.nh
.TH SLIRP4NETNS 1 "August 2019" "Rootless Containers" "User Commands"

.SH NAME
.PP
slirp4netns \- User\-mode networking for unprivileged network namespaces


.SH SYNOPSIS
.PP
slirp4netns [OPTION]... PID|PATH TAPNAME


.SH DESCRIPTION
.PP
slirp4netns provides user\-mode networking ("slirp") for network namespaces.

.PP
Unlike \fBveth\fP(4), slirp4netns does not require the root privileges on the host.

.PP
Default configuration:

.RS
.IP \(bu 2
MTU:               1500
.IP \(bu 2
CIDR:              10.0.2.0/24
.IP \(bu 2
Gateway/Host:      10.0.2.2    (network address + 2)
.IP \(bu 2
DNS:               10.0.2.3    (network address + 3)
.IP \(bu 2
IPv6 CIDR:         fd00::/64
.IP \(bu 2
IPv6 Gateway/Host: fd00::2
.IP \(bu 2
IPv6 DNS:          fd00::3

.RE


.SH OPTIONS
.PP
\fB\-c\fP, \fB\-\-configure\fP
bring up the TAP interface. IP will be set to 10.0.2.100 (network address + 100) by default. IPv6 will be set to a random address.
Starting with v0.4.0, the loopback interface (\fBlo\fP) is brought up as well.

.PP
\fB\-e\fP, \fB\-\-exit\-fd=FD\fP
specify the FD for terminating slirp4netns.
When the FD is specified, slirp4netns exits when a \fBpoll(2)\fP event happens on the FD.

.PP
\fB\-r\fP, \fB\-\-ready\-fd=FD\fP
specify the FD to write to when the initialization steps are finished.
When the FD is specified, slirp4netns writes \fB"1"\fP to the FD and close the FD.
Prior to v0.4.0, the FD was written after the network configuration (\fB\-c\fP)
but before the API socket configuration (\fB\-a\fP).

.PP
\fB\-m\fP, \fB\-\-mtu=MTU\fP (since v0.2.0)
specify MTU (max=65521).

.PP
\fB\-6\fP, \fB\-\-enable\-ipv6\fP (since v0.2.0, EXPERIMENTAL)
enable IPv6

.PP
\fB\-a\fP, \fB\-\-api\-socket\fP (since v0.3.0)
API socket path

.PP
\fB\-\-cidr\fP (since v0.3.0)
specify CIDR, e.g. 10.0.2.0/24

.PP
\fB\-\-disable\-host\-loopback\fP (since v0.3.0)
prohibit connecting to 127.0.0.1:* on the host namespace

.PP
\fB\-\-netns\-type=TYPE\fP (since v0.4.0)
specify network namespace type ([path|pid], default=pid)

.PP
\fB\-\-userns\-path=PATH\fP (since v0.4.0)
specify user namespace path

.PP
\fB\-\-enable\-sandbox\fP (since v0.4.0, EXPERIMENTAL)
enter the user namespace and create a new mount namespace where only /etc and
/run are mounted from the host.

.PP
When running as the root, the process does not enter the user namespace but all
the capabilities except \fB\fCCAP\_NET\_BIND\_SERVICE\fR are dropped.

.PP
\fB\-\-enable\-seccomp\fP (since v0.4.0, EXPERIMENTAL)
enable \fBseccomp(2)\fP to limit syscalls.
Typically used in conjunction with \fB\-\-enable\-sandbox\fP\&.

.PP
\fB\-h\fP, \fB\-\-help\fP (since v0.2.0)
show help and exit

.PP
\fB\-v\fP, \fB\-\-version\fP (since v0.2.0)
show version and exit


.SH EXAMPLE
.PP
Terminal 1: Create user/network/mount namespaces

.PP
.RS

.nf
$ unshare \-\-user \-\-map\-root\-user \-\-net \-\-mount
unshared$ echo $$ > /tmp/pid

.fi
.RE

.PP
Terminal 2: Start slirp4netns

.PP
.RS

.nf
$ slirp4netns \-\-configure \-\-mtu=65520 $(cat /tmp/pid) tap0
starting slirp, MTU=65520
...

.fi
.RE

.PP
Terminal 1: Make sure \fBtap0\fP is configured and connected to the Internet

.PP
.RS

.nf
unshared$ ip a
1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
3: tap0: <BROADCAST,UP,LOWER\_UP> mtu 65520 qdisc fq\_codel state UNKNOWN group default qlen 1000
    link/ether c2:28:0c:0e:29:06 brd ff:ff:ff:ff:ff:ff
    inet 10.0.2.100/24 brd 10.0.2.255 scope global tap0
       valid\_lft forever preferred\_lft forever
    inet6 fe80::c028:cff:fe0e:2906/64 scope link 
       valid\_lft forever preferred\_lft forever
unshared$ echo "nameserver 10.0.2.3" > /tmp/resolv.conf
unshared$ mount \-\-bind /tmp/resolv.conf /etc/resolv.conf
unshared$ curl https://example.com

.fi
.RE

.PP
Bind\-mounting \fB/etc/resolv.conf\fP is only needed when \fB/etc/resolv.conf\fP on
the host refers to loopback addresses (\fB127.0.0.X\fP, typically because of
\fBdnsmasq\fP(8) or \fBsystemd\-resolved.service\fP(8)) that cannot be accessed from
the namespace.

.PP
If your \fB/etc/resolv.conf\fP on the host is managed by \fBnetworkmanager\fP(8)
or \fBsystemd\-resolved.service\fP(8), you might need to mount a new filesystem on
\fB/etc\fP instead, so as to prevent the new \fB/etc/resolv.conf\fP from being
unmounted unexpectedly when \fB/etc/resolv.conf\fP on the host is regenerated.

.PP
.RS

.nf
unshared$ mkdir /tmp/a /tmp/b
unshared$ mount \-\-rbind /etc /tmp/a
unshared$ mount \-\-rbind /tmp/b /etc
unshared$ mkdir /etc/.ro
unshared$ mount \-\-move /tmp/a /etc/.ro
unshared$ cd /etc
unshared$ for f in .ro/*; do ln \-s $f $(basename $f); done
unshared$ rm resolv.conf
unshared$ echo "nameserver 10.0.2.3" > /tmp/resolv.conf
unshared$ curl https://example.com

.fi
.RE


.SH ROUTING PING PACKETS
.PP
To route ping packets, you need to set up \fBnet.ipv4.ping\_group\_range\fP properly
as the root.

.PP
e.g.

.PP
.RS

.nf
$ sudo sh \-c "echo 0   2147483647  > /proc/sys/net/ipv4/ping\_group\_range"

.fi
.RE


.SH FILTERING CONNECTIONS
.PP
By default, ports listening on \fBINADDR\_LOOPBACK\fP (\fB127.0.0.1\fP) on the host are accessible from the child namespace via the gateway (default: \fB10.0.2.2\fP).
\fB\-\-disable\-host\-loopback\fP can be used to prohibit connecting to \fBINADDR\_LOOPBACK\fP on the host.

.PP
However, a host loopback address might be still accessible via the built\-in DNS (default: \fB10.0.2.3\fP) if \fB\fC/etc/resolv.conf\fR on the host refers to a loopback address.
You may want to set up iptables for limiting access to the built\-in DNS in such a case.

.PP
.RS

.nf
unshared$ iptables \-A OUTPUT \-d 10.0.2.3 \-p udp \-\-dport 53 \-j ACCEPT
unshared$ iptables \-A OUTPUT \-d 10.0.2.3 \-j DROP

.fi
.RE


.SH API SOCKET
.PP
slirp4netns can provide QMP\-like API server over an UNIX socket file:

.PP
.RS

.nf
$ slirp4netns \-\-api\-socket /tmp/slirp4netns.sock ...

.fi
.RE

.PP
\fBadd\_hostfwd\fP: Expose a port (IPv4 only)

.PP
.RS

.nf
$ json='{"execute": "add\_hostfwd", "arguments": {"proto": "tcp", "host\_addr": "0.0.0.0", "host\_port": 8080, "guest\_addr": "10.0.2.100", "guest\_port": 80}}'
$ echo \-n $json | nc \-U /tmp/slirp4netns.sock
{ "return": {"id": 42}}

.fi
.RE

.PP
If \fBhost\_addr\fP is not specified, then it defaults to "0.0.0.0".

.PP
If \fBguest\_addr\fP is not specified, then it will be set to the default address that corresponds to \-\-configure.

.PP
\fBlist\_hostfwd\fP: List exposed ports

.PP
.RS

.nf
$ json='{"execute": "list\_hostfwd"}'
$ echo \-n $json | nc \-U /tmp/slirp4netns.sock
{ "return": {"entries": [{"id": 42, "proto": "tcp", "host\_addr": "0.0.0.0", "host\_port": 8080, "guest\_addr": "10.0.2.100", "guest\_port": 80}]}}

.fi
.RE

.PP
\fBremove\_hostfwd\fP: Remove an exposed port

.PP
.RS

.nf
$ json='{"execute": "remove\_hostfwd", "arguments": {"id": 42}}'
$ echo \-n $json | nc \-U /tmp/slirp4netns.sock
{ "return": {}}

.fi
.RE

.PP
Remarks:

.RS
.IP \(bu 2
Client needs to \fBshutdown(2)\fP the socket with \fBSHUT\_WR\fP after sending every request.
i.e. No support for keep\-alive and timeout.
.IP \(bu 2
slirp4netns "stops the world" during processing API requests.
.IP \(bu 2
A request must be less than 4096 bytes.
.IP \(bu 2
JSON responses may contain \fBerror\fP instead of \fBreturn\fP\&.

.RE


.SH DEFINED NAMESPACE PATHS
.PP
A user can define a network namespace path as opposed to the default process ID:

.PP
.RS

.nf
$ slirp4netns \-\-netns\-type=path ... /path/to/netns tap0

.fi
.RE

.PP
Currently, the \fBnetns\-type=TYPE\fP argument supports \fBpath\fP or \fBpid\fP args with the default being \fBpid\fP\&.

.PP
Additionally, a \fB\-\-userns\-path=PATH\fP argument can be included to override any user namespace path defaults

.PP
.RS

.nf
$ slirp4netns \-\-netns\-type=path \-\-userns\-path=/path/to/userns /path/to/netns tap0

.fi
.RE


.SH BUGS
.PP
Kernel 4.20 bumped up the default value of \fB/proc/sys/net/ipv4/tcp\_rmem\fP from 87380 to 131072.
This is known to slow down slirp4netns port forwarding: \fBhttps://github.com/rootless\-containers/slirp4netns/issues/128\fP\&.

.PP
As a workaround, you can adjust the value of \fB/proc/sys/net/ipv4/tcp\_rmem\fP inside the namespace.
No real root privilege is needed to modify the file since kernel 4.15.

.PP
.RS

.nf
unshared$ c=$(cat /proc/sys/net/ipv4/tcp\_rmem); echo $c | sed \-e s/131072/87380/g > /proc/sys/net/ipv4/tcp\_rmem

.fi
.RE


.SH SEE ALSO
.PP
\fBnetwork\_namespaces\fP(7), \fBuser\_namespaces\fP(7), \fBveth\fP(4)


.SH AVAILABILITY
.PP
The slirp4netns command is available from \fBhttps://github.com/rootless\-containers/slirp4netns\fP under GNU GENERAL PUBLIC LICENSE Version 2.