Why

Sometimes, you may want your program use one specific net device, because your net devices have different network access abilities.

For me, I have two demands:

  1. I want to use IRC in company network, and i don’t want company to know that(i need to connect to irc.freenode.net, and IRC uses unique(?) data). But for some program, it need company’s network to access company’s intranet.
  2. Almost all traffic in my home go through VPS, but sometimes i need to download BT resources, these traffic cannot go through, since my VPS have flow limitation and not welcome BT.

What

The approach mentioned here can let different program use different net link. Some programs can use company’s intranet, other programs don’t. All my work is done under Archlinux, you can easily change it to other linux distributions, but not Windows, if you need to do this under Windows, you may need to check reference of this post.

How

Add a net namespace

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ip netns add $netns

If you use wired device

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ip link set $net_link netns $netns

If you use wireless device

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iw phy $phy set netns name $netns    # you can check your $phy using command `iw list`

“Into” this net namespace

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ip netns exec $netns sudo -u $your_user_name bash    # i use zsh

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ip link

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wpa_supplicant -i $net_link -c $wpa_supplicant_config_file &

Note that all commands below is execute under this net namespace

Bind a network address

Since dhcpcd(or dhcpclient) cannot work under net namespace(I cannot, maybe you can, use dhcpcd $net_link then ifconfig to check if this net link have a address now), i need to bind a address for it manually.

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ifconfig $net_link $ip

Check if you can access the extranet

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ping 8.8.8.8

If you cannot, you may need to set route use following command:

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route add default gw $gateway $net_link

Check if dns look up works well

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dig www.google.com    # or just `ping www.google.com`

If ping success, you done. If not, you may need to change dns server manually.(the reason is quite simple, your dns server is mostly point to gateway, but different net link have different gateway address, it can be 192.168.1.1 or 172.17.1.1 or whatever, if your dns lookup fail, it may because your dns server change)

Change dns server if necessary

Since your dns server is written in /etc/resolve.conf, you may want to change it directly, but it won’t work, because dhcpcd will overwrite it. Different linux distribution have different approach to solve this. Under Archlinux, you can edit /etc/resolve.conf.head, add nameserver 8.8.8.8 or something whatever like this into it.

After all this, you mostly done

You may need to check using curl ip.sb to see if your ip address change.

Foreign link(reference)

This post is mostly based on this answer. Check this answer for another approach(I use this approach but for some reason this doesn’t work). This post tells me a lot about linux net namespace. This answer helps me to add a wireless net link into net namespace. Archwiki gives me hint about how to change dns server. For how to connect to a wifi under linux terminal or more wireless network configuration, you may need to check this, this link helps me a lot of time too.