UDP traffic not forwarded from Docker containers -> Docker host

Question

I have a docker container and I am unable to run DNS lookups from inside containers, although it works fine from the docker host.

The configuration management code that builds the Docker host is known to work on a standard RHEL 7 image from the marketplace, therefore the problem is known to be something inside the SOE RHEL 7 image.

RHEL 7.2 / Docker version 1.12.6, build 88a4867/1.12.6. Container is RHEL 7.3. SELinux in enabled/permissive mode. The Docker host is an Amazon EC2 instance.

Some config:

# /etc/sysconfig/docker
OPTIONS='--dns=10.0.0.10 --dns=10.0.0.11 --dns-search=example.com'
DOCKER_CERT_PATH=/etc/docker
ADD_REGISTRY='--add-registry registry.example.com'
no_proxy=169.254.169.254,localhost,127.0.0.1,registory.example.com
http_proxy=http://proxy.example.com:8080
https_proxy=http://proxy.example.com:8080
ftp_proxy=http://proxy.example.com:8080

Resolver config in the container and host is the same:

# /etc/resolv.conf
search example.com
nameserver 10.0.0.10
nameserver 10.0.0.11

If I restart the docker daemon with --debug I see the following in journalctl -u docker.service:

Aug 08 11:44:23 myhost.example.com dockerd-current[17341]: time="2017-08-08T11:44:23.430769581+10:00" level=debug msg="Name To resolve: http://proxy.example.com."
Aug 08 11:44:23 myhost.example.com dockerd-current[17341]: time="2017-08-08T11:44:23.431488213+10:00" level=debug msg="Query http://proxy.example.com.[1] from 172.18.0.6:38189, forwarding to udp:10.162.182.101"
Aug 08 11:44:27 myhost.example.com dockerd-current[17341]: time="2017-08-08T11:44:27.431772666+10:00" level=debug msg="Read from DNS server failed, read udp 172.18.0.6:38189->10.162.182.101:53: i/o timeout"

Following that observation further, it turns out I can get some networking to work if I specify an IP address instead of the DNS name of the proxy; although that really is just a way of avoiding using DNS and not a real fix.

Indeed, (update #3) it turns out I can avoid the issue completely by simply configuring DNS to use TCP instead of UDP, i.e.

# head -1 /etc/sysconfig/docker
OPTIONS="--dns=10.0.0.10 --dns=10.0.0.11 --dns-search=example.com --dns-opt=use-vc"

(Adding a line use-vc tells the resolver to use TCP instead of UDP.)

I did note some suspicious-looking rules in iptables, but these turned out to be normal:

# iptables -n -L DOCKER-ISOLATION -v --line-numbers
Chain DOCKER-ISOLATION (1 references)
num   pkts bytes target     prot opt in     out     source               destination         
1        0     0 DROP       all  --  br-1d6a05c10468 docker0  0.0.0.0/0            0.0.0.0/0           
2        0     0 DROP       all  --  docker0 br-1d6a05c10468  0.0.0.0/0            0.0.0.0/0           
3    34903   11M RETURN     all  --  *      *       0.0.0.0/0            0.0.0.0/0           

After deleting those two DROP rules, I continued to see the issue.

Full iptables:

# iptables -nL -v
Chain INPUT (policy ACCEPT 2518 packets, 1158K bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         
23348 9674K DOCKER-ISOLATION  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
    0     0 DOCKER     all  --  *      docker0  0.0.0.0/0            0.0.0.0/0           
    0     0 ACCEPT     all  --  *      docker0  0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
    0     0 ACCEPT     all  --  docker0 !docker0  0.0.0.0/0            0.0.0.0/0           
    0     0 ACCEPT     all  --  docker0 docker0  0.0.0.0/0            0.0.0.0/0           
23244 9667K DOCKER     all  --  *      br-1d6a05c10468  0.0.0.0/0            0.0.0.0/0           
23232 9667K ACCEPT     all  --  *      br-1d6a05c10468  0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
  104  6230 ACCEPT     all  --  br-1d6a05c10468 !br-1d6a05c10468  0.0.0.0/0            0.0.0.0/0           
   12   700 ACCEPT     all  --  br-1d6a05c10468 br-1d6a05c10468  0.0.0.0/0            0.0.0.0/0           

Chain OUTPUT (policy ACCEPT 2531 packets, 414K bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain DOCKER (2 references)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 ACCEPT     tcp  --  !br-1d6a05c10468 br-1d6a05c10468  0.0.0.0/0            172.18.0.2           tcp dpt:443
    0     0 ACCEPT     tcp  --  !br-1d6a05c10468 br-1d6a05c10468  0.0.0.0/0            172.18.0.2           tcp dpt:80
    0     0 ACCEPT     tcp  --  !br-1d6a05c10468 br-1d6a05c10468  0.0.0.0/0            172.18.0.3           tcp dpt:389

Chain DOCKER-ISOLATION (1 references)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 DROP       all  --  br-1d6a05c10468 docker0  0.0.0.0/0            0.0.0.0/0           
    0     0 DROP       all  --  docker0 br-1d6a05c10468  0.0.0.0/0            0.0.0.0/0           
23348 9674K RETURN     all  --  *      *       0.0.0.0/0            0.0.0.0/0           

Bridge config

# ip addr show docker0
4: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN 
    link/ether 02:42:a8:73:db:bb brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.1/16 scope global docker0
       valid_lft forever preferred_lft forever
# ip addr show br-1d6a05c10468
3: br-1d6a05c10468: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP 
    link/ether 02:42:d5:b6:2d:f5 brd ff:ff:ff:ff:ff:ff
    inet 172.18.0.1/16 scope global br-1d6a05c10468
       valid_lft forever preferred_lft forever

and

# docker network inspect bridge 
[
    {
        "Name": "bridge",
        "Id": "e159ddd37386cac91e0d011ade99a51f9fe887b8d32d212884beace67483af44",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": [
                {
                    "Subnet": "172.17.0.0/16",
                    "Gateway": "172.17.0.1"
                }
            ]
        },
        "Internal": false,
        "Containers": {},
        "Options": {
            "com.docker.network.bridge.default_bridge": "true",
            "com.docker.network.bridge.enable_icc": "true",
            "com.docker.network.bridge.enable_ip_masquerade": "true",
            "com.docker.network.bridge.host_binding_ipv4": "0.0.0.0",
            "com.docker.network.bridge.name": "docker0",
            "com.docker.network.driver.mtu": "1500"
        },
        "Labels": {}
    }
]

In the logs:

Aug 04 17:33:32 myhost.example.com systemd[1]: Starting Docker Application Container Engine...
Aug 04 17:33:33 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:33.056770003+10:00" level=info msg="libcontainerd: new containerd process, pid: 2140"
Aug 04 17:33:34 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:34.740346421+10:00" level=info msg="Graph migration to content-addressability took 0.00 seconds"
Aug 04 17:33:34 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:34.741164354+10:00" level=info msg="Loading containers: start."
Aug 04 17:33:34 myhost.example.com dockerd-current[2131]: .........................time="2017-08-04T17:33:34.903371015+10:00" level=info msg="Firewalld running: true"
Aug 04 17:33:35 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:35.325581993+10:00" level=info msg="Default bridge (docker0) is assigned with an IP address 172.17.0.0/16. Daemon option --bip can be used to set a preferred IP address" 
Aug 04 17:33:36 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:36+10:00" level=info msg="Firewalld running: true"
Aug 04 17:33:37 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:37+10:00" level=info msg="Firewalld running: true"
Aug 04 17:33:37 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:37+10:00" level=info msg="Firewalld running: true"
Aug 04 17:33:38 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:38+10:00" level=info msg="Firewalld running: true"
Aug 04 17:33:39 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:39+10:00" level=info msg="Firewalld running: true"
Aug 04 17:33:40 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:40+10:00" level=info msg="Firewalld running: true"
Aug 04 17:33:40 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:40+10:00" level=info msg="Firewalld running: true"
Aug 04 17:33:42 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:42+10:00" level=info msg="Firewalld running: true"
Aug 04 17:33:42 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:42+10:00" level=info msg="Firewalld running: true"
Aug 04 17:33:43 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:43.541905145+10:00" level=info msg="Loading containers: done."
Aug 04 17:33:43 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:43.541975618+10:00" level=info msg="Daemon has completed initialization"
Aug 04 17:33:43 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:43.541998095+10:00" level=info msg="Docker daemon" commit="88a4867/1.12.6" graphdriver=devicemapper version=1.12.6
Aug 04 17:33:43 myhost.example.com dockerd-current[2131]: time="2017-08-04T17:33:43.548508756+10:00" level=info msg="API listen on /var/run/docker.sock"
Aug 04 17:33:43 myhost.example.com systemd[1]: Started Docker Application Container Engine.

From the container, I can ping the default gateway but all name resolution fails.

I noticed one weird thing in the log (Update #2 I now know that this is a red herring - see discussion below):

# journalctl -u docker.service |grep insmod > /tmp/log # \n's replaced below
Jul 26 23:59:02 myhost.example.com dockerd-current[3185]: time="2017-07-26T23:59:02.056295890+10:00" level=warning msg="Running modprobe bridge br_netfilter failed with message: insmod /lib/modules/3.10.0-514.26.2.el7.x86_64/kernel/net/bridge/bridge.ko 
sysctl: cannot stat /proc/sys/net/bridge/bridge-nf-call-arptables: No such file or directory
sysctl: cannot stat /proc/sys/net/bridge/bridge-nf-call-iptables: No such file or directory
sysctl: cannot stat /proc/sys/net/bridge/bridge-nf-call-ip6tables: No such file or directory
modprobe: ERROR: Error running install command for bridge
modprobe: ERROR: could not insert 'bridge': Unknown error 253
insmod /lib/modules/3.10.0-514.26.2.el7.x86_64/kernel/net/llc/llc.ko 
insmod /lib/modules/3.10.0-514.26.2.el7.x86_64/kernel/net/802/stp.ko 
install /sbin/modprobe --ignore-install bridge && /sbin/sysctl -q -w net.bridge.bridge-nf-call-arptables=0 net.bridge.bridge-nf-call-iptables=0 net.bridge.bridge-nf-call-ip6tables=0 
insmod /lib/modules/3.10.0-514.26.2.el7.x86_64/kernel/net/bridge/br_netfilter.ko 
, error: exit status 1"

Update #1: and this is coming from:

# tail -2 /etc/modprobe.d/dist.conf
# Disable netfilter on bridges when the bridge module is loaded
install bridge /sbin/modprobe --ignore-install bridge && /sbin/sysctl -q -w net.bridge.bridge-nf-call-arptables=0 net.bridge.bridge-nf-call-iptables=0 net.bridge.bridge-nf-call-ip6tables=0

Also:

# cat /proc/sys/net/bridge/bridge-nf-call-{arp,ip,ip6}tables
1
1
1

However, even after I do this:

# for i in /proc/sys/net/bridge/bridge-nf-call-{arp,ip,ip6}tables ; do echo 0 > $i ; done 

Still no luck.

I spent a whole day on this so pulling my hair out by now. Any thoughts on what else I could try or what else the problem might be much appreciated.

Update #4

I performed some experiments using Netcat and I have proved that all UDP packets are not forwarded if sent from any container -> host. I tried using several ports including 53, 2115, and 50000. TCP packets are fine however. This is still true if I flush the iptables rules with iptables -F.

Furthermore, I can send UDP packets from one container to another - only UDP traffic from container -> host is not forwarded.

To set up the test:

On the host, which has IP 10.1.1.10:

# nc -u -l 50000

On the container:

# echo "foo" | nc -w1 -u 10.1.1.10 50000

During a TCP dump capture I see:

17:20:36.761214 IP (tos 0x0, ttl 64, id 48146, offset 0, flags [DF], proto UDP (17), length 32)
    172.17.0.2.41727 > 10.1.1.10.50000: [bad udp cksum 0x2afa -> 0x992f!] UDP, length 4
        0x0000:  4500 0020 bc12 4000 4011 53de ac11 0002  E.....@.@.S.....
        0x0010:  0aa5 7424 a2ff c350 000c 2afa 666f 6f0a  ..t$...P..*.foo.
        0x0020:  0000 0000 0000 0000 0000 0000 0000 0000  ................
17:20:36.761214 IP (tos 0x0, ttl 64, id 48146, offset 0, flags [DF], proto UDP (17), length 32)
    172.17.0.2.41727 > 10.1.1.10.50000: [bad udp cksum 0x2afa -> 0x992f!] UDP, length 4
        0x0000:  4500 0020 bc12 4000 4011 53de ac11 0002  E.....@.@.S.....
        0x0010:  0aa5 7424 a2ff c350 000c 2afa 666f 6f0a  ..t$...P..*.foo.
        0x0020:  0000 0000 0000 0000 0000 0000 0000 0000  ................

I tried unsuccessfully to fix the bad UDP checksums via this.

I noted, however, that the bad UDP checksums are seen even during the successful transmission of UDP packets (host -> host) and container -> container.

In summary, I now know:

• routing is fine

• iptables is flushed

• SELinux is permissive

• all TCP works in all directions

• all UDP from container -> container is fine

• all UDP from host -> host is fine

• all UDP from host -> container is fine

• BUT no UDP packets from container -> host are forwarded

Answer 1

I figured it out.

We had a Trend Micro (anti-virus) agent running in the SOE which I didn't know about.

Fixing it was as simple as:

# systemctl stop ds_agent.service
# pkill ds_agent

Not exactly sure at this point why it is blocking UDP from containers or how to stop it.

Answer 2

It seems you have a modprobe install directive that cannot work. Possibly it's a result of incomplete update to RHEL 7.2 or some manual fixes.

Try grep -r bridge /etc/modprobe.d /lib/modprobe.d for starters, or otherwise dig around /etc/modprobe.d or /lib/modprobe.d and try to find where does it define the install rule that calls sysctl -q -w net.bridge.bridge-nf-call-arptables=0 net.bridge.bridge-nf-call-iptables=0 net.bridge.bridge-nf-call-ip6tables=0

This sysctl is clearly in wrong place. It is either superfluous or should appear after br_netfilter, not before. Why? Recently the /proc/sys/net/bridge handling has been moved from the bridge module to the br_netfilter module. This happens with some version of kernel*.rpm, while the contents of modprobe.d directories are distributed with other individual packages. I've verified on my RHEL 7.2:

# modprobe bridge
# sysctl -q -w net.bridge.bridge-nf-call-iptables=0
sysctl: cannot stat /proc/sys/net/bridge/bridge-nf-call-iptables: No such file or directory
# modprobe br_netfilter
# sysctl -q -w net.bridge.bridge-nf-call-iptables=0    # ok now

I don't see these "broken" rules on my vanilla RHEL 7.1 and their origin is mysterious to me. I've tried:

# modprobe -n -vvv bridge
modprobe: INFO: custom logging function 0x40a130 registered
insmod /lib/modules/3.10.0-229.11.1.el7.x86_64/kernel/net/llc/llc.ko
insmod /lib/modules/3.10.0-229.11.1.el7.x86_64/kernel/net/802/stp.ko
insmod /lib/modules/3.10.0-229.11.1.el7.x86_64/kernel/net/bridge/bridge.ko
modprobe: INFO: context 0xf1c270 released
# echo "install bridge echo example_of_a_modprobe_rule" > /etc/modprobe.d/zzz5.conf
# modprobe -n -vvv bridge
modprobe: INFO: custom logging function 0x40a130 registered
insmod /lib/modules/3.10.0-229.11.1.el7.x86_64/kernel/net/llc/llc.ko
insmod /lib/modules/3.10.0-229.11.1.el7.x86_64/kernel/net/802/stp.ko
install echo example_of_a_modprobe_rule
modprobe: INFO: context 0xeaa270 released
# rm /etc/modprobe.d/zzz5.conf

Update: Looks like xenserver uses a similar modprobe hack. It's a nasty bug to globally change kernel module behavior for everyone whether you actually run xenserver or not; and the bug has fired back at us.

Update 2: Now, you've found that /etc/modprobe.d/dist.conf causes this problem and not docker. Whether you have docker or not, modprobe bridge will always return 1 and print error. Normally dist.conf is a part of module-init-tools package on RHEL6. This file is not supposed to be used on RHEL7. It's not on any of my RHEL7 systems and they run just fine. In RHEL7 the package is kmod and it doesn't contain dist.conf. I would:

rpm -qf /etc/modprobe.d/dist.conf  # what package owns this file?

If dist.conf is not owned by package, backup it and delete any lines that don't give you any obvious benefit (maybe even delete the file altogether).

If dist.conf is owned by a package, consider removing/updating that package, since it became clearly buggy in terms of RHEL 7.2 compatibility.

Answer 3

I had problems with the DNS resolver in our docker containers. I tried a lot of different things, and in the end, I just figured that my VPS in Hostgator didn't have installed by default NetworkManager-tui (nmtui), I just installed and reboot it.

sudo yum install NetworkManager-tui

And reconfigured my resolv.conf with default DNS as 8.8.8.8.

nano /etc/resolv.conf