OpenBSD FAQ - Networking [FAQ Index]

Network Configuration

Network configuration in OpenBSD is done with text files in /etc. Typically, these settings are initially configured during the installation process.

Identifying and Setting Up Network Interfaces

Interfaces are named by the type of card, not the type of connection. For example, here's a dmesg(8) snippet for an Intel Fast Ethernet network card: 
fxp0 at pci0 dev 10 function 0 "Intel 82557" rev 0x0c: irq 5, address 00:02:b3:2b:10:f7
inphy0 at fxp0 phy 1: i82555 10/100 media interface, rev. 4
This device uses the fxp(4) driver and is assigned the number 0 here.

The ifconfig(8) utility will show what network interfaces have been identified on a system. 

$ ifconfig
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 33200
        index 3 priority 0 llprio 3
        groups: lo
        inet 127.0.0.1 netmask 0xff000000
fxp0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        lladdr 00:02:b3:2b:10:f7
        index 1 priority 0 llprio 3
        media: Ethernet autoselect (100baseTX full-duplex)
        status: active
        inet 10.0.0.38 netmask 0xffffff00 broadcast 10.0.0.255
enc0: flags=0<>
        index 2 priority 0 llprio 3
        groups: enc
        status: active
pflog0: flags=141<UP,RUNNING,PROMISC> mtu 33200
        index 4 priority 0 llprio 3
        groups: pflog
This sample shows only one physical Ethernet interface: fxp0. An IP is configured on it, hence the values inet 10.0.0.38 netmask 0xffffff00 broadcast 10.0.0.255. The UP and RUNNING flags are also set on it.

The netstart(8) script configures network interfaces at boot time using hostname.if(5) files, where "if" is replaced by the full name of each interface. The example above would use the file /etc/hostname.fxp0, containing the following options: 

inet 10.0.0.38 255.255.255.0
This hostname.fxp0 file also has an interactive equivalent: 
# ifconfig fxp0 10.0.0.38 255.255.255.0
Several other interfaces come enabled by default. These are virtual interfaces that serve various functions. The following manual pages describe them: Other virtual interfaces can be added with ifconfig(8)'s create subcommand.

Default Hostname and Gateway

The /etc/myname and /etc/mygate files are read by the netstart(8) script. Both of these files consist of a single line, specifying the fully qualified domain name of the system and the address of the gateway host, respectively. The /etc/mygate file need not exist on all systems. See myname(5) for more details.

DNS Resolution

DNS resolution is controlled by the resolv.conf(5) file, which is managed by resolvd(8). 
$ cat /etc/resolv.conf
search example.com
nameserver 125.2.3.4
nameserver 125.2.3.5
lookup file bind
Here the default domain name will be example.com, there will be two name servers (125.2.3.4 and 125.2.3.5) and the hosts(5) file will be consulted before the name servers.

Activating the Changes

From here, either reboot or run the netstart(8) script: 
# sh /etc/netstart
A few warnings may be produced when running this script if the interfaces have already been configured. Use ifconfig(8) to make sure the interfaces were set up correctly.

Even though it's possible to completely reconfigure networking on a running OpenBSD system, a reboot is recommended after any significant changes.

Checking Routes

Routes can be checked via netstat(1) or route(8). 
$ netstat -rn
Routing tables

Internet:
Destination        Gateway            Flags     Refs     Use    Mtu  Prio Iface
default            10.0.0.1           UGS         4       16      -    12 fxp0
224/4              127.0.0.1          URS         0        0  32768     8 lo0
127/8              127.0.0.1          UGRS        0        0  32768     8 lo0
127.0.0.1          127.0.0.1          UH          2       15  32768     1 lo0
10.0.0/24          link#1             UC          1        4      -     4 fxp0
10.0.0.1           aa:0:4:0:81:d      UHL         1       11      -     1 fxp0
10.0.0.38          127.0.0.1          UGHS        0        0      -     1 lo0

$ route show
Routing tables

Internet:
Destination        Gateway            Flags     Refs     Use    Mtu  Prio Iface
default            10.0.0.1           UGS         4       16      -    12 fxp0
base-address.mcast localhost          URS         0        0  32768     8 lo0
loopback           localhost          UGRS        0        0  32768     8 lo0
localhost          localhost          UH          2       15  32768     1 lo0
10.0.0/24          link#1             UC          1        4      -     4 fxp0
10.0.0.1           aa:0:4:0:81:d      UHL         1       11      -     1 fxp0
10.0.0.38          localhost          UGHS        0        0      -     1 lo0

Setting Up Aliases on an Interface

To set up an IP alias on an interface, simply edit its hostname.if(5) file.

In this example, two aliases are added to the interface dc0, which was configured as 192.168.0.2 with a netmask of 255.255.255.0. 

$ cat /etc/hostname.dc0
inet 192.168.0.2 255.255.255.0
inet alias 192.168.0.3 255.255.255.255
inet alias 192.168.0.4 255.255.255.255
Once this file has been created, run netstart or reboot. To view all aliases, use ifconfig -A.

Dynamic Host Configuration Protocol

The Dynamic Host Configuration Protocol (DHCP) is a way to configure network interfaces automatically. OpenBSD can be a DHCP server that configures other machines or a DHCP client that is configured by a DHCP server.

DHCP Client

To use dhcpleased(8), edit the hostname.if(5) file of the network interface. The wireless networking section explains how to set up wireless interfaces. For ethernet interfaces, one line is enough: 
inet autoconf
OpenBSD will gather its IP address, default gateway, and DNS servers from the DHCP server at startup time. Other options can be specified in dhcpleased.conf(5).

To get an IP via DHCP from the command line, run: 

# ifconfig xl0 inet autoconf
Replace xl0 with the interface name.

DHCP Server

To use OpenBSD as a DHCP server, enable the dhcpd(8) daemon at startup: 
# rcctl enable dhcpd
On the next boot, dhcpd will run and attach to all NICs that have valid configurations in dhcpd.conf(5). Individual interfaces may be specified instead by naming them explicitly: 
# rcctl set dhcpd flags em1 em2
An example /etc/dhcpd.conf file might look like this: 
# Home
subnet 192.168.1.0 netmask 255.255.255.0 {
	option domain-name-servers 192.168.1.2;
	option routers 192.168.1.1;
	range 192.168.1.3 192.168.1.50;
}

# Guests
subnet 172.16.0.0 netmask 255.255.255.0 {
	option domain-name-servers 1.2.3.4, 5.6.7.8;
	option routers 172.16.0.1;
	range 172.16.0.2 172.16.0.254;
}
There are two subnets in this example: a home network and a guest network. Clients will automatically be given an IP address and pointed to the gateway and DNS servers specified in their respective sections of the config file. See dhcp-options(5) for more options.

PXE Booting (i386, amd64)

The Preboot Execution Environment (PXE) is a standard method of booting systems using only the network. A client's PXE-capable NIC broadcasts a DHCP request at the start of the boot process and, rather than only receiving basic IP/DNS information, is also given a file to boot from. On OpenBSD, this file is known as pxeboot(8), and is typically served by tftpd(8).

Wireless Networking

OpenBSD has support for a number of wireless chipsets. Further supported devices can be found in usb(4) and pci(4). The precise extent of their support is described in the driver man pages.

The following cards support Host-based Access Point (HostAP) mode, permitting them to be used as a wireless access point:

The ifconfig(8) media subcommand shows media capabilities of network interfaces. For wireless devices, it displays supported 802.11a/b/g/n media modes and the supported operating modes (hostap, ibss, monitor). For example, to see media capabilities of interface ath0, run: 
$ ifconfig ath0 media
Firmware files acquired with fw_update(8) may be needed in order to use certain wireless cards. Some manufacturers refuse to allow free distribution of their firmware, so it can't be included with OpenBSD.

Another option to consider: use a conventional NIC and an external bridging wireless access point for the OpenBSD-based firewall.

Configuring a Wireless Adapter

Adapters based on supported chips can be used like any other network interface. To connect an OpenBSD system to an existing wireless network, use the ifconfig(8) utility.

An example of a hostname.if(5) file for a wireless client might be: 

nwid puffyuberalles wpakey passwordhere
inet autoconf
Or, for multiple access points: 
join home-net wpakey passwordhere
join work-net wpakey passwordhere
join cafe-wifi
inet autoconf
Note that inet autoconf should be after the other configuration lines, as the network adapter will not be able to send a DHCP request until it is configured.

Trunking a Wireless Adapter

Trunks are virtual interfaces consisting of one or more network interfaces. In this section, our example will be a laptop with a wired bge0 interface and a wireless iwn0 interface. We will build a trunk(4) interface using both of them. The wired and wireless interfaces must be connected to the same layer two network.

To do this, we first activate the two physical ports, then assign them to trunk0. 

# echo up > /etc/hostname.bge0
The wireless interface, however, needs a bit more configuration. It will need to attach to our wireless WPA-protected network: 
$ cat /etc/hostname.iwn0
nwid puffynet wpakey mysecretkey
up
Now, our trunk interface is defined like this: 
$ cat /etc/hostname.trunk0
trunkproto failover trunkport bge0
trunkport iwn0
inet autoconf
The trunk is set up in failover mode, so either interface can be used. If both are available, it will prefer the bge0 port, since that is the first one added to the trunk device.

Setting Up a Network Bridge

A bridge(4) is a link between two or more separate networks. Unlike a router, packets go through the bridge transparently: the two network segments appear as one to nodes on either side. Bridges will only forward packets that have to pass from one segment to the other and, as a result, an interface in a bridge may or may not have an IP address of its own. If it does, the interface has effectively two modes of operation: one as part of a bridge, the other as a stand-alone NIC. If neither interface has an IP address, the bridge will pass network data, but will not be externally maintainable (which can be a feature).

A Bridge Acting as a DHCP Server

Let's say we have a system which has four vr(4) interfaces, vr0 through vr3. We want to bridge vr1, vr2 and vr3 together, leaving out vr0 for the uplink. We also want to serve IP addresses through DHCP over the bridged interfaces. Being a DHCP server and an uplink router, the box needs to have an IP address on the bridged network.

It is not possible to assign an IP address directly to a bridge interface. The IP address should be added to one of the member interfaces, but we cannot use a physical interface as the link might be down, in which case the address would not be reachable. Fortunately, there is the vether(4) (virtual Ethernet) driver that can be used for this purpose. We will add it to the bridge, assign the IP address to it and make dhcpd(8) listen there.

First, mark the vr1, vr2 and vr3 interfaces as up: 
# echo up > /etc/hostname.vr1
# echo up > /etc/hostname.vr2
# echo up > /etc/hostname.vr3
Then create the vether0 configuration: 
# echo 'inet 192.168.1.1 255.255.255.0 192.168.1.255' > /etc/hostname.vether0
Configure the bridge interface to contain all the above interfaces: 
$ cat /etc/hostname.bridge0
add vether0
add vr1
add vr2
add vr3
up
And finally we make the DHCP daemon listen on the vether0 interface: 
# rcctl set dhcpd flags vether0
And finally, reboot the system.

Filtering on a Bridge

PF can be used to restrict what traffic goes through a bridge. Keep in mind, by the nature of a bridge, the same data flows through both interfaces, so filtering is only needed on one interface.

Tips on Bridging

Equal-Cost Multipath Routing

Equal-cost multipath routing refers to having multiple routes in the routing table for the same network, such as the default route, 0.0.0.0/0. When the kernel is doing a route lookup to determine where to send packets destined to that network, it can choose from any of the equal-cost routes. In most scenarios, multipath routing is used to provide redundant uplink connections, e.g., redundant connections to the internet.

The route(8) command is used to add/change/delete routes in the routing table. The -mpath argument is used when adding multipath routes. 

# route add -mpath default 10.130.128.1
# route add -mpath default 10.132.0.1
Verify the routes: 
# netstat -rnf inet | grep default
default     10.130.128.1      UGS       2      134      -     fxp1
default     10.132.0.1        UGS       0      172      -     fxp2
In this example we can see that one default route points to 10.130.128.1, which is accessible via the fxp1 interface, and the other points to 10.132.0.1, which is accessible via fxp2.

Since the mygate(5) file does not yet support multipath default routes, the above commands should be added to the bottom of the hostname.if(5) files for the fxp1 and fxp2 interfaces. The /etc/mygate file should then be deleted. 

$ tail -1 /etc/hostname.fxp1
!route add -mpath default 10.130.128.1
$ tail -1 /etc/hostname.fxp2
!route add -mpath default 10.132.0.1
Lastly, don't forget to activate the use of multipath routes by enabling the proper sysctl(8) variable. 
# sysctl net.inet.ip.multipath=1
# sysctl net.inet6.ip6.multipath=1
Be sure to edit sysctl.conf(5) to make the changes permanent.

Now try a traceroute to different destinations. The kernel will load balance the traffic over each multipath route. 

# traceroute -n 154.11.0.4
traceroute to 154.11.0.4 (154.11.0.4), 64 hops max, 60 byte packets
 1  10.130.128.1  19.337 ms  18.194 ms  18.849 ms
 2  154.11.95.170  17.642 ms  18.176 ms  17.731 ms
 3  154.11.5.33  110.486 ms  19.478 ms  100.949 ms
 4  154.11.0.4  32.772 ms  33.534 ms  32.835 ms

# traceroute -n 154.11.0.5
traceroute to 154.11.0.5 (154.11.0.5), 64 hops max, 60 byte packets
 1  10.132.0.1  14.175 ms  14.503 ms  14.58 ms
 2  154.11.95.38  13.664 ms  13.962 ms  13.445 ms
 3  208.38.16.151  13.964 ms  13.347 ms  13.788 ms
 4  154.11.0.5  30.177 ms  30.95 ms  30.593 ms
For more information about how the route is chosen, please refer to RFC2992, "Analysis of an Equal-Cost Multi-Path Algorithm".

It's worth noting that if an interface used by a multipath route goes down (i.e., loses carrier), the kernel will still try to forward packets using the route that points to that interface. This traffic will of course be blackholed and end up going nowhere. It's highly recommended to use ifstated(8) to check for unavailable interfaces and adjust the routing table accordingly.

Using NFS

The Network File System, NFS, is used to share a filesystem over the network.

This section will go through the steps for a simple NFS setup. The example details a server on a LAN, with clients accessing NFS on the LAN. It does not cover securing NFS.

Setting Up an NFS Server

First, enable the portmap(8), mountd(8) and nfsd(8) services on the server: 
# rcctl enable portmap mountd nfsd
Then configure the list of filesystems that will be made available.

In this example, we have a server with the IP address 10.0.0.1. This server will be serving NFS only to clients within its own subnet. This is configured in the following exports(5) file: 

$ cat /etc/exports
/docs -alldirs -ro -network=10.0.0 -mask=255.255.255.0
The local filesystem /docs will be made available via NFS. The -alldirs option specifies that clients will be able to mount at any point under /docs as well as /docs itself. The -ro option specifies that clients will only be granted read-only access. The last two arguments specify that only clients within the 10.0.0.0 network using a netmask of 255.255.255.0 will be authorized to mount this filesystem.

Now start the server-related services: 

# rcctl start portmap mountd nfsd
If changes are made to /etc/exports while NFS is already running, mountd must be made aware of this. 
# rcctl reload mountd

Mounting NFS Filesystems

NFS filesystems should be mounted via mount(8), or more specifically, mount_nfs(8).

To mount the /docs filesystem on host 10.0.0.1 to local filesystem /mnt, run: 

# mount -t nfs 10.0.0.1:/docs /mnt
To have that filesystem mounted at boot, append a line to fstab(5) like so: 
# echo '10.0.0.1:/docs /mnt nfs ro,nodev,nosuid 0 0' >> /etc/fstab
It is important to use 0 0 at the end of this line so that the computer does not try to fsck(8) the NFS filesystem on boot.

When accessing an NFS mount as the root user, the server automatically maps root's access to username nobody and group nobody. This is important to know when considering file permissions. For example, take a file with these permissions: 

-rw-------    1 root     wheel           0 Dec 31 03:00 _daily.B20143
If this file was on an NFS share and the root user tried to access this file from the NFS client, access would be denied.

The user and group that root are mapped to are configurable via the exports(5) file on the NFS server.

Checking Stats on NFS

One thing to check to ensure NFS is operating properly is that all the daemons have properly registered with RPC. To do this, use rpcinfo(8). 
$ rpcinfo -p 10.0.0.1
   program vers proto   port
    100000    2   tcp    111  portmapper
    100000    2   udp    111  portmapper
    100005    1   udp    633  mountd
    100005    3   udp    633  mountd
    100005    1   tcp    916  mountd
    100005    3   tcp    916  mountd
    100003    2   udp   2049  nfs
    100003    3   udp   2049  nfs
    100003    2   tcp   2049  nfs
    100003    3   tcp   2049  nfs
There are a few utilities that allow an administrator to see what is happening with NFS, such as showmount(8) and nfsstat(1).