Pull request: dhcpd: fix possible infinite loop, set timeouts better

Merge in DNS/adguard-home from 2301-dhcp-check to master

Closes #2301.

Squashed commit of the following:

commit bf5c31b8592f909a372fcdaebacc491d310cc3e6
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Nov 13 14:28:54 2020 +0300

    dhcpd: fix possible infinite loop, set timeouts better

CHANGELOG.md

@@ -13,3 +13,7 @@ and this project adheres to
 
 - This changelog :-) (#2294).
 - `HACKING.md`, a guide for developers.
+
+### Fixed
+
+- Infinite loop in `/dhcp/find_active_dhcp` (#2301).

internal/dhcpd/check_other_dhcp.go

@@ -88,13 +88,17 @@ func CheckIfOtherDHCPServersPresentV4(ifaceName string) (bool, error) {
 	for {
 		ok, next, err := tryConn(req, c, iface)
 		if next {
+			if err != nil {
+				log.Debug("dhcpv4: trying a connection: %s", err)
+			}
+
 			continue
 		}
 		if ok {
 			return true, nil
 		}
 		if err != nil {
-			log.Debug("%s", err)
+			return false, err
 		}
 	}
 }
@@ -107,7 +111,11 @@ func tryConn(req *dhcpv4.DHCPv4, c net.PacketConn, iface *net.Interface) (ok, ne
 	log.Tracef("waiting %v for an answer", defaultDiscoverTime)
 
 	b := make([]byte, 1500)
-	_ = c.SetReadDeadline(time.Now().Add(defaultDiscoverTime))
+	err = c.SetDeadline(time.Now().Add(defaultDiscoverTime))
+	if err != nil {
+		return false, false, fmt.Errorf("setting deadline: %w", err)
+	}
+
 	n, _, err := c.ReadFrom(b)
 	if err != nil {
 		if isTimeout(err) {