Merge pull request #773 from rhatdan/ipv6

Add description of slirp options

docs/containers.conf.5.md

@@ -471,7 +471,19 @@ Path to the slirp4netns binary.
 
 Default options to pass to the slirp4netns binary.
 
-Example "allow_host_loopback=true"
+Valid options values are:
+
+  - **allow_host_loopback=true|false**: Allow the slirp4netns to reach the host loopback IP (`10.0.2.2`, which is added to `/etc/hosts` as `host.containers.internal` for your convenience). Default is false.
+  - **mtu=MTU**: Specify the MTU to use for this network. (Default is `65520`).
+  - **cidr=CIDR**: Specify ip range to use for this network. (Default is `10.0.2.0/24`).
+  - **enable_ipv6=true|false**: Enable IPv6. Default is false. (Required for `outbound_addr6`).
+  - **outbound_addr=INTERFACE**: Specify the outbound interface slirp should bind to (ipv4 traffic only).
+  - **outbound_addr=IPv4**: Specify the outbound ipv4 address slirp should bind to.
+  - **outbound_addr6=INTERFACE**: Specify the outbound interface slirp should bind to (ipv6 traffic only).
+  - **outbound_addr6=IPv6**: Specify the outbound ipv6 address slirp should bind to.
+  - **port_handler=rootlesskit**: Use rootlesskit for port forwarding. Default.
+  Note: Rootlesskit changes the source IP address of incoming packets to a IP address in the container network namespace, usually `10.0.2.100`. If your application requires the real source IP address, e.g. web server logs, use the slirp4netns port handler. The rootlesskit port handler is also used for rootless containers when connected to user-defined networks.
+  - **port_handler=slirp4netns**: Use the slirp4netns port forwarding, it is slower than rootlesskit but preserves the correct source IP address. This port handler cannot be used for user-defined networks.
 
 **no_pivot_root**=false