passt¶

passt (Plug A Simple Socket Transport) provides full, quasi-native network connectivity to virtual machines in user-mode without requiring any capabilities or privileges.

The data plane implements a translation layer between a Layer-2 virtual network interface and native Layer-4 (TCP, UDP, ping) sockets on the host, giving the illusion that application processes residing on the guest are running on the local host, from a networking perspective.

Built-in ARP, DHCP, NDP, and DHCPv6 implementations are designed to provide the guest with a network configuration that tightly resembles the host native configuration. With the default options, guest and host share IP addresses, routes, and port bindings.

Port forwarding and translation allow networking services running in the guest to be reachable from both local and remote hosts.

Unlike slirp4netns(1), passt doesn't implement a full TCP stack: the TCP translation layer has no stateful data buffering and operates by reflecting one peer's observed parameters (congestion window size, acknowledged data, etc.) to the corresponding peer.

Currently, the only supported hypervisor is qemu(1), connecting to passt by means of a UNIX domain socket. This is supported starting from qemu 7.2. For older qemu versions, see the qrap(1) wrapper.

pasta¶

pasta (Pack A Subtle Tap Abstraction) provides equivalent functionality to network namespaces, as the one offered by passt for virtual machines.

If PID or --netns are given, pasta associates to an existing user and network namespace. Otherwise, pasta creates a new user and network namespace, and spawns the given command or a default shell within this context. A tap device within the network namespace is created to provide network connectivity.

For local TCP and UDP traffic only, pasta also implements a bypass path directly mapping Layer-4 sockets between init and target namespaces, for performance reasons.

passt-only options¶

-s, --socket-path, --socket path

Path for UNIX domain socket used by qemu(1) or qrap(1) to connect to passt. Default is to probe a free socket, not accepting connections, starting from /tmp/passt_1.socket to /tmp/passt_64.socket.

--vhost-user

Enable vhost-user. The vhost-user command socket is provided by --socket.

--print-capabilities

Print back-end capabilities in JSON format, only meaningful for vhost-user mode.

-F, --fd FD

Pass a pre-opened, connected socket to passt. Usually the socket is opened in the parent process and passt inherits it when run as a child. This allows the parent process to open sockets using another address family or requiring special privileges.

This option implies the behaviour described for --one-off, once this socket is closed.

-1, --one-off

Quit after handling a single client connection, that is, once the client closes the socket, or once we get a socket error.

-t, --tcp-ports spec

Configure TCP port forwarding to guest. spec can be one of:

-u, --udp-ports spec

Configure UDP port forwarding to guest. spec is as described for TCP above.

Note: unless overridden, UDP ports with numbers corresponding to forwarded TCP port numbers are forwarded too, without, however, any port translation. IPv6 bound ports are also forwarded for IPv4.

Default is none.

pasta-only options¶

-I, --ns-ifname name

Name of tap interface to be created in target namespace. By default, the same interface name as the external, routable interface is used. If no such interface exists, the name tap0 will be used instead.

-t, --tcp-ports spec

Configure TCP port forwarding to namespace. spec can be one of:

-u, --udp-ports spec

Configure UDP port forwarding to namespace. spec is as described for TCP above, and the list of ports is derived from listening sockets reported by /proc/net/udp and /proc/net/udp6, see proc(5).

Note: unless overridden, UDP ports with numbers corresponding to forwarded TCP port numbers are forwarded too, without, however, any port translation.

IPv6 bound ports are also forwarded for IPv4.

Default is auto.

-T, --tcp-ns spec

Configure TCP port forwarding from target namespace to init namespace. spec is as described above for TCP.

Default is auto.

-U, --udp-ns spec

Configure UDP port forwarding from target namespace to init namespace. spec is as described above for UDP.

Default is auto.

--host-lo-to-ns-lo (DEPRECATED)

If specified, connections forwarded with -t and -u from the host's loopback address will appear on the loopback address in the guest as well. Without this option such forwarded packets will appear to come from the guest's public address.

--userns spec

Target user namespace to join, as a path. If PID is given, without this option, the user namespace will be the one of the corresponding process.

--netns spec

Target network namespace to join, as a path or a name. A name is treated as with ip-netns(8) as equivalent to a path in /run/netns.

This option can't be specified with a PID.

--netns-only

Join only a target network namespace, not a user namespace, and don't create one for sandboxing purposes either. This is implied if PATH or NAME are given without --userns.

--no-netns-quit

Do not exit once the target namespace reference is removed.

Without this option, pasta will terminate if the target network namespace is bound to the filesystem, and the given path is deleted, or if the target network namespace is represented by a procfs entry, and that entry is deleted, representing the fact that a process with the given PID terminated.

--config-net

Configure networking in the namespace: set up addresses and routes as configured or sourced from the host, and bring up the tap interface.

--no-copy-routes (DEPRECATED)

With --config-net, do not copy all the routes associated to the interface we derive addresses and routes from: set up only the default gateway. Implied by -g, --gateway, for the corresponding IP version only.

Default is to copy all the routing entries from the interface in the outer namespace to the target namespace, translating the output interface attribute to the outbound interface in the namespace.

Note that this configuration option is deprecated and will be removed in a future version. It is not expected to be of any use, and it simply reflects a legacy behaviour. If you have any use for this, refer to REPORTING BUGS below.

--no-copy-addrs (DEPRECATED)

With --config-net, do not copy all the addresses associated to the interface we derive addresses and routes from: set up a single one. Implied by -a, --address, for the corresponding IP version only.

Default is to copy all the addresses, except for link-local ones, from the interface from the outer namespace to the target namespace.

Note that this configuration option is deprecated and will be removed in a future version. It is not expected to be of any use, and it simply reflects a legacy behaviour. If you have any use for this, refer to REPORTING BUGS below.

--ns-mac-addr addr

Configure MAC address addr on the tap interface in the namespace.

Default is to let the tap driver build a pseudorandom hardware address.

--no-splice

Disable the bypass path for inbound, local traffic. See the section Handling of local traffic in pasta in the NOTES for more details.

pasta¶

Create and use a new, connected, user and network namespace

$ iperf3 -s -D
$ ./pasta
Outbound interface: eth0, namespace interface: eth0
ARP:


    address: 28:16:ad:39:a9:ea
DHCP:


    assign: 192.168.1.118


    mask: 255.255.255.0


    router: 192.168.1.1
NDP/DHCPv6:


    assign: 2a02:6d40:3ca5:2001:b81d:fa4a:8cdd:cf17


    router: fe80::62e3:27ff:fe33:2b01
#
# dhclient -4 --no-pid
# dhclient -6 --no-pid
# ip address show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000


    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00


    inet 127.0.0.1/8 scope host lo


       valid_lft forever preferred_lft forever


    inet6 ::1/128 scope host 


       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 65520 qdisc pfifo_fast state UNKNOWN group default qlen 1000


    link/ether 5e:90:02:eb:b0:2a brd ff:ff:ff:ff:ff:ff


    inet 192.168.1.118/24 brd 192.168.1.255 scope global eth0


       valid_lft forever preferred_lft forever


    inet6 2a02:6d40:3ca5:2001:b81d:fa4a:8cdd:cf17/128 scope global 


       valid_lft forever preferred_lft forever


    inet6 2a02:6d40:3ca5:2001:5c90:2ff:feeb:b02a/64 scope global dynamic mngtmpaddr 


       valid_lft 3591sec preferred_lft 3591sec


    inet6 fe80::5c90:2ff:feeb:b02a/64 scope link 


       valid_lft forever preferred_lft forever
# ip route show
default via 192.168.1.1 dev eth0 
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.118 
# ip -6 route show
2a02:6d40:3ca5:2001:b81d:fa4a:8cdd:cf17 dev eth0 proto kernel metric 256 pref medium
2a02:6d40:3ca5:2001::/64 dev eth0 proto kernel metric 256 expires 3584sec pref medium
fe80::/64 dev eth0 proto kernel metric 256 pref medium
default via fe80::62e3:27ff:fe33:2b01 dev eth0 proto ra metric 1024 expires 3584sec pref medium
# iperf3 -c 127.0.0.1 -t1
Connecting to host 127.0.0.1, port 5201
[  5] local 127.0.0.1 port 51938 connected to 127.0.0.1 port 5201
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec  4.46 GBytes  38.3 Gbits/sec    0   3.93 MBytes       
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-1.00   sec  4.46 GBytes  38.3 Gbits/sec    0             sender
[  5]   0.00-1.41   sec  4.45 GBytes  27.1 Gbits/sec                  receiver
iperf Done.
# iperf3 -c ::1 -t1
Connecting to host ::1, port 5201
[  5] local ::1 port 50108 connected to ::1 port 5201
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec  4.35 GBytes  37.4 Gbits/sec    0   4.99 MBytes       
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-1.00   sec  4.35 GBytes  37.4 Gbits/sec    0             sender
[  5]   0.00-1.41   sec  4.35 GBytes  26.4 Gbits/sec                  receiver
iperf Done.
# ping -c1 -4 spaghetti.pizza
PING spaghetti.pizza (172.67.192.217) 56(84) bytes of data.
64 bytes from 172.67.192.217: icmp_seq=1 ttl=255 time=37.3 ms
--- spaghetti.pizza ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
# ping -c1 -6 spaghetti.pizza
PING spaghetti.pizza(2606:4700:3034::6815:147a (2606:4700:3034::6815:147a)) 56 data bytes
64 bytes from 2606:4700:3034::6815:147a: icmp_seq=1 ttl=255 time=35.6 ms
--- spaghetti.pizza ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 35.605/35.605/35.605/0.000 ms
# logout
$

Connect an existing user and network namespace

$ unshare -rUn
# echo $$
2446678

$ ./pasta 2446678
Outbound interface: eth0, namespace interface: eth0
ARP:


    address: 28:16:ad:39:a9:ea
DHCP:


    assign: 192.168.1.118


    mask: 255.255.255.0


    router: 192.168.1.1
NDP/DHCPv6:


    assign: 2a02:6d40:3ca5:2001:b81d:fa4a:8cdd:cf17


    router: fe80::62e3:27ff:fe33:2b01

# dhclient -4 --no-pid
# dhclient -6 --no-pid
# ip address show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000


    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00


    inet 127.0.0.1/8 scope host lo


       valid_lft forever preferred_lft forever


    inet6 ::1/128 scope host 


       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 65520 qdisc pfifo_fast state UNKNOWN group default qlen 1000


    link/ether fa:c1:2a:27:92:a9 brd ff:ff:ff:ff:ff:ff


    inet 192.168.1.118/24 brd 192.168.1.255 scope global eth0


       valid_lft forever preferred_lft forever


    inet6 2a02:6d40:3ca5:2001:b81d:fa4a:8cdd:cf17/128 scope global 


       valid_lft forever preferred_lft forever


    inet6 2a02:6d40:3ca5:2001:f8c1:2aff:fe27:92a9/64 scope global dynamic mngtmpaddr 


       valid_lft 3594sec preferred_lft 3594sec


    inet6 fe80::f8c1:2aff:fe27:92a9/64 scope link 


       valid_lft forever preferred_lft forever

passt¶

Start and connect a guest with basic port forwarding

$ ./passt -f -t 2222:22
Outbound interface: eth0
ARP:


    address: 28:16:ad:39:a9:ea
DHCP:


    assign: 192.168.1.118


    mask: 255.255.255.0


    router: 192.168.1.1


        search:


            redhat.com
NDP/DHCPv6:


    assign: 2a02:6d40:3ca5:2001:b81d:fa4a:8cdd:cf17


    router: fe80::62e3:27ff:fe33:2b01


        search:


            redhat.com
UNIX domain socket bound at /tmp/passt_1.socket
You can now start qrap:


    ./qrap 5 qemu-system-x86_64 ... -net socket,fd=5 -net nic,model=virtio
or directly qemu, patched with:


    qemu/0001-net-Allow-also-UNIX-domain-sockets-to-be-used-as-net.patch
as follows:


    qemu-system-x86_64 ... -net socket,connect=/tmp/passt_1.socket -net nic,model=virtio

$ ./qrap 5 qemu-system-x86_64 test.qcow2 -m 1024 -display none -nodefaults -nographic -net socket,fd=5 -net nic,model=virtio
Connected to /tmp/passt_1.socket

passt: DHCP: ack to request
passt:     from 52:54:00:12:34:56
passt: NDP: received NS, sending NA
passt: NDP: received RS, sending RA
passt: DHCPv6: received SOLICIT, sending ADVERTISE
passt: NDP: received NS, sending NA
passt: DHCPv6: received REQUEST/RENEW/CONFIRM, sending REPLY
passt: NDP: received NS, sending NA

$ ssh -p 2222 root@localhost
root@localhost's password: 

# ip address show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000


    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00


    inet 127.0.0.1/8 scope host lo


       valid_lft forever preferred_lft forever


    inet6 ::1/128 scope host 


       valid_lft forever preferred_lft forever
2: ens2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 65520 qdisc pfifo_fast state UP group default qlen 1000


    link/ether 52:54:00:12:34:56 brd ff:ff:ff:ff:ff:ff


    inet 192.168.1.118/24 brd 192.168.1.255 scope global noprefixroute ens2


       valid_lft forever preferred_lft forever


    inet6 2a02:6d40:3ca5:2001:b81d:fa4a:8cdd:cf17/128 scope global noprefixroute 


       valid_lft forever preferred_lft forever


    inet6 2a02:6d40:3ca5:2001:b019:9ae2:a2fe:e6b4/64 scope global dynamic noprefixroute 


       valid_lft 3588sec preferred_lft 3588sec


    inet6 fe80::1f98:d09f:9309:9e77/64 scope link noprefixroute 


       valid_lft forever preferred_lft forever

Handling of traffic with loopback destination and source addresses¶

Both passt and pasta can bind on ports with a loopback address (127.0.0.0/8 or ::1), depending on the configuration. Loopback destination or source addresses need to be changed before packets are delivered to the guest or target namespace: most operating systems would drop packets received with loopback addresses on non-loopback interfaces, and it would also be impossible for guest or target namespace to route answers back.

For convenience, the source address on these packets is translated to the address specified by the --map-host-loopback option (with some exceptions in pasta mode, see next section below). If not specified this defaults, somewhat arbitrarily, to the address of default IPv4 or IPv6 gateway (if any) -- this is known to be an existing, valid address on the same subnet. If --no-map-gw or --map-host-loopback none are specified this translation is disabled and packets with loopback addresses are simply dropped.

Loopback destination addresses are translated to the observed external address of the guest or target namespace. For IPv6, the observed link-local address is used if the translated source address is link-local, otherwise the observed global address is used. For both IPv4 and IPv6, if no addresses have been seen yet, the configured addresses will be used instead.

For example, if passt or pasta receive a connection from 127.0.0.1, with destination 127.0.0.10, and the default IPv4 gateway is 192.0.2.1, while the last observed source address from guest or namespace is 192.0.2.2, this will be translated to a connection from 192.0.2.1 to 192.0.2.2.

Similarly, for traffic coming from guest or namespace, packets with destination address corresponding to the --map-host-loopback address will have their destination address translated to a loopback address.

Handling of local traffic in pasta¶

Depending on the configuration, pasta can bind to local ports in the init namespace, in the target namespace, or both, and forward connections and packets to corresponding ports in the other namespace.

To avoid unnecessary overhead, these connections and packets are not forwarded through the tap device connecting the namespaces: pasta creates a socket in the destination namespace, with matching Layer-4 protocol, and uses it to forward local data. For TCP, data is forwarded between the originating socket and the new socket using the splice(2) system call, and for UDP, a pair of recvmmsg(2) and sendmmsg(2) system calls deals with packet transfers.

Because it's not possible to bind sockets to foreign addresses, this bypass only applies to local connections and traffic. It also means that the address translation differs slightly from passt mode. Connections from loopback to loopback on the host will appear to come from the target namespace's public address within the guest, unless --host-lo-to-ns-lo is specified, in which case they will appear to come from loopback in the namespace as well. The latter behaviour used to be the default, but is usually undesirable, since it can unintentionally expose namespace local services to the host.

Binding to low numbered ports (well-known or system ports, up to 1023)¶

If the port forwarding configuration requires binding to ports with numbers lower than 1024, passt and pasta will try to bind to them, but will fail, unless, either:

• the sys.net.ipv4.ip_unprivileged_port_start sysctl is set to the number of the lowest port passt and pasta need. For example, as root:

  	sysctl -w net.ipv4.ip_unprivileged_port_start=443

  Note: this is the recommended way of enabling passt and pasta to bind to ports with numbers below 1024.

• or the CAP_NET_BIND_SERVICE Linux capability is granted, see services(5) and capabilities(7).

  This is, in general, not the recommended way, because passt and pasta might be used as vector to effectively use this capability from another process.

  However, if your environment is sufficiently controlled by an LSM (Linux Security Module) such as AppArmor, SELinux, Smack or TOMOYO, and no other processes can interact in such a way in virtue of this, granting this capability to passt and pasta only can effectively prevent other processes from utilising it.

  Note that this will not work for automatic detection and forwarding of ports with pasta, because pasta will relinquish this capability at runtime.

  To grant this capability, you can issue, as root:

  	for p in $(which passt passt.avx2); do
  		setcap 'cap_net_bind_service=+ep' "${p}"
  	done

ICMP/ICMPv6 Echo sockets¶

ICMP and ICMPv6 Echo requests coming from guest or target namespace are handled using so-called "ping" sockets, introduced in Linux 2.6.30. To preserve the original identifier (see RFC 792, page 14, for ICMP, and RFC 4443, section 4.1, for ICMPv6), passt and pasta try to bind these sockets using the observed source identifier as "port" -- that corresponds to Echo identifiers for "ping" sockets.

As bind(2) failures were seen with particularly restrictive SELinux policies, a fall-back mechanism maps different identifiers to different sockets, and identifiers in replies will be mapped back to the original identifier of the request. However, if bind(2) fails and the fall-back mechanism is used, echo requests will be forwarded with different, albeit unique, identifiers.

For ICMP and ICMPv6 Echo requests to work, the ping_group_range parameter needs to include the PID of passt or pasta, see icmp(7).

pasta and loopback interface¶

As pasta connects to an existing namespace, or once it creates a new namespace, it will also ensure that the loopback interface, lo, is brought up. This is needed to bind ports using the loopback address in the namespace.

TCP sending window and TCP_INFO before Linux 5.3¶

To synchronise the TCP sending window from host Layer-4 sockets to the TCP parameters announced in TCP segments sent over the Layer-2 interface, passt and pasta routinely query the size of the sending window seen by the kernel on the corresponding socket using the TCP_INFO socket option, see tcp(7). Before Linux 5.3, i.e. before Linux kernel commit 8f7baad7f035 ("tcp: Add snd_wnd to TCP_INFO"), the sending window (snd_wnd field) is not available.

If the sending window cannot be queried, it will always be announced as the current sending buffer size to guest or target namespace. This might affect throughput of TCP connections.

Local mode for disconnected setups¶

If passt and pasta fail to find a host interface with a configured address, other than loopback addresses, they will, obviously, not attempt to source addresses or routes from the host.

In this case, unless configured otherwise, they will assign the IPv4 link-local address 169.254.2.1 to the guest or target namespace, and no IPv6 address. The notion of the guest or target namespace IPv6 address is derived from the first link-local address observed.

Default gateways will be assigned as the link-local address 169.254.2.2 for IPv4, and as the link-local address fe80::1 for IPv6.