Parse original resolv.conf (#1270)

Handle original search domains in resolv.conf type implementations.

- parse the original resolv.conf file
- merge the search domains
- ignore the domain keyword
- append any other config lines (sortstlist, options)
- fix read origin resolv.conf from bkp in resolvconf implementation
- fix line length validation
- fix number of search domains validation

client/internal/dns/file_linux_test.go

@@ -0,0 +1,62 @@
+package dns
+
+import (
+	"fmt"
+	"testing"
+)
+
+func Test_mergeSearchDomains(t *testing.T) {
+	searchDomains := []string{"a", "b"}
+	originDomains := []string{"a", "b"}
+	mergedDomains := mergeSearchDomains(searchDomains, originDomains)
+	if len(mergedDomains) != 4 {
+		t.Errorf("invalid len of result domains: %d, want: %d", len(mergedDomains), 4)
+	}
+}
+
+func Test_mergeSearchTooMuchDomains(t *testing.T) {
+	searchDomains := []string{"a", "b", "c", "d", "e", "f", "g"}
+	originDomains := []string{"h", "i"}
+	mergedDomains := mergeSearchDomains(searchDomains, originDomains)
+	if len(mergedDomains) != 6 {
+		t.Errorf("invalid len of result domains: %d, want: %d", len(mergedDomains), 6)
+	}
+}
+
+func Test_mergeSearchTooMuchDomainsInOrigin(t *testing.T) {
+	searchDomains := []string{"a", "b"}
+	originDomains := []string{"c", "d", "e", "f", "g"}
+	mergedDomains := mergeSearchDomains(searchDomains, originDomains)
+	if len(mergedDomains) != 6 {
+		t.Errorf("invalid len of result domains: %d, want: %d", len(mergedDomains), 6)
+	}
+}
+
+func Test_mergeSearchTooLongDomain(t *testing.T) {
+	searchDomains := []string{getLongLine()}
+	originDomains := []string{"b"}
+	mergedDomains := mergeSearchDomains(searchDomains, originDomains)
+	if len(mergedDomains) != 1 {
+		t.Errorf("invalid len of result domains: %d, want: %d", len(mergedDomains), 1)
+	}
+
+	searchDomains = []string{"b"}
+	originDomains = []string{getLongLine()}
+
+	mergedDomains = mergeSearchDomains(searchDomains, originDomains)
+	if len(mergedDomains) != 1 {
+		t.Errorf("invalid len of result domains: %d, want: %d", len(mergedDomains), 1)
+	}
+}
+
+func getLongLine() string {
+	x := "search "
+	for {
+		for i := 0; i <= 9; i++ {
+			if len(x) > fileMaxLineCharsLimit {
+				return x
+			}
+			x = fmt.Sprintf("%s%d", x, i)
+		}
+	}
+}