On Sat, Jan 10, 2026 at 05:59:36PM -0500, Jon Maloy wrote: > Extend the -a/--address option to accept addresses in CIDR notation > (e.g., 192.168.1.1/24 or 2001:db8::1/64) as an alternative to using > separate -a and -n options. > > We add a new conf_addr_prefix_len() helper function that: > - Parses address strings with optional /prefix_len suffix > - Validates prefix length based on address family (0-32 for IPv4, > 0-128 for IPv6), including handling of IPv4-to-IPv6 mapping case. > - Returns address family via union inany_addr output parameter > > For IPv4, the prefix length is stored in ip4.prefix_len when provided. > Mixing -n and CIDR notation results in an error to catch likely user > mistakes. > > Also fix a bug in conf_ip4_prefix() that was incorrectly using the > global 'optarg' instead of its 'arg' parameter. > > Signed-off-by: Jon Maloy > > --- > v3: Fixes after feedback from Laurent, David and Stefano > Notably, updated man page for the -a option > > v4: Fixes based on feedback from David G: > - Handling prefix length adjustment when IPv4-to-IPv6 mapping > - Removed redundant !IN6_IS_ADDR_V4MAPPED(&addr.a6) test > - Simplified tests of acceptable address types > - Merged documentation and code commits > - Some documentation text clarifications > --- > conf.c | 91 +++++++++++++++++++++++++++++++++++++++++++++++---------- > inany.c | 29 ++++++++++++++++++ > inany.h | 1 + > passt.1 | 17 +++++++---- > 4 files changed, 118 insertions(+), 20 deletions(-) > > diff --git a/conf.c b/conf.c > index 2942c8c..b36abf3 100644 > --- a/conf.c > +++ b/conf.c > @@ -682,7 +682,7 @@ static int conf_ip4_prefix(const char *arg) > return -1; > } else { > errno = 0; > - len = strtoul(optarg, NULL, 0); > + len = strtoul(arg, NULL, 0); > if (len > 32 || errno) > return -1; > } > @@ -690,6 +690,50 @@ static int conf_ip4_prefix(const char *arg) > return len; > } > > +/** > + * conf_addr_prefix_len() - Parse address with optional prefix length > + * @arg: Address string, optionally with /prefix_len suffix (modified) > + * @addr: Output for parsed address > + * @prefix_len: Output for prefix length (0 if not specified) > + * > + * Return: AF_INET for IPv4, AF_INET6 for IPv6, -1 on error > + */ > +static int conf_addr_prefix_len(char *arg, union inany_addr *addr, > + int *prefix_len) > +{ > + char *slash; > + > + *prefix_len = 0; > + > + /* Check for /prefix_len suffix */ > + slash = strchr(arg, '/'); > + if (slash) { > + unsigned long len; > + char *end; > + > + *slash = '\0'; > + errno = 0; > + len = strtoul(slash + 1, &end, 10); > + if (errno || *end) > + return -1; > + > + *prefix_len = len; > + } > + > + if (!inany_prefix_pton(arg, addr, prefix_len)) > + return -1; The handling of prefix lengths is correct now, but in a really confusing way. This function now returns addresses in a uniform way (essentially as IPv6), but the prefix has a different meaning depending on what type of address it is internally. We parse the prefix length here but the actualy address elsewhere. I think what would make more sense would be to make inany_prefix_pton() parse a full CIDR address itself, with prefix_length becoming a pure output parameter. prefix_length should always be returned in IPv6 form - so add 96 if the input is an IPv4 address. Yes, that means re-correcting it when going from the v4-mapped inany address to the v4-only fields in ctx.ip4, but I think that's preferable to having non-consistent semantics in the interface of a single function. > + > + if (inany_v4(addr)) { > + if (*prefix_len > 32) > + return -1; > + return AF_INET; > + } > + > + if (*prefix_len > 128) > + return -1; > + return AF_INET6; > +} > + > /** > * conf_ip4() - Verify or detect IPv4 support, get relevant addresses > * @ifi: Host interface to attempt (0 to determine one) > @@ -896,7 +940,7 @@ static void usage(const char *name, FILE *f, int status) > " a zero value disables assignment\n" > " default: 65520: maximum 802.3 MTU minus 802.3 header\n" > " length, rounded to 32 bits (IPv4 words)\n" > - " -a, --address ADDR Assign IPv4 or IPv6 address ADDR\n" > + " -a, --address ADDR Assign IPv4 or IPv6 address ADDR[/PREFIXLEN]\n" > " can be specified zero to two times (for IPv4 and IPv6)\n" > " default: use addresses from interface with default route\n" > " -n, --netmask MASK Assign IPv4 MASK, dot-decimal or bits\n" > @@ -1499,6 +1543,7 @@ void conf(struct ctx *c, int argc, char **argv) > const char *logname = (c->mode == MODE_PASTA) ? "pasta" : "passt"; > char userns[PATH_MAX] = { 0 }, netns[PATH_MAX] = { 0 }; > bool copy_addrs_opt = false, copy_routes_opt = false; > + bool prefix_from_cidr = false, prefix_from_opt = false; > enum fwd_ports_mode fwd_default = FWD_NONE; > bool v4_only = false, v6_only = false; > unsigned dns4_idx = 0, dns6_idx = 0; > @@ -1808,23 +1853,36 @@ void conf(struct ctx *c, int argc, char **argv) > c->mtu = mtu; > break; > } > - case 'a': > - if (inet_pton(AF_INET6, optarg, &c->ip6.addr) && > - !IN6_IS_ADDR_UNSPECIFIED(&c->ip6.addr) && > - !IN6_IS_ADDR_LOOPBACK(&c->ip6.addr) && > - !IN6_IS_ADDR_V4MAPPED(&c->ip6.addr) && > - !IN6_IS_ADDR_V4COMPAT(&c->ip6.addr) && > - !IN6_IS_ADDR_MULTICAST(&c->ip6.addr)) { > + case 'a': { > + union inany_addr addr; > + const struct in_addr *a4; > + int prefix_len = 0; > + int af; > + > + af = conf_addr_prefix_len(optarg, &addr, &prefix_len); > + > + if (inany_is_unspecified(&addr) || > + inany_is_multicast(&addr) || > + inany_is_loopback(&addr) || > + IN6_IS_ADDR_V4COMPAT(&addr.a6)) > + die("Invalid address: %s", optarg); > + > + if (af == AF_INET6) { > + c->ip6.addr = addr.a6; > if (c->mode == MODE_PASTA) > c->ip6.no_copy_addrs = true; Is there a reason not to do the prefix_from_cidr / prefix_from_out checks in the IPv6 case? > break; > } > > - if (inet_pton(AF_INET, optarg, &c->ip4.addr) && > - !IN4_IS_ADDR_UNSPECIFIED(&c->ip4.addr) && > - !IN4_IS_ADDR_BROADCAST(&c->ip4.addr) && > - !IN4_IS_ADDR_LOOPBACK(&c->ip4.addr) && > - !IN4_IS_ADDR_MULTICAST(&c->ip4.addr)) { > + a4 = inany_v4(&addr); > + if (af == AF_INET && a4) { > + c->ip4.addr = *a4; > + if (prefix_len) { > + if (prefix_from_opt) > + die("Can't mix CIDR with -n"); > + c->ip4.prefix_len = prefix_len; > + prefix_from_cidr = true; > + } > if (c->mode == MODE_PASTA) > c->ip4.no_copy_addrs = true; > break; > @@ -1832,11 +1890,14 @@ void conf(struct ctx *c, int argc, char **argv) > > die("Invalid address: %s", optarg); > break; > + } > case 'n': > + if (prefix_from_cidr) > + die("Can't use both -n and CIDR prefix length"); > c->ip4.prefix_len = conf_ip4_prefix(optarg); > if (c->ip4.prefix_len < 0) > die("Invalid netmask: %s", optarg); > - > + prefix_from_opt = true; > break; > case 'M': > parse_mac(c->our_tap_mac, optarg); > diff --git a/inany.c b/inany.c > index 7680439..f142a76 100644 > --- a/inany.c > +++ b/inany.c > @@ -57,3 +57,32 @@ int inany_pton(const char *src, union inany_addr *dst) > > return 0; > } > + > +/** inany_prefix_pton - Parse an IPv[46] address with prefix length adjustment > + * @src: IPv[46] address string > + * @dst: output buffer, filled with parsed address > + * @prefix_len: pointer to prefix length > + * > + * Return: on success, 1, if no parseable address is found, 0 > + */ > +int inany_prefix_pton(const char *src, union inany_addr *dst, int *prefix_len) > +{ > + /* First try parsing as plain IPv4 */ > + if (inet_pton(AF_INET, src, &dst->v4mapped.a4)) { > + memset(&dst->v4mapped.zero, 0, sizeof(dst->v4mapped.zero)); > + memset(&dst->v4mapped.one, 0xff, sizeof(dst->v4mapped.one)); > + return 1; > + } > + > + /* Try parsing as IPv6, adjust prefix length if mapped IPv4 address */ > + if (inet_pton(AF_INET6, src, &dst->a6)) { > + if (inany_v4(dst) && prefix_len && *prefix_len > 0) { > + if (*prefix_len < 96) > + return 0; > + *prefix_len -= 96; > + } > + return 1; > + } > + > + return 0; > +} > diff --git a/inany.h b/inany.h > index 61b36fb..36865f9 100644 > --- a/inany.h > +++ b/inany.h > @@ -295,5 +295,6 @@ static inline void inany_siphash_feed(struct siphash_state *state, > > const char *inany_ntop(const union inany_addr *src, char *dst, socklen_t size); > int inany_pton(const char *src, union inany_addr *dst); > +int inany_prefix_pton(const char *src, union inany_addr *dst, int *prefix_len); > > #endif /* INANY_H */ > diff --git a/passt.1 b/passt.1 > index db0d662..7ca03be 100644 > --- a/passt.1 > +++ b/passt.1 > @@ -156,10 +156,14 @@ 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). > > .TP > -.BR \-a ", " \-\-address " " \fIaddr > +.BR \-a ", " \-\-address " " \fIaddr\fR[\fB/\fR\fIprefix_len\fR] > Assign IPv4 \fIaddr\fR via DHCP (\fByiaddr\fR), or \fIaddr\fR via DHCPv6 (option > 5) and an \fIaddr\fR-based prefix via NDP Router Advertisement (option type 3) > for an IPv6 \fIaddr\fR. > +An optional \fB/\fR\fIprefix_len\fR (0-32 for IPv4, 0-128 for IPv6) can be > +appended in CIDR notation (e.g., 192.168.1.1/24). This is an alternative to > +using the \fB-n\fR, \fB--netmask\fR option. Mixing CIDR notation with > +\fB-n\fR results in an error. > 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 > @@ -172,10 +176,13 @@ is assigned for IPv4, and no additional address will be assigned for IPv6. > .TP > .BR \-n ", " \-\-netmask " " \fImask > Assign IPv4 netmask \fImask\fR, 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). > +DHCP (option 1). Alternatively, the prefix length can be specified using CIDR > +notation with the \fB-a\fR, \fB--address\fR option (e.g., \fB-a\fR 192.168.1.1/24). > +Mixing \fB-n\fR with CIDR notation results in an error. > +If no address is indicated, the netmask associated with the adopted host address, > +if any, is used. If an address is indicated, but without a prefix length, the > +netmask is determined based on the corresponding network class. In all other > +cases, the netmask is determined by using the indicated prefix length. > > .TP > .BR \-M ", " \-\-mac-addr " " \fIaddr > -- > 2.52.0 > -- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson