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Modify /etc/resolv.conf when connecting/disconnecting
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The `podman network connect` and `podman network disconnect`
commands give containers access to different networks than the
ones they were created with; these networks can also have DNS
servers associated with them. Until now, however, we did not
modify resolv.conf as network membership changed.

With this PR, `podman network connect` will add any new
nameservers supported by the new network to the container's
/etc/resolv.conf, and `podman network disconnect` command will do
the opposite, removing the network's nameservers from
`/etc/resolv.conf`.

Fixes containers#9603

Signed-off-by: Matthew Heon <matthew.heon@pm.me>
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@mheon
mheon committed Feb 9, 2022
1 parent 6ffd598 commit 8ef6cfa
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Showing 3 changed files with 191 additions and 23 deletions.
135 changes: 114 additions & 21 deletions libpod/container_internal_linux.go
View file
Original file line number Diff line number Diff line change
Expand Up @@ -2045,19 +2045,8 @@ func (c *Container) generateResolvConf() (string, error) {
}
}

ipv6 := false
// If network status is set check for ipv6 and dns namesevers
netStatus := c.getNetworkStatus()
for _, status := range netStatus {
for _, netInt := range status.Interfaces {
for _, netAddress := range netInt.Subnets {
// Note: only using To16() does not work since it also returns a valid ip for ipv4
if netAddress.IPNet.IP.To4() == nil && netAddress.IPNet.IP.To16() != nil {
ipv6 = true
}
}
}

if status.DNSServerIPs != nil {
for _, nsIP := range status.DNSServerIPs {
networkNameServers = append(networkNameServers, nsIP.String())
Expand All @@ -2070,16 +2059,9 @@ 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
ipv6, err := c.checkForIPv6(netStatus)
if err != nil {
return "", err
}

// Ensure that the container's /etc/resolv.conf is compatible with its
Expand Down Expand Up @@ -2160,6 +2142,117 @@ func (c *Container) generateResolvConf() (string, error) {
return destPath, nil
}

// Check if a container uses IPv6.
func (c *Container) checkForIPv6(netStatus map[string]types.StatusBlock) (bool, error) {
ipv6 := false
for _, status := range netStatus {
for _, netInt := range status.Interfaces {
for _, netAddress := range netInt.Subnets {
// Note: only using To16() does not work since it also returns a valid ip for ipv4
if netAddress.IPNet.IP.To4() == nil && netAddress.IPNet.IP.To16() != nil {
ipv6 = true
}
}
}
}

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 false, err
}
ipv6 = slirpOpts.enableIPv6
}

return ipv6, nil
}

// Add a new nameserver to the container's resolv.conf, ensuring that it is the
// first nameserver present.
// Usable only with running containers.
func (c *Container) addNameserver(ips []string) error {
// Take no action if container is not running.
if !c.ensureState(define.ContainerStateRunning, define.ContainerStateCreated) {
return nil
}

// Do we have a resolv.conf at all?
path, ok := c.state.BindMounts["/etc/resolv.conf"]
if !ok {
return nil
}

// Read in full contents, parse out existing nameservers
contents, err := ioutil.ReadFile(path)
if err != nil {
return err
}
ns := resolvconf.GetNameservers(contents)
options := resolvconf.GetOptions(contents)
search := resolvconf.GetSearchDomains(contents)

// We could verify that it doesn't already exist
// but extra nameservers shouldn't harm anything.
// Ensure we are the first entry in resolv.conf though, otherwise we
// might be after user-added servers.
ns = append(ips, ns...)

// We're rewriting the container's resolv.conf as part of this, but we
// hold the container lock, so there should be no risk of parallel
// modification.
if _, err := resolvconf.Build(path, ns, search, options); err != nil {
return errors.Wrapf(err, "error adding new nameserver to container %s resolv.conf", c.ID())
}

return nil
}

// Remove an entry from the existing resolv.conf of the container.
// Usable only with running containers.
func (c *Container) removeNameserver(ips []string) error {
// Take no action if container is not running.
if !c.ensureState(define.ContainerStateRunning, define.ContainerStateCreated) {
return nil
}

// Do we have a resolv.conf at all?
path, ok := c.state.BindMounts["/etc/resolv.conf"]
if !ok {
return nil
}

// Read in full contents, parse out existing nameservers
contents, err := ioutil.ReadFile(path)
if err != nil {
return err
}
ns := resolvconf.GetNameservers(contents)
options := resolvconf.GetOptions(contents)
search := resolvconf.GetSearchDomains(contents)

toRemove := make(map[string]bool)
for _, ip := range ips {
toRemove[ip] = true
}

newNS := make([]string, 0, len(ns))
for _, server := range ns {
if !toRemove[server] {
newNS = append(newNS, server)
}
}

if _, err := resolvconf.Build(path, newNS, search, options); err != nil {
return errors.Wrapf(err, "error removing nameservers from container %s resolv.conf", c.ID())
}

return nil
}

// updateHosts updates the container's hosts file
func (c *Container) updateHosts(path string) error {
var hosts string
Expand Down
48 changes: 46 additions & 2 deletions libpod/networking_linux.go
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1170,6 +1170,7 @@ func (c *Container) NetworkDisconnect(nameOrID, netName string, force bool) erro
}

// update network status if container is running
oldStatus, statusExist := networkStatus[netName]
delete(networkStatus, netName)
c.state.NetworkStatus = networkStatus
err = c.save()
Expand All @@ -1180,8 +1181,26 @@ func (c *Container) NetworkDisconnect(nameOrID, netName string, force bool) erro
// Reload ports when there are still connected networks, maybe we removed the network interface with the child ip.
// Reloading without connected networks does not make sense, so we can skip this step.
if rootless.IsRootless() && len(networkStatus) > 0 {
return c.reloadRootlessRLKPortMapping()
if err := c.reloadRootlessRLKPortMapping(); err != nil {
return err
}
}

// Update resolv.conf if required
if statusExist {
stringIPs := make([]string, 0, len(oldStatus.DNSServerIPs))
for _, ip := range oldStatus.DNSServerIPs {
stringIPs = append(stringIPs, ip.String())
}
if len(stringIPs) == 0 {
return nil
}
logrus.Debugf("Removing DNS Servers %v from resolv.conf", stringIPs)
if err := c.removeNameserver(stringIPs); err != nil {
return err
}
}

return nil
}

Expand Down Expand Up @@ -1263,11 +1282,36 @@ func (c *Container) NetworkConnect(nameOrID, netName string, netOpts types.PerNe
if err != nil {
return err
}

// The first network needs a port reload to set the correct child ip for the rootlessport process.
// Adding a second network does not require a port reload because the child ip is still valid.
if rootless.IsRootless() && len(networks) == 0 {
return c.reloadRootlessRLKPortMapping()
if err := c.reloadRootlessRLKPortMapping(); err != nil {
return err
}
}

ipv6, err := c.checkForIPv6(networkStatus)
if err != nil {
return err
}

// Update resolv.conf if required
stringIPs := make([]string, 0, len(results[netName].DNSServerIPs))
for _, ip := range results[netName].DNSServerIPs {
if (ip.To4() == nil) && !ipv6 {
continue
}
stringIPs = append(stringIPs, ip.String())
}
if len(stringIPs) == 0 {
return nil
}
logrus.Debugf("Adding DNS Servers %v to resolv.conf", stringIPs)
if err := c.addNameserver(stringIPs); err != nil {
return err
}

return nil
}

Expand Down
31 changes: 31 additions & 0 deletions test/e2e/network_connect_disconnect_test.go
View file
Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@ package integration

import (
"os"
"strings"

. "github.com/containers/podman/v4/test/utils"
"github.com/containers/storage/pkg/stringid"
Expand Down Expand Up @@ -77,6 +78,11 @@ var _ = Describe("Podman network connect and disconnect", func() {
Expect(session).Should(Exit(0))
defer podmanTest.removeNetwork(netName)

gw := podmanTest.Podman([]string{"network", "inspect", netName, "--format", "{{(index .Subnets 0).Gateway}}"})
gw.WaitWithDefaultTimeout()
Expect(gw).Should(Exit(0))
ns := gw.OutputToString()

ctr := podmanTest.Podman([]string{"run", "-dt", "--name", "test", "--network", netName, ALPINE, "top"})
ctr.WaitWithDefaultTimeout()
Expect(ctr).Should(Exit(0))
Expand All @@ -85,6 +91,11 @@ var _ = Describe("Podman network connect and disconnect", func() {
exec.WaitWithDefaultTimeout()
Expect(exec).Should(Exit(0))

exec2 := podmanTest.Podman([]string{"exec", "-it", "test", "cat", "/etc/resolv.conf"})
exec2.WaitWithDefaultTimeout()
Expect(exec2).Should(Exit(0))
Expect(strings.Contains(exec2.OutputToString(), ns)).To(BeTrue())

dis := podmanTest.Podman([]string{"network", "disconnect", netName, "test"})
dis.WaitWithDefaultTimeout()
Expect(dis).Should(Exit(0))
Expand All @@ -98,6 +109,11 @@ var _ = Describe("Podman network connect and disconnect", func() {
exec = podmanTest.Podman([]string{"exec", "-it", "test", "ip", "addr", "show", "eth0"})
exec.WaitWithDefaultTimeout()
Expect(exec).Should(ExitWithError())

exec3 := podmanTest.Podman([]string{"exec", "-it", "test", "cat", "/etc/resolv.conf"})
exec3.WaitWithDefaultTimeout()
Expect(exec3).Should(Exit(0))
Expect(strings.Contains(exec3.OutputToString(), ns)).To(BeFalse())
})

It("bad network name in connect should result in error", func() {
Expand Down Expand Up @@ -182,6 +198,16 @@ var _ = Describe("Podman network connect and disconnect", func() {
Expect(session).Should(Exit(0))
defer podmanTest.removeNetwork(newNetName)

gw := podmanTest.Podman([]string{"network", "inspect", newNetName, "--format", "{{(index .Subnets 0).Gateway}}"})
gw.WaitWithDefaultTimeout()
Expect(gw).Should(Exit(0))
ns := gw.OutputToString()

exec2 := podmanTest.Podman([]string{"exec", "-it", "test", "cat", "/etc/resolv.conf"})
exec2.WaitWithDefaultTimeout()
Expect(exec2).Should(Exit(0))
Expect(strings.Contains(exec2.OutputToString(), ns)).To(BeFalse())

ip := "10.11.100.99"
mac := "44:11:44:11:44:11"
connect := podmanTest.Podman([]string{"network", "connect", "--ip", ip, "--mac-address", mac, newNetName, "test"})
Expand All @@ -206,6 +232,11 @@ var _ = Describe("Podman network connect and disconnect", func() {
Expect(exec.OutputToString()).Should(ContainSubstring(ip))
Expect(exec.OutputToString()).Should(ContainSubstring(mac))

exec3 := podmanTest.Podman([]string{"exec", "-it", "test", "cat", "/etc/resolv.conf"})
exec3.WaitWithDefaultTimeout()
Expect(exec3).Should(Exit(0))
Expect(strings.Contains(exec3.OutputToString(), ns)).To(BeTrue())

// make sure no logrus errors are shown https://github.com/containers/podman/issues/9602
rm := podmanTest.Podman([]string{"rm", "--time=0", "-f", "test"})
rm.WaitWithDefaultTimeout()
Expand Down

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