Docker network through host IPSec / Strongswan VPN

vpn-docker-fix

#!/usr/bin/env bash

# Link up docker network via IPSec VPN on docker-host.
#
# NOTE: This script can either be "sourced" into your .bashrc or executed directly. Be
# it sourced or executed, the usage syntax below is the same.
#
# Usage: [dry_run=1] [debug=1] vpn-docker-fix [docker-network-1 [docker-network-2 ...]]
#
# Env Variables:
#   dry_run - Set to 1 to have a dry run, just printing out the iptables command
#   debug   - Set to 1 to see bash substitutions
vpn-docker-fix() {
    # subshell to reduce "pollution" of the terminal shell when this script is "sourced"
    # into .bashrc
    (
        : "${dry_run:=0}"
        : "${debug:=0}"

        _grep() {
            /usr/bin/env grep "$@"
        }

        _awk() {
            /usr/bin/env awk "$@"
        }

        _cut() {
            /usr/bin/env cut "$@"
        }

        _log_stderr() {
            echo "$*" >&2
        }

        _dry_run() {
            if [ "$dry_run" = 1 ] || [ "$debug" = 1 ]; then
                _log_stderr "$*"
            fi

            if [ "$dry_run" = 1 ]; then
                return
            fi

            eval "$@"
        }

        _bash_debug_on() {
            if [[ "$debug" != 1 ]]; then
                return 0
            fi

            : "${bash_debug_level:=0}"
            ((bash_debug_level++))

            [ -n "$bash_debug_state" ] || {
                bash_debug_state=$(set +o | _grep xtrace)
                set -x
            }
        }

        _bash_debug_off() {
            if [[ "$debug" != 1 ]]; then
                return 0
            fi

            : "${bash_debug_level:=1}"
            ((bash_debug_level--))

            if [ "$bash_debug_level" -le 0 ]; then
                if [ -n "$bash_debug_state" ]; then
                    eval "$bash_debug_state"
                    unset bash_debug_state
                fi
            fi
        }

        # get docker network (if not specified)
        get_docker_networks() {
            local nets=()
            if [[ $# == 0 ]]; then
                nets=("$default_docker_network")
            else
                nets=("$@")
            fi

            printf '%s\n' "${nets[@]}"
        }

        # get docker network's CIDR
        get_docker_network_subnet() {
            if [ $# -lt 1 ] || [[ "$1" == "" ]]; then
                _log_stderr "No docker network specified"
                return 1
            fi

            local docker_network="$default_docker_network"
            if [ "$1" != "$default_docker_network" ]; then
                local net_hash
                if ! net_hash=$(docker network ls -qf "Name=${1}") || [ -z "$net_hash" ]; then
                    _log_stderr "Docker network '$1' not found"
                    return 1
                fi

                docker_network="br-$net_hash"
            fi

            ip route show | _grep -v '169.254' | _grep " dev $docker_network" | _awk '{ print $1 }'
        }

        connect_docker_networks_to_vpn() {
            if [ $# -lt 1 ] || [[ "$1" == "" ]]; then
                _log_stderr "No docker network(s) specified"
                return 1
            fi

            # vpnSubnet - VPN's CIDR
            mapfile -t vpnSubnets < <(ip route list table 220 | _grep -o '^[0-9.]*/[0-9]*')
            if [ "${#vpnSubnets}" -eq 0 ]; then
                _log_stderr "VPN not active"
                return 1
            fi

            # defaultRouteInterface - the active host network interface connecting to the VPN
            defaultRouteInterface="$(ip route show | _grep -e "^default" | _awk '{ print $5 }')"

            # virtualIP -  virtual IP address of the host in the VPN
            # - ignore dockerX and lo iterfaces
            # - IP from interfaces that have valid-lifetime 'forever'
            virtualIP="$(ip -4 -o addr show | _awk '/ (lo|docker|br-|tun)/ {next;} /valid_lft forever/ {print $4}' | _cut -d/ -f1)"

            local docker_net docker_subnet vpn_subnet
            for docker_net in "$@"; do
                if docker_subnet="$(get_docker_network_subnet "$docker_net")"; then
                    for vpn_subnet in "${vpnSubnets[@]}"; do
                        _dry_run sudo iptables \
                            -j SNAT -t nat -I POSTROUTING 1 \
                            -o "$defaultRouteInterface" \
                            -d "$vpn_subnet" \
                            -s "$docker_subnet" \
                            --to-source "$virtualIP"
                    done
                fi
            done
        }

        _bash_debug_on

        mapfile -t nets_chosen < <(get_docker_networks "$@")
        connect_docker_networks_to_vpn "${nets_chosen[@]}"

        _bash_debug_off
    )
}

: "${default_docker_network:=docker0}"

# check if this script is being sourced or executed
if ! (return 0 2>/dev/null); then
    # if not sourced, execute vpn-docker-fix with CLI agrs
    vpn-docker-fix "$@"
else
    _vpn-docker-fix_completions() {
        local net nets=()
        while read -r net; do
            if [[ " ${COMP_WORDS[*]} " =~ " $net " ]]; then
                continue
            fi
            nets+=("$net")
        done < <(command docker network ls --format '{{.Name}}' --filter Driver=bridge | command grep -v ^bridge$)
        
        mapfile -t COMPREPLY < <(compgen -W "${nets[*]}" -- ${COMP_WORDS[COMP_CWORD]})
    }

    complete -F _vpn-docker-fix_completions vpn-docker-fix
fi

This gist was developed based on this SO SF answer.
Once sourced into your bash shell, you can invoke command vpn-docker-fix in the below forms.

# setup an iptables rule to connect default network docker0 to VPN
vpn-docker-fix

# setup an iptables rule to connect the specific-network created via docker-compose or
# using 'docker network create' to VPN
vpn-docker-fix <specific-network>

# turn on debug
debug=1 vpn-docker-fix [...]

# dry run without applying any changes
dry_run=1 vpn-docker-fix [...]

# dry run and debug on
dry_run=1 debug=1 vpn-docker-fix [...]

do you run it on the host or in a docker container?

@rszczypka
You run it on the host.

Thank you! That helped me!