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Use static IP address for dhcpd

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For containers with user-defined bridge networks, Docker creates an
"embedded" DNS server, that listens on the loopback interface.
`generate-dhcpd-conf` was trying to pass this local DNS server to the VM,
causing it to fail all DNS queries.

This change makes `generate-dhcpd-conf` return static IP addresses, by
default `8.8.8.8`,`8.8.4.4`. This seems like a reasonable enough middle
ground between "no DNS" and trying to proxy VM DNS requests to the
embedded docker DNS server.

This has the unfortunate side-effect that the VM cannot resolve the
names of containers on the same user-defined network (though it can
still connect to them by IP). Hopefully I'll eventually get around to
fixing this.
This commit is contained in:
Josh Kunz 2019-03-11 23:41:58 -07:00
parent e78aac3b9c
commit a8e258e5e2
1 changed files with 11 additions and 17 deletions
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28
generate-dhcpd-conf
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@ -7,11 +7,10 @@ import re
import socket import socket
import subprocess import subprocess
from typing import List from typing import List, Iterable
DEFAULT_ROUTE = "default" DEFAULT_ROUTE = 'default'
NS_IP_RE = re.compile(r'^nameserver\s+(\S+)$') DEFAULT_DNS_IPS = ('8.8.8.8', '8.8.4.4')
RESOLV_CONF_PATH = '/etc/resolv.conf'
DHCP_CONF_TEMPLATE = """ DHCP_CONF_TEMPLATE = """
start {host_addr} start {host_addr}
@ -29,16 +28,6 @@ option subnet {subnet}
option hostname {hostname} option hostname {hostname}
""" """
def nameservers() -> List[str]:
"""Returns the list of nameserver IPs in resolv.conf"""
result = []
with open(RESOLV_CONF_PATH) as resolv_f:
for line in resolv_f:
match = NS_IP_RE.match(line)
if match:
result.append(match.group(1))
return result
def default_route(routes): def default_route(routes):
"""Returns the host's default route""" """Returns the host's default route"""
for route in routes: for route in routes:
@ -57,7 +46,7 @@ def addr_of(addrs, dev : str) -> ipaddress.IPv4Interface:
return ipaddress.IPv4Interface((info['local'], info['prefixlen'])) return ipaddress.IPv4Interface((info['local'], info['prefixlen']))
raise ValueError('dev {0} not found'.format(dev)) raise ValueError('dev {0} not found'.format(dev))
def generate_conf(intf_name : str) -> str: def generate_conf(intf_name : str, dns : Iterable[str]) -> str:
"""Generates a dhcpd config. `intf_name` is the interface to listen on.""" """Generates a dhcpd config. `intf_name` is the interface to listen on."""
with subprocess.Popen(['ip', '-json', 'route'], with subprocess.Popen(['ip', '-json', 'route'],
stdout=subprocess.PIPE) as proc: stdout=subprocess.PIPE) as proc:
@ -71,7 +60,7 @@ def generate_conf(intf_name : str) -> str:
return DHCP_CONF_TEMPLATE.format( return DHCP_CONF_TEMPLATE.format(
dhcp_intf = intf_name, dhcp_intf = intf_name,
dns = ' '.join(nameservers()), dns = ' '.join(dns),
gateway = droute['gateway'], gateway = droute['gateway'],
host_addr = host_addr.ip, host_addr = host_addr.ip,
hostname = socket.gethostname(), hostname = socket.gethostname(),
@ -81,6 +70,11 @@ def generate_conf(intf_name : str) -> str:
if __name__ == '__main__': if __name__ == '__main__':
parser = argparse.ArgumentParser() parser = argparse.ArgumentParser()
parser.add_argument('intf_name') parser.add_argument('intf_name')
parser.add_argument('dns_ips', nargs='*')
args = parser.parse_args() args = parser.parse_args()
print(generate_conf(args.intf_name)) dns_ips = args.dns_ips
if not dns_ips:
dns_ips = DEFAULT_DNS_IPS
print(generate_conf(args.intf_name, dns_ips))