add ipv6 nameservers only when the container has ipv6 enabled

The containers /etc/resolv.conf allways preserved the ipv6 nameserves
from the host even when the container did not supported ipv6. Check
if the cni result contains an ipv6 address or slirp4netns has ipv6
support enabled and only add the ipv6 nameservers when this is the case.

The test needs to have an ipv6 nameserver in the hosts /etc/hosts but we
should never mess with this file on the host. Therefore the test is
skipped when no ipv6 is detected.

Fixes #10158

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

libpod/container_internal_linux.go

@@ -1666,17 +1666,16 @@ func (c *Container) generateResolvConf() (string, error) {
 		return "", err
 	}
 
-	// Ensure that the container's /etc/resolv.conf is compatible with its
+	ipv6 := false
-	// network configuration.
-	// TODO: set ipv6 enable bool more sanely
-	resolv, err := resolvconf.FilterResolvDNS(contents, true, c.config.CreateNetNS)
-	if err != nil {
-		return "", errors.Wrapf(err, "error parsing host resolv.conf")
-	}
-
 	// Check if CNI gave back and DNS servers for us to add in
 	cniResponse := c.state.NetworkStatus
 	for _, i := range cniResponse {
+		for _, ip := range i.IPs {
+			// Note: only using To16() does not work since it also returns a vaild ip for ipv4
+			if ip.Address.IP.To4() == nil && ip.Address.IP.To16() != nil {
+				ipv6 = true
+			}
+		}
 		if i.DNS.Nameservers != nil {
 			cniNameServers = append(cniNameServers, i.DNS.Nameservers...)
 			logrus.Debugf("adding nameserver(s) from cni response of '%q'", i.DNS.Nameservers)
@@ -1687,6 +1686,25 @@ func (c *Container) generateResolvConf() (string, error) {
 		}
 	}
 
+	if c.config.NetMode.IsSlirp4netns() {
+		ctrNetworkSlipOpts := []string{}
+		if c.config.NetworkOptions != nil {
+			ctrNetworkSlipOpts = append(ctrNetworkSlipOpts, c.config.NetworkOptions["slirp4netns"]...)
+		}
+		slirpOpts, err := parseSlirp4netnsNetworkOptions(c.runtime, ctrNetworkSlipOpts)
+		if err != nil {
+			return "", err
+		}
+		ipv6 = slirpOpts.enableIPv6
+	}
+
+	// Ensure that the container's /etc/resolv.conf is compatible with its
+	// network configuration.
+	resolv, err := resolvconf.FilterResolvDNS(contents, ipv6, c.config.CreateNetNS)
+	if err != nil {
+		return "", errors.Wrapf(err, "error parsing host resolv.conf")
+	}
+
 	dns := make([]net.IP, 0, len(c.runtime.config.Containers.DNSServers))
 	for _, i := range c.runtime.config.Containers.DNSServers {
 		result := net.ParseIP(i)

test/system/500-networking.bats

@@ -329,4 +329,62 @@ load helpers
     run_podman network rm -f $mynetname
 }
 
+@test "podman ipv6 in /etc/resolv.conf" {
+    ipv6_regex='([0-9A-Fa-f]{0,4}:){2,7}([0-9A-Fa-f]{0,4})(%\w+)?'
+
+    # Make sure to read the correct /etc/resolv.conf file in case of systemd-resolved.
+    resolve_file=$(readlink -f /etc/resolv.conf)
+    if [[ "$resolve_file" == "/run/systemd/resolve/stub-resolv.conf" ]]; then
+        resolve_file="/run/systemd/resolve/resolv.conf"
+    fi
+
+    # If the host doesn't have an ipv6 in resolv.conf skip this test.
+    # We should never modify resolv.conf on the host.
+    if ! grep -E "$ipv6_regex" "$resolve_file"; then
+        skip "This test needs an ipv6 nameserver in $resolve_file"
+    fi
+
+    # ipv4 slirp
+    run_podman run --rm --network slirp4netns:enable_ipv6=false $IMAGE cat /etc/resolv.conf
+    if grep -E "$ipv6_regex" <<< $output; then
+        die "resolv.conf contains a ipv6 nameserver"
+    fi
+
+    # ipv6 slirp
+    run_podman run --rm --network slirp4netns:enable_ipv6=true $IMAGE cat /etc/resolv.conf
+    # "is" does not like the ipv6 regex
+    if ! grep -E "$ipv6_regex" <<< $output; then
+        die "resolv.conf does not contain a ipv6 nameserver"
+    fi
+
+    # ipv4 cni
+    local mysubnet=$(random_rfc1918_subnet)
+    local netname=testnet-$(random_string 10)
+
+    run_podman network create --subnet $mysubnet.0/24 $netname
+    is "$output" ".*/cni/net.d/$netname.conflist" "output of 'network create'"
+
+    run_podman run --rm --network $netname $IMAGE cat /etc/resolv.conf
+    if grep -E "$ipv6_regex" <<< $output; then
+        die "resolv.conf contains a ipv6 nameserver"
+    fi
+
+    run_podman network rm -f $netname
+
+    # ipv6 cni
+    mysubnet=fd00:4:4:4:4::/64
+    netname=testnet-$(random_string 10)
+
+    run_podman network create --subnet $mysubnet $netname
+    is "$output" ".*/cni/net.d/$netname.conflist" "output of 'network create'"
+
+    run_podman run --rm --network $netname $IMAGE cat /etc/resolv.conf
+    # "is" does not like the ipv6 regex
+    if ! grep -E "$ipv6_regex" <<< $output; then
+        die "resolv.conf does not contain a ipv6 nameserver"
+    fi
+
+    run_podman network rm -f $netname
+}
+
 # vim: filetype=sh