passt(1)

NAME
passt - Unprivileged user-mode network connectivity for virtual machines
pasta - Unprivileged user-mode network connectivity for network namespaces

SYNOPSIS
passt [OPTION]...
pasta [OPTION]... [COMMAND [ARG]...]
pasta [OPTION]... PID
pasta [OPTION]... --netns [PATH|NAME]

DESCRIPTION

  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.

OPTIONS

-d, --debug
    Be verbose, don't log to the system logger.

--trace
    Be extra verbose, show single packets. Implies --debug.

-q, --quiet
    Don't print informational messages.

-f, --foreground
    Don't run in background. This implies that the process is not moved to a detached PID namespace after starting, because the PID itself cannot change. Default is to fork into background.

-e, --stderr
    Log to standard error too. Default is to log to the system logger only, if started from an interactive terminal, and to both system logger and standard error otherwise.

-l, --log-file PATH
    Log to file PATH, not to standard error, and not to the system logger.

--log-size SIZE
    Limit log file size to SIZE bytes. When the log file is full, make room for new entries by removing old ones at the beginning. This limit is mandatory. Default is 1048576 (1 MiB).

--runas UID|UID:GID|LOGIN|LOGIN:GROUP
    Attempt to change to given UID and corresponding group if UID is given, or to given UID and given GID if both are given. Alternatively, login name, or login name and group name can be passed. This requires privileges (either initial effective UID 0 or CAP_SETUID capability) to work. Default is to change to user nobody if started as root.

-h, --help
    Display a help message and exit.

--version
    Show version and exit.

-p, --pcap file
    Capture tap-facing (that is, guest-side or namespace-side) network packets to file in pcap format.

-P, --pid file
    Write own PID to file once initialisation is done, before forking to background (if configured to do so).

-m, --mtu mtu
    Assign mtu via DHCP (option 26) and NDP (option type 5). A zero value disables assignment. By default, the advertised MTU is 65520 bytes, that is, the maximum 802.3 MTU minus the length of a 802.3 header, rounded to 32 bits (IPv4 words).

-a, --address addr
    Assign IPv4 addr via DHCP (yiaddr), or addr via DHCPv6 (option 5) and an addr-based prefix via NDP Router Advertisement (option type 3) for an IPv6 addr. This option can be specified zero (for defaults) to two times (once for IPv4, once for IPv6). By default, assigned IPv4 and IPv6 addresses are taken from the host interfaces with the first default route for the corresponding IP version.

-n, --netmask mask
    Assign IPv4 netmask mask, expressed as dot-decimal or number of bits, via DHCP (option 1). By default, the netmask associated to the host address matching the assigned one is used. If there's no matching address on the host, the netmask is determined according to the CIDR block of the assigned address (RFC 4632).

-M, --mac-addr addr
    Use source MAC address addr when communicating to the guest or to the target namespace. Default is to use the MAC address of the interface with the first IPv4 default route on the host.

-g, --gateway addr
    Assign IPv4 addr as default gateway via DHCP (option 3), or IPv6 addr as source for NDP Router Advertisement and DHCPv6 messages. This option can be specified zero (for defaults) to two times (once for IPv4, once for IPv6). By default, IPv4 and IPv6 gateways are taken from the host interface with the first default route for the corresponding IP version. If the default route is a multipath one, the gateway is the first nexthop router returned by the kernel which has the highest weight in the set of paths. Note: these addresses are also used as source address for packets directed to the guest or to the target namespace having a loopback or local source address, to allow mapping of local traffic to guest and target namespace. See the NOTES below for more details about this mechanism.

-i, --interface name
    Use host interface name to derive addresses and routes. Default is to use the interfaces specified by --outbound-if4 and --outbound-if6, for IPv4 and IPv6 addresses and routes, respectively. If no interfaces are given, the interface with the first default routes for each IP version is selected.

-o, --outbound addr
    Use an IPv4 addr as source address for IPv4 outbound TCP connections, UDP flows, ICMP requests, or an IPv6 addr for IPv6 ones, by binding outbound sockets to it. This option can be specified zero (for defaults) to two times (once for IPv4, once for IPv6). By default, the source address is selected by the routing tables.

--outbound-if4 name
    Bind IPv4 outbound sockets to host interface name, and, unless another interface is specified via -i, --interface, use this interface to derive IPv4 addresses and routes. By default, the interface given by the default route is selected.

--outbound-if6 name
    Bind IPv6 outbound sockets to host interface name, and, unless another interface is specified via -i, --interface, use this interface to derive IPv6 addresses and routes. By default, the interface given by the default route is selected.

-D, --dns addr
    Use addr (IPv4 or IPv6) for DHCP, DHCPv6, NDP or DNS forwarding, as configured (see options --no-dhcp-dns, --dhcp-dns, --dns-forward) instead of reading addresses from /etc/resolv.conf. This option can be specified multiple times. Specifying -D none disables usage of DNS addresses altogether.

--dns-forward addr
    Map addr (IPv4 or IPv6) as seen from guest or namespace to the first configured DNS resolver (with corresponding IP version). Mapping is limited to UDP traffic directed to port 53, and DNS answers are translated back with a reverse mapping. This option can be specified zero to two times (once for IPv4, once for IPv6).

-S, --search list
    Use space-separated list for DHCP, DHCPv6, and NDP purposes, instead of reading entries from /etc/resolv.conf. See options --no-dhcp-search and --dhcp-search. --search none disables the DNS domain search list altogether (if you need to search a domain called "none" you can use --search none.).

--no-dhcp-dns addr
    In passt mode, do not assign IPv4 addresses via DHCP (option 23) or IPv6 addresses via NDP Router Advertisement (option type 25) and DHCPv6 (option 23) as DNS resolvers. By default, all the configured addresses are passed.

--dhcp-dns addr
    In pasta mode, assign IPv4 addresses via DHCP (option 23) or IPv6 addresses via NDP Router Advertisement (option type 25) and DHCPv6 (option 23) as DNS resolvers. By default, configured addresses, if any, are not passed.

--no-dhcp-search addr
    In passt mode, do not send the DNS domain search list addresses via DHCP (option 119), via NDP Router Advertisement (option type 31) and DHCPv6 (option 24). By default, the DNS domain search list resulting from configuration is passed.

--dhcp-search addr
    In pasta mode, send the DNS domain search list addresses via DHCP (option 119), via NDP Router Advertisement (option type 31) and DHCPv6 (option 24). By default, the DNS domain search list resulting from configuration is not passed.

--no-tcp
    Disable the TCP protocol handler. No TCP connections will be accepted host-side, and TCP packets coming from guest or target namespace will be silently dropped.

--no-udp
    Disable the UDP protocol handler. No UDP traffic coming from the host side will be forwarded, and UDP packets coming from guest or target namespace will be silently dropped.

--no-icmp
    Disable the ICMP/ICMPv6 echo handler. ICMP and ICMPv6 echo requests coming from guest or target namespace will be silently dropped.

--no-dhcp
    Disable the DHCP server. DHCP client requests coming from guest or target namespace will be silently dropped. Implied if there is no gateway on the selected IPv4 default route.

--no-ndp
    Disable NDP responses. NDP messages coming from guest or target namespace will be ignored.

--no-dhcpv6
    Disable the DHCPv6 server. DHCPv6 client requests coming from guest or target namespace will be silently dropped.

--no-ra
    Disable Router Advertisements. Router Solicitations coming from guest or target namespace will be ignored.

--no-map-gw
    Don't remap TCP connections and untracked UDP traffic, with the gateway address as destination, to the host. Implied if there is no gateway on the selected default route for any of the enabled address families.

-4, --ipv4-only
    Enable IPv4-only operation. IPv6 traffic will be ignored. By default, IPv6 operation is enabled as long as at least an IPv6 default route and an interface address are configured on a given host interface.

-6, --ipv6-only
    Enable IPv6-only operation. IPv4 traffic will be ignored. By default, IPv4 operation is enabled as long as at least an IPv4 default route and an interface address are configured on a given host interface.

  passt-only options

-s, --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.

-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:

    none
        Don't forward any ports

    all Forward all unbound, non-ephemeral ports, as permitted by current capabilities. For low (< 1024) ports, see NOTES. No failures are reported for unavailable ports, unless no ports could be forwarded at all.

    ports
        A comma-separated list of ports, optionally ranged with -, and, optionally, with target ports after :, if they differ. Specific addresses can be bound as well, separated by /, and also, since Linux 5.7, limited to specific interfaces, prefixed by %. Within given ranges, selected ports and ranges can be excluded by an additional specification prefixed by ~. Specifying excluded ranges only implies that all other ports are forwarded. In this case, no failures are reported for unavailable ports, unless no ports could be forwarded at all. Examples:

        -t 22
            Forward local port 22 to port 22 on the guest

        -t 22:23
            Forward local port 22 to port 23 on the guest

        -t 22,25
            Forward local ports 22 and 25 to ports 22 and 25 on the guest

        -t 22-80
            Forward local ports between 22 and 80 to corresponding ports on the guest

        -t 22-80:32-90
            Forward local ports between 22 and 80 to ports between 32 and 90 on the guest

        -t 192.0.2.1/22
            Forward local port 22, bound to 192.0.2.1, to port 22 on the guest

        -t 192.0.2.1%eth0/22
            Forward local port 22, bound to 192.0.2.1 and interface eth0, to port 22

        -t %eth0/22
            Forward local port 22, bound to any address on interface eth0, to port 22

        -t 2000-5000,~3000-3010
            Forward local ports between 2000 and 5000, except for those between 3000 and 3010

        -t 192.0.2.1/20-30,~25
            For the local address 192.0.2.1, forward ports between 20 and 24 and between 26 and 30

        -t ~20000-20010
            Forward all ports to the guest, except for the range from 20000 to 20010
    Default is none.

-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.

-t, --tcp-ports spec
    Configure TCP port forwarding to namespace. spec can be one of:

    none
        Don't forward any ports

    auto
        Dynamically forward ports bound in the namespace. The list of ports is periodically derived (every second) from listening sockets reported by /proc/net/tcp and /proc/net/tcp6, see proc(5).

    ports
        A comma-separated list of ports, optionally ranged with -, and, optionally, with target ports after :, if they differ. Specific addresses can be bound as well, separated by /, and also, since Linux 5.7, limited to specific interfaces, prefixed by %. Within given ranges, selected ports and ranges can be excluded by an additional specification prefixed by ~. Specifying excluded ranges only implies that all other ports are forwarded. In this case, no failures are reported for unavailable ports, unless no ports could be forwarded at all. Examples:

        -t 22
            Forward local port 22 to 22 in the target namespace

        -t 22:23
            Forward local port 22 to port 23 in the target namespace

        -t 22,25
            Forward local ports 22 and 25 to ports 22 and 25 in the target namespace

        -t 22-80
            Forward local ports between 22 and 80 to corresponding ports in the target namespace

        -t 22-80:32-90
            Forward local ports between 22 and 80 to ports between 32 and 90 in the target namespace

        -t 192.0.2.1/22
            Forward local port 22, bound to 192.0.2.1, to port 22 in the target namespace

        -t 192.0.2.1%eth0/22
            Forward local port 22, bound to 192.0.2.1 and interface eth0, to port 22

        -t %eth0/22
            Forward local port 22, bound to any address on interface eth0, to port 22

        -t 2000-5000,~3000-3010
            Forward local ports between 2000 and 5000, except for those between 3000 and 3010

        -t 192.0.2.1/20-30,~25
            For the local address 192.0.2.1, forward ports between 20 and 24 and between 26 and 30

        -t ~20000-20010
            Forward all ports to the namespace, except for those between 20000 and 20010
    IPv6 bound ports are also forwarded for IPv4. Default is auto.

-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.

--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
    If the target network namespace is bound to the filesystem (that is, if PATH or NAME are given as target), do not exit once the network namespace is deleted.

--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. 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. 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.

EXAMPLES

  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
    [From another terminal]
    $ ./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
    [Back to the original terminal]
    # 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
    [From another terminal]
    $ ./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
    [Back to the original terminal]
    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
    [From yet another terminal]
    $ 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

NOTES

  Handling of traffic with local destination and source addresses
Both passt and pasta can bind on ports with a local address, depending on the configuration. Local 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 from non-loopback interfaces with local addresses, and it would also be impossible for guest or target namespace to route answers back. For convenience, and somewhat arbitrarily, the source address on these packets is translated to the address of the default IPv4 or IPv6 gateway -- this is known to be an existing, valid address on the same subnet. Loopback destination addresses are instead translated to the observed external address of the guest or target namespace. For IPv6 packets, if usage of a link-local address by guest or namespace has ever been observed, and the original destination address is also a link-local address, the observed link-local address is used. 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 default gateway will have their destination address translated to a loopback address, if and only if a packet, in the opposite direction, with a loopback destination or source address, port-wise matching for UDP, or connection-wise for TCP, has been recently forwarded to guest or namespace. This behaviour can be disabled with --no-map-gw.

  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. This bypass only applies to local connections and traffic, because it's not possible to bind sockets to foreign addresses.

  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.

LIMITATIONS
Currently, IGMP/MLD proxying (RFC 4605) and support for SCTP (RFC 4960) are not implemented. TCP Selective Acknowledgment (RFC 2018), as well as Protection Against Wrapped Sequences (PAWS) and Round-Trip Time Measurement (RTTM), both described by RFC 7232, are currently not implemented.

AUTHORS
Stefano Brivio <sbrivio@redhat.com>, David Gibson <david@gibson.dropbear.id.au>.

REPORTING BUGS
Please report issues on the bug tracker at https://passt.top/passt/bugs, or send a message to the passt-user@passt.top mailing list, see https://passt.top/passt/lists.

COPYRIGHT
Copyright (c) 2020-2022 Red Hat GmbH. passt and pasta are free software: you can redistribute them and/or modify them under the terms of the GNU Affero General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

SEE ALSO
namespaces(7), qemu(1), qrap(1), slirp4netns(1). High-level documentation is available at https://passt.top/passt/about/.