proposal: net: populate source address on OpError when available #44127
Description
What version of Go are you using (go version)?
$ go version go version go1.16rc1 darwin/amd64
Does this issue reproduce with the latest release?
Yeppers
What operating system and processor architecture are you using (go env)?
go env Output
$ go env GO111MODULE="" GOARCH="amd64" GOBIN="" GOCACHE="/Users/andybons/Library/Caches/go-build" GOENV="/Users/andybons/Library/Application Support/go/env" GOEXE="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="darwin" GOINSECURE="" GOMODCACHE="/Users/andybons/go/pkg/mod" GONOPROXY="" GONOSUMDB="" GOOS="darwin" GOPATH="/Users/andybons/go" GOPRIVATE="" GOPROXY="https://proxy.golang.org,direct" GOROOT="/usr/local/go" GOSUMDB="sum.golang.org" GOTMPDIR="" GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64" GOVCS="" GOVERSION="go1.16rc1" GCCGO="gccgo" AR="ar" CC="clang" CXX="clang++" CGO_ENABLED="1" GOMOD="/dev/null" CGO_CFLAGS="-g -O2" CGO_CPPFLAGS="" CGO_CXXFLAGS="-g -O2" CGO_FFLAGS="-g -O2" CGO_LDFLAGS="-g -O2" PKG_CONFIG="pkg-config" GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/4w/rc3rq2s55w5fswy2n8kphy5r0000gn/T/go-build575792972=/tmp/go-build -gno-record-gcc-switches -fno-common"
What did you do?
When using net.Dial or its variants and a connection fails, it can be extremely helpful to know the source IP/port for diagnostic purposes, but it isn’t populated within the OpError. The below code shows the behavior in action:
// The Go Failing TCP Connection Challenge!
//
// Print the source IP address and port number of an outgoing TCP connection
// that times out and doesn't connect to the remote host.
//
// Rules:
//
// 1. You can't set the local address or port number. The local address must
// be 0.0.0.0 (INADDR_ANY), and the port number must be 0 (so the kernel
// selects an ephemeral port).
//
// 2. You can't use the "unsafe" package.
//
// 3. Whatever you do needs to return a net.Conn.
package main
import (
"fmt"
"net"
"time"
)
func main() {
if conn, err := net.DialTimeout("tcp4", "google.com:7357", time.Second); err != nil {
fmt.Printf("Connection timed out:\n%#v\n%q\n", err, err)
// &net.OpError{Op:"dial", Net:"tcp4", Source:net.Addr(nil), Addr:(*net.TCPAddr)(0xc000112630), Err:(*net.timeoutError)(0x1209ce0)}
// "dial tcp4 172.217.10.46:7357: i/o timeout" <-- shows destination but not source
} else {
fmt.Printf("Connection succeeded:\n%#v\n%v -> %v\n", conn, conn.LocalAddr(), conn.RemoteAddr())
}
}
What did you expect to see?
It would be nice if the OpError populated the source IP/port when it could, or allowed for clients to get access to it (say, if there were security concerns with exposing the source IP/port in the OpError by default if it may be blindly bubbled up to clients).
For reference, this is achieved through the C program below.
#include <arpa/inet.h>
#include <errno.h>
#include <fcntl.h>
#include <netdb.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>
int main() {
struct addrinfo hints;
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM;
struct addrinfo *results;
getaddrinfo("google.com", "7357", &hints, &results);
int fd;
struct addrinfo *r = results;
fd = socket(r->ai_family, r->ai_socktype, r->ai_protocol);
fcntl(fd, F_SETFL, fcntl(fd, F_GETFL, NULL) | O_NONBLOCK);
connect(fd, r->ai_addr, r->ai_addrlen);
struct sockaddr_in sal;
socklen_t sal_len = sizeof sal;
getsockname(fd, (struct sockaddr *)&sal, &sal_len);
char la[256], ra[256];
strlcpy(la, inet_ntoa(sal.sin_addr), 256);
strlcpy(ra, inet_ntoa(((struct sockaddr_in *)(r->ai_addr))->sin_addr), 256);
printf(
"%s:%u -> %s:%u\n",
la, ntohs(sal.sin_port),
ra, ntohs(((struct sockaddr_in *)(r->ai_addr))->sin_port)
);
return 0;
}
What did you see instead?
Only a destination IP/port in the error.