There's a std lib function for that

config/parser.go

@@ -4,7 +4,6 @@ import (
 	"errors"
 	"fmt"
 	"os"
-	"strings"
 
 	"github.com/mholt/caddy/middleware"
 )
@@ -183,15 +182,3 @@ func (p *parser) err(kind, msg string) error {
 	msg = fmt.Sprintf("%s:%d - %s error: %s", p.filename, p.line(), kind, msg)
 	return errors.New(msg)
 }
-
-// parseAddress takes a host:port string (val), and returns the host
-// and port as separate values. Empty strings can be returned if
-// either is missing.
-func parseAddress(val string) (string, string) {
-	parts := strings.SplitN(val, ":", 3)
-	if len(parts) == 1 {
-		return parts[0], ""
-	} else {
-		return parts[0], parts[1]
-	}
-}

config/parsing.go

 package config
 
-import "errors"
+import (
+	"errors"
+	"net"
+)
 
 // This file contains the recursive-descent parsing
 // functions.
@@ -24,12 +27,12 @@ func (p *parser) begin() error {
 
 // address expects that the current token is a host:port
 // combination.
-func (p *parser) address() error {
+func (p *parser) address() (err error) {
 	if p.tkn() == "}" || p.tkn() == "{" {
 		return p.err("Syntax", "'"+p.tkn()+"' is not EOF or address")
 	}
-	p.cfg.Host, p.cfg.Port = parseAddress(p.tkn())
-	return nil
+	p.cfg.Host, p.cfg.Port, err = net.SplitHostPort(p.tkn())
+	return
 }
 
 // addressBlock leads into parsing directives, including

config/parsing_test.go

 package config
-
-import "testing"
-
-func TestParseAddress(t *testing.T) {
-	type addr struct {
-		host string
-		port string
-	}
-	testCases := []struct {
-		input    string
-		expected addr
-	}{
-		{input: "host:port", expected: addr{host: "host", port: "port"}},
-		{input: "localhost:1234", expected: addr{host: "localhost", port: "1234"}},
-		{input: "127.0.0.1:0", expected: addr{host: "127.0.0.1", port: "0"}},
-		{input: "127.0.0.1", expected: addr{host: "127.0.0.1", port: ""}},
-		{input: "somedomain.com", expected: addr{host: "somedomain.com", port: ""}},
-		{input: "somedomain.com:", expected: addr{host: "somedomain.com", port: ""}},
-		{input: ":80", expected: addr{host: "", port: "80"}},
-		{input: "localhost:8080", expected: addr{host: "localhost", port: "8080"}},
-		{input: "", expected: addr{host: "", port: ""}},
-	}
-	for _, test := range testCases {
-		actualHost, actualPort := parseAddress(test.input)
-		if actualHost != test.expected.host {
-			t.Errorf("For '%s' expected host '%s' but got '%s'", test.input, test.expected.host, actualHost)
-		}
-		if actualPort != test.expected.port {
-			t.Errorf("For '%s' expected port '%s' but got '%s'", test.input, test.expected.port, actualPort)
-		}
-	}
-}