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pkg/net: add ip arithmetic functions #3815

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pkg/net: add ip arithmetic functions #3815

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62 changes: 62 additions & 0 deletions pkg/net/ip.go
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Original file line number Diff line number Diff line change
Expand Up @@ -16,7 +16,9 @@
package net

import (
"errors"
"fmt"
"math/big"
"net/netip"

"cuelang.org/go/cue"
Expand Down Expand Up @@ -242,3 +244,63 @@ func IPString(ip cue.Value) (string, error) {
}
return ipdata.String(), nil
}

func netIPAdd(addr netip.Addr, offset *big.Int) (netip.Addr, error) {
i := big.NewInt(0).SetBytes(addr.AsSlice())
i = i.Add(i, offset)

if i.Sign() < 0 {
return netip.Addr{}, errors.New("IP address arithmetic resulted in out-of-range address (underflow)")
}

b := i.Bytes()
size := addr.BitLen() / 8

if len(b) > size {
return netip.Addr{}, errors.New("IP address arithmetic resulted in out-of-range address (overflow)")
}

if len(b) < size {
b = append(make([]byte, size-len(b), size), b...)
}
addr, _ = netip.AddrFromSlice(b)
return addr, nil
}

// AddIP adds a numerical offset to a given IP address.
// The address can be provided as a string, byte array, or CIDR subnet notation.
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It seems a bit odd that we can provide a CIDR string here. What's the use case for that?
For example, net.AddIPOffset("100.0.0.0/8", 1) returns "100.0.0.1/8" but that's not generally useful, as all the bits after the prefix are conventionally zero and ignored.

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@takonomura takonomura Jun 26, 2025 •
edited
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My use case is generating network interface configurations, where IP addresses are written in CIDR notation with a prefix length. Since many configuration formats support this form, allowing AddIPOffset to take a CIDR string makes it easier to produce such values.

// It returns the resulting IP address or CIDR subnet notation as a string.
func AddIP(ip cue.Value, offset *big.Int) (string, error) {
prefix, err := netGetIPCIDR(ip)
if err == nil {
addr, err := netIPAdd(prefix.Addr(), offset)
if err != nil {
return "", err
}
return netip.PrefixFrom(addr, prefix.Bits()).String(), nil
}
ipdata := netGetIP(ip)
if !ipdata.IsValid() {
return "", fmt.Errorf("invalid IP %q", ip)
}
addr, err := netIPAdd(ipdata, offset)
if err != nil {
return "", err
}
return addr.String(), nil
}

// AddIPCIDR adds a numerical offset to a given CIDR subnet
// string, returning a CIDR string.
func AddIPCIDR(ip cue.Value, offset *big.Int) (string, error) {
prefix, err := netGetIPCIDR(ip)
if err != nil {
return "", err
}
shifted := big.NewInt(0).Lsh(offset, (uint)(prefix.Addr().BitLen()-prefix.Bits()))
addr, err := netIPAdd(prefix.Addr(), shifted)
if err != nil {
return "", err
}
return netip.PrefixFrom(addr, prefix.Bits()).String(), nil
}
26 changes: 26 additions & 0 deletions pkg/net/pkg.go
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38 changes: 38 additions & 0 deletions pkg/net/testdata/gen.txtar
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Original file line number Diff line number Diff line change
Expand Up @@ -37,6 +37,19 @@ t31: net.URL & "https://foo.com/bar"
t32: net.AbsURL & "/foo/bar"
t33: net.AbsURL & "https://foo.com/bar"
t34: net.AbsURL & "%"
t35: net.AddIP("127.0.0.1", 1)
t36: net.AddIP("127.0.0.1/8", 2)
t37: net.AddIP("2001:db8::", 1)
t38: net.AddIP("2001:db8::/64", 2)
t39: net.AddIP("invalid ip", 1)
t40: net.AddIP("0.0.0.0", -1)
t41: net.AddIP("255.255.255.255", 1)
t42: net.AddIP("::ffff:127.0.0.1", 1)
t43: net.AddIPCIDR("192.168.0.0/23", 1)
t44: net.AddIPCIDR("2001:db8:1111:2222::/64", 1)
t45: net.AddIPCIDR("192.168.0.0", 1)
t46: net.AddIPCIDR("10.0.0.0/8", -11)
t47: net.AddIPCIDR("255.0.0.0/8", 1)
-- out/net --
Errors:
t25: invalid value "2001:db8::1234567" (does not satisfy net.IPv6):
Expand Down Expand Up @@ -64,6 +77,18 @@ t20: error in call to net.IPCIDR: netip.ParsePrefix("172.16.12.3"): no '/':
t27: invalid value "23.23.23.23" (does not satisfy net.IPv6):
./in.cue:29:6
./in.cue:29:19
t39: error in call to net.AddIP: invalid IP "invalid ip":
./in.cue:41:6
t40: error in call to net.AddIP: IP address arithmetic resulted in out-of-range address (underflow):
./in.cue:42:6
t41: error in call to net.AddIP: IP address arithmetic resulted in out-of-range address (overflow):
./in.cue:43:6
t45: error in call to net.AddIPCIDR: netip.ParsePrefix("192.168.0.0"): no '/':
./in.cue:47:6
t46: error in call to net.AddIPCIDR: IP address arithmetic resulted in out-of-range address (underflow):
./in.cue:48:6
t47: error in call to net.AddIPCIDR: IP address arithmetic resulted in out-of-range address (overflow):
./in.cue:49:6

Result:
t1: "foo.bar."
Expand Down Expand Up @@ -100,3 +125,16 @@ t31: "https://foo.com/bar"
t32: _|_ // t32: invalid value "/foo/bar" (does not satisfy net.AbsURL): t32: error in call to net.AbsURL: URL is not absolute
t33: "https://foo.com/bar"
t34: _|_ // t34: invalid value "%" (does not satisfy net.AbsURL): t34: error in call to net.AbsURL: parse "%": invalid URL escape "%"
t35: "127.0.0.2"
t36: "127.0.0.3/8"
t37: "2001:db8::1"
t38: "2001:db8::2/64"
t39: _|_ // t39: error in call to net.AddIP: invalid IP "invalid ip"
t40: _|_ // t40: error in call to net.AddIP: IP address arithmetic resulted in out-of-range address (underflow)
t41: _|_ // t41: error in call to net.AddIP: IP address arithmetic resulted in out-of-range address (overflow)
t42: "::ffff:127.0.0.2"
t43: "192.168.2.0/23"
t44: "2001:db8:1111:2223::/64"
t45: _|_ // t45: error in call to net.AddIPCIDR: netip.ParsePrefix("192.168.0.0"): no '/'
t46: _|_ // t46: error in call to net.AddIPCIDR: IP address arithmetic resulted in out-of-range address (underflow)
t47: _|_ // t47: error in call to net.AddIPCIDR: IP address arithmetic resulted in out-of-range address (overflow)