Termux .bashrc snippet to override resolv.conf with Wi-Fi DHCP DNS config

.bashrc

# This is to help address this Termux/Android issue:
#  https://github.com/termux/termux-packages/issues/1174

function _getAndroidAllDNS()
{
    local -n dnsMapRef=$1
    local -n domainMapRef=$2

    # The logic here is from experimentation based on:
    #  https://www.reddit.com/r/termux/comments/1249mqx/use_dhcp_provided_dns_in_resolveconf/
    # This is currently an oversimplification. Each network has a complicated
    #  "Score" field which seems to govern which network is used, and it is
    #  totally ignored here.

    local connectivityDump=$(/system/bin/dumpsys connectivity)
    if [[ $? -ne 0 ]]; then
        echo 'ERROR: Failed to get connectivity dump!' >&2
        return 1
    fi

    local re='\<android\.permission\.DUMP\>'
    if [[ ${connectivityDump} =~ ${re} ]]; then
        echo 'ERROR: Termux needs permission to dump connectivity; run on adb client:' >&2
        echo '        adb shell pm grant com.termux android.permission.DUMP' >&2
        echo '       If desired, can be undone using "revoke" instead of "grant".' >&2
        return 1
    fi

    local line
    re='\<NetworkAgentInfo\>.*\<ni *\{ *([^ \{\}]+) *CONNECTED *extra: *([^ \{\}]*) *\}.*\<DnsAddresses: *\[ *([^][ ]*) *\].*\<Domains: *([^ ]*)'
    local name
    local -A networks
    local -a dnsList
    local dns
    while IFS= read line; do
        if [[ ${line} =~ ${re} ]]; then
            name=${BASH_REMATCH[1]}
            [[ -n ${BASH_REMATCH[2]} ]] && name+=".${BASH_REMATCH[2]}"
            if [[ -v networks[${name}] ]]; then
                echo "ERROR: Duplicate network \"${name}\"!" >&2
                return 1
            fi
            networks[${name}]=

            dnsList=()
            for dns in ${BASH_REMATCH[3]//,/ }; do
                if [[ ${dns} == /* ]]; then
                    dnsList+=("${dns#/}")
                fi
            done
            if [[ ${#dnsList[@]} -gt 0 ]]; then
                dnsMapRef[${name}]=${dnsList[*]}
            fi

            if [[ -n ${BASH_REMATCH[4]} && ${BASH_REMATCH[4]} != 'null' ]]; then
                domainMapRef[${name}]=${BASH_REMATCH[4]//,/ }
            fi
        fi
    done <<<"${connectivityDump}"
}

function _prettyPrintAndroidAllDNS()
{
    local -A dnsMap domainMap
    local name
    _getAndroidAllDNS dnsMap domainMap || return $?
    for name in ${!dnsMap[@]}; do
        echo "${name} DNS Servers: ${dnsMap[${name}]}"
        if [[ -v domainMap[${name}] ]]; then
            echo "${name} Domains: ${domainMap[${name}]}"
        fi
    done
}

function _getAndroidWiFiDNS()
{
    local -n dnsListRef=$1
    local -n domainListRef=$2

    # The logic here is just based on some experimentation.

    local -A dnsMap domainMap
    _getAndroidAllDNS dnsMap domainMap || return $?
    if [[ ! -v dnsMap[WIFI] ]]; then
        return 1
    fi
    dnsListRef=(${dnsMap[WIFI]})
    if [[ -v domainMap[WIFI] ]]; then
        domainListRef=(${domainMap[WIFI]})
    fi
}

function _backupResolvConfIfNotAlready()
{
    [[ -f "${HOME}/.backup.etc.resolv.conf" ]] && return
    [[ ! -e "${HOME}/.backup.etc.resolv.conf" ]] || return $?
    cp -a "${PREFIX}/etc/resolv.conf" "${HOME}/.backup.etc.resolv.conf"
}

function _restoreResolvConf()
{
    [[ ! -f "${HOME}/.backup.etc.resolv.conf" ]] && return
    mv "${HOME}/.backup.etc.resolv.conf" "${PREFIX}/etc/resolv.conf"
}

function _replaceOrRestoreResolvConfAccordingToWiFi()
{
    local -a wiFiDNS wiFiDomains
    _getAndroidWiFiDNS wiFiDNS wiFiDomains
    if [[ $? -eq 0 && ${#wiFiDNS[@]} -gt 0 ]]; then
        echo "DNS servers found for Wi-Fi network, will override Termux's resolv.conf."
        if ! _backupResolvConfIfNotAlready; then
            echo 'ERROR: Failed to backup resolv.conf!' >&2
            return 1
        fi
        (
            echo "# Generated by ~/.bashrc using Wi-Fi network's DNS config:"
            if [[ ${#wiFiDomains[@]} -gt 0 ]]; then
                echo -n 'search'
                for domain in "${wiFiDomains[@]}"; do
                    echo -n " ${domain}"
                done
                echo
            fi
            for dns in "${wiFiDNS[@]:0:3}"; do # resolv.conf can have max 3 nameservers
                echo "nameserver ${dns}"
            done
        ) >"${PREFIX}/etc/resolv.conf"
    else
        echo "No DNS servers found for Wi-Fi network, will use Termux's resolv.conf."
        if ! _restoreResolvConf; then
            echo 'ERROR: Failed to restore resolv.conf!' >&2
            return 1
        fi
    fi
}

# NOTE: There's nothing intrinsically correct about doing this, it's just a choice.
_replaceOrRestoreResolvConfAccordingToWiFi

function _resolvWarnRun()
{
    local sep=$(printf '%.0s*' $(seq "${COLUMNS}"))
    echo "${sep}" >&2
    echo "WARNING: $1 uses Termux's static ${PREFIX}/etc/resolv.conf" >&2
    echo "          instead of Android's DNS!" >&2
    echo "${sep}" >&2
    echo >&2
    command "$@"
}
alias dig='_resolvWarnRun dig'
alias host='_resolvWarnRun host'
alias nmap='_resolvWarnRun nmap'
alias nslookup='_resolvWarnRun nslookup'

Thank you SO MUCH for this.