 address but the connection then later fails.

If I change this line to have SocketAddr::V4 instead of SocketAddr::V6 to prefer IPv4 addresses first, the connection succeeds.

This seems to be the same behavior as curl:

$ curl -v http://localhost:27017/
* Host localhost:27017 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying [::1]:27017...
* Established connection to localhost (::1 port 27017) from ::1 port 46504 
* using HTTP/1.x
> GET / HTTP/1.1
> Host: localhost:27017
> User-Agent: curl/8.19.0
> Accept: */*
> 
* Request completely sent off
* Recv failure: Connection reset by peer
* closing connection #0
curl: (56) Recv failure: Connection reset by peer

$ curl -v http://localhost:27017/ --ipv4
* Host localhost:27017 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying 127.0.0.1:27017...
* Established connection to localhost (127.0.0.1 port 27017) from 127.0.0.1 port 40290 
* using HTTP/1.x
> GET / HTTP/1.1
> Host: localhost:27017
> User-Agent: curl/8.19.0
> Accept: */*
> 
* Request completely sent off
* HTTP 1.0, assume close after body
< HTTP/1.0 200 OK
< Connection: close
< Content-Type: text/plain
< Content-Length: 85
< 
It looks like you are trying to access MongoDB over HTTP on the native driver port.
* shutting down connection #0
$ 

However, I think it is unfortunate that this means that I can't use mongodb with https://github.com/testcontainers/testcontainers-rs-modules-community without doing a localhost -> 127.0.0.1 replacement.

But I actually think this is podman's fault. If you use docker it also listens on the [::]:27017 and everything works:

$ docker ps
CONTAINER ID   IMAGE          COMMAND                  CREATED         STATUS         PORTS                                             NAMES
106f6c024c24   mongo:8.0.20   "docker-entrypoint.s…"   2 seconds ago   Up 2 seconds   0.0.0.0:27017->27017/tcp, [::]:27017->27017/tcp   crazy_dhawan

---

See podman-container-tools/podman#14768

[tyilo]

Changing podman's default network to be a dual-stack one fixes this. I wrote a section on the Arch Linux on how to set this up: https://wiki.archlinux.org/title/Podman#Dual-stack_IPv4/IPv6