Pull request: create aghnet package

Merge in DNS/adguard-home from mk-aghnet to master

Updates #2829.

Squashed commit of the following:

commit 519806c04b8d0517aacce9c31f2d06ab56631937
Merge: 92376c86 97361234
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Mar 16 19:13:56 2021 +0300

    Merge branch 'master' into mk-aghnet

commit 92376c8665e529191aa482432f9d5e3e2e3afdc8
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Mar 16 18:37:22 2021 +0300

    aghnet: fix linux

commit 7f36d19b0e650d4e4fc5cf9ea4b501a7f636abeb
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Mar 16 18:08:30 2021 +0300

    aghnet: mv network utils from util

commit aa68c70c1146b8c32303c6e037953a41aa7d72f9
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Mar 16 17:30:06 2021 +0300

    aghnet: mv ipDetector here

commit b033657f5c5ee91f869c36508a5eb15976a174a0
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Mar 16 17:24:07 2021 +0300

    all: mk aghnet package, rename sysutil package

internal/aghnet/net_linux.go

@@ -0,0 +1,176 @@
+// +build linux
+
+package aghnet
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"net"
+	"os"
+	"strings"
+
+	"github.com/AdguardTeam/AdGuardHome/internal/aghio"
+	"github.com/AdguardTeam/golibs/file"
+)
+
+// maxConfigFileSize is the maximum length of interfaces configuration file.
+const maxConfigFileSize = 1024 * 1024
+
+func ifaceHasStaticIP(ifaceName string) (has bool, err error) {
+	// TODO(a.garipov): Currently, this function returns the first
+	// definitive result.  So if /etc/dhcpcd.conf has a static IP while
+	// /etc/network/interfaces doesn't, it will return true.  Perhaps this
+	// is not the most desirable behavior.
+
+	for _, check := range []struct {
+		checker  func(io.Reader, string) (bool, error)
+		filePath string
+	}{{
+		checker:  dhcpcdStaticConfig,
+		filePath: "/etc/dhcpcd.conf",
+	}, {
+		checker:  ifacesStaticConfig,
+		filePath: "/etc/network/interfaces",
+	}} {
+		var f *os.File
+		f, err = os.Open(check.filePath)
+		if err != nil {
+			// ErrNotExist can happen here if there is no such file.
+			// This is normal, as not every system uses those files.
+			if errors.Is(err, os.ErrNotExist) {
+				err = nil
+
+				continue
+			}
+
+			return false, err
+		}
+		defer f.Close()
+
+		var fileReadCloser io.ReadCloser
+		fileReadCloser, err = aghio.LimitReadCloser(f, maxConfigFileSize)
+		if err != nil {
+			return false, err
+		}
+		defer fileReadCloser.Close()
+
+		has, err = check.checker(fileReadCloser, ifaceName)
+		if err != nil {
+			return false, err
+		}
+
+		return has, nil
+	}
+
+	return false, ErrNoStaticIPInfo
+}
+
+// dhcpcdStaticConfig checks if interface is configured by /etc/dhcpcd.conf to
+// have a static IP.
+func dhcpcdStaticConfig(r io.Reader, ifaceName string) (has bool, err error) {
+	s := bufio.NewScanner(r)
+	var withinInterfaceCtx bool
+
+	for s.Scan() {
+		line := strings.TrimSpace(s.Text())
+
+		if withinInterfaceCtx && len(line) == 0 {
+			// An empty line resets our state.
+			withinInterfaceCtx = false
+		}
+
+		if len(line) == 0 || line[0] == '#' {
+			continue
+		}
+
+		fields := strings.Fields(line)
+
+		if withinInterfaceCtx {
+			if len(fields) >= 2 && fields[0] == "static" && strings.HasPrefix(fields[1], "ip_address=") {
+				return true, nil
+			}
+			if len(fields) > 0 && fields[0] == "interface" {
+				// Another interface found.
+				withinInterfaceCtx = false
+			}
+			continue
+		}
+
+		if len(fields) == 2 && fields[0] == "interface" && fields[1] == ifaceName {
+			// The interface found.
+			withinInterfaceCtx = true
+		}
+	}
+
+	return false, s.Err()
+}
+
+// ifacesStaticConfig checks if interface is configured by
+// /etc/network/interfaces to have a static IP.
+func ifacesStaticConfig(r io.Reader, ifaceName string) (has bool, err error) {
+	s := bufio.NewScanner(r)
+	for s.Scan() {
+		line := strings.TrimSpace(s.Text())
+
+		if len(line) == 0 || line[0] == '#' {
+			continue
+		}
+
+		fields := strings.Fields(line)
+		// Man page interfaces(5) declares that interface definition
+		// should consist of the key word "iface" followed by interface
+		// name, and method at fourth field.
+		if len(fields) >= 4 && fields[0] == "iface" && fields[1] == ifaceName && fields[3] == "static" {
+			return true, nil
+		}
+	}
+	return false, s.Err()
+}
+
+func ifaceSetStaticIP(ifaceName string) (err error) {
+	ipNet := GetSubnet(ifaceName)
+	if ipNet.IP == nil {
+		return errors.New("can't get IP address")
+	}
+
+	gatewayIP := GatewayIP(ifaceName)
+	add := updateStaticIPdhcpcdConf(ifaceName, ipNet.String(), gatewayIP, ipNet.IP)
+
+	body, err := ioutil.ReadFile("/etc/dhcpcd.conf")
+	if err != nil {
+		return err
+	}
+
+	body = append(body, []byte(add)...)
+	err = file.SafeWrite("/etc/dhcpcd.conf", body)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// updateStaticIPdhcpcdConf sets static IP address for the interface by writing
+// into dhcpd.conf.
+func updateStaticIPdhcpcdConf(ifaceName, ip string, gatewayIP, dnsIP net.IP) string {
+	var body []byte
+
+	add := fmt.Sprintf("\ninterface %s\nstatic ip_address=%s\n",
+		ifaceName, ip)
+	body = append(body, []byte(add)...)
+
+	if gatewayIP != nil {
+		add = fmt.Sprintf("static routers=%s\n",
+			gatewayIP)
+		body = append(body, []byte(add)...)
+	}
+
+	add = fmt.Sprintf("static domain_name_servers=%s\n\n",
+		dnsIP)
+	body = append(body, []byte(add)...)
+
+	return string(body)
+}