net/dns/resolvconffile: fix handling of multiple search domains

Fixes #6875

Change-Id: I57eb9312c9a1c81792ce2b5a0a0f254213b05df2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/resolvconffile/resolvconffile.go

@@ -83,17 +83,26 @@ func Parse(r io.Reader) (*Config, error) {
 		}
 
 		if s, ok := strs.CutPrefix(line, "search"); ok {
-			domain := strings.TrimSpace(s)
-			if len(domain) == len(s) {
+			domains := strings.TrimSpace(s)
+			if len(domains) == len(s) {
 				// No leading space?!
-				return nil, fmt.Errorf("missing space after \"domain\" in %q", line)
+				return nil, fmt.Errorf("missing space after \"search\" in %q", line)
+			}
+			for len(domains) > 0 {
+				domain := domains
+				i := strings.IndexAny(domain, " \t")
+				if i != -1 {
+					domain = domain[:i]
+					domains = strings.TrimSpace(domains[i+1:])
+				} else {
+					domains = ""
 				}
 				fqdn, err := dnsname.ToFQDN(domain)
 				if err != nil {
-				return nil, fmt.Errorf("parsing search domains %q: %w", line, err)
+					return nil, fmt.Errorf("parsing search domain %q in %q: %w", domain, line, err)
 				}
 				config.SearchDomains = append(config.SearchDomains, fqdn)
-			continue
+			}
 		}
 	}
 	return config, nil

net/dns/resolvconffile/resolvconffile_test.go

@@ -57,6 +57,31 @@ func TestParse(t *testing.T) {
 		},
 		{in: `searchtailsacle.com`, wantErr: true},
 		{in: `search`, wantErr: true},
+
+		// Issue 6875: there can be multiple search domains, and even if they're
+		// over 253 bytes long total.
+		{
+			in: "search search-01.example search-02.example search-03.example search-04.example search-05.example search-06.example search-07.example search-08.example search-09.example search-10.example search-11.example search-12.example search-13.example search-14.example search-15.example\n",
+			want: &Config{
+				SearchDomains: []dnsname.FQDN{
+					"search-01.example.",
+					"search-02.example.",
+					"search-03.example.",
+					"search-04.example.",
+					"search-05.example.",
+					"search-06.example.",
+					"search-07.example.",
+					"search-08.example.",
+					"search-09.example.",
+					"search-10.example.",
+					"search-11.example.",
+					"search-12.example.",
+					"search-13.example.",
+					"search-14.example.",
+					"search-15.example.",
+				},
+			},
+		},
 	}
 
 	for _, tt := range tests {