Pull request: aghnet: do not expect dhcpdc.conf to exist

Updates #3257.

Squashed commit of the following:

commit f3c3359
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Jun 15 19:56:23 2021 +0300

    aghnet: fix docs, names

commit 513ade2
Merge: aa58f1d 9f5a015
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Jun 15 19:43:24 2021 +0300

    Merge branch 'master' into 3257-dhcpdc-conf

commit aa58f1d
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Jun 15 17:18:19 2021 +0300

    aghnet: do not expect dhcpdc.conf to exist

CHANGELOG.md

@@ -48,6 +48,7 @@ released by then.
 
 ### Fixed
 
+- Errors when setting static IP on Linux ([#3257]).
 - Treatment of domain names and FQDNs in custom rules with `$dnsrewrite` that
   use the `PTR` type ([#3256]).
 - Redundant hostname generating while loading static leases with empty hostname
@@ -75,6 +76,7 @@ released by then.
 [#3194]: https://github.com/AdguardTeam/AdGuardHome/issues/3194
 [#3198]: https://github.com/AdguardTeam/AdGuardHome/issues/3198
 [#3256]: https://github.com/AdguardTeam/AdGuardHome/issues/3256
+[#3257]: https://github.com/AdguardTeam/AdGuardHome/issues/3257
 
 
 

internal/aghnet/net_linux.go

@@ -16,7 +16,8 @@ import (
 	"github.com/google/renameio/maybe"
 )
 
-// maxConfigFileSize is the maximum length of interfaces configuration file.
+// maxConfigFileSize is the maximum assumed length of the interface
+// configuration file.
 const maxConfigFileSize = 1024 * 1024
 
 func ifaceHasStaticIP(ifaceName string) (has bool, err error) {
@@ -130,17 +131,19 @@ func ifacesStaticConfig(r io.Reader, ifaceName string) (has bool, err error) {
 	return false, s.Err()
 }
 
+// ifaceSetStaticIP configures the system to retain its current IP on the
+// interface through dhcpdc.conf.
 func ifaceSetStaticIP(ifaceName string) (err error) {
 	ipNet := GetSubnet(ifaceName)
 	if ipNet.IP == nil {
 		return errors.Error("can't get IP address")
 	}
 
 	gatewayIP := GatewayIP(ifaceName)
-	add := updateStaticIPdhcpcdConf(ifaceName, ipNet.String(), gatewayIP, ipNet.IP)
+	add := dhcpcdConfIface(ifaceName, ipNet, gatewayIP, ipNet.IP)
 
 	body, err := os.ReadFile("/etc/dhcpcd.conf")
-	if err != nil {
+	if err != nil && !errors.Is(err, os.ErrNotExist) {
 		return err
 	}
 
@@ -153,23 +156,23 @@ func ifaceSetStaticIP(ifaceName string) (err error) {
 	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 {
+// dhcpcdConfIface returns configuration lines for the dhcpdc.conf files that
+// configure the interface to have a static IP.
+func dhcpcdConfIface(ifaceName string, ipNet *net.IPNet, gatewayIP, dnsIP net.IP) (conf string) {
 	var body []byte
 
-	add := fmt.Sprintf("\ninterface %s\nstatic ip_address=%s\n",
-		ifaceName, ip)
+	add := fmt.Sprintf(
+		"\n# %[1]s added by AdGuard Home.\ninterface %[1]s\nstatic ip_address=%s\n",
+		ifaceName,
+		ipNet)
 	body = append(body, []byte(add)...)
 
 	if gatewayIP != nil {
-		add = fmt.Sprintf("static routers=%s\n",
-			gatewayIP)
+		add = fmt.Sprintf("static routers=%s\n", gatewayIP)
 		body = append(body, []byte(add)...)
 	}
 
-	add = fmt.Sprintf("static domain_name_servers=%s\n\n",
-		dnsIP)
+	add = fmt.Sprintf("static domain_name_servers=%s\n\n", dnsIP)
 	body = append(body, []byte(add)...)
 
 	return string(body)

internal/aghnet/net_linux_test.go

@@ -102,22 +102,29 @@ func TestSetStaticIPdhcpcdConf(t *testing.T) {
 		routers    net.IP
 	}{{
 		name: "with_gateway",
-		dhcpcdConf: nl + `interface wlan0` + nl +
+		dhcpcdConf: nl + `# wlan0 added by AdGuard Home.` + nl +
+			`interface wlan0` + nl +
 			`static ip_address=192.168.0.2/24` + nl +
 			`static routers=192.168.0.1` + nl +
 			`static domain_name_servers=192.168.0.2` + nl + nl,
 		routers: net.IP{192, 168, 0, 1},
 	}, {
 		name: "without_gateway",
-		dhcpcdConf: nl + `interface wlan0` + nl +
+		dhcpcdConf: nl + `# wlan0 added by AdGuard Home.` + nl +
+			`interface wlan0` + nl +
 			`static ip_address=192.168.0.2/24` + nl +
 			`static domain_name_servers=192.168.0.2` + nl + nl,
 		routers: nil,
 	}}
 
+	ipNet := &net.IPNet{
+		IP:   net.IP{192, 168, 0, 2},
+		Mask: net.IPMask{255, 255, 255, 0},
+	}
+
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
-			s := updateStaticIPdhcpcdConf("wlan0", "192.168.0.2/24", tc.routers, net.IP{192, 168, 0, 2})
+			s := dhcpcdConfIface("wlan0", ipNet, tc.routers, net.IP{192, 168, 0, 2})
 			assert.Equal(t, tc.dhcpcdConf, s)
 		})
 	}