Query hosts dns servers and populate nodes config with it

clab/clab.go

@@ -25,6 +25,7 @@ import (
 	"github.com/srl-labs/containerlab/runtime/docker"
 	"github.com/srl-labs/containerlab/runtime/ignite"
 	"github.com/srl-labs/containerlab/types"
+	"github.com/srl-labs/containerlab/utils"
 	"golang.org/x/crypto/ssh"
 	"golang.org/x/exp/slices"
 )
@@ -760,3 +761,31 @@ func (c *CLab) ResolveLinks() error {
 
 	return nil
 }
+
+func (c *CLab) PrepareDNSServers() error {
+	// extract DNS servers from relevant resolv.conf files
+	DNSServers, err := utils.ExtractDNSServerFromResolvConf([]string{"/etc/resolv.conf", "/run/systemd/resolve/resolv.conf"})
+	if err != nil {
+		return err
+	}
+
+	// no DNS Servers found, return
+	if len(DNSServers) == 0 {
+		return nil
+	}
+
+	// if no dns servers are explicitly configured,
+	// we set the DNS servers that we've discovered.
+	for _, n := range c.Nodes {
+		config := n.Config()
+		if config.DNS == nil {
+			config.DNS = &types.DNSConfig{}
+		}
+
+		if n.Config().DNS.Servers == nil {
+			n.Config().DNS.Servers = DNSServers
+		}
+	}
+
+	return nil
+}

cmd/deploy.go

@@ -184,6 +184,10 @@ func deployFn(_ *cobra.Command, _ []string) error {
 		return err
 	}
 
+	if err := c.PrepareDNSServers(); err != nil {
+		return err
+	}
+
 	// determine the number of node and link worker
 	nodeWorkers, _, err := countWorkers(uint(len(c.Nodes)), uint(len(c.Links)), maxWorkers)
 	if err != nil {

nodes/srl/srl.go

@@ -59,6 +59,10 @@ set / system gnmi-server admin-state enable network-instance mgmt admin-state en
 set / system gnmi-server rate-limit 65000
 set / system gnmi-server trace-options [ request response common ]
 set / system gnmi-server unix-socket admin-state enable
+{{- if .DNSServers }}
+set / system dns network-instance mgmt
+set / system dns server-list [ {{ range $dnsserver := .DNSServers}}{{$dnsserver}} {{ end }}]
+{{- end }}
 set / system json-rpc-server admin-state enable network-instance mgmt http admin-state enable
 set / system json-rpc-server admin-state enable network-instance mgmt https admin-state enable tls-profile clab-profile
 set / system snmp community public
@@ -553,6 +557,7 @@ type srlTemplateData struct {
 	IFaces     map[string]tplIFace
 	SSHPubKeys string
 	MgmtMTU    int
+	DNSServers []string
 }
 
 // tplIFace template interface struct.
@@ -573,12 +578,13 @@ func (n *srl) addDefaultConfig(ctx context.Context) error {
 	// struct that holds data used in templating of the default config snippet
 
 	tplData := srlTemplateData{
-		TLSKey:    n.Cfg.TLSKey,
-		TLSCert:   n.Cfg.TLSCert,
-		TLSAnchor: n.Cfg.TLSAnchor,
-		Banner:    b,
-		IFaces:    map[string]tplIFace{},
-		MgmtMTU:   0,
+		TLSKey:     n.Cfg.TLSKey,
+		TLSCert:    n.Cfg.TLSCert,
+		TLSAnchor:  n.Cfg.TLSAnchor,
+		Banner:     b,
+		IFaces:     map[string]tplIFace{},
+		MgmtMTU:    0,
+		DNSServers: n.Config().DNS.Servers,
 	}
 
 	n.filterSSHPubKeys()

utils/resolve.go

@@ -0,0 +1,56 @@
+package utils
+
+import (
+	"bufio"
+	"os"
+	"regexp"
+	"strings"
+
+	log "github.com/sirupsen/logrus"
+)
+
+// ExtractDNSServerFromResolvConf takes a list of filenames
+// the files are supposed to be formated in the resolv.conf format.
+// it will extract ipv4 DNS server addresses and return these
+func ExtractDNSServerFromResolvConf(filenames []string) ([]string, error) {
+	DNSServersMap := map[string]struct{}{}
+
+	for _, filename := range filenames {
+		readFile, err := os.Open(filename)
+		if err != nil {
+			log.Debugf("Error opening host DNS config %s: %v", filename, err)
+			continue
+		}
+
+		fileScanner := bufio.NewScanner(readFile)
+		fileScanner.Split(bufio.ScanLines)
+		ipPattern := `\s*nameserver\s+((\d{1,3}\.){3}\d{1,3})`
+		// Compile the regular expression.
+		re := regexp.MustCompile(ipPattern)
+
+		// check line by line for a match
+		for fileScanner.Scan() {
+			if match := re.FindStringSubmatch(fileScanner.Text()); match != nil {
+				// skip 127.x.y.z addresses
+				if strings.HasPrefix(match[1], "127") {
+					continue
+				}
+				DNSServersMap[match[1]] = struct{}{}
+			}
+		}
+		// close the file
+		readFile.Close()
+	}
+
+	// if we've not found any DNS Servers we return
+	if len(DNSServersMap) == 0 {
+		return nil, nil
+	}
+
+	// convert the map into a slice
+	DNSServers := make([]string, 0, len(DNSServersMap))
+	for k := range DNSServersMap {
+		DNSServers = append(DNSServers, k)
+	}
+	return DNSServers, nil
+}